Tailored and universal parallel transmit broadband pulses for homogeneous 3D excitation of the human heart at 7T.

PURPOSE: To research and evaluate the performance of broadband tailored kT-point pulses (TP) and universal pulses (UP) for homogeneous excitation of the human heart at 7T. METHODS: Relative 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ -maps of the thorax were acquired from 29 healthy volunteers. TP and UP were designed using the small-tip-angle approximation for a different composition of up to seven resonance frequencies. TP were computed for each of the 29 B 1 + $$ {\mathrm{B}}_1^{+} $$ -maps, and UPs were calculated using 22 B 1 + $$ {\mathrm{B}}_1^{+} $$ -maps and tested in seven testcases. The performance of the pulses was analyzed using the coefficient of variation (CV) in the 3D heart volumes. The 3D gradient-echo (GRE) scans were acquired for the seven testcases to qualitatively validate the B 1 + $$ {\mathrm{B}}_1^{+} $$ -predictions. RESULTS: Single- and double-frequency optimized pulses achieved homogeneity in flip angle (FA) for the frequencies they were optimized for, while the broadband pulses achieved uniformity in FA across a 1300 Hz frequency range. CONCLUSION: Broadband TP and UP can be used for homogeneous excitation of the heart volume across a 1300 Hz frequency range, including the water and the main six fat peaks, or with longer pulse durations and higher FAs for a smaller transmit bandwidth. Moreover, despite large inter-volunteer variations, broadband UP can be used for calibration-free 3D heart FA homogenization in time-critical situations.

Calibration-free parallel transmission of the cervical, thoracic, and lumbar spinal cord at 7T.

PURPOSE: To address the limitations of spinal cord imaging at ultra-high field (UHF) due to time-consuming parallel transmit (pTx) adjustments. This study introduces calibration-free offline computed universal shim modes that can be applied seamlessly for different pTx RF coils and spinal cord target regions, substantially enhancing spinal cord imaging efficiency at UHF. METHODS: A library of channel-wise relative B 1 + $$ {B}_1^{+} $$ maps for the cervical spinal cord (six datasets) and thoracic and lumbar spinal cord (nine datasets) was constructed to optimize transmit homogeneity and efficiency for these regions. A tailored B0 shim was optimized for the cervical spine to enhance spatial magnetic field homogeneity further. The performance of the universal shims was validated using absolute saturation based B 1 + $$ {B}_1^{+} $$ mapping and high-resolution 2D and 3D multi-echo gradient-recalled echo (GRE) data to assess the image quality. RESULTS: The proposed universal shims demonstrated a 50% improvement in B 1 + $$ {B}_1^{+} $$ efficiency compared to the default (zero phase) shim mode. B 1 + $$ {B}_1^{+} $$ homogeneity was also improved by 20%. The optimized universal shims achieved performance comparable to subject-specific pTx adjustments, while eliminating the need for lengthy pTx calibration times, saving about 10 min per experiment. CONCLUSION: The development of universal shims represents a significant advance by eliminating time-consuming subject-specific pTx adjustments. This approach is expected to make UHF spinal cord imaging more accessible and user-friendly, particularly for non-pTx experts.

Respiration induced B1+ changes and their impact on universal and tailored 3D kT-point parallel transmission pulses for 7T cardiac imaging.

PURPOSE: Human heart imaging at ultra-high fields is highly challenging because of respiratory motion-induced artefacts and spatially heterogeneous B1+ profiles. This work demonstrates that respiration resolved 3D B1+ -maps can be used with a dedicated tailored and universal parallel transmission (pTx) pulse design to compensate respiration related B1+ changes in subjects performing shallow and deep breathing (SB/DB). METHODS: Three-dimensional (3D) B1+ -maps of the thorax were acquired in 31 subjects under SB and in 15 subjects under SB and DB. Different universal and tailored non-selective pTx pulses were designed from non-respiration resolved (NRR) and respiration resolved (RR) reconstructions of the SB/DB B1+ -maps. The performance of all pulses was tested with RR-SB/DB B1+ -maps. Respiration-robust tailored and universal pulses were applied in vivo in 5 subjects at 7T in 3D gradient-echo free-breathing scans. RESULTS: All optimized pTx pulses performed well for SB. For DB, however, only the universal and the tailored respiration-robust pulses achieved homogeneous flip angles (FAs) in all subjects and across all respiration states, whereas the tailored respiration-specific pulses resulted in a higher FA variation. The respiration-robust universal pulse resulted in an average coefficient of variation in the FA maps of 12.6% compared to 8.2% achieved by tailored respiration-robust pulses. In vivo measurements at 7T demonstrate the benefits of using respiration-robust pulses for DB. CONCLUSION: Universal and tailored respiration-robust pTx pulses based on RR B1+ -maps are highly preferred to achieve 3D heart FA homogenization at 7T when subjects perform DB, whereas universal and tailored pulses based on NRR B1+ -maps are sufficient when subjects perform SB.

