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Tušar K, Serša I. Use of nonlinear pulsed magnetic fields for spatial encoding in magnetic resonance imaging. Sci Rep 2024; 14:7521. [PMID: 38553559 PMCID: PMC10980706 DOI: 10.1038/s41598-024-58229-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2023] [Accepted: 03/26/2024] [Indexed: 04/02/2024] Open
Abstract
This study examines the use of nonlinear magnetic field coils for spatial encoding in magnetic resonance imaging. Existing theories on imaging with such coils share a complex reconstruction process that originates from a suboptimal signal interpretation in the spatial-frequency domain (k-space). In this study, a new solution to this problem is proposed, namely a two-step reconstruction process, in which in the first step, the image signal is converted into a frequency spectrum, and in the second step, the spectrum, which represents the distorted image, is geometrically and intensity corrected to obtain an undistorted image. This theory has been verified by numerical simulations and experimentally using a straight wire as a coil model for an extremely nonlinear magnetic field. The results of this study facilitate the use of simple encoding coil designs that can feature low inductance, allowing for much faster switching times and higher magnetic field gradients.
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Affiliation(s)
- Kaja Tušar
- Jožef Stefan International Postgraduate School, Jamova 39, 1000, Ljubljana, Slovenia
| | - Igor Serša
- Jožef Stefan Institute, Jamova 39, 1000, Ljubljana, Slovenia.
- Faculty of Medicine, University of Ljubljana, Vrazov trg 2, 1000, Ljubljana, Slovenia.
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2
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Tian R, Uecker M, Davids M, Thielscher A, Buckenmaier K, Holder O, Steffen T, Scheffler K. Accelerated 2D Cartesian MRI with an 8-channel local B 0 coil array combined with parallel imaging. Magn Reson Med 2024; 91:443-465. [PMID: 37867407 DOI: 10.1002/mrm.29799] [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: 03/10/2023] [Revised: 06/26/2023] [Accepted: 06/30/2023] [Indexed: 10/24/2023]
Abstract
PURPOSE In MRI, the magnetization of nuclear spins is spatially encoded with linear gradients and radiofrequency receivers sensitivity profiles to produce images, which inherently leads to a long scan time. Cartesian MRI, as widely adopted for clinical scans, can be accelerated with parallel imaging and rapid magnetic field modulation during signal readout. Here, by using an 8-channel localB 0 $$ {\mathrm{B}}_0 $$ coil array, the modulation scheme optimized for sampling efficiency is investigated to speed up 2D Cartesian scans. THEORY AND METHODS An 8-channel localB 0 $$ {\mathrm{B}}_0 $$ coil array is made to carry sinusoidal currents during signal readout to accelerate 2D Cartesian scans. An MRI sampling theory based on reproducing kernel Hilbert space is exploited to visualize the efficiency of nonlinear encoding in arbitrary sampling duration. A field calibration method using current monitors for localB 0 $$ {\mathrm{B}}_0 $$ coils and the ESPIRiT algorithm is proposed to facilitate image reconstruction. Image acceleration with various modulation field shapes, aliasing control, and distinct modulation frequencies are scrutinized to find an optimized modulation scheme. A safety evaluation is conducted. In vivo 2D Cartesian scans are accelerated by the localB 0 $$ {\mathrm{B}}_0 $$ coils. RESULTS For 2D Cartesian MRI, the optimal modulation field by this localB 0 $$ {\mathrm{B}}_0 $$ array converges to a nearly linear gradient field. With the field calibration technique, it accelerates the in vivo scans (i.e., proved safe) by threefold and eightfold free of visible artifacts, without and with SENSE, respectively. CONCLUSION The nonlinear encoding analysis tool, the field calibration method, the safety evaluation procedures, and the in vivo reconstructed scans make significant steps to push MRI speed further with the localB 0 $$ {\mathrm{B}}_0 $$ coil array.
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Affiliation(s)
- Rui Tian
- High-Field MR center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Martin Uecker
- Institute of Biomedical Imaging, Graz University of Technology, Graz, Austria
- Institute for Diagnostic and Interventional Radiology, University Medical Center Göttingen, Göttingen, Germany
- German Centre for Cardiovascular Research (DZHK), Partner Site Göttingen, Göttingen, Germany
- BioTechMed-Graz, Graz, Austria
| | - Mathias Davids
- A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA
- Harvard Medical School, Boston, Massachusetts, USA
| | - Axel Thielscher
- Department of Health Technology, Technical University of Denmark, Kongens Lyngby, Denmark
- Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark
| | - Kai Buckenmaier
- High-Field MR center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Oliver Holder
- High-Field MR center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Theodor Steffen
- High-Field MR center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Klaus Scheffler
- High-Field MR center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
- Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany
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3
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Gudino N, Littin S. Advancements in Gradient System Performance for Clinical and Research MRI. J Magn Reson Imaging 2023; 57:57-70. [PMID: 36073722 DOI: 10.1002/jmri.28421] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Revised: 08/18/2022] [Accepted: 08/18/2022] [Indexed: 02/03/2023] Open
Abstract
In magnetic resonance imaging (MRI), spatial field gradients are applied along each axis to encode the location of the nuclear spin in the frequency domain. During recent years, the development of new gradient technologies has been focused on the generation of stronger and faster gradient fields for imaging with higher spatial and temporal resolution. This benefits imaging methods, such as brain diffusion and functional MRI, and enables human imaging at ultra-high field MRI. In addition to improving gradient performance, new technologies have been presented to minimize peripheral nerve stimulation and gradient-related acoustic noise, both generated by the rapid switching of strong gradient fields. This review will provide a general background on the gradient system and update on the state-of-the-art gradient technology. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.
