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Ouyang B, Yang Q, Wang X, He H, Ma L, Yang Q, Zhou Z, Cai S, Chen Z, Wu Z, Zhong J, Cai C. Single-shot T 2 mapping via multi-echo-train multiple overlapping-echo detachment planar imaging and multitask deep learning. Med Phys 2022; 49:7095-7107. [PMID: 35765150 DOI: 10.1002/mp.15820] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2022] [Revised: 05/02/2022] [Accepted: 06/13/2022] [Indexed: 12/13/2022] Open
Abstract
BACKGROUND Quantitative magnetic resonance imaging provides robust biomarkers in clinics. Nevertheless, the lengthy scan time reduces imaging throughput and increases the susceptibility of imaging results to motion. In this context, a single-shot T2 mapping method based on multiple overlapping-echo detachment (MOLED) planar imaging was presented, but the relatively small echo time range limits its accuracy, especially in tissues with large T2 . PURPOSE In this work we proposed a novel single-shot method, Multi-Echo-Train Multiple OverLapping-Echo Detachment (METMOLED) planar imaging, to accommodate a large range of T2 quantification without additional measurements to rectify signal degeneration arisen from refocusing pulse imperfection. METHODS Multiple echo-train techniques were integrated into the MOLED sequence to capture larger TE information. Maps of T2 , B1 , and spin density were reconstructed synchronously from acquired METMOLED data via multitask deep learning. A typical U-Net was trained with 3000/600 synthetic data with geometric/brain patterns to learn the mapping relationship between METMOLED signals and quantitative maps. The refocusing pulse imperfection was settled through the inherent information of METMOLED data and auxiliary tasks. RESULTS Experimental results on the digital brain (structural similarity (SSIM) index = 0.975/0.991/0.988 for MOLED/METMOLED-2/METMOLED-3, hyphenated number denotes the number of echo-trains), physical phantom (the slope of linear fitting with reference T2 map = 1.047/1.017/1.006 for MOLED/METMOLED-2/METMOLED-3), and human brain (Pearson's correlation coefficient (PCC) = 0.9581/0.9760/0.9900 for MOLED/METMOLED-2/METMOLED-3) demonstrated that the METMOLED improved the quantitative accuracy and the tissue details in contrast to the MOLED. These improvements were more pronounced in tissues with large T2 and in application scenarios with high temporal resolution (PCC = 0.8692/0.9465/0.9743 for MOLED/METMOLED-2/METMOLED-3). Moreover, the METMOLED could rectify the signal deviations induced by the non-ideal slice profiles of refocusing pulses without additional measurements. A preliminary measurement also demonstrated that the METMOLED is highly repeatable (mean coefficient of variation (CV) = 1.65%). CONCLUSIONS METMOLED breaks the restriction of echo-train length to TE and implements unbiased T2 estimates in an extensive range. Furthermore, it corrects the effect of refocusing pulse inaccuracy without additional measurements or signal post-processing, thus retaining its single-shot characteristic. This technique would be beneficial for accurate T2 quantification.
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Affiliation(s)
- Binyu Ouyang
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Qizhi Yang
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Xiaoyin Wang
- Center for Brain Imaging Science and Technology, College of Biomedical Engineering and Instrumental Science, Zhejiang University, Hangzhou, Zhejiang, 310058, China
| | - Hongjian He
- Center for Brain Imaging Science and Technology, College of Biomedical Engineering and Instrumental Science, Zhejiang University, Hangzhou, Zhejiang, 310058, China
| | - Lingceng Ma
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Qinqin Yang
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Zihan Zhou
- Center for Brain Imaging Science and Technology, College of Biomedical Engineering and Instrumental Science, Zhejiang University, Hangzhou, Zhejiang, 310058, China
| | - Shuhui Cai
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Zhong Chen
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
| | - Zhigang Wu
- MSC Clinical and Technical Solutions, Philips Healthcare, Shenzhen, Guangdong, 518005, China
| | - Jianhui Zhong
- Center for Brain Imaging Science and Technology, College of Biomedical Engineering and Instrumental Science, Zhejiang University, Hangzhou, Zhejiang, 310058, China.,Department of Imaging Sciences, University of Rochester, Rochester, New York, 14642, USA
| | - Congbo Cai
- Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Xiamen University, Xiamen, Fujian, 361005, China
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Geldschläger O, Bosch D, Henning A. OTUP workflow: target specific optimization of the transmit k-space trajectory for flexible universal parallel transmit RF pulse design. NMR IN BIOMEDICINE 2022; 35:e4728. [PMID: 35297104 DOI: 10.1002/nbm.4728] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2021] [Revised: 02/09/2022] [Accepted: 03/14/2022] [Indexed: 06/14/2023]
Abstract
