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

Abstract

PurposeTo 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.MethodsRelative 3D ‐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 ‐maps, and UPs were calculated using 22 ‐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 ‐predictions.ResultsSingle‐ 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.ConclusionBroadband 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

“…24,25 However, dynamic pTx is more sensitive to 𝛥B 0 variations and often exhibits lower temporal B + 1 efficiency compared to static pTx (RF shimming) due to the use of gradients to traverse through excitation k-space. 15,26,27 These challenges are particularly pronounced in SC imaging due to the lower B + 1 efficiency of SC coils compared to brain coils. Moreover, the curved SC anatomy makes it challenging to effectively compensate for ΔB 0 variations using vendor-provided tailored ΔB 0 shimming tools that are typically limited to cuboids.…”

“…Otherwise, the measured ΔB 0 variations can be incorporated into the pTx pulse design 15 or prospectively compensated for using broadband pTx RF pulses. 26 In this in-vivo study, we investigate the feasibility and benefits of using universal RF shims (US) to optimize the B + 1 transmit and B + 1 efficiency profiles in the cervical, thoracic, and lumbar spine. The US solutions are successfully validated at 7T in three previously unexamined subjects, covering the cervical and the thoracic/lumbar SC, respectively.…”