Standardized universal pulse: A fast RF calibration approach to improve flip angle accuracy in parallel transmission.

PURPOSE: In parallel transmission (pTX), subject-tailored RF pulses allow achieving excellent flip angle (FA) accuracy but often require computationally extensive online optimizations, precise characterization of the static field ( ΔB0 ), and the transmit RF field ( B1+ ) distributions. This costs time and requires expertise from the MR user. Universal pulses (UPs) have been proposed to reduce this burden, yet, with a penalty in FA accuracy. This study introduces the concept of standardized universal pulses (SUPs), where pulses are designed offline and adjusted to the subject through a fast online calibration scan. METHODS: A SUP is designed offline using a so-called standardized database, wherein each B1+ map has been normalized to a reference transmit RF field distribution. When scanning a new subject, a 3-slice B1+ acquisition (scan time <10  s) is performed and used to adjust the SUP to the subject through a linear transform. SUP performance was assessed at 7T with simulations by computing the FA-normalized root mean square error (FA-NRMSE) and the FA pattern stability as measured by the average and coefficient of variation of the FA across 15 control subjects, along with in vivo experiments using an MP2RAGE sequence implementing the SUP variant for the FLASH readout. RESULTS: Adjusted SUP improved the FA-NRMSE (8.8 % for UP vs. 7.1 % for adjusted SUP). Experimentally in vivo, this translated in an improved signal homogeneity and more accurate T1 quantification using MP2RAGE. CONCLUSION: The proposed SUP approach improves excitation accuracy (FA-NRMSE) while preserving the same offline pulse design principle as offered by UPs.

Calibration-free pTx of the human heart at 7T via 3D universal pulses.

PURPOSE: MRI at ultra-high fields in the human body is highly challenging and requires lengthy calibration times to compensate for spatially heterogeneous B1+ profiles. This study investigates the feasibility of using pre-computed universal pulses for calibration-free homogeneous 3D flip angle distribution in the human heart at 7T. METHODS: Twenty-two channel-wise 3D B1+ data sets were acquired under free-breathing in 19 subjects to generate a library for an offline universal pulse (UP) design (group 1: 12 males [M] and 7 females [F], 21-66 years, 19.8-28.3 kg/m2 ). Three of these subjects (2M/1F, 21-33 years, 20.8-23.6 kg/m2 ) were re-scanned on different days. A 4kT-points UP optimized for the 22 channel-wise 3D B1+ data sets in group 1 (UP22-4kT) is proposed and applied at 7T in 9 new and unseen subjects (group 2: 4M/5F, 25-56 years, 19.5-35.3 kg/m2 ). Multiple tailored and universal static and dynamic parallel-transmit (pTx) pulses were designed and evaluated for different permutations of the B1+ data sets in group 1 and 2. RESULTS: The proposed UP22-4kT provides low B1+ variation in all subjects, seen and unseen, without severe signal drops. Experimental data at 7T acquired with UP22-4kT shows comparable image quality as data acquired with tailored-4kT pulses and demonstrates successful calibration-free pTx of the human heart. CONCLUSION: UP22-4kT allows for calibration-free homogeneous flip angle distributions across the human heart at 7T. Large inter-subject variations because of sex, age, and body mass index are well tolerated. The proposed universal pulse removes the need for lengthy (10-15 min) calibration scans and therefore has the potential to bring body imaging at 7T closer to the clinical application.

Optimization of pseudo-continuous arterial spin labeling at 7T with parallel transmission B1 shimming.

PURPOSE: To optimize pseudo-continuous arterial spin labeling (pCASL) for 7 T, and to further improve the labeling efficiency with parallel RF transmission transmit B1 ( B1+ ) shimming. METHODS: pCASL parameters were optimized based on B1+/B0 field distributions at 7 T with simulation. To increase labeling efficiency, the B1+ amplitude at inflowing arteries was increased with parallel RF transmission B1+ shimming. The "indv-shim" with shimming weights calculated for each individual subject, and the "univ-shim" with universal weights calculated on a group of 12 subjects, were compared with circular polarized (CP) shim. The optimized pCASL sequences with three B1+ shimming modes (indv-shim, univ-shim, and CP-shim) were evaluated in 6 subjects who underwent two repeated scans 24 hours apart, along with a pulsed ASL sequence. Quantitative metrics including mean B1+ amplitude, perfusion, and intraclass correlation coefficient were calculated. The optimized 7T pCASL was compared with standard 3T pCASL on 5 subjects, using spatial SNR and temporal SNR. RESULTS: The optimal pCASL parameter set (RF duration/gap = 300/250 us, Gave=0.6mT/m,gRatio=10 ) achieved robust perfusion measurement in the presence of B1+/B0 inhomogeneities. Both indv-shim and univ-shim significantly increased B1+ amplitude compared with CP-shim in simulation and in vivo experiment (P < .01). Compared with CP-shim, perfusion signal was increased by 9.5% with indv-shim (P < .05) and by 5.3% with univ-shim (P = .35). All three pCASL sequences achieved fair to good repeatability (intraclass correlation coefficient ≥ 0.5). Compared with 3T pCASL, the optimized 7T pCASL achieved 78.3% higher spatial SNR and 200% higher temporal SNR. CONCLUSION: The optimized pCASL achieved robust perfusion imaging at 7 T, while both indv-shim and univ-shim further increased labeling efficiency.