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Affiliation(s)
- Natalia Gudino
- MRI Engineering Core, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA
| | - Sebastian Littin
- Medical Physics, Department of Radiology, Faculty of Medicine, University Freiburg, Freiburg, Germany
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Kang L, Tang F, Xia L, Liu F. Design of an insertable cone-shaped gradient coil matrix for head imaging with a volumetric finite-difference method. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2021; 92:124709. [PMID: 34972446 DOI: 10.1063/5.0060194] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/16/2021] [Accepted: 12/07/2021] [Indexed: 06/14/2023]
Abstract
Matrix gradient coils have received increasing interest in generating arbitrary-shaped magnetic fields for various magnetic resonance imaging applications. In this paper, a novel cone-shaped matrix gradient coil is proposed to design a multifunctional insertable system for head imaging. Using a volumetric finite-difference-based method, the matrix coil is designed to have comprised several coil elements that can implement localized imaging and control eddy current, dissipated power, and minimum wire gap. With the lowest total dissipated power, various current configurations are selected to generate multiple gradient fields within a large, spheroidal region of interest (ROI) and two small spherical sub-ROIs. The numerical computation results show that the designed matrix coil offers high flexibility in generating a local gradient field capable of improving the local resolution. In addition, with enhanced coil performance, the cone-shaped structure provides a patient-friendly solution for head imaging.
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Affiliation(s)
- Liyi Kang
- Department of Biomedical Engineering, Zhejiang University, Hangzhou 310027, China
| | - Fangfang Tang
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
| | - Ling Xia
- Department of Biomedical Engineering, Zhejiang University, Hangzhou 310027, China
| | - Feng Liu
- School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia
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Kopanoglu E, Güngör A, Kilic T, Saritas EU, Oguz KK, Çukur T, Güven HE. Simultaneous use of individual and joint regularization terms in compressive sensing: Joint reconstruction of multi-channel multi-contrast MRI acquisitions. NMR IN BIOMEDICINE 2020; 33:e4247. [PMID: 31970849 DOI: 10.1002/nbm.4247] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/02/2019] [Revised: 11/18/2019] [Accepted: 11/22/2019] [Indexed: 06/10/2023]
Abstract
Multi-contrast images are commonly acquired together to maximize complementary diagnostic information, albeit at the expense of longer scan times. A time-efficient strategy to acquire high-quality multi-contrast images is to accelerate individual sequences and then reconstruct undersampled data with joint regularization terms that leverage common information across contrasts. However, these terms can cause features that are unique to a subset of contrasts to leak into the other contrasts. Such leakage-of-features may appear as artificial tissues, thereby misleading diagnosis. The goal of this study is to develop a compressive sensing method for multi-channel multi-contrast magnetic resonance imaging (MRI) that optimally utilizes shared information while preventing feature leakage. Joint regularization terms group sparsity and colour total variation are used to exploit common features across images while individual sparsity and total variation are also used to prevent leakage of distinct features across contrasts. The multi-channel multi-contrast reconstruction problem is solved via a fast algorithm based on Alternating Direction Method of Multipliers. The proposed method is compared against using only individual and only joint regularization terms in reconstruction. Comparisons were performed on single-channel simulated and multi-channel in-vivo datasets in terms of reconstruction quality and neuroradiologist reader scores. The proposed method demonstrates rapid convergence and improved image quality for both simulated and in-vivo datasets. Furthermore, while reconstructions that solely use joint regularization terms are prone to leakage-of-features, the proposed method reliably avoids leakage via simultaneous use of joint and individual terms, thereby holding great promise for clinical use.
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Affiliation(s)
- Emre Kopanoglu
- Cardiff University, Brain Research Imaging Centre (CUBRIC), School of Psychology, Cardiff University, Cardiff, UK
- ASELSAN Research Center, Ankara, Turkey
| | - Alper Güngör
- ASELSAN Research Center, Ankara, Turkey
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
| | - Toygan Kilic
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
| | - Emine Ulku Saritas
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
- Neuroscience Program, Sabuncu Brain Research Center, Bilkent University, Ankara, Turkey
| | - Kader K Oguz
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
- Department of Radiology, Hacettepe University, Ankara, Turkey
| | - Tolga Çukur
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
- Neuroscience Program, Sabuncu Brain Research Center, Bilkent University, Ankara, Turkey
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Kroboth S, Layton KJ, Jia F, Littin S, Yu H, Hennig J, Zaitsev M. Switching Circuit Optimization for Matrix Gradient Coils. ACTA ACUST UNITED AC 2020; 5:248-259. [PMID: 31245546 PMCID: PMC6588200 DOI: 10.18383/j.tom.2018.00056] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
Matrix gradient coils with up to 84 coil elements were recently introduced for magnetic resonance imaging. Ideally, each element is driven by a dedicated amplifier, which may be technically and financially infeasible. Instead, several elements can be connected in series (called a “cluster”) and driven by a single amplifier. In previous works, a set of clusters, called a “configuration,” was sought to approximate a target field shape. Because a magnetic resonance pulse sequence requires several distinct field shapes, a mechanism to switch between configurations is needed. This can be achieved by a hypothetical switching circuit connecting all terminals of all elements with each other and with the amplifiers. For a predefined set of configurations, a switching circuit can be designed to require only a limited amount of switches. Here we introduce an algorithm to minimize the number of switches without affecting the ability of the configurations to accurately create the desired fields. The problem is modeled using graph theory and split into 2 sequential combinatorial optimization problems that are solved using simulated annealing. For the investigated cases, the results show that compared to unoptimized switching circuits, the reduction of switches in optimized circuits ranges from 8% to up to 44% (average of 31%). This substantial reduction is achieved without impeding circuit functionality. This study shows how technical effort associated with implementation and operation of a matrix gradient coil is related to different hardware setups and how to reduce this effort.
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Affiliation(s)
- Stefan Kroboth
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
| | - Kelvin J Layton
- Institute for Telecommunications Research, University of South Australia, Adelaide, Australia
| | - Feng Jia
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
| | - Sebastian Littin
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
| | - Huijun Yu
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
| | - Jürgen Hennig
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
| | - Maxim Zaitsev
- Department of Radiology, Medical Physics, Medical Center University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany and
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Ye JC. Compressed sensing MRI: a review from signal processing perspective. BMC Biomed Eng 2019; 1:8. [PMID: 32903346 PMCID: PMC7412677 DOI: 10.1186/s42490-019-0006-z] [Citation(s) in RCA: 62] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2018] [Accepted: 02/04/2019] [Indexed: 11/27/2022] Open
Abstract
Magnetic resonance imaging (MRI) is an inherently slow imaging modality, since it acquires multi-dimensional k-space data through 1-D free induction decay or echo signals. This often limits the use of MRI, especially for high resolution or dynamic imaging. Accordingly, many investigators has developed various acceleration techniques to allow fast MR imaging. For the last two decades, one of the most important breakthroughs in this direction is the introduction of compressed sensing (CS) that allows accurate reconstruction from sparsely sampled k-space data. The recent FDA approval of compressed sensing products for clinical scans clearly reflect the maturity of this technology. Therefore, this paper reviews the basic idea of CS and how this technology have been evolved for various MR imaging problems.