PURPOSE To optimize transmit k-space trajectories for a wide range of excitation targets and to design "universal pTx RF pulses" based on these trajectories. METHODS Transmit k-space trajectories (stack of spirals and SPINS) were optimized to best match different excitation targets using the parameters of the analytical equations of spirals and SPINS. The performances of RF pulses designed based on optimized and non-optimized trajectories were compared. The optimized trajectories were utilized for universal pulse design. The universal pulse performances were compared with subject specific tailored pulse performances. The OTUP workflow (optimization of transmit k-space trajectories and universal pulse calculation) was tested on three test target excitation patterns. For one target (local excitation of a central area in the human brain) the pulses were tested in vivo at 9.4 T. RESULTS The workflow produced appropriate transmit k-space trajectories for each test target. Utilization of an optimized trajectory was crucial for the pulse performance. Using unsuited trajectories diminished the performance. It was possible to create target specific universal pulses. However, not every test target is equally well suited for universal pulse design. There was no significant difference in the in vivo performance between subject specific tailored pulses and a universal pulse at 9.4 T. CONCLUSIONS The proposed workflow further exploited and improved the universal pulse concept by combining it with gradient trajectory optimization for stack of spirals and SPINS. It emphasized the importance of a well suited trajectory for pTx RF pulse design. Universal and tailored pulses performed with a sufficient degree of similarity in simulations and a high degree of similarity in vivo. The implemented OTUP workflow and the B0 /B1+ map data from 18 subjects measured at 9.4 T are available as open source (https://github.com/ole1965/workflow_OTUP.git).
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Affiliation(s)
- Ole Geldschläger
- High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Dario Bosch
- High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
- Biomedical Magnetic Resonance, University Hospital Tübingen, Tübingen, Germany
| | - Anke Henning
- High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
- Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, USA
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Froidevaux R, Weiger M, Pruessmann KP. Pulse encoding for ZTE imaging: RF excitation without dead-time penalty. Magn Reson Med 2021; 87:1360-1374. [PMID: 34775617 PMCID: PMC9299067 DOI: 10.1002/mrm.29056] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2021] [Revised: 09/09/2021] [Accepted: 10/04/2021] [Indexed: 11/23/2022]
Abstract
Purpose To overcome limitations in the duration of RF excitation in zero‐TE (ZTE) MRI by exploiting intrinsic encoding properties of RF pulses to retrieve data missed during the dead time caused by the pulse. Methods An enhanced ZTE signal model was developed using multiple RF pulses, which enables accessing information hidden in the pulse‐induced dead time via encoding intrinsically applied by the RF pulses. Such ZTE with pulse encoding was implemented by acquisition of two ZTE data sets using excitation with similar frequency‐swept pulses differing only by a small off‐resonance in their center frequency. In this way, the minimum scan time is doubled but each acquisition contributes equally to the SNR, as with ordinary averaging. The method was demonstrated on long‐T2 and short‐T2 phantoms as well as in in vivo experiments. Results ZTE with pulse encoding provided good image quality at unprecedented dead‐time gaps, demonstrated here up to 6 Nyquist dwells. In head imaging, the ability to use longer excitation pulses led to approximately 2‐fold improvements in SNR efficiency as compared with conventional ZTE and allowed the creation of T1 contrast. Conclusion Exploiting intrinsic encoding properties of RF pulses in a new signal model enables algebraic reconstruction of ZTE data sets with large dead‐time gaps. This permits larger flip angles, which can be used to achieve enhanced T1 contrast and significant improvements in SNR efficiency in case the Ernst angle can be better approached, thus broadening the range of application of ZTE MRI.
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Affiliation(s)
- Romain Froidevaux
- Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland
| | - Markus Weiger
- Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland
| | - Klaas P Pruessmann
- Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland
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Landes VL, Nayak KS. Iterative correction of RF envelope distortion with GRATER-measured waveforms. Magn Reson Med 2019; 83:188-194. [PMID: 31441540 DOI: 10.1002/mrm.27930] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2019] [Revised: 07/06/2019] [Accepted: 07/12/2019] [Indexed: 11/11/2022]
Abstract
PURPOSE To develop and evaluate a method for RF envelope correction without extra hardware or synchronization. METHODS Transmitted RF waveforms are measured through a simple pulse sequence called the gradient reversal approach to evaluate RF (GRATER). The measured RF waveforms are used to compute predistorted RF waveforms. This process is repeated until a stopping criterion is met, for example, based on pulse performance or a maximum number of iterations. Excitation profiles and simultaneous multi-slice (SMS) image quality are compared before and after RF predistortion. RESULTS The proposed method improved the accuracy of multiband RF pulses, reducing normalized root mean squared error (NRMSE) by >12-fold, and reducing spurious side-lobe excitation by >6-fold. The reduction in unwanted side-lobe signal is demonstrated using SMS bSSFP imaging at 3T in phantoms and in the heart. CONCLUSION Iterative GRATER-based predistortion is a practical, hardware-free way to boost performance of short duration, low flip angle RF pulses, such as those used in SMS bSSFP imaging. Because of its efficiency, this technique could be included as part of an initial scan setup or for use with subsequent scans.
