Publications

2011
Yogesh Rathi, O Michailovich, K Setsompop, S Bouix, ME Shenton, and CF Westin. 2011. “Sparse multi-shell diffusion imaging.” Medical image computing and computer-assisted intervention : MICCAI .. International Conference on Medical Image Computing and Computer-Assisted Intervention, 14, Pt 2, Pp. 58–65. Publisher's VersionAbstract
Diffusion magnetic resonance imaging (dMRI) is an important tool that allows non-invasive investigation of neural architecture of the brain. The data obtained from these in-vivo scans provides important information about the integrity and connectivity of neural fiber bundles in the brain. A multi-shell imaging (MSI) scan can be of great value in the study of several psychiatric and neurological disorders, yet its usability has been limited due to the long acquisition times required. A typical MSI scan involves acquiring a large number of gradient directions for the 2 (or more) spherical shells (several b-values), making the acquisition time significantly long for clinical application. In this work, we propose to use results from the theory of compressive sampling and determine the minimum number of gradient directions required to attain signal reconstruction similar to a traditional MSI scan. In particular, we propose a generalization of the single shell spherical ridgelets basis for sparse representation of multi shell signals. We demonstrate its efficacy on several synthetic and in-vivo data sets and perform quantitative comparisons with solid spherical harmonics based representation. Our preliminary results show that around 20-24 directions per shell are enough for robustly recovering the diffusion propagator.
2009
Kawin Setsompop, Vijayanand Alagappan, Borjan A Gagoski, Andreas Potthast, Franz Hebrank, Ulrich Fontius, Franz Schmitt, LL Wald, and E Adalsteinsson. 2009. “Broadband slab selection with B1+ mitigation at 7T via parallel spectral-spatial excitation.” Magn Reson Med, 61, 2, Pp. 493-500.Abstract
Chemical shift imaging benefits from signal-to-noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B1+ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B1+ mitigation methods, which are based on a type of echovolumnar trajectory referred to as "spokes" or "fast-kz". Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B1+ inhomogeneities and achieve relatively short pulse durations with slice-selective excitations, they exhibit a narrow-band off-resonance response and may not be suitable for applications that require B1+ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral-spatial excitation that achieves a target-error minimization simultaneously over a bandwidth of frequencies and a specified spatial-domain. The technique is demonstrated for slab-selective excitation with in-plane B1+ mitigation over a 600-Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight-channel transmit array system. The results show significant increases in the pulse's spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B1+ mitigation compared to the standard spoke-based parallel RF design.
Kawin Setsompop, Vijayanand Alagappan, Borjan A Gagoski, Andreas Potthast, Franz Hebrank, Ulrich Fontius, Franz Schmitt, LL Wald, and E Adalsteinsson. 2009. “Broadband slab selection with B1+ mitigation at 7T via parallel spectral-spatial excitation.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 61, 2, Pp. 493–500. Publisher's VersionAbstract
Chemical shift imaging benefits from signal-to-noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B1+ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B1+ mitigation methods, which are based on a type of echovolumnar trajectory referred to as "spokes" or "fast-kz". Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B1+ inhomogeneities and achieve relatively short pulse durations with slice-selective excitations, they exhibit a narrow-band off-resonance response and may not be suitable for applications that require B1+ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral-spatial excitation that achieves a target-error minimization simultaneously over a bandwidth of frequencies and a specified spatial-domain. The technique is demonstrated for slab-selective excitation with in-plane B1+ mitigation over a 600-Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight-channel transmit array system. The results show significant increases in the pulse's spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B1+ mitigation compared to the standard spoke-based parallel RF design.
2008
Adam C Zelinski, Lawrence L Wald, Kawin Setsompop, Vijayanand Alagappan, Borjan A Gagoski, Vivek K Goyal, and Elfar Adalsteinsson. 2008. “Fast slice-selective radio-frequency excitation pulses for mitigating B+1 inhomogeneity in the human brain at 7 Tesla.” Magn Reson Med, 59, 6, Pp. 1355-64.Abstract
A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate B+1 inhomogeneity present in the human brain at high field. The method is provided an estimate of the B+1 field in an axial slice of the brain and then optimizes the placement of sinc-like "spokes" in kz via an L1-norm penalty on candidate (kx, ky) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly nonuniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.