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Affiliation(s)
- Jong Chul Ye
- Department of Bio and Brain Engineering, Korea Adv. Inst. of Science & Technology (KAIST), 291 Daehak-ro, Daejeon, Korea
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8
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Dispenza NL, Littin S, Zaitsev M, Constable RT, Galiana G. Clinical Potential of a New Approach to MRI Acceleration. Sci Rep 2019; 9:1912. [PMID: 30760731 PMCID: PMC6374397 DOI: 10.1038/s41598-018-36802-5] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2018] [Accepted: 11/20/2018] [Indexed: 11/11/2022] Open
Abstract
Fast ROtary Nonlinear Spatial ACquisition (FRONSAC) was recently introduced as a new strategy that applies nonlinear gradients as a small perturbation to improve image quality in highly undersampled MRI. In addition to experimentally showing the previously simulated improvement to image quality, this work introduces the insight that Cartesian-FRONSAC retains many desirable features of Cartesian imaging. Cartesian-FRONSAC preserves the existing linear gradient waveforms of the Cartesian sequence while adding oscillating nonlinear gradient waveforms. Experiments show that performance is essentially identical to Cartesian imaging in terms of (1) resilience to experimental imperfections, like timing errors or off-resonance spins, (2) accommodating scan geometry changes without the need for recalibration or additional field mapping, (3) contrast generation, as in turbo spin echo. Despite these similarities to Cartesian imaging, which provides poor parallel imaging performance, Cartesian-FRONSAC consistently shows reduced undersampling artifacts and better response to advanced reconstruction techniques. A final experiment shows that hardware requirements are also flexible. Cartesian-FRONSAC improves accelerated imaging while retaining the robustness and flexibility critical to real clinical use.
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Affiliation(s)
- Nadine L Dispenza
- Department of Biomedical Engineering, Yale University, New Haven, CT, USA
| | - Sebastian Littin
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Maxim Zaitsev
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - R Todd Constable
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT, 06520, USA
- Department of Neurosurgery, Yale University, New Haven, CT, 06520, USA
| | - Gigi Galiana
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT, 06520, USA.
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Ertan K, Taraghinia S, Sadeghi A, Atalar E. A z-gradient array for simultaneous multi-slice excitation with a single-band RF pulse. Magn Reson Med 2017; 80:400-412. [PMID: 29205480 DOI: 10.1002/mrm.27031] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2017] [Revised: 11/13/2017] [Accepted: 11/13/2017] [Indexed: 12/25/2022]
Abstract
PURPOSE Multi-slice radiofrequency (RF) pulses have higher specific absorption rates, more peak RF power, and longer pulse durations than single-slice RF pulses. Gradient field design techniques using a z-gradient array are investigated for exciting multiple slices with a single-band RF pulse. THEORY AND METHODS Two different field design methods are formulated to solve for the required current values of the gradient array elements for the given slice locations. The method requirements are specified, optimization problems are formulated for the minimum current norm and an analytical solution is provided. A 9-channel z-gradient coil array driven by independent, custom-designed gradient amplifiers is used to validate the theory. RESULTS Performance measures such as normalized slice thickness error, gradient strength per unit norm current, power dissipation, and maximum amplitude of the magnetic field are provided for various slice locations and numbers of slices. Two and 3 slices are excited by a single-band RF pulse in simulations and phantom experiments. CONCLUSION The possibility of multi-slice excitation with a single-band RF pulse using a z-gradient array is validated in simulations and phantom experiments. Magn Reson Med 80:400-412, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Koray Ertan
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.,Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
| | - Soheil Taraghinia
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.,Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
| | - Alireza Sadeghi
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.,Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
| | - Ergin Atalar
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey.,Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
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Ertan K, Atalar E. Simultaneous use of linear and nonlinear gradients for B 1+ inhomogeneity correction. NMR IN BIOMEDICINE 2017; 30:e3742. [PMID: 28543797 DOI: 10.1002/nbm.3742] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/22/2016] [Revised: 03/17/2017] [Accepted: 04/04/2017] [Indexed: 06/07/2023]
Abstract
The simultaneous use of linear spatial encoding magnetic fields (L-SEMs) and nonlinear spatial encoding magnetic fields (N-SEMs) in B1+ inhomogeneity problems is formulated and demonstrated with both simulations and experiments. Independent excitation k-space variables for N-SEMs are formulated for the simultaneous use of L-SEMs and N-SEMs by assuming a small tip angle. The formulation shows that, when N-SEMs are considered as an independent excitation k-space variable, numerous different k-space trajectories and frequency weightings differing in dimension, length, and energy can be designed for a given target transverse magnetization distribution. The advantage of simultaneous use of L-SEMs and N-SEMs is demonstrated by B1+ inhomogeneity correction with spoke excitation. To fully utilize the independent k-space formulations, global optimizations are performed for 1D, 2D RF power limited, and 2D RF power unlimited simulations and experiments. Three different cases are compared: L-SEMs alone, N-SEMs alone, and both used simultaneously. In all cases, the simultaneous use of L-SEMs and N-SEMs leads to a decreased standard deviation in the ROI compared with using only L-SEMs or N-SEMs. The simultaneous use of L-SEMs and N-SEMs results in better B1+ inhomogeneity correction than using only L-SEMs or N-SEMs due to the increased number of degrees of freedom.