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Affiliation(s)
- Vanessa L Landes
- Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, California
| | - Krishna S Nayak
- Ming Hsieh Department of Electrical and Computer Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, California
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Landes VL, Nayak KS. Simple method for RF pulse measurement using gradient reversal. Magn Reson Med 2017; 79:2642-2651. [PMID: 28905516 DOI: 10.1002/mrm.26920] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2017] [Revised: 08/21/2017] [Accepted: 08/23/2017] [Indexed: 11/08/2022]
Abstract
PURPOSE To develop and evaluate a simple method for measuring the envelope of small-tip radiofrequency (RF) excitation waveforms in MRI, without extra hardware or synchronization. THEORY AND METHODS Gradient reversal approach to evaluate RF (GRATER) involves RF excitation with a constant gradient and reversal of that gradient during signal reception to acquire the time-reversed version of an RF envelope. An outer-volume suppression prepulse is used optionally to preselect a uniform volume. GRATER was evaluated in phantom and in vivo experiments. It was compared with the programmed waveform and the traditional pick-up coil method. RESULTS In uniform phantom experiments, pick-up coil, GRATER, and outer-volume suppression + GRATER matched the programmed waveforms to less than 2.1%, less than 6.1%, and less than 2.4% normalized root mean square error, respectively, for real RF pulses with flip angle less than or equal to 30°, time-bandwidth product 2 to 8, and two to five excitation bands. For flip angles greater than 30°, GRATER measurement error increased as predicted by Bloch simulation. Fat-water phantom and in vivo experiments with outer-volume suppression + GRATER demonstrated less than 6.4% normalized root mean square error. CONCLUSIONS The GRATER sequence measures small-tip RF envelopes without extra hardware or synchronization in just over two times the RF duration. The sequence may be useful in prescan calibration and for measurement and precompensation of RF amplifier nonlinearity. Magn Reson Med 79:2642-2651, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Vanessa L Landes
- Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, Los Angles, California, USA
| | - Krishna S Nayak
- Ming Hsieh Department of Electrical Engineering, Viterbi School of Engineering, University of Southern California, Los Angles, California, USA
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Çavuşoğlu M, Dietrich BE, Brunner DO, Weiger M, Pruessmann KP. Correction of parallel transmission using concurrent RF and gradient field monitoring. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2017; 30:473-488. [DOI: 10.1007/s10334-017-0620-5] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/13/2016] [Revised: 03/27/2017] [Accepted: 04/10/2017] [Indexed: 10/19/2022]
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Brunner DO, Dietrich BE, Çavuşoğlu M, Wilm BJ, Schmid T, Gross S, Barmet C, Pruessmann KP. Concurrent recording of RF pulses and gradient fields - comprehensive field monitoring for MRI. NMR IN BIOMEDICINE 2016; 29:1162-1172. [PMID: 26269210 DOI: 10.1002/nbm.3359] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/06/2014] [Revised: 05/26/2015] [Accepted: 06/18/2015] [Indexed: 06/04/2023]
Abstract
Reconstruction of MRI data is based on exact knowledge of all magnetic field dynamics, since the interplay of RF and gradient pulses generates the signal, defines the contrast and forms the basis of resolution in spatial and spectral dimensions. Deviations caused by various sources, such as system imperfections, delays, eddy currents, drifts or externally induced fields, can therefore critically limit the accuracy of MRI examinations. This is true especially at ultra-high fields, because many error terms scale with the main field strength, and higher available SNR renders even smaller errors relevant. Higher baseline field also often requires higher acquisition bandwidths and faster signal encoding, increasing hardware demands and the severity of many types of hardware imperfection. To address field imperfections comprehensively, in this work we propose to expand the concept of magnetic field monitoring to also encompass the recording of RF fields. In this way, all dynamic magnetic fields relevant for spin evolution are covered, including low- to audio-frequency magnetic fields as produced by main magnets, gradients and shim systems, as well as RF pulses generated with single- and multiple-channel transmission systems. The proposed approach permits field measurements concurrently with actual MRI procedures on a strict common time base. The combined measurement is achieved with an array of miniaturized field probes that measure low- to audio-frequency fields via (19) F NMR and simultaneously pick up RF pulses in the MRI system's (1) H transmit band. Field recordings can form the basis of system calibration, retrospective correction of imaging data or closed-loop feedback correction, all of which hold potential to render MRI more robust and relax hardware requirements. The proposed approach is demonstrated for a range of imaging methods performed on a 7 T human MRI system, including accelerated multiple-channel RF pulses. Copyright © 2015 John Wiley & Sons, Ltd.
