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Ivanov D, De Martino F, Formisano E, Fritz FJ, Goebel R, Huber L, Kashyap S, Kemper VG, Kurban D, Roebroeck A, Sengupta S, Sorger B, Tse DHY, Uludağ K, Wiggins CJ, Poser BA. Magnetic resonance imaging at 9.4 T: the Maastricht journey. MAGMA (NEW YORK, N.Y.) 2023; 36:159-173. [PMID: 37081247 PMCID: PMC10140139 DOI: 10.1007/s10334-023-01080-4] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2022] [Revised: 03/18/2023] [Accepted: 03/20/2023] [Indexed: 04/22/2023]
Abstract
The 9.4 T scanner in Maastricht is a whole-body magnet with head gradients and parallel RF transmit capability. At the time of the design, it was conceptualized to be one of the best fMRI scanners in the world, but it has also been used for anatomical and diffusion imaging. 9.4 T offers increases in sensitivity and contrast, but the technical ultra-high field (UHF) challenges, such as field inhomogeneities and constraints set by RF power deposition, are exacerbated compared to 7 T. This article reviews some of the 9.4 T work done in Maastricht. Functional imaging experiments included blood oxygenation level-dependent (BOLD) and blood-volume weighted (VASO) fMRI using different readouts. BOLD benefits from shorter T2* at 9.4 T while VASO from longer T1. We show examples of both ex vivo and in vivo anatomical imaging. For many applications, pTx and optimized coils are essential to harness the full potential of 9.4 T. Our experience shows that, while considerable effort was required compared to our 7 T scanner, we could obtain high-quality anatomical and functional data, which illustrates the potential of MR acquisitions at even higher field strengths. The practical challenges of working with a relatively unique system are also discussed.
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Affiliation(s)
- Dimo Ivanov
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands.
| | - Federico De Martino
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Elia Formisano
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Francisco J Fritz
- Institute of Systems Neuroscience, Center for Experimental Medicine, University Medical Center Hamburg-Eppendorf (UKE), Hamburg, Germany
| | - Rainer Goebel
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Laurentius Huber
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Sriranga Kashyap
- Krembil Brain Institute, University Health Network, Toronto, ON, Canada
| | - Valentin G Kemper
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Denizhan Kurban
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Alard Roebroeck
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | | | - Bettina Sorger
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Desmond H Y Tse
- Scannexus BV, Oxfordlaan 55, 6229 EV, Maastricht, The Netherlands
| | - Kâmil Uludağ
- Krembil Brain Institute, Koerner Scientist in MR Imaging, University Health Network Toronto, Toronto, ON, Canada
- Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada
- Center for Neuroscience Imaging Research, Institute for Basic Science and Department of Biomedical Engineering, Sungkyunkwan University, Suwon, Republic of Korea
| | - Christopher J Wiggins
- Imaging Core Facility (INM-ICF), Institut für Neurowissenschaften und Medizin, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany
| | - Benedikt A Poser
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
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Seo JH, Jo YS, Oh CH, Chung JY. A New Combination of Radio-Frequency Coil Configurations Using High-Permittivity Materials and Inductively Coupled Structures for Ultrahigh-Field Magnetic Resonance Imaging. SENSORS (BASEL, SWITZERLAND) 2022; 22:8968. [PMID: 36433565 PMCID: PMC9694602 DOI: 10.3390/s22228968] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/28/2022] [Revised: 11/03/2022] [Accepted: 11/17/2022] [Indexed: 06/16/2023]
Abstract
In ultrahigh-field (UHF) magnetic resonance imaging (MRI) system, the RF power required to excite the nuclei of the target object increases. As the strength of the main magnetic field (B0 field) increases, the improvement of the RF transmit field (B1+ field) efficiency and receive field (B1- field) sensitivity of radio-frequency (RF) coils is essential to reduce their specific absorption rate and power deposition in UHF MRI. To address these problems, we previously proposed a method to simultaneously improve the B1+ field efficiency and B1- field sensitivity of 16-leg bandpass birdcage RF coils (BP-BC RF coils) by combining a multichannel wireless RF element (MCWE) and segmented cylindrical high-permittivity material (scHPM) comprising 16 elements in 7.0 T MRI. In this work, we further improved the performance of transmit/receive RF coils. A new combination of RF coil with wireless element and HPM was proposed by comparing the BP-BC RF coil with the MCWE and the scHPM proposed in the previous study and the multichannel RF coils with a birdcage RF coil-type wireless element (BCWE) and the scHPM proposed in this study. The proposed 16-ch RF coils with the BCWE and scHPM provided excellent B1+ field efficiency and B1- field sensitivity improvement.
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Affiliation(s)
- Jeung-Hoon Seo
- Neuroscience Research Institute, Gachon University, Incheon 21988, Republic of Korea
| | - Young-Seung Jo
- Neuroscience Research Institute, Gachon University, Incheon 21988, Republic of Korea
- Department of Electronics and Information Engineering, Korea University, Sejong 30019, Republic of Korea
| | - Chang-Hyun Oh
- Department of Electronics and Information Engineering, Korea University, Sejong 30019, Republic of Korea
| | - Jun-Young Chung
- Department of Neuroscience, College of Medicine, Gachon University, Incheon 21565, Republic of Korea
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3
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Lu M, Sengupta S, Gore JC, Grissom WA, Yan X. High-Density MRI RF Arrays Using Mixed Dipole Antennas and Microstrip Transmission Line Resonators. IEEE Trans Biomed Eng 2022; 69:3243-3252. [PMID: 35404807 PMCID: PMC9587496 DOI: 10.1109/tbme.2022.3166279] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
OBJECTIVE High-density multi-coil arrays are desirable in MRI because they provide high signal-to-noise ratios (SNR), enable highly accelerated parallel imaging, and provide more uniform transmit fields at high fields. For high-density arrays such as a head array with 16 elements in a row, popular dipole antennas and microstrip transmission line (also referred to as "MTL") resonators both have severe coupling issues. METHODS In this work, we show that dipoles and MTLs have naturally low coupling and propose a novel array configuration in which they are interleaved. We first show the electromagnetic (EM) coupling between a single dipole and a single MTL across different separations in bench tests. Then we validate and analyze this through EM simulations. Finally, we construct a 16-channel mixed dipole and MTL array and evaluate its performance on the bench and through MRI experiments. RESULTS Without any decoupling treatments, the worst coupling between a dipole and an MTL was only -15.8 dB when their center-to-center distance was 4.7 cm (versus -5.4 dB for two dipole antennas and -6.0 dB for two MTL resonators). Even in a dense 16-channel mixed array, the inter-element isolation among all elements was better than -14 dB. CONCLUSION This study reveals, analyzes, and validates a novel finding that the popular dipole antennas and MTL resonators used in ultrahigh field MRI have naturally low coupling. SIGNIFICANCE These findings will simplify the construction of high-density arrays, enable new applications, and benefit imaging performance in ultrahigh field MRI.
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4
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Vaidya MV, Zhang B, Hong D, Brown R, Batsios G, Viswanath P, Paska J, Wulf G, Grant AK, Ronen SM, Larson PEZ. A 13C/ 31P surface coil to visualize metabolism and energetics in the rodent brain at 3 Tesla. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2022; 343:107286. [PMID: 36075133 PMCID: PMC9721620 DOI: 10.1016/j.jmr.2022.107286] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/20/2022] [Revised: 08/04/2022] [Accepted: 08/24/2022] [Indexed: 06/15/2023]
Abstract
PURPOSE We constructed a 13C/31P surface coil at 3 T for studying cancer metabolism and bioenergetics. In a single scan session, hyperpolarized 13C-pyruvate MRS and 31P MRS was carried out for a healthy rat brain. METHODS All experiments were carried out at 3 Tesla. The multinuclear surface coil was designed as two coplanar loops each tuned to either the 13C or 31P operating frequency with an LCC trap on the 13C loop. A commercial volume proton coil was used for anatomical localization and B0 shimming. Single tuned coils operating at either the 13C or 31P frequency were built to evaluate the relative performance of the multinuclear coil. Coil performance metrics consisted of measuring Q factor ratio, calculating system input power using a single-pulse acquisition, and acquiring SNR and flip angle maps using 2D CSI sequences. To observe in vivo spectra, a bolus of hyperpolarized [1-13C] pyruvate was administered via tail vein. In vivo13C and endogenous 31P spectra were obtained in a single scan session using 1D slice selective acquisitions. RESULTS When compared with single tuned surface coils, the multinuclear coil performance showed a decrease in Q factor ratio, SNR, and transmit efficiency. Flip angle maps showed adequate flip angles within the phantom when the transmit voltage was set using an external phantom. Results show good detection of 13C labeled lactate, alanine, and bicarbonate in addition to ATP from 31P MRS. CONCLUSIONS The coil enables obtaining complementary information within a scan session, thus reducing the number of trials and minimizing biological variability for studies of metabolism and bioenergetics.
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Affiliation(s)
- Manushka V Vaidya
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA.
| | - Bei Zhang
- Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, USA
| | - DongHyun Hong
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA
| | - Ryan Brown
- Center for Advanced Imaging Innovation and Research, and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, USA
| | - Georgios Batsios
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA
| | - Pavithra Viswanath
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA
| | - Jan Paska
- Center for Advanced Imaging Innovation and Research, and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, USA
| | - Gerburg Wulf
- Department of Hematology-Oncology, Beth Israel Deaconess Medical Center, Boston, MA, USA
| | - Aaron K Grant
- Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA, USA
| | - Sabrina M Ronen
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA
| | - Peder E Z Larson
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA
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A Comparative Study of Birdcage RF Coil Configurations for Ultra-High Field Magnetic Resonance Imaging. SENSORS 2022; 22:s22051741. [PMID: 35270889 PMCID: PMC8914904 DOI: 10.3390/s22051741] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 01/04/2022] [Revised: 02/14/2022] [Accepted: 02/21/2022] [Indexed: 12/26/2022]
Abstract
Improvements in transmission and reception sensitivities of radiofrequency (RF) coils used in ultra-high field (UHF) magnetic resonance imaging (MRI) are needed to reduce specific absorption rates (SAR) and RF power deposition, albeit without applying high-power RF. Here, we propose a method to simultaneously improve transmission efficiency and reception sensitivity of a band-pass birdcage RF coil (BP-BC RF coil) by combining a multi-channel wireless RF element (MCWE) with a high permittivity material (HPM) in a 7.0 T MRI. Electromagnetic field (EM-field) simulations, performed using two types of phantoms, viz., a cylindrical phantom filled with oil and a human head model, were used to compare the effects of MCWE and HPM on BP-BC RF coils. EM-fields were calculated using the finite difference time-domain (FDTD) method and analyzed using Matlab software. Next, to improve RF transmission efficiency, we compared two HPM structures, namely, a hollow cylinder shape HPM (hcHPM) and segmented cylinder shape HPM (scHPM). The scHPM and MCWE model comprised 16 elements (16-rad BP-BC RF coil) and this coil configuration demonstrated superior RF transmission efficiency and reception sensitivity along with an acceptable SAR. We expect wider clinical application of this combination in 7.0 T MRIs, which were recently approved by the United States Food and Drug Administration.