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Affiliation(s)
- Koray Ertan
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey
- Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
| | - Ergin Atalar
- National Magnetic Resonance Research Center (UMRAM), Bilkent University, Bilkent, Ankara, Turkey
- Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara, Turkey
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Littin S, Jia F, Layton KJ, Kroboth S, Yu H, Hennig J, Zaitsev M. Development and implementation of an 84-channel matrix gradient coil. Magn Reson Med 2017; 79:1181-1191. [PMID: 28444778 DOI: 10.1002/mrm.26700] [Citation(s) in RCA: 36] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2016] [Revised: 02/27/2017] [Accepted: 03/13/2017] [Indexed: 01/07/2023]
Abstract
PURPOSE Design, implement, integrate, and characterize a customized coil system that allows for generating spatial encoding magnetic fields (SEMs) in a highly-flexible fashion. METHODS A gradient coil with a high number of individual elements was designed. Dimensions of the coil were chosen to mimic a whole-body gradient system, scaled down to a head insert. Mechanical shape and wire layout of each element were optimized to increase the local gradient strength while minimizing eddy current effects and simultaneously considering manufacturing constraints. RESULTS Resulting wire layout and mechanical design is presented. A prototype matrix gradient coil with 12 × 7 = 84 elements consisting of two element types was realized and characterized. Measured eddy currents are <1% of the original field. The coil is shown to be capable of creating nonlinear, and linear SEMs. In a DSV of 0.22 m gradient strengths between 24 mT∕m and 78 mT∕m could be realized locally with maximum currents of 150 A. Initial proof-of-concept imaging experiments using linear and nonlinear encoding fields are demonstrated. CONCLUSION A shielded matrix gradient coil setup capable of generating encoding fields in a highly-flexible manner was designed and implemented. The presented setup is expected to serve as a basis for validating novel imaging techniques that rely on nonlinear spatial encoding fields. Magn Reson Med 79:1181-1191, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Sebastian Littin
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Feng Jia
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Kelvin J Layton
- Institute for Telecommunications Research, University of South Australia, Mawson Lakes, South Australia, Australia
| | - Stefan Kroboth
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Huijun Yu
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Jürgen Hennig
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
| | - Maxim Zaitsev
- Department of Radiology, Medical Physics, Medical Center - University of Freiburg, Faculty of Medicine, University of Freiburg, Freiburg, Germany
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Salajeghe S, Babyn P, Sarty GE. Composite pulses for RF phase encoded MRI: A simulation study. Magn Reson Imaging 2017; 36:40-48. [DOI: 10.1016/j.mri.2016.10.007] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2016] [Revised: 10/02/2016] [Accepted: 10/05/2016] [Indexed: 10/20/2022]
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13
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Wang H, Tam L, Kopanoglu E, Peters DC, Constable RT, Galiana G. O-space with high resolution readouts outperforms radial imaging. Magn Reson Imaging 2016; 37:107-115. [PMID: 27876569 DOI: 10.1016/j.mri.2016.11.012] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2015] [Revised: 11/17/2016] [Accepted: 11/17/2016] [Indexed: 11/25/2022]
Abstract
PURPOSE While O-Space imaging is well known to accelerate image acquisition beyond traditional Cartesian sampling, its advantages compared to undersampled radial imaging, the linear trajectory most akin to O-Space imaging, have not been detailed. In addition, previous studies have focused on ultrafast imaging with very high acceleration factors and relatively low resolution. The purpose of this work is to directly compare O-Space and radial imaging in their potential to deliver highly undersampled images of high resolution and minimal artifacts, as needed for diagnostic applications. We report that the greatest advantages to O-Space imaging are observed with extended data acquisition readouts. THEORY AND METHODS A sampling strategy that uses high resolution readouts is presented and applied to compare the potential of radial and O-Space sequences to generate high resolution images at high undersampling factors. Simulations and phantom studies were performed to investigate whether use of extended readout windows in O-Space imaging would increase k-space sampling and improve image quality, compared to radial imaging. RESULTS Experimental O-Space images acquired with high resolution readouts show fewer artifacts and greater sharpness than radial imaging with equivalent scan parameters. Radial images taken with longer readouts show stronger undersampling artifacts, which can cause small or subtle image features to disappear. These features are preserved in a comparable O-Space image. CONCLUSIONS High resolution O-Space imaging yields highly undersampled images of high resolution and minimal artifacts. The additional nonlinear gradient field improves image quality beyond conventional radial imaging.
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Affiliation(s)
- Haifeng Wang
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA
| | - Leo Tam
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA
| | - Emre Kopanoglu
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA
| | - Dana C Peters
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA
| | - R Todd Constable
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA; Department of Neurosurgery, Yale University, New Haven, CT 06520, USA
| | - Gigi Galiana
- Department of Radiology and Biomedical Imaging, Yale University, New Haven, CT 06520, USA.
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Stockmann JP, Cooley CZ, Guerin B, Rosen MS, Wald LL. Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2016; 268:36-48. [PMID: 27155906 PMCID: PMC4909507 DOI: 10.1016/j.jmr.2016.04.005] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Revised: 03/17/2016] [Accepted: 04/07/2016] [Indexed: 06/01/2023]
Abstract
Transmit Array Spatial Encoding (TRASE) is a promising new MR encoding method that uses transmit RF (B1(+)) phase gradients over the field-of-view to perform Fourier spatial encoding. Acquisitions use a spin echo train in which the transmit coil phase ramp is modulated to jump from one k-space point to the next. This work extends the capability of TRASE by using swept radiofrequency (RF) pulses and a quadratic phase removal method to enable TRASE where it is arguably most needed: portable imaging systems with inhomogeneous B0 fields. The approach is particularly well-suited for portable MR scanners where (a) inhomogeneous B0 fields are a byproduct of lightweight magnet design, (b) heavy, high power-consumption gradient coil systems are a limitation to siting the system in non-conventional locations and (c) synergy with the use of spin echo trains is required to overcome intra-voxel dephasing (short T2(∗)) in the inhomogeneous field. TRASE does not use a modulation of the B0 field to encode, but it does suffer from secondary effects of the inhomogeneous field. Severe artifacts arise in TRASE images due to off-resonance effects when the RF pulse does not cover the full bandwidth of spin resonances in the imaging FOV. Thus, for highly inhomogeneous B0 fields, the peak RF power needed for high-bandwidth refocusing hard pulses becomes very expensive, in addition to requiring RF coils that can withstand thousands of volts. In this work, we use swept WURST RF pulse echo trains to achieve TRASE imaging in a highly inhomogeneous magnetic field (ΔB0/B0∼0.33% over the sample). By accurately exciting and refocusing the full bandwidth of spins, the WURST pulses eliminate artifacts caused by the limited bandwidth of the hard pulses used in previous realizations of TRASE imaging. We introduce a correction scheme to remove the unwanted quadratic phase modulation caused by the swept pulses. Also, a phase alternation scheme is employed to mitigate artifacts caused by mixture of the even and odd-echo coherence pathways due to defects in the refocusing pulse. In this paper, we describe this needed methodology and demonstrate the ability of TRASE to Fourier encode in an inhomogeneous field (ΔB0/B0∼1% over the full FOV).