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Affiliation(s)
- David O Brunner
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
| | - Benjamin E Dietrich
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
| | - Mustafa Çavuşoğlu
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
| | - Bertram J Wilm
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
- Skope Magnetic Resonance Technologies, Zurich, Switzerland
| | - Thomas Schmid
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
| | - Simon Gross
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
| | - Christoph Barmet
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
- Skope Magnetic Resonance Technologies, Zurich, Switzerland
| | - Klaas P Pruessmann
- Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland
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Lau AZ, Miller JJ, Tyler DJ. Mapping of intracellular pH in the in vivo rodent heart using hyperpolarized [1-13C]pyruvate. Magn Reson Med 2016; 77:1810-1817. [PMID: 27173806 PMCID: PMC5412837 DOI: 10.1002/mrm.26260] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Revised: 03/14/2016] [Accepted: 04/06/2016] [Indexed: 12/22/2022]
Abstract
Purpose To demonstrate the feasibility of mapping intracellular pH within the in vivo rodent heart. Alterations in cardiac acid‐base balance can lead to acute contractile depression and alterations in Ca2+ signaling. The transient reduction in adenosine triphosphate (ATP) consumption and cardiac contractility may be initially beneficial; however, sustained pH changes can be maladaptive, leading to myocardial damage and electrical arrhythmias. Methods Spectrally selective radiofrequency (RF) pulses were used to excite the
HCO3− and CO2 resonances individually while preserving signal from the injected hyperpolarized [1‐13C]pyruvate. The large flip angle pulses were placed within a three‐dimensional (3D) imaging acquisition, which exploited CA‐mediated label exchange between
HCO3− and CO2. Images at 4.5 × 4.5 × 5 mm3 resolution were obtained in the in vivo rodent heart. The technique was evaluated in healthy rodents scanned at baseline and during high cardiac workload induced by dobutamine infusion. Results The intracellular pH was measured to be 7.15 ± 0.04 at baseline, and decreased to 6.90 ± 0.06 following 15 min of continuous β‐adrenergic stimulation. Conclusions Volumetric maps of intracellular pH can be obtained following an injection of hyperpolarized [1‐13C]pyruvate. The new method is anticipated to enable assessment of stress‐inducible ischemia and potential ventricular arrythmogenic substrates within the ischemic heart. Magn Reson Med 77:1810–1817, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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Affiliation(s)
- Angus Z Lau
- Division of Cardiovascular Medicine, Radcliffe Department of Medicine, University of Oxford, UK.,Department of Physiology, Anatomy, and Genetics, University of Oxford, UK
| | - Jack J Miller
- Department of Physiology, Anatomy, and Genetics, University of Oxford, UK.,Department of Physics, Clarendon Laboratory, University of Oxford, UK
| | - Damian J Tyler
- Division of Cardiovascular Medicine, Radcliffe Department of Medicine, University of Oxford, UK.,Department of Physiology, Anatomy, and Genetics, University of Oxford, UK
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Leung G, Norquay G, Schulte RF, Wild JM. Radiofrequency pulse design for the selective excitation of dissolved 129Xe. Magn Reson Med 2014; 73:21-30. [PMID: 24395490 DOI: 10.1002/mrm.25089] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2013] [Revised: 10/30/2013] [Accepted: 11/27/2013] [Indexed: 12/25/2022]
Abstract
PURPOSE To optimize radiofrequency (RF) pulses for the selective excitation of dissolved phase (129)Xe that take into account the very short T2*, while simultaneously, minimally exciting the much larger gas signal. METHODS Numerical simulations of Shinnar le-Roux pulses and binomial coefficient composite-element pulses were performed and experimentally implemented on a 1.5 Tesla (T) clinical scanner. These were compared with pulses commonly used for short T2* imaging from the literature. The pulses were then experimentally tested in vivo with healthy volunteers inhaling hyperpolarized (129)Xe using nuclear MR spectroscopy on a 1.5T clinical scanner. RESULTS Standard RF excitation pulses inadvertently excite the gas compartment, or are long enough that the T2* of the dissolved compartment deteriorates the received signal. Amplitude modulated binomial composite pulses perform well being short and having high selectivity, however, deteriorate at high amplifier gain setting. Composite pulses using pulse width modulation provide the desired frequency response even in these nonlinear, high gain regimes. CONCLUSION Composite pulses provide a means of very narrow band frequency selectivity in a short duration pulse that is well suited to dissolved (129)Xe imaging. Pulse width modulation maintains the desired frequency response even in the presence of amplitude distortion.