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Priovoulos N, Roos T, Ipek Ö, Meliado EF, Nkrumah RO, Klomp DWJ, van der Zwaag W. A local multi-transmit coil combined with a high-density receive array for cerebellar fMRI at 7 T. NMR IN BIOMEDICINE 2021; 34:e4586. [PMID: 34231292 PMCID: PMC8519055 DOI: 10.1002/nbm.4586] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/14/2020] [Revised: 06/09/2021] [Accepted: 06/22/2021] [Indexed: 06/13/2023]
Abstract
The human cerebellum is involved in a wide array of functions, ranging from motor control to cognitive control, and as such is of great neuroscientific interest. However, its function is underexplored in vivo, due to its small size, its dense structure and its placement at the bottom of the brain, where transmit and receive fields are suboptimal. In this study, we combined two dense coil arrays of 16 small surface receive elements each with a transmit array of three antenna elements to improve BOLD sensitivity in the human cerebellum at 7 T. Our results showed improved B1+ and SNR close to the surface as well as g-factor gains compared with a commercial coil designed for whole-head imaging. This resulted in improved signal stability and large gains in the spatial extent of the activation close to the surface (<3.5 cm), while good performance was retained deeper in the cerebellum. Modulating the phase of the transmit elements of the head coil to constructively interfere in the cerebellum improved the B1+ , resulting in a temporal SNR gain. Overall, our results show that a dedicated transmit array along with the SNR gains of surface coil arrays can improve cerebellar imaging, at the cost of a decreased field of view and increased signal inhomogeneity.
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Affiliation(s)
- Nikos Priovoulos
- Spinoza Center for NeuroimagingRoyal Netherlands Academy of Arts and Sciences (KNAW)AmsterdamThe Netherlands
| | - Thomas Roos
- Spinoza Center for NeuroimagingRoyal Netherlands Academy of Arts and Sciences (KNAW)AmsterdamThe Netherlands
| | - Özlem Ipek
- Department of Biomedical Engineering, School of Biomedical Engineering & Imaging SciencesKing's College LondonLondonUK
| | - Ettore F. Meliado
- Image Sciences InstituteUniversity Medical Center UtrechtUtrechtNetherlands
| | - Richard O. Nkrumah
- Department of Biomedical Engineering, School of Biomedical Engineering & Imaging SciencesKing's College LondonLondonUK
| | - Dennis W. J. Klomp
- Image Sciences InstituteUniversity Medical Center UtrechtUtrechtNetherlands
| | - Wietske van der Zwaag
- Spinoza Center for NeuroimagingRoyal Netherlands Academy of Arts and Sciences (KNAW)AmsterdamThe Netherlands
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7
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Delgado PR, Kuehne A, Periquito JS, Millward JM, Pohlmann A, Waiczies S, Niendorf T. B 1 inhomogeneity correction of RARE MRI with transceive surface radiofrequency probes. Magn Reson Med 2020; 84:2684-2701. [PMID: 32447779 DOI: 10.1002/mrm.28307] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2020] [Revised: 03/27/2020] [Accepted: 04/13/2020] [Indexed: 12/17/2022]
Abstract
PURPOSE The use of surface radiofrequency (RF) coils is common practice to boost sensitivity in (pre)clinical MRI. The number of transceive surface RF coils is rapidly growing due to the surge in cryogenically cooled RF technology and ultrahigh-field MRI. Consequently, there is an increasing need for effective correction of the excitation field ( B 1 + ) inhomogeneity inherent in these coils. Retrospective B1 correction permits quantitative MRI, but this usually requires a pulse sequence-specific analytical signal intensity (SI) equation. Such an equation is not available for fast spin-echo (Rapid Acquisition with Relaxation Enhancement, RARE) MRI. Here we present, test, and validate retrospective B1 correction methods for RARE. METHODS We implemented the commonly used sensitivity correction and developed an empirical model-based method and a hybrid combination of both. Tests and validations were performed with a cryogenically cooled RF probe and a single-loop RF coil. Accuracy of SI quantification and T1 contrast were evaluated after correction. RESULTS The three described correction methods achieved dramatic improvements in B1 homogeneity and significantly improved SI quantification and T1 contrast, with mean SI errors reduced from >40% to >10% following correction in all cases. Upon correction, images of phantoms and mouse heads demonstrated homogeneity comparable to that of images acquired with a volume resonator. This was quantified by SI profile, SI ratio (error < 10%), and percentage of integral uniformity (PIU > 80% in vivo and ex vivo compared to PIU > 87% with the reference RF coil). CONCLUSION This work demonstrates the efficacy of three B1 correction methods tailored for transceive surface RF probes and RARE MRI. The corrected images are suitable for quantification and show comparable results between the three methods, opening the way for T1 measurements and X-nuclei quantification using surface transceiver RF coils. This approach is applicable to other MR techniques for which no analytical SI exists.
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Affiliation(s)
- Paula Ramos Delgado
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.,Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
| | | | - João S Periquito
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.,Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
| | - Jason M Millward
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
| | - Andreas Pohlmann
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
| | - Sonia Waiczies
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany
| | - Thoralf Niendorf
- Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.,Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.,MRI.TOOLS GmbH, Berlin, Germany
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Hoff MN, McKinney A, Shellock FG, Rassner U, Gilk T, Watson RE, Greenberg TD, Froelich J, Kanal E. Safety Considerations of 7-T MRI in Clinical Practice. Radiology 2019; 292:509-518. [PMID: 31310177 DOI: 10.1148/radiol.2019182742] [Citation(s) in RCA: 49] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Although 7-T MRI has recently received approval for use in clinical patient care, there are distinct safety issues associated with this relatively high magnetic field. Forces on metallic implants and radiofrequency power deposition and heating are safety considerations at 7 T. Patient bioeffects such as vertigo, dizziness, false feelings of motion, nausea, nystagmus, magnetophosphenes, and electrogustatory effects are more common and potentially more pronounced at 7 T than at lower field strengths. Herein the authors review safety issues associated with 7-T MRI. The rationale for safety concerns at this field strength are discussed as well as potential approaches to mitigate risk to patients and health care professionals.
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Affiliation(s)
- Michael N Hoff
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Alexander McKinney
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Frank G Shellock
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Ulrich Rassner
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Tobias Gilk
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Robert E Watson
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Todd D Greenberg
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Jerry Froelich
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
| | - Emanuel Kanal
- From the Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7117 (M.N.H.); Department of Radiology, University of Minnesota, Minneapolis, Minn (A.M., J.F.); Department of Clinical Physical Therapy, University of Southern California, Los Angeles, Calif (F.G.S.); Department of Radiology, University of Utah Health Sciences Center, Salt Lake City, Utah (U.R.); RADIOLOGY-Planning, Kansas City, Mo (T.G.); Department of Radiology, Mayo Clinic, Rochester, Minn (R.E.W.); G3 Global Group, Boulder, Colo, Mo (T.D.G.); and Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pa (E.K.)
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9
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Ianniello C, Madelin G, Moy L, Brown R. A dual-tuned multichannel bilateral RF coil for 1 H/ 23 Na breast MRI at 7 T. Magn Reson Med 2019; 82:1566-1575. [PMID: 31148249 DOI: 10.1002/mrm.27829] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2019] [Revised: 05/06/2019] [Accepted: 05/07/2019] [Indexed: 12/25/2022]
Abstract
PURPOSE Sodium MRI has shown promise for monitoring neoadjuvant chemotherapy response in breast cancer. The purpose of this work was to build a dual-tuned bilateral proton/sodium breast coil for 7T MRI that provides sufficient SNR to enable sodium breast imaging in less than 10 minutes. METHODS The proton/sodium coil consists of 2 shielded unilateral units: 1 for each breast. Each unit consists of 3 nested layers: (1) a 3-loop solenoid for sodium excitation, (2) a 3-loop solenoid for proton excitation and signal reception, and (3) a 4-channel receive array for sodium signal reception. Benchmark measurements were performed in phantoms with and without the sodium receive array insert. In vivo images were acquired on a healthy volunteer. RESULTS The sodium receive array boosted 1.5 to 3 times the SNR compared with the solenoid. Proton SNR loss due to residual interaction with the sodium array was less than 10%. The coil enabled sodium imaging in vivo with 2.8-mm isotropic nominal resolution (~5-mm real resolution) in 9:36 minutes. CONCLUSION The coil design that we propose addresses challenges associated with sodium's low SNR from a hardware perspective and offers the opportunity to investigate noninvasively breast tumor metabolism as a function of sodium concentration in patients undergoing neoadjuvant chemotherapy.
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Affiliation(s)
- Carlotta Ianniello
- Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Science, New York University School of Medicine, New York, New York
| | - Guillaume Madelin
- Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Science, New York University School of Medicine, New York, New York
| | - Linda Moy
- Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Science, New York University School of Medicine, New York, New York
| | - Ryan Brown
- Center for Advanced Imaging Innovation and Research (CAI2R) and Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Science, New York University School of Medicine, New York, New York
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10
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Vergara Gomez TS, Dubois M, Glybovski S, Larrat B, de Rosny J, Rockstuhl C, Bernard M, Abdeddaim R, Enoch S, Kober F. Wireless coils based on resonant and nonresonant coupled-wire structure for small animal multinuclear imaging. NMR IN BIOMEDICINE 2019; 32:e4079. [PMID: 30773725 PMCID: PMC6594360 DOI: 10.1002/nbm.4079] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/16/2018] [Revised: 01/11/2019] [Accepted: 01/15/2019] [Indexed: 06/09/2023]
Abstract
Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as whole-body imaging and dual-frequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupled-wire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupled-wire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for 1 H and 282.6 MHz for 19 F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signal-to-noise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance.
