Robust RF shimming and small-tip-angle multispoke pulse design with finite-difference regularization.

PURPOSE: A new regularizer is proposed for the magnitude least-squares optimization algorithm, to ensure robust parallel transmit RF shimming and small-tip-angle multispoke pulse designs for ultrahigh-field MRI. METHODS: A finite-difference regularization term is activated as an additional regularizer in the iterative magnitude-least-squares based pulse design algorithm when an unwanted flip angle null distribution is detected. Both simulated and experimental B1+ maps from different transmit arrays and different human subjects at 7 T were used to evaluate the proposed algorithm. The algorithm was further demonstrated in experiment with dynamic multislice RF shimming for a single-shot gradient-echo EPI for human functional MRI at 7 T. RESULTS: The proposed finite-difference regularizer effectively prevented excitation null to be formed for RF shimming and small-tip-angle multispoke pulses, and improved the latter with a monotonic trade-off relationship between flip angle error and RF power. The proposed algorithm was demonstrated to be effective with several head-array geometries by simulation and with a commercial head array with 12 healthy human subjects by experiment. During a functional MRI scan at 7 T with dynamic RF shimming, the proposed algorithm ensured high image SNR throughout the human brain, compared with near-complete local signal loss by the conventional magnitude-least-squares algorithm. CONCLUSION: Using finite-difference regularization to avoid unwanted solutions, the robustness of RF shimming and small-tip-angle multispoke pulse design algorithms are improved, with better flip angle homogeneity and a monotonic trade-off relationship between flip angle error and RF power.

High-resolution gradient-recalled echo imaging at 9.4T using 16-channel parallel transmit simultaneous multislice spokes excitations with slice-by-slice flip angle homogenization.

PURPOSE: In order to fully benefit from the improved signal-to-noise and contrast-to-noise ratios at 9.4T, the challenges of B1+ inhomogeneity and the long acquisition time of high-resolution 2D gradient-recalled echo (GRE) imaging were addressed. THEORY AND METHODS: Flip angle homogenized excitations were achieved by parallel transmission (pTx) of 3-spoke pulses, designed by magnitude least-squares optimization in a slice-by-slice fashion; the acquisition time reduction was achieved by simultaneous multislice (SMS) pulses. The slice-specific spokes complex radiofrequency scaling factors were applied to sinc waveforms on a per-channel basis and combined with the other pulses in an SMS slice group to form the final SMS-pTX pulse. Optimal spokes locations were derived from simulations. RESULTS: Flip angle maps from presaturation TurboFLASH showed improvement of flip angle homogenization with 3-spoke pulses over CP-mode excitation (normalized root-mean-square error [NRMSE] 0.357) as well as comparable excitation homogeneity across the single-band (NRMSE 0.119), SMS-2 (NRMSE 0.137), and SMS-3 (NRMSE 0.132) 3-spoke pulses. The application of the 3-spoke SMS-3 pulses in a 48-slice GRE protocol, which has an in-plane resolution of 0.28 × 0.28 mm, resulted in a 50% reduction of scan duration (total acquisition time 6:52 min including reference scans). CONCLUSION: Time-efficient flip angle homogenized high-resolution GRE imaging at 9.4T was accomplished by using slice-specific SMS-pTx spokes excitations. Magn Reson Med 78:1050-1058, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency-gradient delay: Example of bipolar spokes pulses in parallel transmission.

PURPOSE: To eliminate a slice-position-dependent excitation error commonly observed in bipolar-gradient composite excitations such as spokes pulses in parallel transmission. THEORY AND METHODS: An undesired timing delay between subpulses in the composite pulse and their bipolar slice-selective gradient is hypothesized to cause the error. A mathematical model is presented here to relate this mismatch to an induced slice-position-dependent phase difference between the subpulses. A new navigator method is proposed to measure the timing mismatch and eliminate the error. This is demonstrated at 7 Tesla with flip-angle maps measured by a presaturation turbo-flash sequence and in vivo images acquired by a simultaneous multislice/echo-planar imaging (SMS-EPI) sequence. RESULTS: Error-free flip-angle maps were obtained in two ways: 1) by correcting the time delay directly and 2) by applying the corresponding slice-position-dependent phase differences to the subpulses. This confirms the validity of the mathematical description. The radiofrequency (RF)-gradient delay measured by the navigator method was of 6.3 µs, which agreed well with the estimate from flip-angle maps at different delay times. By applying the timing correction, accurately excited EPI images were acquired with bipolar dual-spokes SMS-2 excitations. CONCLUSION: An effective correction is proposed to mitigate slice-position-dependent errors in bipolar composite excitations caused by undesired RF-gradient timing delays. Magn Reson Med 78:1883-1890, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Joint design of large-tip-angle parallel RF pulses and blipped gradient trajectories.