Adam C Zelinski, Lawrence L Wald, Kawin Setsompop, Vijayanand Alagappan, Borjan A Gagoski, Vivek K Goyal, and Elfar Adalsteinsson. 2008. “Fast slice-selective radio-frequency excitation pulses for mitigating B+1 inhomogeneity in the human brain at 7 Tesla.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 59, 6, Pp. 1355–64. Publisher's VersionAbstract
A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate B+1 inhomogeneity present in the human brain at high field. The method is provided an estimate of the B+1 field in an axial slice of the brain and then optimizes the placement of sinc-like "spokes" in kz via an L1-norm penalty on candidate (kx, ky) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly nonuniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.
Kawin Setsompop, Vijayanand Alagappan, Adam C Zelinski, Andreas Potthast, Ulrich Fontius, Franz Hebrank, Franz Schmitt, Lawrence L Wald, and Elfar Adalsteinsson. 2008. “High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T.” Journal of magnetic resonance (San Diego, Calif. : 1997), 195, 1, Pp. 76–84. Publisher's VersionAbstract
At high magnetic field, B(1)(+) non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90 degrees and 180 degrees spin-echo slice-selective excitations that mitigate severe B(1)(+) inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B(1)(+) inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B(0) field strengths where large B(1)(+) inhomogeneity is present.
Kawin Setsompop, Vijayanand Alagappan, Adam C Zelinski, Andreas Potthast, Ulrich Fontius, Franz Hebrank, Franz Schmitt, Lawrence L Wald, and Elfar Adalsteinsson. 2008. “High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T.” J Magn Reson, 195, 1, Pp. 76-84.Abstract
At high magnetic field, B(1)(+) non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90 degrees and 180 degrees spin-echo slice-selective excitations that mitigate severe B(1)(+) inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B(1)(+) inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B(0) field strengths where large B(1)(+) inhomogeneity is present.
K Setsompop, LL Wald, V Alagappan, B a Gagoski, and E Adalsteinsson. 2008. “Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 59, 4, Pp. 908–15. Publisher's VersionAbstract
Spatially tailored radio frequency (RF) excitations accelerated with parallel transmit systems provide the opportunity to create shaped volume excitations or mitigate inhomogeneous B(1) excitation profiles with clinically relevant pulse lengths. While such excitations are often designed as a least-squares optimized approximation to a target magnitude and phase profile, adherence to the target phase profile is usually not important as long as the excitation phase is slowly varying compared with the voxel dimension. In this work, we demonstrate a method for a magnitude least squares optimization of the target magnetization profile for multichannel parallel excitation to improve the magnitude profile and reduce the RF power at the cost of a less uniform phase profile. The method enables the designer to trade off the allowed spatial phase variation for the improvement in magnitude profile and reduction in RF power. We validate the method with simulation studies and demonstrate its performance in fourfold accelerated two-dimensional spiral excitations, as well as for uniform in-plane slice selective parallel excitations using an eight-channel transmit array on a 7T human MRI scanner. The experimental results are in good agreement with the simulations, which show significant improvement in the magnitude profile and reductions in the required RF power while still maintaining negligible intravoxel phase variation.
K Setsompop, LL Wald, V Alagappan, B a Gagoski, and E Adalsteinsson. 2008. “Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels.” Magn Reson Med, 59, 4, Pp. 908-15.Abstract
Spatially tailored radio frequency (RF) excitations accelerated with parallel transmit systems provide the opportunity to create shaped volume excitations or mitigate inhomogeneous B(1) excitation profiles with clinically relevant pulse lengths. While such excitations are often designed as a least-squares optimized approximation to a target magnitude and phase profile, adherence to the target phase profile is usually not important as long as the excitation phase is slowly varying compared with the voxel dimension. In this work, we demonstrate a method for a magnitude least squares optimization of the target magnetization profile for multichannel parallel excitation to improve the magnitude profile and reduce the RF power at the cost of a less uniform phase profile. The method enables the designer to trade off the allowed spatial phase variation for the improvement in magnitude profile and reduction in RF power. We validate the method with simulation studies and demonstrate its performance in fourfold accelerated two-dimensional spiral excitations, as well as for uniform in-plane slice selective parallel excitations using an eight-channel transmit array on a 7T human MRI scanner. The experimental results are in good agreement with the simulations, which show significant improvement in the magnitude profile and reductions in the required RF power while still maintaining negligible intravoxel phase variation.