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Affiliation(s)
- Tania S. Vergara Gomez
- Aix Marseille Univ, CNRS, Centrale Marseille, Institut FresnelMarseilleFrance
- Aix Marseille Univ, CNRS, CRMBMMarseilleFrance
| | - Marc Dubois
- Aix Marseille Univ, CNRS, Centrale Marseille, Institut FresnelMarseilleFrance
| | - Stanislav Glybovski
- Department of Nanophotonics and MetamaterialsITMO UniversitySt. PetersburgRussia
| | - Benoit Larrat
- Commissariat à l'Energie Atomique et aux Energies Alternatives, Direction de la recherche Fondamentale, NeuroSpinUniversité Paris SaclayGif‐sur‐YvetteFrance
| | - Julien de Rosny
- ESPCI Paris, PSL Research University, CNRS, Institut LangevinParisFrance
| | - Carsten Rockstuhl
- Institute of Theoretical Solid State PhysicsKarlsruhe Institute of TechnologyKarlsruheGermany
- Institute of NanotechnologyKarlsruhe Institute of TechnologyKarlsruheGermany
| | | | - Redha Abdeddaim
- Aix Marseille Univ, CNRS, Centrale Marseille, Institut FresnelMarseilleFrance
| | - Stefan Enoch
- Aix Marseille Univ, CNRS, Centrale Marseille, Institut FresnelMarseilleFrance
| | - Frank Kober
- Aix Marseille Univ, CNRS, CRMBMMarseilleFrance
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11
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Vaidya MV, Sodickson DK, Collins CM, Lattanzi R. Disentangling the effects of high permittivity materials on signal optimization and sample noise reduction via ideal current patterns. Magn Reson Med 2018; 81:2746-2758. [PMID: 30426554 DOI: 10.1002/mrm.27554] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2018] [Revised: 08/14/2018] [Accepted: 09/06/2018] [Indexed: 11/06/2022]
Abstract
PURPOSE To investigate how high-permittivity materials (HPMs) can improve SNR when placed between MR detectors and the imaged body. METHODS We used a simulation framework based on dyadic Green's functions to calculate the electromagnetic field inside a uniform dielectric sphere at 7 Tesla, with and without a surrounding layer of HPM. SNR-optimizing (ideal) current patterns were expressed as the sum of signal-optimizing (signal-only) current patterns and dark mode current patterns that minimize sample noise while contributing nothing to signal. We investigated how HPM affects the shape and amplitude of these current patterns, sample noise, and array SNR. RESULTS Ideal and signal-only current patterns were identical for a central voxel. HPMs introduced a phase shift into these patterns, compensating for signal propagation delay in the HPMs. For an intermediate location within the sphere, dark mode current patterns were present and illustrated the mechanisms by which HPMs can reduce sample noise. High-amplitude signal-only current patterns were observed for HPM configurations that shield the electromagnetic field from the sample. For coil arrays, these configurations corresponded to poor SNR in deep regions but resulted in large SNR gains near the surface due to enhanced fields in the vicinity of the HPM. For very high relative permittivity values, HPM thicknesses corresponding to even multiples of λ/4 resulted in coil SNR gains throughout the sample. CONCLUSION HPMs affect both signal sensitivity and sample noise. Lower amplitude signal-only optimal currents corresponded to higher array SNR performance and could guide the design of coils integrated with HPM.
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Affiliation(s)
- Manushka V Vaidya
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York
| | - Daniel K Sodickson
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York
| | - Christopher M Collins
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York
| | - Riccardo Lattanzi
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York
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12
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Budinger TF, Bird MD. MRI and MRS of the human brain at magnetic fields of 14 T to 20 T: Technical feasibility, safety, and neuroscience horizons. Neuroimage 2018; 168:509-531. [DOI: 10.1016/j.neuroimage.2017.01.067] [Citation(s) in RCA: 52] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2016] [Revised: 01/23/2017] [Accepted: 01/27/2017] [Indexed: 11/16/2022] Open
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13
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Vaidya MV, Lazar M, Deniz CM, Haemer GG, Chen G, Bruno M, Sodickson DK, Lattanzi R, Collins CM. Improved detection of fMRI activation in the cerebellum at 7T with dielectric pads extending the imaging region of a commercial head coil. J Magn Reson Imaging 2018; 48:431-440. [PMID: 29357200 DOI: 10.1002/jmri.25936] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2017] [Accepted: 12/09/2017] [Indexed: 12/12/2022] Open
Abstract
BACKGROUND There is growing interest in detecting cerebro-cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal-to-noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum. PURPOSE To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil. STUDY TYPE Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI. SUBJECTS Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM. FIELD STRENGTH/SEQUENCE Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1 -weighted MPRAGE were acquired with and without HPM pads at 7T. ASSESSMENT Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads. STATISTICAL TESTS: fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t-test of acquisitions with and without HPM. RESULTS Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1 -weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group-level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere. DATA CONCLUSION HPM pads can improve cerebellar fMRI at 7T with a commonly-used head coil without compromising RF safety. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2018;48:431-440.
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Affiliation(s)
- Manushka V Vaidya
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Mariana Lazar
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Cem M Deniz
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA
| | - Gillian G Haemer
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Gang Chen
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Mary Bruno
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA
| | - Daniel K Sodickson
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Riccardo Lattanzi
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
| | - Christopher M Collins
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA
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14
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Kalidasan V, Liu XL, Li Y, Sugumaran PJ, Liu AH, Ren L, Ding J. Examining the effect of ions and proteins on the heat dissipation of iron oxide nanocrystals. RSC Adv 2018; 8:1443-1450. [PMID: 35540917 PMCID: PMC9077098 DOI: 10.1039/c7ra11472a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2017] [Accepted: 11/29/2017] [Indexed: 01/05/2023] Open
Abstract
In this paper, the effect and contribution of physiological components like ions and proteins under an applied alternating magnetic field (AMF) towards heat dissipation of superparamagnetic iron oxide nanoparticles (SPIONs) are discussed. Our results have shown that under an applied AMF, magnetic hyperthermia efficiency could be significantly enhanced if SPIONs were suspended in 1× phosphate buffered saline (PBS) compared to a suspension in de-ionized (DI) water. However, no heat enhancement was found when SPIONs were suspended in blood which is an amalgamation of physiological ions and proteins. Closer investigations have revealed that the presence of physiological ions can contribute positively to heating efficiency, and the heating efficiency increases with concentration of ions, ionic mass and solubility. However, the heating efficiency of ions can be suppressed to an insignificant level (comparable with measurement error), in the presence of physiological proteins in 1×PBS. Our electrochemical studies also showed that ionic mobility can be reduced significantly if proteins were present in the solution, thus retarding the heating efficiency.
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Affiliation(s)
- V Kalidasan
- Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore 7 Engineering Drive 1 117574 Singapore
| | - X L Liu
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology Beijing 100190 People's Republic of China
| | - Y Li
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanoscience and Technology Beijing 100190 People's Republic of China .,Department of Biomaterials, Fujian Provincial Key Laboratory of Fire Retardant Materials, College of Materials, Xiamen University Xiamen 361005 People's Republic of China
| | - P J Sugumaran
- Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore 7 Engineering Drive 1 117574 Singapore
| | - A H Liu
- Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore 7 Engineering Drive 1 117574 Singapore
| | - L Ren
- Department of Biomaterials, Fujian Provincial Key Laboratory of Fire Retardant Materials, College of Materials, Xiamen University Xiamen 361005 People's Republic of China
| | - J Ding
- Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore 7 Engineering Drive 1 117574 Singapore
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15
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Rispoli JV, Wilcox MD, By S, Wright SM, McDougall MP. Effects of coplanar shielding for high field MRI. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2017; 2016:6250-6253. [PMID: 28269680 DOI: 10.1109/embc.2016.7592157] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
This work investigates the efficacy of "coplanar shielding," in which copper shields are oriented concentric and coplanar to the RF coils rather than implemented as a full ground plane behind them. Following FDTD simulations to determine optimal shielding parameters, two coil geometries were constructed: a circular loop surface coil and a half-volume five-element receive array. Each was evaluated using bench measurements with and without coplanar shielding. Imaging, including accelerated SENSE imaging, was performed with the shielded and unshielded receive arrays on a whole-body 7T scanner. Results from modeled and fabricated coils showed good agreement with improvements in Q factors for all cases. Imaging showed substantial improvements in SNR and g-factors for the coplanar shielded array.
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16
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Hsu YC, Lattanzi R, Chu YH, Cloos MA, Sodickson DK, Lin FH. Mitigation of B1+ inhomogeneity using spatially selective excitation with jointly designed quadratic spatial encoding magnetic fields and RF shimming. Magn Reson Med 2017; 78:577-587. [PMID: 27696518 PMCID: PMC5538365 DOI: 10.1002/mrm.26397] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2016] [Revised: 08/05/2016] [Accepted: 08/06/2016] [Indexed: 11/05/2022]
Abstract
PURPOSE The inhomogeneity of flip angle distribution is a major challenge impeding the application of high-field MRI. We report a method combining spatially selective excitation using generalized spatial encoding magnetic fields (SAGS) with radiofrequency (RF) shimming to achieve homogeneous excitation. This method can be an alternative approach to address the challenge of B1+ inhomogeneity using nonlinear gradients. METHODS We proposed a two-step algorithm that jointly optimizes the combination of nonlinear spatial encoding magnetic fields and the combination of multiple RF transmitter coils and then optimizes the locations, RF amplitudes, and phases of the spokes. RESULTS Our results show that jointly designed SAGS and RF shimming can provide a more homogeneous flip angle distribution than using SAGS or RF shimming alone. Compared with RF shimming alone, our approach can reduce the relative standard deviation of flip angle by 56% and 52% using phantom and human head data, respectively. CONCLUSION The jointly designed SAGS and RF shimming method can be used to achieve homogeneous flip angle distributions when fully parallel RF transmission is not available. Magn Reson Med 78:577-587, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Yi-Cheng Hsu
- Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
| | - Riccardo Lattanzi
- Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, 660 1 Ave. New York, NY 10016 USA
| | - Ying-Hua Chu
- Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
| | - Martijn A. Cloos
- Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, 660 1 Ave. New York, NY 10016 USA
| | - Daniel K. Sodickson
- Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, 660 1 Ave. New York, NY 10016 USA
| | - Fa-Hsuan Lin
- Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
- Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Espoo, Finland
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17
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Lattanzi R, Wiggins GC, Zhang B, Duan Q, Brown R, Sodickson DK. Approaching ultimate intrinsic signal-to-noise ratio with loop and dipole antennas. Magn Reson Med 2017; 79:1789-1803. [PMID: 28675512 DOI: 10.1002/mrm.26803] [Citation(s) in RCA: 38] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2016] [Revised: 05/26/2017] [Accepted: 05/27/2017] [Indexed: 11/08/2022]
Abstract
PURPOSE Previous work with body-size objects suggested that loops are optimal MR detectors at low fields, whereas electric dipoles are required to maximize signal-to-noise ratio (SNR) at ultrahigh fields ( ≥ 7 T). Here we investigated how many loops and/or dipoles are needed to approach the ultimate intrinsic SNR (UISNR) at various field strengths. METHODS We calculated the UISNR inside dielectric cylinders mimicking different anatomical regions. We assessed the performance of various arrays with respect to the UISNR. We validated our results by comparing simulated and experimental coil performance maps. RESULTS Arrays with an increasing number of loops can rapidly approach the UISNR at fields up to 3 T, but are suboptimal at ultrahigh fields for body-size objects. The opposite is true for dipole arrays. At 7 T and above, 16 dipoles provide considerably larger central SNR than any possible loop array, and minimal g factor penalty for parallel imaging. CONCLUSIONS Electric dipoles can be advantageous at ultrahigh fields because they can produce both curl-free and divergence-free currents, whereas loops are limited to divergence-free contributions only. Combining loops and dipoles may be optimal for body imaging at 3 T, whereas arrays of loops or dipoles alone may perform better at lower or higher field strengths, respectively. Magn Reson Med 79:1789-1803, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Riccardo Lattanzi
- Center for Advanced Imaging Innovation and Research and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York, USA
| | - Graham C Wiggins
- Center for Advanced Imaging Innovation and Research and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA
| | - Bei Zhang
- Center for Advanced Imaging Innovation and Research and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA
| | - Qi Duan
- Laboratory of Functional and Molecular Imaging, NINDS, National Institutes of Health, Bethesda, Maryland, USA
| | - Ryan Brown
- Center for Advanced Imaging Innovation and Research and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York, USA
| | - Daniel K Sodickson
- Center for Advanced Imaging Innovation and Research and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.,The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.,NYU WIRELESS, New York University Tandon School of Engineering, Brooklyn, New York, USA
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18
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Fiedler TM, Ladd ME, Bitz AK. SAR Simulations & Safety. Neuroimage 2017; 168:33-58. [PMID: 28336426 DOI: 10.1016/j.neuroimage.2017.03.035] [Citation(s) in RCA: 59] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2016] [Revised: 02/28/2017] [Accepted: 03/16/2017] [Indexed: 01/19/2023] Open
Abstract
At ultra-high fields, the assessment of radiofrequency (RF) safety presents several new challenges compared to low-field systems. Multi-channel RF transmit coils in combination with parallel transmit techniques produce time-dependent and spatially varying power loss densities in the tissue. Further, in ultra-high-field systems, localized field effects can be more pronounced due to a transition from the quasi stationary to the electromagnetic field regime. Consequently, local information on the RF field is required for reliable RF safety assessment as well as for monitoring of RF exposure during MR examinations. Numerical RF and thermal simulations for realistic exposure scenarios with anatomical body models are currently the only practical way to obtain the requisite local information on magnetic and electric field distributions as well as tissue temperature. In this article, safety regulations and the fundamental characteristics of RF field distributions in ultra-high-field systems are reviewed. Numerical methods for computation of RF fields as well as typical requirements for the analysis of realistic multi-channel RF exposure scenarios including anatomical body models are highlighted. In recent years, computation of the local tissue temperature has become of increasing interest, since a more accurate safety assessment is expected because temperature is directly related to tissue damage. Regarding thermal simulation, bio-heat transfer models and approaches for taking into account the physiological response of the human body to RF exposure are discussed. In addition, suitable methods are presented to validate calculated RF and thermal results with measurements. Finally, the concept of generalized simulation-based specific absorption rate (SAR) matrix models is discussed. These models can be incorporated into local SAR monitoring in multi-channel MR systems and allow the design of RF pulses under constraints for local SAR.