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Affiliation(s)
- General Leung
- Unit of Academic Radiology, University of Sheffield, Sheffield, United Kingdom
| | - Graham Norquay
- Unit of Academic Radiology, University of Sheffield, Sheffield, United Kingdom
| | | | - Jim M Wild
- Unit of Academic Radiology, University of Sheffield, Sheffield, United Kingdom
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Kaushik SS, Freeman MS, Cleveland ZI, Davies J, Stiles J, Virgincar RS, Robertson SH, He M, Kelly KT, Foster WM, McAdams HP, Driehuys B. Probing the regional distribution of pulmonary gas exchange through single-breath gas- and dissolved-phase 129Xe MR imaging. J Appl Physiol (1985) 2013; 115:850-60. [PMID: 23845983 DOI: 10.1152/japplphysiol.00092.2013] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Although some central aspects of pulmonary function (ventilation and perfusion) are known to be heterogeneous, the distribution of diffusive gas exchange remains poorly characterized. A solution is offered by hyperpolarized 129Xe magnetic resonance (MR) imaging, because this gas can be separately detected in the lung's air spaces and dissolved in its tissues. Early dissolved-phase 129Xe images exhibited intensity gradients that favored the dependent lung. To quantitatively corroborate this finding, we developed an interleaved, three-dimensional radial sequence to image the gaseous and dissolved 129Xe distributions in the same breath. These images were normalized and divided to calculate "129Xe gas-transfer" maps. We hypothesized that, for healthy volunteers, 129Xe gas-transfer maps would retain the previously observed posture-dependent gradients. This was tested in nine subjects: when the subjects were supine, 129Xe gas transfer exhibited a posterior-anterior gradient of -2.00 ± 0.74%/cm; when the subjects were prone, the gradient reversed to 1.94 ± 1.14%/cm (P < 0.001). The 129Xe gas-transfer maps also exhibited significant heterogeneity, as measured by the coefficient of variation, that correlated with subject total lung capacity (r = 0.77, P = 0.015). Gas-transfer intensity varied nonmonotonically with slice position and increased in slices proximal to the main pulmonary arteries. Despite substantial heterogeneity, the mean gas transfer for all subjects was 1.00 ± 0.01 while supine and 1.01 ± 0.01 while prone (P = 0.25), indicating good "matching" between gas- and dissolved-phase distributions. This study demonstrates that single-breath gas- and dissolved-phase 129Xe MR imaging yields 129Xe gas-transfer maps that are sensitive to altered gas exchange caused by differences in lung inflation and posture.
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Affiliation(s)
- S Sivaram Kaushik
- Department of Biomedical Engineering, Duke University, Durham, North Carolina
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Balezeau F, Eliat PA, Cayamo AB, Saint-Jalmes H. Mapping of low flip angles in magnetic resonance. Phys Med Biol 2011; 56:6635-47. [PMID: 21941028 DOI: 10.1088/0031-9155/56/20/008] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Errors in the flip angle have to be corrected in many magnetic resonance imaging applications, especially for T1 quantification. However, the existing methods of B1 mapping fail to measure lower values of the flip angle despite the fact that these are extensively used in dynamic acquisition and 3D imaging. In this study, the nonlinearity of the radiofrequency (RF) transmit chain, especially for very low flip angles, is investigated and a simple method is proposed to accurately determine both the gain of the RF transmitter and the B1 field map for low flip angles. The method makes use of the spoiled gradient echo sequence with long repetition time (TR), such as applied in the double-angle method. It uses an image acquired with a flip angle of 90° as a reference image that is robust to B1 inhomogeneity. The ratio of the image at flip angle alpha to the image at a flip angle of 90° enables us to calculate the actual value of alpha. This study was carried out at 1.5 and 4.7 T, showing that the linearity of the RF supply system is highly dependent on the hardware. The method proposed here allows us to measure the flip angle from 1° to 60° with a maximal uncertainty of 10% and to correct T1 maps based on the variable flip angle method.