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Affiliation(s)
- Jason P Stockmann
- A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129, United States.
| | - Clarissa Z Cooley
- A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129, United States
| | - Bastien Guerin
- A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129, United States; Harvard Medical School, Boston, MA, United States
| | - Matthew S Rosen
- A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129, United States; Department of Physics, Harvard University, Cambridge, MA 02141, United States; Harvard Medical School, Boston, MA, United States
| | - Lawrence L Wald
- A.A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA 02129, United States; Harvard Medical School, Boston, MA, United States
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15
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Jia F, Schultz G, Testud F, Welz AM, Weber H, Littin S, Yu H, Hennig J, Zaitsev M. Performance evaluation of matrix gradient coils. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2015; 29:59-73. [PMID: 26667966 DOI: 10.1007/s10334-015-0519-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/05/2015] [Revised: 10/19/2015] [Accepted: 12/06/2015] [Indexed: 01/06/2023]
Abstract
OBJECTIVE In this paper, we present a new performance measure of a matrix coil (also known as multi-coil) from the perspective of efficient, local, non-linear encoding without explicitly considering target encoding fields. MATERIALS AND METHODS An optimization problem based on a joint optimization for the non-linear encoding fields is formulated. Based on the derived objective function, a figure of merit of a matrix coil is defined, which is a generalization of a previously known resistive figure of merit for traditional gradient coils. RESULTS A cylindrical matrix coil design with a high number of elements is used to illustrate the proposed performance measure. The results are analyzed to reveal novel features of matrix coil designs, which allowed us to optimize coil parameters, such as number of coil elements. A comparison to a scaled, existing multi-coil is also provided to demonstrate the use of the proposed performance parameter. CONCLUSIONS The assessment of a matrix gradient coil profits from using a single performance parameter that takes the local encoding performance of the coil into account in relation to the dissipated power.
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Affiliation(s)
- Feng Jia
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany.
| | - Gerrit Schultz
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Frederik Testud
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Anna Masako Welz
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Hans Weber
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Sebastian Littin
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Huijun Yu
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Jürgen Hennig
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Maxim Zaitsev
- Medical Physics, Department of Radiology, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
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16
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Weber H, Schultz G, Gallichan D, Hennig J, Zaitsev M. Local field of view imaging for alias-free undersampling with nonlinear spatial encoding magnetic fields. Magn Reson Med 2015; 71:1002-14. [PMID: 23649975 DOI: 10.1002/mrm.24754] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
PURPOSE Nonlinear spatial encoding magnetic fields result in an inhomogeneous image resolution. Within this study, this characteristic property of nonlinear encoding is investigated with regard to its potential to accelerate MRI acquisitions. THEORY A dependency between k-space coverage and local resolvability of the image causes k-space samples to have a spatially localized contribution to the reconstruction of the spin density. On the basis of this observation, a concept for alias-free data undersampling is developed, which is referred to as the local field of view concept. METHODS On the basis of this concept, a fast sampling trajectory is developed. It is evaluated with simulations and experiments (both using a phantom and in vivo) for MRI with, as an example, pure quadrupolar encoding fields. To demonstrate that the concept is only applicable to (spatially) nonlinear encoding, a comparison with linear encoding is provided. RESULTS Application of the local field of view concept results in a localized adaptation of the image resolution by undersampling higher frequency k-space samples without introducing aliasing. CONCLUSIONS A new effect of nonlinear spatial encoding magnetic fields was found, which allows more efficient data sampling and at the same time counterbalancing the natural variation in image resolution.
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Affiliation(s)
- Hans Weber
- Department of Radiology-Medical Physics, University Medical Center Freiburg, Freiburg, Germany
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17
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Kopanoglu E, Constable RT. Radiofrequency pulse design using nonlinear gradient magnetic fields. Magn Reson Med 2015; 74:826-39. [PMID: 25203286 PMCID: PMC4362804 DOI: 10.1002/mrm.25423] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2014] [Revised: 07/15/2014] [Accepted: 08/01/2014] [Indexed: 11/10/2022]
Abstract
PURPOSE An iterative k-space trajectory and radiofrequency (RF) pulse design method is proposed for excitation using nonlinear gradient magnetic fields. THEORY AND METHODS The spatial encoding functions (SEFs) generated by nonlinear gradient fields are linearly dependent in Cartesian coordinates. Left uncorrected, this may lead to flip angle variations in excitation profiles. In the proposed method, SEFs (k-space samples) are selected using a matching pursuit algorithm, and the RF pulse is designed using a conjugate gradient algorithm. Three variants of the proposed approach are given: the full algorithm, a computationally cheaper version, and a third version for designing spoke-based trajectories. The method is demonstrated for various target excitation profiles using simulations and phantom experiments. RESULTS The method is compared with other iterative (matching pursuit and conjugate gradient) and noniterative (coordinate-transformation and Jacobian-based) pulse design methods as well as uniform density spiral and EPI trajectories. The results show that the proposed method can increase excitation fidelity. CONCLUSION An iterative method for designing k-space trajectories and RF pulses using nonlinear gradient fields is proposed. The method can either be used for selecting the SEFs individually to guide trajectory design, or can be adapted to design and optimize specific trajectories of interest.