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Affiliation(s)
- Thomas M Fiedler
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
| | - Mark E Ladd
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Erwin L. Hahn Institute for MRI, University Duisburg-Essen, Essen, Germany
| | - Andreas K Bitz
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Electromagnetic Theory and Applied Mathematics, Faculty of Electrical Engineering and Information Technology, FH Aachen - University of Applied Sciences, 52066 Aachen, Germany
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19
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Lai M, Gruetter R, Lanz B. Progress towards in vivo brain 13C-MRS in mice: Metabolic flux analysis in small tissue volumes. Anal Biochem 2017; 529:229-244. [PMID: 28119064 DOI: 10.1016/j.ab.2017.01.019] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2016] [Revised: 01/19/2017] [Accepted: 01/20/2017] [Indexed: 01/08/2023]
Abstract
The combination of dynamic 13C MRS data under infusion of 13C-labelled substrates and compartmental models of cerebral metabolism enabled in vivo measurement of metabolic fluxes with a quantitative and distinct determination of cellular-specific activities. The non-invasive nature and the chemical specificity of the 13C dynamic data obtained in those tracer experiments makes it an attractive approach offering unique insights into cerebral metabolism. Genetically engineered mice present a wealth of disease models particularly interesting for the neuroscience community. Nevertheless, in vivo13C NMR studies of the mouse brain are only recently appearing in the field due to the numerous challenges linked to the small mouse brain volume and the difficulty to follow the mouse physiological parameters within the NMR system during the infusion experiment. This review will present the progresses in the quest for a higher in vivo13C signal-to-noise ratio up to the present state of the art techniques, which made it feasible to assess glucose metabolism in different regions of the mouse brain. We describe how experimental results were integrated into suitable compartmental models and how a deep understanding of cerebral metabolism depends on the reliable detection of 13C in the different molecules and carbon positions.
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Affiliation(s)
- Marta Lai
- Laboratory for Functional and Metabolic Imaging (LIFMET), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
| | - Rolf Gruetter
- Laboratory for Functional and Metabolic Imaging (LIFMET), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland; Department of Radiology, University of Geneva, 1205 Geneva, Switzerland; Department of Radiology, University of Lausanne, 1015 Lausanne, Switzerland
| | - Bernard Lanz
- Laboratory for Functional and Metabolic Imaging (LIFMET), École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland; Sir Peter Mansfield Imaging Centre, School of Physics and Astronomy, University of Nottingham, Nottingham, United Kingdom
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20
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Sengupta S, Roebroeck A, Kemper VG, Poser BA, Zimmermann J, Goebel R, Adriany G. A Specialized Multi-Transmit Head Coil for High Resolution fMRI of the Human Visual Cortex at 7T. PLoS One 2016; 11:e0165418. [PMID: 27911950 PMCID: PMC5135047 DOI: 10.1371/journal.pone.0165418] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2016] [Accepted: 10/11/2016] [Indexed: 11/19/2022] Open
Abstract
PURPOSE To design, construct and validate radiofrequency (RF) transmit and receive phased array coils for high-resolution visual cortex imaging at 7 Tesla. METHODS A 4 channel transmit and 16 channel receive array was constructed on a conformal polycarbonate former. Transmit field efficiency and homogeneity were simulated and validated, along with the Specific Absorption Rate, using [Formula: see text] mapping techniques and electromagnetic simulations. Receiver signal-to-noise ratio (SNR), temporal SNR (tSNR) across EPI time series, g-factors for accelerated imaging and noise correlations were evaluated and compared with a commercial 32 channel whole head coil. The performance of the coil was further evaluated with human subjects through functional MRI (fMRI) studies at standard and submillimeter resolutions of upto 0.8mm isotropic. RESULTS The transmit and receive sections were characterized using bench tests and showed good interelement decoupling, preamplifier decoupling and sample loading. SNR for the 16 channel coil was ∼ 1.5 times that of the commercial coil in the human occipital lobe, and showed better g-factor values for accelerated imaging. fMRI tests conducted showed better response to Blood Oxygen Level Dependent (BOLD) activation, at resolutions of 1.2mm and 0.8mm isotropic. CONCLUSION The 4 channel phased array transmit coil provides homogeneous excitation across the visual cortex, which, in combination with the dual row 16 channel receive array, makes for a valuable research tool for high resolution anatomical and functional imaging of the visual cortex at 7T.
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Affiliation(s)
- Shubharthi Sengupta
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- Maastricht Brain Imaging Center, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- * E-mail:
| | - Alard Roebroeck
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- Maastricht Brain Imaging Center, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
| | - Valentin G. Kemper
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- Maastricht Brain Imaging Center, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
| | - Benedikt A. Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- Maastricht Brain Imaging Center, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
| | - Jan Zimmermann
- Center for Neural Science, New York University, NY, United States of America
| | - Rainer Goebel
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
- Maastricht Brain Imaging Center, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
| | - Gregor Adriany
- Department of Radiology, Center for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis, MN, United States of America
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21
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Tse DHY, Wiggins CJ, Ivanov D, Brenner D, Hoffmann J, Mirkes C, Shajan G, Scheffler K, Uludağ K, Poser BA. Volumetric imaging with homogenised excitation and static field at 9.4 T. MAGMA (NEW YORK, N.Y.) 2016; 29:333-45. [PMID: 26995492 PMCID: PMC4891373 DOI: 10.1007/s10334-016-0543-6] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/17/2015] [Revised: 02/26/2016] [Accepted: 02/29/2016] [Indexed: 11/29/2022]
Abstract
OBJECTIVES To overcome the challenges of B0 and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B0 and flip-angle homogenisation was implemented on a human 9.4 T scanner. MATERIALS AND METHODS Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B0- and B1-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B0 shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T1- and T 2 (*) -weighted images with MPRAGE and 3D EPI, respectively. RESULTS Over six in vivo acquisitions, the B0-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10 ± 0.01 ppm (39 ± 2 Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode's 30.5 ± 0.9 to 9.2 ± 0.7 % with all B 1 (+) voids eliminated. The SNR inhomogeneities and contrast variations in the T1- and T 2 (*) -weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T1-weighted image. CONCLUSION A 15-minute B0- and flip-angle homogenisation workflow, including the B0- and B1-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images.