Can 7T MPRAGE match MP2RAGE for gray-white matter contrast?

Ultra-High Field (UHF) MRI provides a significant increase in Signal-to-Noise Ratio (SNR) and gains in contrast weighting in several functional and structural acquisitions. Unfortunately, an increase in field strength also induces non-uniformities in the transmit field (B1+) that can compromise image contrast non-uniformly. The MPRAGE is one of the most common T1 weighted (T1w) image acquisitions for structural imaging. It provides excellent contrast between gray and white matter and is widely used for brain segmentation. At 7T, the signal non-uniformities tend to complicate this and therefore, the self-bias-field corrected MP2RAGE is often used there. In both MPRAGE and MP2RAGE, more homogeneous image contrast can be achieved with adiabatic pulses, like the TR-FOCI inversion pulse, or special pulse design on parallel transmission systems, like Universal Pulses (UP). In the present study, we investigate different strategies to improve the bias-field for MPRAGE at 7T, comparing the contrast and GM/WM segmentability against MP2RAGE. The higher temporal efficiency of MPRAGE combined with the potential of the user-friendly UPs was the primary motivation for this MPRAGE-MP2RAGE comparison. We acquired MPRAGE data in six volunteers, adding a k-space shutter to reduce scan time, a kt-point UP approach for homogeneous signal excitation, and a TR-FOCI pulse for homogeneous inversion. Our results show remarkable signal contrast improvement throughout the brain, including regions of low B1+ such as the cerebellum. The improvements in the MPRAGE were largest following the introduction of the UPs. In addition to the CNR, both SNR and GM/WM segmentability were also assessed. Among the MPRAGEs, the combined strategy (UP + TR-FOCI) yielded highest SNR and showed highest spatial similarity between GM segments to the MP2RAGE. Interestingly, the distance between gray and white matter peaks in the intensity histograms did not increase, as better pulses and higher SNR especially benefitted the (cerebellar) gray matter. Overall, the gray-white matter contrast from MP2RAGE is higher, with higher CNR and higher intensity peak distances, even when scaled to scan time. Hence, the extra acquisition time for MP2RAGE is justified by the improved segmentability.

Optimizing BOLD sensitivity in the 7T Human Connectome Project resting-state fMRI protocol using plug-and-play parallel transmission.

The Human Connectome Project (HCP) has a 7T component that aims to study the human brain's organization and function with high spatial and temporal resolution fMRI and diffusion-weighted acquisitions. For whole brain applications at 7T, a major weakness however remains the heterogeneity of the radiofrequency transmission field (B1+ ), which prevents from achieving an optimal signal and contrast homogeneously throughout the brain. In this work, we use parallel transmission (pTX) Universal Pulses (UP) to improve the flip angle homogeneity and demonstrate their application to highly accelerated multi-band EPI (MB5 and GRAPPA2, as prescribed in the 7T HCP protocol) sequence, but also to acquire at 7T B1+ -artefact-free T1 - and T2 -weighted anatomical scans used in the pre-processing pipeline of the HCP protocol. As compared to typical implementations of pTX, the proposed solution is fully operator-independent and allows "plug and play" exploitation of the benefits offered by multi-channel transmission. Validation in five healthy adults shows that the proposed technique achieves a flip angle homogeneity comparable to that of a clinical 3 T system. Compared to standard single-channel transmission, the use of UPs at 7T yielded up to a two-fold increase of the temporal signal-to-noise ratio in the temporal lobes as well as improved detection of functional connectivity in the brain regions most strongly affected by B1+ inhomogeneity.

Robust nonadiabatic T2 preparation using universal parallel-transmit kT -point pulses for 3D FLAIR imaging at 7 T.