Kawin Setsompop, Vijayanand Alagappan, Borjan Gagoski, Thomas Witzel, Jonathan R. Polimeni, Andreas Potthast, Franz Hebrank, Ulrich Fontius, Franz Schmitt, Lawrence L Wald, and Elfar Adalsteinsson. 2008. “Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 60, 6, Pp. 1422–32. Publisher's VersionAbstract
Slice-selective RF waveforms that mitigate severe B1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B1+ on the excitation FA. The optimized parallel RF pulses for human B1+ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in-plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse-duration penalty.
Kawin Setsompop, Vijayanand Alagappan, Borjan Gagoski, Thomas Witzel, Jonathan Polimeni, Andreas Potthast, Franz Hebrank, Ulrich Fontius, Franz Schmitt, Lawrence L Wald, and Elfar Adalsteinsson. 2008. “Slice-selective RF pulses for in vivo B1+ inhomogeneity mitigation at 7 tesla using parallel RF excitation with a 16-element coil.” Magn Reson Med, 60, 6, Pp. 1422-32.Abstract
Slice-selective RF waveforms that mitigate severe B1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B1+ on the excitation FA. The optimized parallel RF pulses for human B1+ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in-plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse-duration penalty.
Adam C Zelinski, Lawrence L Wald, Kawin Setsompop, Vivek K Goyal, and Elfar Adalsteinsson. 2008. “Sparsity-enforced slice-selective MRI RF excitation pulse design.” IEEE transactions on medical imaging, 27, 9, Pp. 1213–29. Publisher's VersionAbstract
We introduce a novel algorithm for the design of fast slice-selective spatially-tailored magnetic resonance imaging (MRI) excitation pulses. This method, based on sparse approximation theory, uses a second-order cone optimization to place and modulate a small number of slice-selective sinc-like radio-frequency (RF) pulse segments ("spokes") in excitation k-space, enforcing sparsity on the number of spokes allowed while simultaneously encouraging those that remain to be placed and modulated in a way that best forms a user-defined in-plane target magnetization. Pulses are designed to mitigate B(1) inhomogeneity in a water phantom at 7 T and to produce highly-structured excitations in an oil phantom on an eight-channel parallel excitation system at 3 T. In each experiment, pulses generated by the sparsity-enforced method outperform those created via conventional Fourier-based techniques, e.g., when attempting to produce a uniform magnetization in the presence of severe B(1) inhomogeneity, a 5.7-ms 15-spoke pulse generated by the sparsity-enforced method produces an excitation with 1.28 times lower root mean square error than conventionally-designed 15-spoke pulses. To achieve this same level of uniformity, the conventional methods need to use 29-spoke pulses that are 7.8 ms long.
Adam C Zelinski, Lawrence L Wald, Kawin Setsompop, Vivek K Goyal, and Elfar Adalsteinsson. 2008. “Sparsity-enforced slice-selective MRI RF excitation pulse design.” IEEE Trans Med Imaging, 27, 9, Pp. 1213-29.Abstract
We introduce a novel algorithm for the design of fast slice-selective spatially-tailored magnetic resonance imaging (MRI) excitation pulses. This method, based on sparse approximation theory, uses a second-order cone optimization to place and modulate a small number of slice-selective sinc-like radio-frequency (RF) pulse segments ("spokes") in excitation k-space, enforcing sparsity on the number of spokes allowed while simultaneously encouraging those that remain to be placed and modulated in a way that best forms a user-defined in-plane target magnetization. Pulses are designed to mitigate B(1) inhomogeneity in a water phantom at 7 T and to produce highly-structured excitations in an oil phantom on an eight-channel parallel excitation system at 3 T. In each experiment, pulses generated by the sparsity-enforced method outperform those created via conventional Fourier-based techniques, e.g., when attempting to produce a uniform magnetization in the presence of severe B(1) inhomogeneity, a 5.7-ms 15-spoke pulse generated by the sparsity-enforced method produces an excitation with 1.28 times lower root mean square error than conventionally-designed 15-spoke pulses. To achieve this same level of uniformity, the conventional methods need to use 29-spoke pulses that are 7.8 ms long.