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Affiliation(s)
- Desmond H Y Tse
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
| | | | - Dimo Ivanov
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - Daniel Brenner
- German Centre for Neurodegenerative Diseases (DZNE), Bonn, Germany
| | - Jens Hoffmann
- High Field MR Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany
| | - Christian Mirkes
- High Field MR Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany
- Department for Biomedical Magnetic Resonance, University of Tuebingen, Tuebingen, Germany
| | - Gunamony Shajan
- High Field MR Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany
| | - Klaus Scheffler
- High Field MR Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany
- Department for Biomedical Magnetic Resonance, University of Tuebingen, Tuebingen, Germany
| | - Kâmil Uludağ
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
- Scannexus BV, Maastricht, The Netherlands
| | - Benedikt A Poser
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
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22
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Ertürk MA, Raaijmakers AJE, Adriany G, Uğurbil K, Metzger GJ. A 16-channel combined loop-dipole transceiver array for 7 Tesla body MRI. Magn Reson Med 2016; 77:884-894. [PMID: 26887533 DOI: 10.1002/mrm.26153] [Citation(s) in RCA: 111] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2015] [Revised: 12/21/2015] [Accepted: 01/17/2016] [Indexed: 12/17/2022]
Abstract
PURPOSE To develop a 16-channel transceive body imaging array at 7.0 T with improved transmit, receive, and specific absorption rate (SAR) performance by combining both loop and dipole elements and using their respective and complementary near and far field characteristics. METHODS A 16-channel radiofrequency (RF) coil array consisting of eight loop-dipole blocks (16LD) was designed and constructed. Transmit and receive performance was quantitatively investigated in phantom and human model simulations, and experiments on five healthy volunteers inside the prostate. Comparisons were made with 16-channel microstrip line (16ML) and 10-channel fractionated dipole antenna (10DA) arrays. The 16LD was used to acquire anatomic and functional images of the prostate, kidneys, and heart. RESULTS The 16LD provided > 14% improvements in the signal-to-noise ratio (SNR), peak B1+, B1+ transmit, and SAR efficiencies over the 16ML and 10DA in simulations inside the prostate. Experimentally, the 16LD had > 20% higher SNR and B1+ transmit efficiency compared with other arrays, and achieved up to 51.8% higher peak B1+ compared with 10DA. CONCLUSION Combining loop and dipole elements provided a body imaging array with high channel count and density while limiting inter-element coupling. The 16LD improved both near and far-field performance compared with existing 7.0T body arrays and provided high-quality MRI of the prostate kidneys and heart. Magn Reson Med 77:884-894, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- M Arcan Ertürk
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | | | - Gregor Adriany
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Kâmil Uğurbil
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Gregory J Metzger
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
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23
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Vaidya MV, Collins CM, Sodickson DK, Brown R, Wiggins GC, Lattanzi R. Dependence of B1+ and B1- Field Patterns of Surface Coils on the Electrical Properties of the Sample and the MR Operating Frequency. CONCEPTS IN MAGNETIC RESONANCE. PART B, MAGNETIC RESONANCE ENGINEERING 2016; 46:25-40. [PMID: 27795697 PMCID: PMC5082994 DOI: 10.1002/cmr.b.21319] [Citation(s) in RCA: 49] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/19/2023]
Abstract
In high field MRI, the spatial distribution of the radiofrequency magnetic ( B1) field is usually affected by the presence of the sample. For hardware design and to aid interpretation of experimental results, it is important both to anticipate and to accurately simulate the behavior of these fields. Fields generated by a radiofrequency surface coil were simulated using dyadic Green's functions, or experimentally measured over a range of frequencies inside an object whose electrical properties were varied to illustrate a variety of transmit [Formula: see text] and receive [Formula: see text] field patterns. In this work, we examine how changes in polarization of the field and interference of propagating waves in an object can affect the B1 spatial distribution. Results are explained conceptually using Maxwell's equations and intuitive illustrations. We demonstrate that the electrical conductivity alters the spatial distribution of distinct polarized components of the field, causing "twisted" transmit and receive field patterns, and asymmetries between [Formula: see text] and [Formula: see text]. Additionally, interference patterns due to wavelength effects are observed at high field in samples with high relative permittivity and near-zero conductivity, but are not present in lossy samples due to the attenuation of propagating EM fields. This work provides a conceptual framework for understanding B1 spatial distributions for surface coils and can provide guidance for RF engineers.
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Affiliation(s)
- Manushka V Vaidya
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201
| | - Christopher M Collins
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201
| | - Daniel K Sodickson
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201
| | - Ryan Brown
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201
| | - Graham C Wiggins
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAI R) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016
| | - Riccardo Lattanzi
- Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201
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24
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Ferrauto G, Delli Castelli D, Di Gregorio E, Terreno E, Aime S. LipoCEST and cellCEST imaging agents: opportunities and challenges. WILEY INTERDISCIPLINARY REVIEWS-NANOMEDICINE AND NANOBIOTECHNOLOGY 2016; 8:602-18. [PMID: 26810631 DOI: 10.1002/wnan.1385] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/30/2015] [Revised: 11/10/2015] [Accepted: 11/19/2015] [Indexed: 01/01/2023]
Abstract
From the early days of CEST agents' disclosure, it was evident that their potential for in vivo applications was strongly hampered by the intrinsic low sensitivity. Therefore, much work has been devoted to seek out suitable routes to achieve strong CEST contrast enhancement. The use of nanosized systems turned out to be a strategic choice, because a very large amount of CEST agents can be delivered at the site of interest. However, the breakthrough innovation in term of increase of sensitivity was found by designing the lipoCEST agents. The naturally inspired, liposomes vesicles, when loaded with paramagnetic lanthanide-based shift reagents, can be transformed into CEST probes. The large number of water molecules entrapped inside the inner cavity of the nanovesicles represents an enormous pool of exchanging protons for the generation of CEST contrast, whereas the presence of the shift reagent increases the separation in chemical shift of their nuclear magnetic resonance signal from that of the bulk water, thus allowing for a proper exchange regime for the activation of CEST contrast. From lipoCEST, it has been rather straightforward to evolve to cellCEST in order to exploit the cytoplasmatic water molecules as source of the CEST effect, once cells have been loaded with the proper shift reagent. The red blood cells were found to be particularly suitable for the development of the cellCEST concept. Finally, an understanding of the main determinants of the CEST effects in nanosized and cellular-sized agents has allowed the design of innovative lipoCEST/RBC aggregates for potential theranostic applications. WIREs Nanomed Nanobiotechnol 2016, 8:602-618. doi: 10.1002/wnan.1385 For further resources related to this article, please visit the WIREs website.
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Affiliation(s)
- Giuseppe Ferrauto
- Molecular & Preclinical Imaging Centers, Department of Molecular Biotechnology and Health Sciences, University of Torino, Turin, Italy
| | - Daniela Delli Castelli
- Molecular & Preclinical Imaging Centers, Department of Molecular Biotechnology and Health Sciences, University of Torino, Turin, Italy
| | - Enza Di Gregorio
- Molecular & Preclinical Imaging Centers, Department of Molecular Biotechnology and Health Sciences, University of Torino, Turin, Italy
| | - Enzo Terreno
- Molecular & Preclinical Imaging Centers, Department of Molecular Biotechnology and Health Sciences, University of Torino, Turin, Italy.,IBB-CNR-UOS, University of Torino (IT), Turin, Italy
| | - Silvio Aime
- Molecular & Preclinical Imaging Centers, Department of Molecular Biotechnology and Health Sciences, University of Torino, Turin, Italy.,IBB-CNR-UOS, University of Torino (IT), Turin, Italy
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25
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Pohmann R, Speck O, Scheffler K. Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays. Magn Reson Med 2015; 75:801-9. [PMID: 25820458 DOI: 10.1002/mrm.25677] [Citation(s) in RCA: 224] [Impact Index Per Article: 24.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2014] [Revised: 02/04/2015] [Accepted: 02/09/2015] [Indexed: 12/18/2022]
Abstract
PURPOSE Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays. METHODS The same human subjects were scanned at all three field strengths, using identical sequence parameters and similar 31- or 32-channel receive coil arrays. The SNR of three-dimensional (3D) gradient echo images was determined using a multiple replica approach and corrected with measured flip angle and T2 (*) distributions and the T1 of white matter to obtain the intrinsic SNR. The g-factor maps were derived from 3D gradient echo images with several GRAPPA accelerations. RESULTS As expected, T1 values increased, T2 (*) decreased and the B1 -homogeneity deteriorated with increasing field. The SNR showed a distinctly supralinear increase with field strength by a factor of 3.10 ± 0.20 from 3T to 7T, and 1.76 ± 0.13 from 7T to 9.4T over the entire cerebrum. The g-factors did not show the expected decrease, indicating a dominating role of coil design. CONCLUSION In standard experimental conditions, SNR increased supralinearly with field strength (SNR ∼ B0 (1.65) ). To take full advantage of this gain, the deteriorating B1 -homogeneity and the decreasing T2 (*) have to be overcome.
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Affiliation(s)
- Rolf Pohmann
- Max Planck Institute for Biological Cybernetics, Magnetic Resonance Center, Tübingen, Germany
| | - Oliver Speck
- Department of Biomedical Magnetic Resonance, Otto-von-Guericke University Magdeburg, Germany.,German Centre for Neurodegenerative Diseases (DZNE), Site Magdeburg, Germany.,Leibniz Institute for Neurobiology, Magdeburg, Germany.,Center for Behavioral Brain Sciences, Magdeburg, Germany
| | - Klaus Scheffler
- Max Planck Institute for Biological Cybernetics, Magnetic Resonance Center, Tübingen, Germany.,Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany
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26
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Shajan G, Mirkes C, Buckenmaier K, Hoffmann J, Pohmann R, Scheffler K. Three‐layered radio frequency coil arrangement for sodium MRI of the human brain at 9.4 Tesla. Magn Reson Med 2015; 75:906-16. [DOI: 10.1002/mrm.25666] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2014] [Revised: 01/22/2015] [Accepted: 02/02/2015] [Indexed: 11/09/2022]
Affiliation(s)
- G. Shajan
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
| | - Christian Mirkes
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
- Department for Biomedical Magnetic ResonanceUniversity of TübingenTübingen Germany
| | - Kai Buckenmaier
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
| | - Jens Hoffmann
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
| | - Rolf Pohmann
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
| | - Klaus Scheffler
- High Field MR Center, Max Planck Institute for Biological CyberneticsTübingen Germany
- Department for Biomedical Magnetic ResonanceUniversity of TübingenTübingen Germany
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27
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Rancan G, Nguyen TT, Glaser SJ. Gradient ascent pulse engineering for rapid exchange saturation transfer. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2015; 252:1-9. [PMID: 25635353 DOI: 10.1016/j.jmr.2014.12.016] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/11/2014] [Revised: 12/17/2014] [Accepted: 12/19/2014] [Indexed: 06/04/2023]
Abstract
Efforts in the clinical translation of the paraCEST contrast agent Yb-HPDO3A have prompted an investigation into saturation pulse optimality under energy constraints. The GRAPE algorithm has been adapted and implemented for saturation pulse optimization with chemical exchange. The flexibility of the methodology, both in extracting the microscopical parameter ensemble for the algorithm as well as in determining the characteristics of this new class of rising amplitude waveforms allows rapid testing and implementation. Optimal pulses achieve higher saturation efficiencies than the continuous wave gold standard for rapid and especially for variable exchange rates, as brought about by pH-catalysis. Gains of at least 5-15% without any tradeoff have been confirmed both on a spectrometer and on a clinical imager. Pool specific solutions, with pulses optimized for a specific exchange rate value, additionally increase the flexibility of the CEST ratiometric analysis. A simple experimental approach to determine close to optimal triangular pulses is presented.
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Affiliation(s)
- G Rancan
- Department of Chemistry, Technical University Munich, 4, Lichtenbergstr., Garching, Germany; Klinikum rechts der Isar, Technical University Munich, 22, Ismaningerstr., München, Germany.
| | - T T Nguyen
- Department of Chemistry, Technical University Munich, 4, Lichtenbergstr., Garching, Germany
| | - S J Glaser
- Department of Chemistry, Technical University Munich, 4, Lichtenbergstr., Garching, Germany.