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Ying L, Jianqi L, Gengying L. A novel selective-excitation RF pulse for magnetic resonance imaging. MAGMA (NEW YORK, N.Y.) 2008; 21:199-205. [PMID: 18446395 DOI: 10.1007/s10334-008-0111-9] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/27/2007] [Revised: 01/25/2008] [Accepted: 03/06/2008] [Indexed: 11/30/2022]
Abstract
OBJECT A selective-excitation radiofrequency (RF) pulse that uses hard pulses composed of a sequence of composite pulses with positive and negative phases (P/N pulse) is proposed herein. Because the amplitude of the RF signal is unchanged during the excitation, RF amplification can be accomplished using a nonlinear RF power amplifier (i.e., class C or D type). MATERIALS AND METHODS In this article, Fourier series have been first used to analyze the equivalence between the proposed P/N pulse and the conventional soft pulse on selective excitation. Subsequently, computer simulations based on density-matrix theory are used to compare the excitation profiles of both the soft and the P/N pulses. RESULTS The excitation profiles of the P/N pulses have been measured experimentally through a home-built 0.3-T magnetic resonance imaging (MRI) system. In addition, several slices of images have been obtained as proofs by using the multislice two-dimensional spin echo sequence through replacement of the conventional soft pulse by the proposed P/N pulse. CONCLUSION Because the perfect selectivity of the proposed P/N pulse, it can be used for imaging studies to improve the efficiency of amplification at the lowest cost.
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Affiliation(s)
- Liu Ying
- Shanghai Key Laboratory of Functional Magnetic Resonance Imaging, Department of Physics, East China Normal University, Shanghai 200062, People's Republic of China.
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Abstract
Over the past two decades, proton magnetic resonance spectroscopy (proton MRS) of the brain has made the transition from research tool to a clinically useful modality. In this review, we first describe the localization methods currently used in MRS studies of the brain and discuss the technical and practical factors that determine the applicability of the methods to particular clinical studies. We also describe each of the resonances detected by localized solvent-suppressed proton MRS of the brain and discuss the metabolic and biochemical information that can be derived from an analysis of their concentrations. We discuss spectral quantitation and summarize the reproducibility of both single-voxel and multivoxel methods at 1.5 and 3-4 T. We have selected three clinical neurologic applications in which there has been a consensus as to the diagnostic value of MRS and summarize the information relevant to clinical applications. Finally, we speculate about some of the potential technical developments, either in progress or in the future, that may lead to improvements in the performance of proton MRS.
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Affiliation(s)
- Yael Rosen
- Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 02215 Boston, Massachusetts
| | - Robert E. Lenkinski
- Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 02215 Boston, Massachusetts
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Wang J, Mao W, Qiu M, Smith MB, Constable RT. Factors influencing flip angle mapping in MRI: RF pulse shape, slice-select gradients, off-resonance excitation, andB0 inhomogeneities. Magn Reson Med 2006; 56:463-8. [PMID: 16773653 DOI: 10.1002/mrm.20947] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
To understand the various effects that influence actual flip angles, and correct for these effects, it is important to precisely quantify the MRI parameters (such as T1, T2, and perfusion). In this paper actual flip angle maps are calculated using a conventional gradient-echo (GRE) sequence with different radiofrequency (RF) pulse shapes (Gaussian, sinc, and truncated-sinc), slice-selection gradients, off-resonance excitations, and B0 field inhomogeneities. The experimental results demonstrate that RF pulse shapes significantly affect the flip angle distribution and calibration factors. Off-resonance RF excitations, B0 nonuniformities, and slice-selection gradients can lead to degradations in the signal intensities of the images used to map the flip angle, and potentially introduce a bias and increased variance in the measured flip angles.
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Affiliation(s)
- Jinghua Wang
- Department of Diagnostic Radiology, Yale University School Medical Center, New Haven, Connecticut 06520-8042, USA.
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16
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17
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Abstract
Spatial encoding with multiband selective excitation (e.g., Hadamard encoding) has been restricted to a small number of slices because the RF pulse becomes unacceptably long when more than about eight slices are encoded. In this work, techniques to shorten multiband RF pulses, and thus allow larger numbers of slices, are investigated. A method for applying the techniques while retaining the capability of adaptive slice thickness is outlined. A tradeoff between slice thickness and pulse duration is shown. Simulations and experiments with the shortened pulses confirmed that motion-induced excitation profile blurring and phase accrual were reduced. The connection between gradient hardware limitations, slice thickness, and flow sensitivity is shown. Excitation profiles for encoding 32 contiguous slices of 1-mm thickness were measured experimentally, and the artifact resulting from errors in timing of RF pulse relative to gradient was investigated. A multiband technique for imaging 32 contiguous 2-mm slices, with adaptive slice thickness, was developed and demonstrated for coronary artery imaging in healthy subjects. With the ability to image high numbers of contiguous slices, using relatively short (1-2 ms) RF pulses, multiband encoding has been advanced further toward practical application.
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Affiliation(s)
- Charles H Cunningham
- Department of Medical Biophysics, University of Toronto and Sunnybrook and Women's College, Health Sciences Center, Canada.