PURPOSE: To design multichannel large-tip-angle kT-points and spokes radiofrequency (RF) pulses and gradient waveforms for transmit field inhomogeneity compensation in high field magnetic resonance imaging. THEORY AND METHODS: An algorithm to design RF subpulse weights and gradient blip areas is proposed to minimize a magnitude least-squares cost function that measures the difference between realized and desired state parameters in the spin domain, and penalizes integrated RF power. The minimization problem is solved iteratively with interleaved target phase updates, RF subpulse weights updates using the conjugate gradient method with optimal control-based derivatives, and gradient blip area updates using the conjugate gradient method. Two-channel parallel transmit simulations and experiments were conducted in phantoms and human subjects at 7 T to demonstrate the method and compare it to small-tip-angle-designed pulses and circularly polarized excitations. RESULTS: The proposed algorithm designed more homogeneous and accurate 180° inversion and refocusing pulses than other methods. It also designed large-tip-angle pulses on multiple frequency bands with independent and joint phase relaxation. Pulses designed by the method improved specificity and contrast-to-noise ratio in a finger-tapping spin echo blood oxygen level dependent functional magnetic resonance imaging study, compared with circularly polarized mode refocusing. CONCLUSION: A joint RF and gradient waveform design algorithm was proposed and validated to improve large-tip-angle inversion and refocusing at ultrahigh field.

Array-compressed parallel transmit pulse design.

PURPOSE: To design array-compressed parallel transmit radiofrequency (RF) pulses and compare them to pulses designed with existing transmit array compression strategies. THEORY AND METHODS: Array-compressed parallel RF pulse design is proposed as the joint optimization of a matrix of complex-valued compression weights that relate a full-channel physical array to a reduced-channel virtual array, along with a set of RF pulses for the virtual array. In this way, the physics of the RF pulse application determine the coil combination weights. Array-compressed pulse design algorithms are described for four parallel transmit applications: accelerated two-dimensional spiral excitation, multislice RF shimming, small-tip-angle kT -points excitation, and slice-selective spokes refocusing. Array-compressed designs are compared in simulations and an experiment to pulses designed using four existing array compression strategies. RESULTS: In all cases, array-compressed pulses achieved the lowest root-mean-square excitation error among the array compression approaches. Low errors were generally achieved without increasing root-mean-square RF amplitudes or maximum local 10-gram specific absorption rate. Leave-one-out multisubject shimming simulations demonstrated that array-compressed RF shimming can identify useful fixed coil combination weights that perform well across a population. CONCLUSION: Array-compressed pulse design jointly identifies the transmit coil array compression weights and RF pulses that perform best for a specific parallel excitation application. Magn Reson Med 76:1158-1169, 2016. © 2015 Wiley Periodicals, Inc.

Low peak power multiband spokes pulses for B1 (+) inhomogeneity-compensated simultaneous multislice excitation in high field MRI.

PURPOSE: To design low peak and integrated power simultaneous multislice excitation radiofrequency pulses with transmit field inhomogeneity compensation in high field MRI. THEORY AND METHODS: The "interleaved greedy and local optimization" algorithm for small-tip-angle spokes pulses is extended to design multiband (MB) spokes pulses that simultaneously excite multiple slices, with independent spokes weight optimization for each slice. The peak power of the pulses is controlled using a slice phase optimization technique. Simulations were performed at 7T to compare the peak power of optimized MB spokes pulses to unoptimized pulses, and to compare the proposed slice-independent spokes weight optimization to a joint approach. In vivo experiments were performed at 7T to validate the pulse's function and compare them to conventional MB pulses. RESULTS: Simulations showed that the peak power-minimized pulses had lower peak power than unregularized and integrated power-regularized pulses, and that the slice-independent spokes weight optimization consistently produced lower flip angle inhomogeneity and lower peak and integrated power pulses. In the brain imaging experiments, the MB spokes pulses showed significant improvement in excitation flip angle and subsequently signal homogeneity compared to conventional MB pulses. CONCLUSION: The proposed MB spokes pulses improve flip angle homogeneity in simultaneous multislice acquisitions at ultrahigh field, with minimal increase in integrated and peak radiofrequency power.