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28
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Rancan G, Delli Castelli D, Aime S. MRI CEST at 1T with large µeff Ln(3+) complexes T m(3+)-HPDO3A: An efficient MRI pH reporter. Magn Reson Med 2015; 75:329-36. [PMID: 25651986 DOI: 10.1002/mrm.25589] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2014] [Revised: 11/24/2014] [Accepted: 11/29/2014] [Indexed: 12/24/2022]
Abstract
PURPOSE Chemical exchange saturation transfer (CEST) sensitivity relies on the prototropic exchange rate kex between the agent and the "bulk" water protons. To exploit large kex, a large frequency separation (Δω) between the pools of exchanging protons is necessary. For this reason, high magnetic fields are preferred. Herein it is shown that the use of paramagnetic CEST agents based on lanthanide (III) ions with large effective magnetic moments allows the carrying out of CEST experiments at the relatively low field strength of 1 tesla (T). METHODS Measurements were performed on a 1T MR-scanner using continuous wave (cw)-presaturation with a spin echo sequence. ParaCEST complexes have been synthetized by mixing the ligand and Ln(III)Cl3 in a stoichiometric ratio at room temperature and pH 7. RESULTS Different lanthanide chelates were investigated (Tm-, Dy-, Yb-, Eu-HPDO3A, and Eu-DOTAMGly). Ratiometric (Tm-HPDO3A) and selective detection (Eu-DOTAMGly and Tm-HPDO3A) experiments have been proven feasible in vivo. CONCLUSION In vitro experiments demonstrated the feasibility of the CEST methodology at 1T for nearly every paraCEST candidate under investigation, except for Eu-HPDO3A. Among the studied compounds, Tm-HPDO3A proved suitable for the application of a ratiometric method for assessing pH both in vitro and in vivo.
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Affiliation(s)
- Giaime Rancan
- Technical University Munich, Klinikum rechts der Isar, München, Germany
| | | | - Silvio Aime
- University of Turin, Molecular Imaging Center, Torino, Italy
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29
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Tse DHY, Poole MS, Magill AW, Felder J, Brenner D, Jon Shah N. Encoding methods for B1(+) mapping in parallel transmit systems at ultra high field. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2014; 245:125-32. [PMID: 25036294 DOI: 10.1016/j.jmr.2014.06.006] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2013] [Revised: 06/13/2014] [Accepted: 06/16/2014] [Indexed: 05/05/2023]
Abstract
Parallel radiofrequency (RF) transmission, either in the form of RF shimming or pulse design, has been proposed as a solution to the B1(+) inhomogeneity problem in ultra high field magnetic resonance imaging. As a prerequisite, accurate B1(+) maps from each of the available transmit channels are required. In this work, four different encoding methods for B1(+) mapping, namely 1-channel-on, all-channels-on-except-1, all-channels-on-1-inverted and Fourier phase encoding, were evaluated using dual refocusing acquisition mode (DREAM) at 9.4 T. Fourier phase encoding was demonstrated in both phantom and in vivo to be the least susceptible to artefacts caused by destructive RF interference at 9.4 T. Unlike the other two interferometric encoding schemes, Fourier phase encoding showed negligible dependency on the initial RF phase setting and therefore no prior B1(+) knowledge is required. Fourier phase encoding also provides a flexible way to increase the number of measurements to increase SNR, and to allow further reduction of artefacts by weighted decoding. These advantages of Fourier phase encoding suggest that it is a good choice for B1(+) mapping in parallel transmit systems at ultra high field.
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Affiliation(s)
- Desmond H Y Tse
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany
| | - Michael S Poole
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany
| | - Arthur W Magill
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany
| | - Jörg Felder
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany
| | - Daniel Brenner
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany; German Center for Neurodegenerative Diseases (DZNE), Ernst-Robert-Curtius-Straße 12, 53117 Bonn, Germany
| | - N Jon Shah
- Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany; Department of Neurology, Faculty of Medicine, RWTH Aachen University, JARA, Aachen, Germany.
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Brown R, Deniz CM, Zhang B, Chang G, Sodickson DK, Wiggins GC. Design and application of combined 8-channel transmit and 10-channel receive arrays and radiofrequency shimming for 7-T shoulder magnetic resonance imaging. Invest Radiol 2014; 49:35-47. [PMID: 24056112 DOI: 10.1097/rli.0b013e3182a5662d] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
OBJECTIVE The objective of the study was to investigate the feasibility of 7-T shoulder magnetic resonance imaging by developing transmit and receive radiofrequency (RF) coil arrays and exploring RF shim methods. MATERIALS AND METHODS A mechanically flexible 8-channel transmit array and an anatomically conformable 10-channel receive array were designed and implemented. The transmit performance of various RF shim methods was assessed through local flip angle measurements in the right and left shoulders of 6 subjects. The receive performance was assessed through signal-to-noise ratio measurements using the developed 7-T coil and a baseline commercial 3-T coil. RESULTS The 7-T transmit array driven with phase-coherent RF shim weights provided adequate B₁⁺ efficiency and uniformity for turbo spin echo shoulder imaging. B₁⁺ twisting that is characteristic of high-field loop coils necessitates distinct RF shim weights in the right and left shoulders. The 7-T receive array provided a 2-fold signal-to-noise ratio improvement over the 3-T array in the deep articular shoulder cartilage. CONCLUSIONS Shoulder imaging at 7-T is feasible with a custom transmit/receive array either in a single-channel transmit mode with a fixed RF shim or in a parallel transmit mode with a subject-specific RF shim.
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Affiliation(s)
- Ryan Brown
- From the Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University Langone Medical Center, NY
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31
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Spatial phase encoding exploiting the Bloch–Siegert shift effect. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2013; 27:363-71. [PMID: 24254040 DOI: 10.1007/s10334-013-0417-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/24/2013] [Revised: 10/28/2013] [Accepted: 10/29/2013] [Indexed: 10/26/2022]
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32
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Hsu YC, Chu YH, Chern IL, Lattanzi R, Huang TY, Lin FH. Mitigate B₁(+) inhomogeneity by nonlinear gradients and RF shimming. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2013; 2013:1085-8. [PMID: 24109880 DOI: 10.1109/embc.2013.6609693] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
High-field MRI has the challenge of inhomogeneous B1(+) and consequently an inhomogeneous flip angle distribution. This causes spatially dependent contrast and makes clinical diagnosis difficult. Under the small flip angle approximation and using nonlinear spatial encoding magnetic fields (SEMs), we propose a method to remap the B1(+) map into a lower dimension coordinate system. Combining with RF shimming method, a simple pulse sequence design using nonlinear SEMs can achieve a homogenous flip angle distribution efficiently. Using simulations, we demonstrate that combining RF shimming and spatially selective RF excitation using generalized SEMs (SAGS) using linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B1(+) inhomogeneity at 7T efficiently without using parallel RF transmission.
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Zhao W, Cohen-Adad J, Polimeni JR, Keil B, Guerin B, Setsompop K, Serano P, Mareyam A, Hoecht P, Wald LL. Nineteen-channel receive array and four-channel transmit array coil for cervical spinal cord imaging at 7T. Magn Reson Med 2013; 72:291-300. [PMID: 23963998 DOI: 10.1002/mrm.24911] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2013] [Revised: 06/12/2013] [Accepted: 07/15/2013] [Indexed: 11/10/2022]
Abstract
PURPOSE To design and validate a radiofrequency (RF) array coil for cervical spinal cord imaging at 7T. METHODS A 19-channel receive array with a four-channel transmit array was developed on a close-fitting coil former at 7T. Transmit efficiency and specific absorption rate were evaluated in a B1 (+) mapping study and an electromagnetic model. Receive signal-to-noise ratio (SNR) and noise amplification for parallel imaging were evaluated and compared with a commercial 3T 19-channel head-neck array and a 7T four-channel spine array. The performance of the array was qualitatively demonstrated in human volunteers using high-resolution imaging (down to 300 μm in-plane). RESULTS The transmit and receive arrays showed good bench performance. The SNR was approximately 4.2-fold higher in the 7T receive array at the location of the cord with respect to the 3T coil. The g-factor results showed an additional acceleration was possible with the 7T array. In vivo imaging was feasible and showed high SNR and tissue contrast. CONCLUSION The highly parallel transmit and receive arrays were demonstrated to be fit for spinal cord imaging at 7T. The high sensitivity of the receive coil combined with ultra-high field will likely improve investigations of microstructure and tissue segmentation in the healthy and pathological spinal cord.
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Affiliation(s)
- Wei Zhao
- A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA; Harvard Medical School, Boston, Massachusetts, USA
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Sbrizzi A, Hoogduin H, Lagendijk JJ, Luijten P, van den Berg CAT. Robust reconstruction ofB1+maps by projection into a spherical functions space. Magn Reson Med 2013; 71:394-401. [DOI: 10.1002/mrm.24640] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2012] [Revised: 12/19/2012] [Accepted: 12/20/2012] [Indexed: 11/09/2022]
Affiliation(s)
- Alessandro Sbrizzi
- Imaging Division; University Medical Center Utrecht; Utrecht The Netherlands
| | - Hans Hoogduin
- Imaging Division; University Medical Center Utrecht; Utrecht The Netherlands
- Rudolf Magnus Institute; University Medical Center Utrecht; Utrecht The Netherlands
| | - Jan J. Lagendijk
- Imaging Division; University Medical Center Utrecht; Utrecht The Netherlands
| | - Peter Luijten
- Imaging Division; University Medical Center Utrecht; Utrecht The Netherlands
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35
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Liu W, Kao CP, Collins CM, Smith MB, Yang QX. On consideration of radiated power in RF field simulations for MRI. Magn Reson Med 2012; 69:290-4. [PMID: 22473620 DOI: 10.1002/mrm.24244] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2011] [Revised: 02/14/2012] [Accepted: 02/14/2012] [Indexed: 11/11/2022]
Abstract
In numerical analyses of radiofrequency (RF) fields for MRI, RF power is often permitted to radiate out of the problem region. In reality, RF power will be confined by the magnet bore and RF screen enclosing the magnet room. We present numerical calculations at different frequencies for various surface and volume coils, with samples from simple spheres to the human body in environments from free space to a shielded RF room. Results for calculations within a limited problem region show radiated power increases with frequency. When the magnet room RF screen is included, nearly all the power is dissipated in the human subject. For limited problem regions, inclusion of a term for radiation loss results in an underestimation of transmit efficiency compared to results including the complete bore and RF screen. If the term for radiated power is not included, calculated coil efficiencies are slightly overestimated compared to the complete case.