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Affiliation(s)
- Emre Kopanoglu
- Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, CT, USA 06520
| | - R. Todd Constable
- Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, CT, USA 06520
- Department of Neurosurgery, Yale University School of Medicine, New Haven, CT, USA 06520
- Department of Biomedical Engineering, Yale University School of Medicine, New Haven, CT, USA 06520
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18
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Layton KJ, Kroboth S, Jia F, Littin S, Yu H, Zaitsev M. Trajectory optimization based on the signal-to-noise ratio for spatial encoding with nonlinear encoding fields. Magn Reson Med 2015; 76:104-17. [PMID: 26243290 DOI: 10.1002/mrm.25859] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2014] [Revised: 07/03/2015] [Accepted: 07/07/2015] [Indexed: 11/06/2022]
Abstract
PURPOSE Multiple nonlinear gradient fields offer many potential benefits for spatial encoding including reduced acquisition time, fewer artefacts and region-specific imaging, although designing a suitable trajectory for such a setup is difficult. This work aims to optimize encoding trajectories for multiple nonlinear gradient fields based on the image signal-to-noise ratio. THEORY AND METHODS Image signal-to-noise ratio is directly linked to the covariance of the reconstructed pixels, which can be calculated recursively for each projection of the trajectory under a Bayesian formulation. An evolutionary algorithm is used to find the higher-dimensional projections that minimize the pixel covariance, incorporating receive coil profiles, intravoxel dephasing, and reconstruction regularization. The resulting trajectories are tested through simulations and experiments. RESULTS The optimized trajectories produce images with higher resolution and fewer artefacts compared with traditional approaches, particularly for high undersampling. However, higher-dimensional projection experiments strongly depend on accurate hardware and calibration. CONCLUSION Computer-based optimization provides an efficient means to explore the large trajectory space created by the use of multiple nonlinear encoding fields. The optimization framework, as presented here, is necessary to fully exploit the advantages of nonlinear fields. Magn Reson Med 76:104-117, 2016. © 2015 Wiley Periodicals, Inc.
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Affiliation(s)
- Kelvin J Layton
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
| | - Stefan Kroboth
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
| | - Feng Jia
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
| | - Sebastian Littin
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
| | - Huijun Yu
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
| | - Maxim Zaitsev
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
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19
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Littin S, Gallichan D, Welz AM, Jia F, Dewdney A, Weber H, Schultz G, Hennig J, Zaitsev M. Monoplanar gradient system for imaging with nonlinear gradients. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2015; 28:447-57. [PMID: 25684133 DOI: 10.1007/s10334-015-0481-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/08/2014] [Revised: 01/05/2015] [Accepted: 01/22/2015] [Indexed: 11/27/2022]
Abstract
OBJECT In this paper we present a monoplanar gradient system capable of imaging a volume comparable with that covered by linear gradient systems. Such a system has been designed and implemented. MATERIALS AND METHODS Building such a system was made possible by relaxing the constraint of global linearity and replacing it with a requirement for local orthogonality. A framework was derived for optimization of local orthogonality within the physical boundaries and geometric constraints. Spatial encoding of magnetic fields was optimized for their local orthogonality over a large field of view. RESULTS A coil design consisting of straight wire segments was optimized, implemented, and integrated into a 3T human scanner to show the feasibility of this approach. Initial MR images are shown and further applications of the derived optimization method and the nonlinear planar gradient system are discussed. CONCLUSION Encoding fields generated by the prototype encoding system were shown to be locally orthogonal and able to encode a cylindrical volume sufficient for some abdomen imaging applications for humans.
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Affiliation(s)
- Sebastian Littin
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany.
| | | | - Anna Masako Welz
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Feng Jia
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | | | - Hans Weber
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Gerrit Schultz
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Jürgen Hennig
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
| | - Maxim Zaitsev
- Department of Diagnostic Radiology, Medical Physics, University Medical Center Freiburg, Breisacher Str. 60a, 79106, Freiburg, Germany
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20
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Tam LK, Galiana G, Stockmann JP, Tagare H, Peters DC, Constable RT. Pseudo-random center placement O-space imaging for improved incoherence compressed sensing parallel MRI. Magn Reson Med 2014; 73:2212-24. [PMID: 25042143 DOI: 10.1002/mrm.25364] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2013] [Revised: 06/23/2014] [Accepted: 06/24/2014] [Indexed: 11/09/2022]
Abstract
PURPOSE Nonlinear spatial encoding magnetic (SEM) field strategies such as O-space imaging have previously reported dispersed artifacts during accelerated scans. Compressed sensing (CS) has shown a sparsity-promoting convex program allows image reconstruction from a reduced data set when using the appropriate sampling. The development of a pseudo-random center placement (CP) O-space CS approach optimizes incoherence through SEM field modulation to reconstruct an image with reduced error. THEORY AND METHODS The incoherence parameter determines the sparsity levels for which CS is valid and the related transform point spread function measures the maximum interference for a single point. The O-space acquisition is optimized for CS by perturbing the Z(2) strength within 30% of the nominal value and demonstrated on a human 3T scanner. RESULTS Pseudo-random CP O-space imaging is shown to improve incoherence between the sensing and sparse domains. Images indicate pseudo-random CP O-space has reduced mean squared error compared with a typical linear SEM field acquisition method. CONCLUSION Pseudo-random CP O-space imaging, with a nonlinear SEM field designed for CS, is shown to reduce mean squared error of images at high acceleration over linear encoding methods for a 2D slice when using an eight channel circumferential receiver array for parallel imaging.
