OTUP workflow: target specific optimization of the transmit k-space trajectory for flexible universal parallel transmit RF pulse design

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 B-0/B-1(+) map data from 18 subjects measured at 9.4 T are available as open source ().

Automated summary

The purpose of this study was to optimize transmit k-space trajectories and design universal pTx RF pulses. By optimizing trajectories and comparing the performance of different RF pulse designs, it was found that using optimized trajectories was crucial for pulse performance and that target specific universal pulses could be created. However, not every test target was suitable for universal pulse design. In vivo experiments showed a high degree of similarity between subject specific tailored pulses and universal pulses.