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Affiliation(s)
- Wanzhan Liu
- Medtronic, Inc., Minneapolis, Minnesota, USA
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36
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Zhu Y, Alon L, Deniz CM, Brown R, Sodickson DK. System and SAR characterization in parallel RF transmission. Magn Reson Med 2011; 67:1367-78. [PMID: 22139808 DOI: 10.1002/mrm.23126] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2011] [Revised: 07/04/2011] [Accepted: 07/11/2011] [Indexed: 11/09/2022]
Abstract
The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.
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Affiliation(s)
- Yudong Zhu
- Department of Radiology, Center for Biomedical Imaging, New York University School of Medicine, New York, New York 10016, USA.
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37
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Lattanzi R, Sodickson DK. Ideal current patterns yielding optimal signal-to-noise ratio and specific absorption rate in magnetic resonance imaging: computational methods and physical insights. Magn Reson Med 2011; 68:286-304. [PMID: 22127735 DOI: 10.1002/mrm.23198] [Citation(s) in RCA: 87] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2011] [Revised: 06/03/2011] [Accepted: 08/02/2011] [Indexed: 11/11/2022]
Abstract
At high and ultra-high magnetic field strengths, understanding interactions between tissues and the electromagnetic fields generated by radiofrequency coils becomes crucial for safe and effective coil design as well as for insight into limits of performance. In this work, we present a rigorous electrodynamic modeling framework, using dyadic Green's functions, to derive the electromagnetic field in homogeneous spherical and cylindrical samples resulting from arbitrary surface currents in the presence or absence of a surrounding radiofrequency shield. We show how to calculate ideal current patterns that result in the highest possible signal-to-noise ratio (ultimate intrinsic signal-to-noise ratio) or the lowest possible radiofrequency power deposition (ultimate intrinsic specific absorption rate) compatible with electrodynamic principles. We identify familiar coil designs within optimal current patterns at low to moderate field strength, thereby establishing and explaining graphically the near-optimality of traditional surface and volume quadrature designs. We also document the emergence of less familiar patterns, e.g., involving substantial electric--as well as magnetic-dipole contributions, at high field strength. Performance comparisons with particular coil array configurations demonstrate that optimal performance may be approached with finite arrays if ideal current patterns are used as a guide for coil design.
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Affiliation(s)
- Riccardo Lattanzi
- The Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University Langone Medical Center, New York, New York 10016, USA.
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38
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Reischauer C, Vorburger RS, Wilm BJ, Jaermann T, Boesiger P. Optimizing signal-to-noise ratio of high-resolution parallel single-shot diffusion-weighted echo-planar imaging at ultrahigh field strengths. Magn Reson Med 2011; 67:679-90. [DOI: 10.1002/mrm.23057] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2011] [Revised: 05/04/2011] [Accepted: 05/24/2011] [Indexed: 11/06/2022]
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39
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Collins CM, Wang Z. Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects. Magn Reson Med 2011; 65:1470-82. [PMID: 21381106 DOI: 10.1002/mrm.22845] [Citation(s) in RCA: 81] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2010] [Revised: 11/23/2010] [Accepted: 01/05/2011] [Indexed: 11/11/2022]
Abstract
Radiofrequency magnetic fields are critical to nuclear excitation and signal reception in magnetic resonance imaging. The interactions between these fields and human tissues in anatomical geometries results in a variety of effects regarding image integrity and safety of the human subject. In recent decades, numerical methods of calculation have been used increasingly to understand the effects of these interactions and aid in engineering better, faster, and safer equipment and methods. As magnetic resonance imaging techniques and technology have evolved through the years, so to have the requirements for meaningful interpretation of calculation results. Here, we review the basic physics of radiofrequency electromagnetics in magnetic resonance imaging and discuss a variety of ways radiofrequency field calculations are used in magnetic resonance imaging in engineering and safety assurance from simple systems and sequences through advanced methods of development for the future.
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Affiliation(s)
- Christopher M Collins
- Department of Radiology, The Pennsylvania State University, Hershey, Pennsylvania, USA.
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40
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Shrivastava D, Abosch A, Hanson T, Tian J, Gupte A, Iaizzo PA, Vaughan JT. Effect of the extracranial deep brain stimulation lead on radiofrequency heating at 9.4 Tesla (400.2 MHz). J Magn Reson Imaging 2011; 32:600-7. [PMID: 20815057 DOI: 10.1002/jmri.22292] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
PURPOSE To study the effect of the extracranial portion of a deep brain stimulation (DBS) lead on radiofrequency (RF) heating with a transmit and receive 9.4 Tesla head coil. MATERIALS AND METHODS The RF heating was studied in four excised porcine heads (mean animal head weight = 5.46 +/- 0.14 kg) for each of the following two extracranial DBS lead orientations: one, parallel to the coil axial direction; two, perpendicular to the coil axial direction (i.e., azimuthal). Temperatures were measured using fluoroptic probes at four locations: one, scalp; two, near the second DBS lead electrode-brain contact; three, near the distal tip of the DBS lead; and four, air surrounding the head. A continuous wave RF power was delivered to each head for 15 min using the coil. Net, delivered RF power was measured at the coil (mean whole head average specific absorption rate = 2.94 +/- 0.08 W/kg). RESULTS RF heating was significantly reduced when the extracranial DBS lead was placed in the axial direction (temperature change = 0-5 degrees C) compared with the azimuthal direction (temperature change = 1-27 degrees C). CONCLUSION Development of protocols seems feasible to keep RF heating near DBS electrodes clinically safe during ultra-high field head imaging.
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Affiliation(s)
- Devashish Shrivastava
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota 55455, USA.
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41
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Adriany G, Auerbach EJ, Snyder CJ, Gözübüyük A, Moeller S, Ritter J, Van de Moortele PF, Vaughan T, Uğurbil K. A 32-channel lattice transmission line array for parallel transmit and receive MRI at 7 tesla. Magn Reson Med 2010; 63:1478-85. [PMID: 20512850 DOI: 10.1002/mrm.22413] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Transmit and receive RF coil arrays have proven to be particularly beneficial for ultra-high-field MR. Transmit coil arrays enable such techniques as B(1) (+) shimming to substantially improve transmit B(1) homogeneity compared to conventional volume coil designs, and receive coil arrays offer enhanced parallel imaging performance and SNR. Concentric coil arrangements hold promise for developing transceiver arrays incorporating large numbers of coil elements. At magnetic field strengths of 7 tesla and higher where the Larmor frequencies of interest can exceed 300 MHz, the coil array design must also overcome the problem of the coil conductor length approaching the RF wavelength. In this study, a novel concentric arrangement of resonance elements built from capacitively-shortened half-wavelength transmission lines is presented. This approach was utilized to construct an array with whole-brain coverage using 16 transceiver elements and 16 receive-only elements, resulting in a coil with a total of 16 transmit and 32 receive channels.
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Affiliation(s)
- Gregor Adriany
- Center for Magnetic Resonance Research, Department of Radiology, School of Medicine, University of Minnesota, Minneapolis, Minnesota 55455, USA.
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42
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Tkác I, Oz G, Adriany G, Uğurbil K, Gruetter R. In vivo 1H NMR spectroscopy of the human brain at high magnetic fields: metabolite quantification at 4T vs. 7T. Magn Reson Med 2010; 62:868-79. [PMID: 19591201 DOI: 10.1002/mrm.22086] [Citation(s) in RCA: 270] [Impact Index Per Article: 19.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
A comprehensive comparative study of metabolite quantification from the human brain was performed on the same 10 subjects at 4T and 7T using MR scanners with identical consoles, the same type of RF coils, and identical pulse sequences and data analysis. Signal-to-noise ratio (SNR) was increased by a factor of 2 at 7T relative to 4T in a volume of interest selected in the occipital cortex using half-volume quadrature radio frequency (RF) coils. Spectral linewidth was increased by 50% at 7T, which resulted in a 14% increase in spectral resolution at 7T relative to 4T. Seventeen brain metabolites were reliably quantified at both field strengths. Metabolite quantification at 7T was less sensitive to reduced SNR than at 4T. The precision of metabolite quantification and detectability of weakly represented metabolites were substantially increased at 7T relative to 4T. Because of the increased spectral resolution at 7T, only one-half of the SNR of a 4T spectrum was required to obtain the same quantification precision. The Cramér-Rao lower bounds (CRLB), a measure of quantification precision, of several metabolites were lower at both field strengths than the intersubject variation in metabolite concentrations, which resulted in a strong correlation between metabolite concentrations of individual subjects measured at 4T and 7T.
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Affiliation(s)
- Ivan Tkác
- Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
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43
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Collins CM. Numerical field calculations considering the human subject for engineering and safety assurance in MRI. NMR IN BIOMEDICINE 2009; 22:919-26. [PMID: 18384179 PMCID: PMC2836719 DOI: 10.1002/nbm.1251] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Numerical calculations of static, switched, and radiofrequency (RF) electromagnetic (EM) fields considering the geometry and EM properties of the human body are used increasingly in MRI to explain observed phenomena, explore the limitations of various approaches, engineer improved techniques and technology, and assure safety. As the static field strengths and RF field frequencies in MRI have increased in recent years, the value of these methods has become more pronounced and their use has become more widespread. With the recent growth in parallel reception techniques and the advent of transmit RF arrays, the utility of these calculations will become only more critical to continued progress of MRI. Proper relation of field calculation results to the MRI experiment can require significant understanding of MRI physics, EM field principles, MRI coil hardware, and EM field safety. Here some fundamental principles are reviewed and current approaches and applications are catalogued to aid the reader in finding resources valuable in beginning field calculations for their own applications in MR, with an eye to the current needs and future utility of numerical field calculations in MRI.
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Affiliation(s)
- Christopher M Collins
- Department of Radiology, The Pennsylvania State University, Hershey, PA 17033-0850, USA.
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44
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Wu B, Wang C, Krug R, Kelley DA, Xu D, Pang Y, Banerjee S, Vigneron DB, Nelson SJ, Majumdar S, Zhang X. 7T human spine imaging arrays with adjustable inductive decoupling. IEEE Trans Biomed Eng 2009; 57:397-403. [PMID: 19709956 DOI: 10.1109/tbme.2009.2030170] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Ultrahigh-field human spine RF transceiver coil arrays face daunting technical challenges in achieving large imaging coverage with sufficient B(1) penetration and sensitivity, and in attaining robust decoupling among coil elements. In this paper, human spine coil arrays for ultrahigh field were built and studied. Transceiver arrays with loop-shaped microstrip transmission line were designed, fabricated, and tested for 7-tesla (7T) MRI. With the proposed adjustable inductive decoupling technique, the isolation between adjacent coil elements is easily addressed. Preliminary results of human spine images acquired using the transceiver arrays demonstrate the feasibility of the design for ultrahigh-field MR applications and its robust performance for parallel imaging.