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Affiliation(s)
- Leo K Tam
- Yale University, Department of Diagnostic Radiology, New Haven, Connecticut, USA
| | - Gigi Galiana
- Yale University, Department of Diagnostic Radiology, New Haven, Connecticut, USA
| | - Jason P Stockmann
- Massachusetts General Hospital Martinos Center for Imaging, Boston, Massachusetts, USA
| | - Hemant Tagare
- Yale University, Department of Diagnostic Radiology, New Haven, Connecticut, USA.,Yale University, Department of Electrical Engineering, New Haven, Connecticut, USA.,Yale University, Department of Biomedical Engineering, New Haven, Connecticut, USA
| | - Dana C Peters
- Yale University, Department of Diagnostic Radiology, New Haven, Connecticut, USA
| | - R Todd Constable
- Yale University, Department of Diagnostic Radiology, New Haven, Connecticut, USA.,Yale University, Department of Biomedical Engineering, New Haven, Connecticut, USA.,Yale University, Department of Neurosurgery, New Haven, Connecticut, USA
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21
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Zaitsev M, Schultz G, Hennig J, Gruetter R, Gallichan D. Parallel imaging with phase scrambling. Magn Reson Med 2014; 73:1407-19. [DOI: 10.1002/mrm.25252] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2013] [Revised: 03/20/2014] [Accepted: 03/24/2014] [Indexed: 11/11/2022]
Affiliation(s)
- Maxim Zaitsev
- Medical Physics; Department of Radiology; University Medical Center Freiburg; Freiburg Germany
| | - Gerrit Schultz
- Medical Physics; Department of Radiology; University Medical Center Freiburg; Freiburg Germany
| | - Juergen Hennig
- Medical Physics; Department of Radiology; University Medical Center Freiburg; Freiburg Germany
| | - Rolf Gruetter
- Laboratory for Functional and Metabolic Imaging, EPFL; Lausanne Switzerland
- Department of Radiology; University of Lausanne; Lausanne Switzerland
- Department of Radiology; University of Geneva; Geneva Switzerland
| | - Daniel Gallichan
- Laboratory for Functional and Metabolic Imaging, EPFL; Lausanne Switzerland
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22
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Witschey WR, Littin S, Cocosco CA, Gallichan D, Schultz G, Weber H, Welz A, Hennig J, Zaitsev M. Stages: sub-Fourier dynamic shim updating using nonlinear magnetic field phase preparation. Magn Reson Med 2013; 71:57-66. [PMID: 23440677 DOI: 10.1002/mrm.24625] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2012] [Revised: 11/05/2012] [Accepted: 12/11/2012] [Indexed: 11/09/2022]
Abstract
Heterogeneity of the static magnetic field in magnetic resonance imaging may cause image artifacts and degradation in image quality. The field heterogeneity can be reduced by dynamically adjusting shim fields or dynamic shim updating, in which magnetic field homogeneity is optimized for each tomographic slice to improve image quality. A limitation of this approach is that a new magnetic field can be applied only once for each slice, otherwise image quality would improve somewhere to its detriment elsewhere in the slice. The motivation of this work is to overcome this limitation and develop a technique using nonlinear magnetic fields to dynamically shim the static magnetic field within a single Fourier-encoded volume or slice, called sub-Fourier dynamic shim updating. However, the nonlinear magnetic fields are not used as shim fields; instead, they impart a strong spatial dependence to the acquired MR signal by nonlinear phase preparation, which may be exploited to locally improve magnetic field homogeneity during acquisition. A theoretical description of the method is detailed, simulations and a proof-of-principle experiment are performed using a magnet coil with a known field geometry. The method is shown to remove artifacts associated with magnetic field homogeneity in balanced steady-state free-precession pulse sequences. We anticipate that this method will be useful to improve the quality of magnetic resonance images by removing deleterious artifacts associated with a heterogeneous static magnetic field.
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Affiliation(s)
- Walter Rt Witschey
- University of Pennsylvania, Philadelphia, PA, USA.,University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Sebastian Littin
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Chris A Cocosco
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Daniel Gallichan
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Gerrit Schultz
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Hans Weber
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Anna Welz
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Jürgen Hennig
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
| | - Maxim Zaitsev
- University Medical Center Freiburg, Freiburg i. Breisgau, Germany
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23
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Stockmann JP, Galiana G, Tam L, Juchem C, Nixon TW, Constable RT. In vivo O-Space imaging with a dedicated 12 cm Z2 insert coil on a human 3T scanner using phase map calibration. Magn Reson Med 2013; 69:444-55. [PMID: 22585546 PMCID: PMC3491108 DOI: 10.1002/mrm.24282] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2011] [Revised: 01/28/2012] [Accepted: 03/03/2012] [Indexed: 11/11/2022]
Abstract
Recently, spatial encoding with nonlinear magnetic fields has drawn attention for its potential to achieve faster gradient switching within safety limits, tailored resolution in regions of interest, and improved parallel imaging using encoding fields that complement the sensitivity profiles of radio frequency receive arrays. Proposed methods can broadly be divided into those that use phase encoding (Cartesian-trajectory PatLoc and COGNAC) and those that acquire nonlinear projections (O-Space, Null space imaging, radial PatLoc, and 4D-RIO). Nonlinear projection data are most often reconstructed with iterative algorithms that backproject data using the full encoding matrix. Just like conventional radial sequences that use linear spatial encoding magnetic fields, nonlinear projection methods are more sensitive than phase encoding methods to imperfect calibration of the encoding fields. In this work, voxel-wise phase evolution is mapped at each acquired point in an O-Space trajectory using a variant of chemical shift imaging, capturing all spin dynamics caused by encoding fields, eddy currents, and pulse timing. Phase map calibration is then applied to data acquired from a high-power, 12 cm, Z2 insert coil with an eight-channel radio frequency transmit-receive array on a 3T human scanner. We show the first experimental proof-of-concept O-Space images on in vivo and phantom samples, paving the way for more in-depth exploration of O-Space and similar imaging methods.
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Affiliation(s)
- Jason P Stockmann
- Department of Biomedical Engineering, Yale University, New Haven, Connecticut 06520, USA.
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24
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Haas M, Ullmann P, Schneider JT, Post H, Ruhm W, Hennig J, Zaitsev M. PexLoc-Parallel excitation using local encoding magnetic fields with nonlinear and nonbijective spatial profiles. Magn Reson Med 2012. [PMID: 23203228 DOI: 10.1002/mrm.24559] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
With the recent proposal of using magnetic fields that are nonlinear by design for spatial encoding, new flexibility has been introduced to MR imaging. The new degrees of freedom in shaping the spatially encoding magnetic fields (SEMs) can be used to locally adapt the imaging resolution to features of the imaged object, e.g., anatomical structures, to reduce peripheral nerve stimulation during in vivo experiments or to increase the gradient switching speed by reducing the inductance of the coils producing the SEMs and thus accelerate the imaging process. In this work, the potential of nonlinear and nonbijective SEMs for spatial encoding during transmission in multidimensional spatially selective excitation is explored. Methods for multidimensional spatially selective excitation radiofrequency pulse design based on nonlinear encoding fields are introduced, and it is shown how encoding ambiguities can be resolved using parallel transmission. In simulations and phantom experiments, the feasibility of selective excitation using nonlinear, nonbijective SEMs is demonstrated, and it is shown that the spatial resolution with which the target distribution of the transverse magnetization can be realized varies locally. Thus, the resolution of the target pattern can be increased in some regions compared with conventional linear encoding. Furthermore, experimental proof of principle of accelerated two-dimensional spatially selective excitation using nonlinear SEMs is provided in this study.