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18
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Lebsack ET, Wright SM. Iterative RF pulse refinement for magnetic resonance imaging. IEEE Trans Biomed Eng 2002; 49:41-8. [PMID: 11794770 DOI: 10.1109/10.972838] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Selective RF pulses are needed for many applications in magnetic resonance imaging (MRI). The waveform required to produce a desired excitation profile is, to first-order, its Fourier transform. This approximation is most valid for small tip angles and the quality and accuracy of such excitations decreases with increasing tip angle. Since large-tip-angle excitations are required in most types of imaging, a better synthesis technique is necessary. While a variety of analytical and numerical synthesis techniques based on solution of the Bloch equations are available, these techniques fail to consider the effect of the physical scanner hardware and are often accompanied by computational complexity. We present a technique for selective RF pulse refinement which uses real-time feedback techniques in lieu of a solution to the Bloch equations. Physical experiments are conducted to demonstrate the effectiveness of this algorithm and an extension to pulses of 90 degrees is investigated.
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Affiliation(s)
- Eliot T Lebsack
- Department of Electrical Engineering, Texas A&M University, College Station 77843-3128, USA
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19
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Star-Lack JM, Spielman DM. Zero-quantum filter offering single-shot lipid suppression and simultaneous detection of lactate, choline, and creatine resonances. Magn Reson Med 2001; 46:1233-7. [PMID: 11746591 DOI: 10.1002/mrm.1321] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
A zero-quantum (ZQ) filter offering single-shot lipid suppression and providing for simultaneous detection of the lactate methyl doublet (1.3 ppm) and nonoverlapping singlets including choline (Cho, 3.2 ppm) and creatine (Cr, 3.0 ppm) is described. Filtering is provided by soft mixing and reading pulses (RF(mix), RF(rd)) that are selective for the lactate methine quartet (4.1 ppm), Cho, and Cr resonances but exclude the 1.3 ppm lactate component and overlapping lipids. Surrounding RF(mix) and RF(rd) are magnetic field gradient pulses of equal magnitude but opposite signs to enable the rephasing of the zero-quantum lactate coherence and the creation of a stimulated echo for singlets within the pulse passbands. The sequence is designed to retain half the original lactate and singlet signal intensities. Theoretical predictions were confirmed experimentally at 1.5T using phantom acquisitions. The lipid suppression factor was measured to be over 10(3).
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Affiliation(s)
- J M Star-Lack
- Lucas MRS Imaging Center, Department of Radiology, Stanford University, Stanford, California 94305, USA
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20
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Abstract
For conventional multiband encoding techniques such as Hadamard encoding, scan time scales linearly with the number of slices encoded simultaneously. In this work, a new multiband encoding technique called partial discrete Fourier transform (PDFT) encoding is introduced, which overcomes this restriction. This technique incorporates the principle of partial Fourier imaging, allowing the tradeoff of SNR and imaging time without changing the number of slices. The theory behind PDFT encoding and its inherent sensitivity to phase errors are outlined. The theory was validated through simulations, showing that phase errors result in degraded slice localization. The feasibility of PDFT encoding of 12 slices was tested with experimental excitation profile measurements and heart images of a human subject using commercial MRI equipment. Imaging time was reduced to 66% with SNR reduced to 82%. Magn Reson Med 45:118-127, 2001.
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Affiliation(s)
- C H Cunningham
- Department of Medical Biophysics, University of Toronto, Ontario, Canada.
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21
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De Deenet Y, De Wagter C, De Neve W, Achten E. Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: II. Analysis of B1-field inhomogeneity. Phys Med Biol 2000; 45:1825-39. [PMID: 10943921 DOI: 10.1088/0031-9155/45/7/308] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to radiation dose that has been delivered to a gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B1-field inhomogeneities as a source of dose inaccuracy. First, the influence of B1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is used to map the B1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition, both experiments and computer simulations revealed the effects of B1 field inhomogeneities on the measured R2. This work offers a method to correct R2 maps for B1 -field inhomogeneities.
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Affiliation(s)
- Y De Deenet
- Radiotherapy and Nuclear Medicine, University Hospital of Gent, Belgium.
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22
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Abstract
Investigation of the cause of image artifacts generated by a magnetization transfer (MT) sequence using binomial-pulse trains led to findings of imperfections in the pulses. These imperfections caused anomalous direct saturation of the free water, which was localized due to the static magnetic field inhomogeneity. In the case of single binomial pulses a loss of overall MT response across the field of view results. Two methods of correcting the imperfections and removing the artifact have been established using interactive adjustment of sub-pulse lobes and phase swapping of pulse trains. These imperfections may be present in many systems and may have led to erroneous judgements of the value of binomial pulses for MT imaging. A technique for interrogating the frequency spectrum of the binomial-pulse train has been utilized, allowing its optimization. The use of accurate and optimized binomial pulses may yet prove to be preferable to pulsed off-resonance methods for quantitative, clinical MT imaging.