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Affiliation(s)
- Bing Wu
- Department of Radiology and Biomedical Imaging, University of California, San Francisco, CA 94158, USA
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45
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Van de Moortele PF, Auerbach EJ, Olman C, Yacoub E, Uğurbil K, Moeller S. T1 weighted brain images at 7 Tesla unbiased for Proton Density, T2* contrast and RF coil receive B1 sensitivity with simultaneous vessel visualization. Neuroimage 2009; 46:432-46. [PMID: 19233292 DOI: 10.1016/j.neuroimage.2009.02.009] [Citation(s) in RCA: 238] [Impact Index Per Article: 15.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2007] [Revised: 01/20/2009] [Accepted: 02/07/2009] [Indexed: 11/24/2022] Open
Abstract
At high magnetic field, MR images exhibit large, undesirable signal intensity variations commonly referred to as "intensity field bias". Such inhomogeneities mostly originate from heterogeneous RF coil B(1) profiles and, with no appropriate correction, are further pronounced when utilizing rooted sum of square reconstruction with receive coil arrays. These artifacts can significantly alter whole brain high resolution T(1)-weighted (T(1)w) images that are extensively utilized for clinical diagnosis, for gray/white matter segmentation as well as for coregistration with functional time series. In T(1) weighted 3D-MPRAGE sequences, it is possible to preserve a bulk amount of T(1) contrast through space by using adiabatic inversion RF pulses that are insensitive to transmit B(1) variations above a minimum threshold. However, large intensity variations persist in the images, which are significantly more difficult to address at very high field where RF coil B(1) profiles become more heterogeneous. Another characteristic of T(1)w MPRAGE sequences is their intrinsic sensitivity to Proton Density and T(2)(*) contrast, which cannot be removed with post-processing algorithms utilized to correct for receive coil sensitivity. In this paper, we demonstrate a simple technique capable of producing normalized, high resolution T(1)w 3D-MPRAGE images that are devoid of receive coil sensitivity, Proton Density and T(2)(*) contrast. These images, which are suitable for routinely obtaining whole brain tissue segmentation at 7 T, provide higher T(1) contrast specificity than standard MPRAGE acquisitions. Our results show that removing the Proton Density component can help in identifying small brain structures and that T(2)(*) induced artifacts can be removed from the images. The resulting unbiased T(1)w images can also be used to generate Maximum Intensity Projection angiograms, without additional data acquisition, that are inherently registered with T(1)w structural images. In addition, we introduce a simple technique to reduce residual signal intensity variations induced by transmit B(1) heterogeneity. Because this approach requires two 3D images, one divided with the other, head motion could create serious problems, especially at high spatial resolution. To alleviate such inter-scan motion problems, we developed a new sequence where the two contrast acquisitions are interleaved within a single scan. This interleaved approach however comes with greater risk of intra-scan motion issues because of a longer single scan time. Users can choose between these two trade offs depending on specific protocols and patient populations. We believe that the simplicity and the robustness of this double contrast based approach to address intensity field bias at high field and improve T(1) contrast specificity, together with the capability of simultaneously obtaining angiography maps, advantageously counter balance the potential drawbacks of the technique, mainly a longer acquisition time and a moderate reduction in signal to noise ratio.
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Affiliation(s)
- Pierre-François Van de Moortele
- Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, 2021 Sixth Street S.E., Minneapolis, MN 55455, USA.
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46
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Lattanzi R, Sodickson DK, Grant AK, Zhu Y. Electrodynamic constraints on homogeneity and radiofrequency power deposition in multiple coil excitations. Magn Reson Med 2009; 61:315-34. [PMID: 19165885 PMCID: PMC2749671 DOI: 10.1002/mrm.21782] [Citation(s) in RCA: 87] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2007] [Accepted: 07/17/2008] [Indexed: 11/12/2022]
Abstract
The promise of increased signal-to-noise ratio and spatial/spectral resolution continues to drive MR technology toward higher magnetic field strengths. SAR management and B1 inhomogeneity correction become critical issues at the high frequencies associated with high field MR. In recent years, multiple coil excitation techniques have been recognized as potentially powerful tools for controlling specific absorption rate (SAR) while simultaneously compensating for B1 inhomogeneities. This work explores electrodynamic constraints on transmit homogeneity and SAR, for both fully parallel transmission and its time-independent special case known as radiofrequency shimming. Ultimate intrinsic SAR--the lowest possible SAR consistent with electrodynamics for a particular excitation profile but independent of transmit coil design--is studied for different field strengths, object sizes, and pulse acceleration factors. The approach to the ultimate intrinsic limit with increasing numbers of finite transmit coils is also studied, and the tradeoff between homogeneity and SAR is explored for various excitation strategies. In the case of fully parallel transmission, ultimate intrinsic SAR shows flattening or slight reduction with increasing field strength, in contradiction to the traditionally cited quadratic dependency, but consistent with established electrodynamic principles.
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Affiliation(s)
- Riccardo Lattanzi
- Division of Magnetic Resonance Research, Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA
- Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA
| | - Daniel K. Sodickson
- Center for Biomedical Imaging, Department of Radiology, New York University Medical Center, New York, NY
| | - Aaron K. Grant
- Division of Magnetic Resonance Research, Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA
- Harvard Medical School, Boston, MA
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47
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Adriany G, Van de Moortele PF, Ritter J, Moeller S, Auerbach EJ, Akgün C, Snyder CJ, Vaughan T, Uğurbil K. A geometrically adjustable 16-channel transmit/receive transmission line array for improved RF efficiency and parallel imaging performance at 7 Tesla. Magn Reson Med 2008; 59:590-7. [PMID: 18219635 DOI: 10.1002/mrm.21488] [Citation(s) in RCA: 163] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
A novel geometrically adjustable transceiver array system is presented. A key feature of the geometrically adjustable array was the introduction of decoupling capacitors that allow for automatic change in capacitance dependent on neighboring resonant element distance. The 16-element head array version of such an adjustable coil based on transmission line technology was compared to fixed geometry transmission line arrays (TLAs) of various sizes at 7T. The focus of this comparison was on parallel imaging performance, RF transmit efficiency, and signal-to-noise ratio (SNR). Significant gains in parallel imaging performance and SNR were observed for the new coil and attributed to its adjustability and to the design of the individual elements with a three-sided ground plane.
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Affiliation(s)
- Gregor Adriany
- Center for Magnetic Resonance Research, Department of Radiology, School of Medicine, University of Minnesota, Minneapolis, USA 55455, USA.
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48
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Metzger GJ, Snyder C, Akgun C, Vaughan T, Ugurbil K, Van de Moortele PF. Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements. Magn Reson Med 2008; 59:396-409. [PMID: 18228604 DOI: 10.1002/mrm.21476] [Citation(s) in RCA: 242] [Impact Index Per Article: 15.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B(1) (B(1) (+)) shimming to minimize B(1) (+) losses resulting from destructive interferences. B(1) (+) shimming was performed by altering the input phase of individual RF channels based on relative B(1) (+) phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B(1) (+) coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B(1) (+) shimming provided a gain in transmit efficiency of 4.2 +/- 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B(1) (+) shimming decreased B(1) (+) nonuniformity within the prostate from (24 +/- 9%) to (5 +/- 4%). This study demonstrates the tremendous impact of fast local B(1) (+) phase shimming on ultrahigh magnetic field body imaging.
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Affiliation(s)
- Gregory J Metzger
- Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA.
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49
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Shrivastava D, Hanson T, Schlentz R, Gallaghar W, Snyder C, Delabarre L, Prakash S, Iaizzo P, Vaughan JT. Radiofrequency heating at 9.4T: in vivo temperature measurement results in swine. Magn Reson Med 2008; 59:73-8. [PMID: 17969077 PMCID: PMC2754718 DOI: 10.1002/mrm.21425] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2007] [Accepted: 08/31/2007] [Indexed: 11/11/2022]
Abstract
In vivo temperatures were correlated to the whole head average specific absorption rate (SAR(avg)) at 9.4T using 12 anesthetized swine (mean animal weight = 52 kg, standard deviation = 6.7 kg). Correlating the temperatures and SAR(avg) is necessary to ensure safe levels of human heating during ultra-high field MR exams. The temperatures were measured at three depths inside the brain, in the rectum, and at the head-skin of swine. A 400 MHz, continuous wave RF power was deposited to the head using a volume coil. The SAR(avg) values were varied between 2.7-5.8 W/kg. The RF power exposure durations were varied between 1.4-3.7 hr. To differentiate the temperature response caused by the RF from that of the anesthesia, the temperatures were recorded in four unheated swine. To study the effect of the spatial distribution of the RF and tissue properties, the temperature probes were placed at two brain locations (n = 4 swine for each location). Results showed that the in vivo brain temperatures correlated to the SAR(avg) in a geometry-dependent manner. Additionally, 1) the skin temperature change was not the maximum temperature change; 2) the RF heating caused an inhomogeneous brain temperature distribution; and 3) the maximum temperature occurred inside the brain.
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Affiliation(s)
- Devashish Shrivastava
- Center for Magnetic Resonance Research, Univesity of Minnesota, Minneapolis, Minnesota 55455, USA.
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50
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Kumar A, Bottomley PA. Optimized quadrature surface coil designs. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2007; 21:41-52. [PMID: 18057975 DOI: 10.1007/s10334-007-0090-2] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/09/2007] [Revised: 09/21/2007] [Accepted: 10/26/2007] [Indexed: 11/25/2022]
Abstract
BACKGROUND Quadrature surface MRI/MRS detectors comprised of circular loop and figure-8 or butterfly-shaped coils offer improved signal-to-noise-ratios (SNR) compared to single surface coils, and reduced power and specific absorption rates (SAR) when used for MRI excitation. While the radius of the optimum loop coil for performing MRI at depth d in a sample is known, the optimum geometry for figure-8 and butterfly coils is not. MATERIALS AND METHODS The geometries of figure-8 and square butterfly detector coils that deliver the optimum SNR are determined numerically by the electromagnetic method of moments. Figure-8 and loop detectors are then combined to create SNR-optimized quadrature detectors whose theoretical and experimental SNR performance are compared with a novel quadrature detector comprised of a strip and a loop, and with two overlapped loops optimized for the same depth at 3 T. The quadrature detection efficiency and local SAR during transmission for the three quadrature configurations are analyzed and compared. RESULTS The SNR-optimized figure-8 detector has loop radius r8 approximately 0.6d, so r8/r0 approximately 1.3 in an optimized quadrature detector at 3 T. The optimized butterfly coil has side length approximately d and crossover angle of > or = 150 degrees at the center. CONCLUSIONS These new design rules for figure-8 and butterfly coils optimize their performance as linear and quadrature detectors.
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Affiliation(s)
- Ananda Kumar
- Department of Radiology and Electrical and Computer Engineering, Johns Hopkins University, Division of MR research, 601 N. Caroline Street, JHOC room 4243, Baltimore, MD 21287, USA.
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