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Affiliation(s)
- M Haas
- Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany
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25
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Lin FH, Witzel T, Schultz G, Gallichan D, Kuo WJ, Wang FN, Hennig J, Zaitsev M, Belliveau JW. Reconstruction of MRI data encoded by multiple nonbijective curvilinear magnetic fields. Magn Reson Med 2012; 68:1145-56. [PMID: 22246786 PMCID: PMC3736833 DOI: 10.1002/mrm.24115] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2011] [Revised: 10/18/2011] [Accepted: 11/18/2011] [Indexed: 11/07/2022]
Abstract
Parallel imaging technique using localized gradients (PatLoc) uses the combination of surface gradient coils generating nonbijective curvilinear magnetic fields for spatial encoding. PatLoc imaging using one pair of multipolar spatial encoding magnetic fields (SEMs) has two major caveats: (1) The direct inversion of the encoding matrix requires exact determination of multiple locations which are ambiguously encoded by the SEMs. (2) Reconstructed images have a prominent loss of spatial resolution at the center of field-of-view using a symmetric coil array for signal detection. This study shows that a PatLoc system actually has a higher degree of freedom in spatial encoding to mitigate the two challenges mentioned above. Specifically, a PatLoc system can generate not only multipolar but also linear SEMs, which can be used to reduce the loss of spatial resolution at the field-of-view center. Here, we present an efficient and generalized image reconstruction method for PatLoc imaging using multiple SEMs without explicitly identifying the locations where SEM encoding is not unique. Reconstructions using simulations and empirical experimental data are compared with those using conventional linear gradients to demonstrate that the general combination of SEMs can improve image reconstructions.
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Affiliation(s)
- Fa-Hsuan Lin
- Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan
- Harvard Medical School - Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA
| | - Thomas Witzel
- Harvard Medical School - Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA
| | - Gerrit Schultz
- Department of Radiology, University Medical Center Freiburg, Freiburg, Germany
| | - Daniel Gallichan
- Department of Radiology, University Medical Center Freiburg, Freiburg, Germany
| | - Wen-Jui Kuo
- Institute of Neuroscience, National Yang-Ming University, Taipei, Taiwan
| | - Fu-Nien Wang
- Department of Biomedical Engineering and Environmental Sciences, National Tsing-Hua University, Hsinchu, Taiwan
| | - Juergen Hennig
- Department of Radiology, University Medical Center Freiburg, Freiburg, Germany
| | - Maxim Zaitsev
- Department of Radiology, University Medical Center Freiburg, Freiburg, Germany
| | - John W. Belliveau
- Harvard Medical School - Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA
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26
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Galiana G, Stockmann JP, Tam L, Peters D, Tagare H, Constable RT. The Role of Nonlinear Gradients in Parallel Imaging: A k-Space Based Analysis. CONCEPTS IN MAGNETIC RESONANCE. PART A, BRIDGING EDUCATION AND RESEARCH 2012; 40A:253-267. [PMID: 26604857 PMCID: PMC4655121 DOI: 10.1002/cmr.a.21243] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Sequences that encode the spatial information of an object using nonlinear gradient fields are a new frontier in MRI, with potential to provide lower peripheral nerve stimulation, windowed fields of view, tailored spatially-varying resolution, curved slices that mirror physiological geometry, and, most importantly, very fast parallel imaging with multichannel coils. The acceleration for multichannel images is generally explained by the fact that curvilinear gradient isocontours better complement the azimuthal spatial encoding provided by typical receiver arrays. However, the details of this complementarity have been more difficult to specify. We present a simple and intuitive framework for describing the mechanics of image formation with nonlinear gradients, and we use this framework to review some the main classes of nonlinear encoding schemes.
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Affiliation(s)
- Gigi Galiana
- Department of Diagnostic Radiology, Yale University, New Haven, CT
| | - Jason P Stockmann
- Department of Biomedical Engineering, Yale University, New Haven, CT
| | - Leo Tam
- Department of Biomedical Engineering, Yale University, New Haven, CT
| | - Dana Peters
- Department of Diagnostic Radiology, Yale University, New Haven, CT
| | - Hemant Tagare
- Department of Diagnostic Radiology, Yale University, New Haven, CT ; Department of Biomedical Engineering, Yale University, New Haven, CT
| | - R Todd Constable
- Department of Diagnostic Radiology, Yale University, New Haven, CT ; Department of Biomedical Engineering, Yale University, New Haven, CT ; Department of Neurosurgery, Yale University, New Haven, CT
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Knoll F, Schultz G, Bredies K, Gallichan D, Zaitsev M, Hennig J, Stollberger R. Reconstruction of undersampled radial PatLoc imaging using total generalized variation. Magn Reson Med 2012; 70:40-52. [PMID: 22847824 DOI: 10.1002/mrm.24426] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2012] [Revised: 06/05/2012] [Accepted: 06/29/2012] [Indexed: 11/11/2022]
Abstract
In the case of radial imaging with nonlinear spatial encoding fields, a prominent star-shaped artifact has been observed if a spin distribution is encoded with an undersampled trajectory. This work presents a new iterative reconstruction method based on the total generalized variation, which reduces this artifact. For this approach, a sampling operator (as well as its adjoint) is needed that maps data from PatLoc k-space to the final image space. It is shown that this can be realized as a type-3 nonuniform fast Fourier transform, which is implemented by a combination of a type-1 and type-2 nonuniform fast Fourier transform. Using this operator, it is also possible to implement an iterative conjugate gradient SENSE based method for PatLoc reconstruction, which leads to a significant reduction of computation time in comparison to conventional PatLoc image reconstruction methods. Results from numerical simulations and in vivo PatLoc measurements with as few as 16 radial projections are presented, which demonstrate significant improvements in image quality with the total generalized variation-based approach.
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Affiliation(s)
- Florian Knoll
- Institute of Medical Engineering, Graz University of Technology, Kronesgasse 5, A-8010 Graz, Austria.
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28
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Gallichan D, Cocosco CA, Schultz G, Weber H, Welz AM, Hennig J, Zaitsev M. Practical considerations for in vivo MRI with higher dimensional spatial encoding. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2012; 25:419-31. [DOI: 10.1007/s10334-012-0314-y] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2011] [Revised: 03/06/2012] [Accepted: 03/14/2012] [Indexed: 11/29/2022]
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