PURPOSE: The fluid attenuated inversion recovery sequence is a pillar technique to detect brain lesions in MRI. At ultrahigh field, the lengthening of T1 often advocates a T2 -weighting preparation module to regain signal and contrast between tissues, which can be affected by transmit RF field inhomogeneity. In this note, we report an extension of a previous fluid attenuated inversion recovery study that now incorporates the T2 preparation with parallel transmission calibration-free universal pulses to mitigate the problem. METHODS: The preparation consisted of a 90°-τ-180° -τ-90° module to implement an effective inversion in the CSF and a saturation in the brain tissues. Care was taken for the pulses to have the desired phase relationship in every voxel by appropriate pulse design. The RF pulse design made use of the kT -point parametrization and was based on a database of 20 B1+ and ΔB0 maps previously acquired on different subjects at 7 T. Simulations and experiments on 5 volunteers, not contained in the database, were performed for validation. RESULTS: Simulations reported very good inversion efficiency for the preparation module with 8% variation, with respectively 4 and 6 times less power and specific absorption rate than for the adiabatic version. Experiments revealed fluid attenuated inversion recovery images free of B1+ artifacts. CONCLUSION: This work contributes further to the panel of 3D sequences validated and now available with universal pulses at 7 T. The drop in power and specific absorption rate demand compared with adiabatic pulses in the T2 preparation leads to more freedom for the design of the readout train.

Design of universal parallel-transmit refocusing kT -point pulses and application to 3D T2 -weighted imaging at 7T.

PURPOSE: T2 -weighted sequences are particularly sensitive to the radiofrequency (RF) field inhomogeneity problem at ultra-high-field because of the errors accumulated by the imperfections of the train of refocusing pulses. As parallel transmission (pTx) has proved particularly useful to counteract RF heterogeneities, universal pulses were recently demonstrated to save precious time and computational efforts by skipping B1 calibration and online RF pulse tailoring. Here, we report a universal RF pulse design for non-selective refocusing pulses to mitigate the RF inhomogeneity problem at 7T in turbo spin-echo sequences with variable flip angles. METHOD: Average Hamiltonian theory was used to synthetize a single non-selective refocusing pulse with pTx while optimizing its scaling properties in the presence of static field offsets. The design was performed under explicit power and specific absorption rate constraints on a database of 10 subjects using a 8Tx-32Rx commercial coil at 7T. To validate the proposed design, the RF pulses were tested in simulation and applied in vivo on 5 additional test subjects. RESULTS: The root-mean-square rotation angle error (RA-NRMSE) evaluation and experimental data demonstrated great improvement with the proposed universal pulses (RA-NRMSE ∼8%) compared to the standard circularly polarized mode of excitation (RA-NRMSE ∼26%). CONCLUSION: This work further completes the spectrum of 3D universal pulses to mitigate RF field inhomogeneity throughout all 3D MRI sequences without any pTx calibration. The approach returns a single pulse that can be scaled to match the desired flip angle train, thereby increasing the modularity of the proposed plug and play approach. Magn Reson Med 80:53-65, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Universal pulses for homogeneous excitation using single channel coils.

PURPOSE: Universal Pulses (UPs) are excitation pulses that reduce the flip angle inhomogeneity in high field MRI systems without subject-specific optimization, originally developed for parallel transmit (PTX) systems at 7 T. We investigated the potential benefits of UPs for single channel (SC) transmit systems at 3 T, which are widely used for clinical and research imaging, and for which flip angle inhomogeneity can still be problematic. METHODS: SC-UPs were designed using a spiral nonselective k-space trajectory for brain imaging at 3 T using transmit field maps (B1+) and off-resonance maps (B0) acquired on two different scanner types: a 'standard' single channel transmit system and a system with a PTX body coil. The effect of training group size was investigated using data (200 subjects) from the standard system. The PTX system was used to compare SC-UPs to PTX-UPs (15 subjects). In two additional subjects, prospective imaging using SC-UP was studied. RESULTS: Average flip angle homogeneity error fell from 9.5 ± 0.5 % for 'default' excitation to 3.0 ± 0.6 % using SC-UPs trained over 50 subjects. Performance of the UPs was found to steadily improve as training group size increased, but stabilized after ~15 subjects. On the PTX-enabled system, SC-UPs again outperformed default excitation in simulations (4.8 ± 0.6 % error versus 10.6 ± 0.8 % respectively) though greater homogenization could be achieved with PTX-UPs (3.9 ± 0.6 %) and personalized pulses (SC-PP 3.6 ± 1.0 %, PTX-PP 2.9 ± 0.6 %). MP-RAGE imaging using SC-UP resulted in greater separation between grey and white matter signal intensities than default excitation. CONCLUSIONS: SC-UPs can improve excitation homogeneity in standard 3 T systems without further calibration and could be used instead of a default excitation pulse for nonselective neuroimaging at 3 T.