2007
Adam C. Zelinski, Lawrence L. Wald, Kawin Setsompop, Vijayanand Alagappan, Borjan A. Gagoski, Vivek K Goyal, Franz Hebrank, Ulrich Fontius, Franz Schmitt, and Elfar Adalsteinsson. 2007. “Comparison of three algorithms for solving linearized systems of parallel excitation RF waveform design equations: Experiments on an eight-channel system at 3 Tesla.” Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 31B, 3, Pp. 176–190. Publisher's Version
Vijayanand Alagappan, Juergen Nistler, Elfar Adalsteinsson, Kawin Setsompop, Ulrich Fontius, Adam Zelinski, Markus Vester, Graham C Wiggins, Franz Hebrank, Wolfgang Renz, Franz Schmitt, and Lawrence L Wald. 2007. “Degenerate mode band-pass birdcage coil for accelerated parallel excitation.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 57, 6, Pp. 1148–58. Publisher's VersionAbstract
An eight-rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this "loop coil" basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B(1) field pattern, and the higher-order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one-, four-, six-, and eightfold-accelerated spatially tailored RF excitations with 2D and 3D k-space excitation trajectories. Transmit accelerations of up to six-fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B(1) transmit inhomogeneities using parallel excitation.
Vijayanand Alagappan, Juergen Nistler, Elfar Adalsteinsson, Kawin Setsompop, Ulrich Fontius, Adam Zelinski, Markus Vester, Graham C Wiggins, Franz Hebrank, Wolfgang Renz, Franz Schmitt, and Lawrence L Wald. 2007. “Degenerate mode band-pass birdcage coil for accelerated parallel excitation.” Magn Reson Med, 57, 6, Pp. 1148-58.Abstract
An eight-rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this "loop coil" basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B(1) field pattern, and the higher-order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one-, four-, six-, and eightfold-accelerated spatially tailored RF excitations with 2D and 3D k-space excitation trajectories. Transmit accelerations of up to six-fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B(1) transmit inhomogeneities using parallel excitation.
2006
Kawin Setsompop, Lawrence L Wald, Vijayanand Alagappan, Borjan Gagoski, Franz Hebrank, Ulrich Fontius, Franz Schmitt, and Elfar Adalsteinsson. 2006. “Parallel RF transmission with eight channels at 3 Tesla.” Magn Reson Med, 56, 5, Pp. 1163-71.Abstract
Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight-coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low-flip-angle pulse design to achieve desired spatial target profiles with two- (2D) and three-dimensional (3D) k-space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high-resolution spatial pattern in-plane, while the 3D trajectory produced high-quality slice selection with a uniform in-plane excitation despite the highly nonuniform individual spatial profiles of the coil array. The multichannel parallel RF excitation was used to accelerate the 2D excitation by factors of 2-8, and experimental results were in excellent agreement with simulations based on the measured coil maps. Parallel RF transmission may become critical for robust and routine human studies at very high field strengths where B(1) inhomogeneity is commonly severe.
Kawin Setsompop, Lawrence L Wald, Vijayanand Alagappan, Borjan Gagoski, Franz Hebrank, Ulrich Fontius, Franz Schmitt, and Elfar Adalsteinsson. 2006. “Parallel RF transmission with eight channels at 3 Tesla.” Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 56, 5, Pp. 1163–71. Publisher's VersionAbstract
Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight-coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low-flip-angle pulse design to achieve desired spatial target profiles with two- (2D) and three-dimensional (3D) k-space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high-resolution spatial pattern in-plane, while the 3D trajectory produced high-quality slice selection with a uniform in-plane excitation despite the highly nonuniform individual spatial profiles of the coil array. The multichannel parallel RF excitation was used to accelerate the 2D excitation by factors of 2-8, and experimental results were in excellent agreement with simulations based on the measured coil maps. Parallel RF transmission may become critical for robust and routine human studies at very high field strengths where B(1) inhomogeneity is commonly severe.

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