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Affiliation(s)
- N P Davies
- Medical Physics Group, School of Physics, University of Exeter, Exeter EX4 4QL, United Kingdom.
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23
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Abstract
A method to design multiband RF pulses for magnetic resonance imaging is described. The method is based on the Shinnar-Le Roux transform and involves a phase correction that provides control over the phase of the excited bands. The theory behind the method and this phase correction is outlined. The method is demonstrated with the design of RF pulses for Hadamard encoding and Haar wavelet encoding. Experimentally measured excitation profiles and images for RF pulses designed with the new method are compared to those designed by the conventional method. The conventional method is shown to result in distortion of the excitation profile when the bands are closely spaced. A 78% reduction in this distortion is attributed to the new method. This translates into a 52% reduction of out-of-slice signal in Haar wavelet encoding. Magn Reson Med 42:577-584, 1999.
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Affiliation(s)
- C H Cunningham
- Department of Medical Biophysics, University of Toronto and Sunnybrook and Women's College Health Sciences Centre, Toronto, Canada.
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24
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Abstract
Optimized Shinnar-Le Roux (SLR) pulses have very good slice profiles with no sidelobes and very small out-of-slice ripples with nearly constant amplitude. In multislice fast spin-echo (FSE) imaging, a better slice definition changes the stimulated echo (STE) and magnetization transfer (MT) contribution on contrast and signal. We investigated the effects of STE and MT on signal and contrast enhancement when optimized 180 degrees SLR pulses are used in multislice FSE measurements. From the results obtained in both phantom and volunteer studies, it can be inferred that using optimized 180 degrees SLR pulses with band-widths larger than that of the conventional 180 degrees sinc-pulse allows significant signal improvement and contrast enhancement.
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Affiliation(s)
- A Raddi
- Department of Neuroradiology, University of Tübingen, Germany.
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25
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Abstract
The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90 degrees linear-phase Shinnar-LeRoux pulse; a 90 degrees self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material.
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Affiliation(s)
- D P Lewis
- Department of Biomedical Engineering, University of North Carolina, Chapel Hill 27599-7575, USA.
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Hua J, Hurst GC. Analysis of on- and off-resonance magnetization transfer techniques. J Magn Reson Imaging 1995; 5:113-20. [PMID: 7696801 DOI: 10.1002/jmri.1880050120] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Three methods of performing magnetization transfer (MT) MR imaging are analyzed: (a) off-resonance continuous wave, (b) off-resonance shaped pulses, and (c) on-resonance binomial pulses. With two-pool Bloch-model simulations, signal levels from "MT active" spin systems were calculated, with reference to direct saturation of "MT inactive" systems, allowing calculation of contrast due to MT. Simulations demonstrate several trends with variation of excitation amplitude and offset frequency for the off-resonance methods and with variation of excitation amplitude and pulse shape "order" for binomial pulses. The simulations show that nominally optimized versions of each of these approaches provide essentially equivalent contrast at a given level of applied MT power, contrary to previous claims. Experiments with an MT-inactive phantom, with a whole-body system, show results with off-resonance pulses to be in good agreement with simulations, whereas binomial-pulse experiments show anomalously large direct saturation.
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Affiliation(s)
- J Hua
- Department of Radiology, MetroHealth Medical Center, Cleveland, OH, USA
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27
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Abstract
A pulse synthesis algorithm is described that allows for the synthesis of selective radio frequency pulses requiring less peak power. The pulses thus synthesized have the same duration, total energy, and frequency response as those synthesized by the standard algorithm. Their phase function is, however, different. The power reduction is typically on the order of 60-70%. This modification will increase the utility of optimized selective pulses.
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Affiliation(s)
- M Shinnar
- Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia 19104
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28
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Pearlman JD, Wieczorek TJ. Relaxivity corrected response modulated excitation (RME): a T2-corrected technique achieving specified magnetization patterns from an RF pulse and a time-varying magnetic field. Magn Reson Med 1994; 32:388-95. [PMID: 7984071 DOI: 10.1002/mrm.1910320314] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
We have reworked the theory of RF excitation to enable correction for relaxivity while designing response-modulated excitation (RME) to achieve specified magnetization targets. This results in a significant improvement in the ability to achieve a specified target magnetization, especially if excitation time is long or T2 is short. The methods presented may also be used to improve the quality of spatial-spectral pulses as well as localized spectroscopy, real-time imaging, real-time localized velocity, and noninvasive pressure measurement.
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