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

• Published in: NMR in biomedicine Vol. 35; no. 8; pp. e4728 - n/a
• Main Authors: Geldschläger, Ole, Bosch, Dario, Henning, Anke
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.08.2022
• Subjects: 9.4 T; Biological products; Design; Design optimization; Excitation; gradient optimization; high‐field MRI; In vivo methods and tests; local excitation; Optimization; parallel transmit; pTx; Similarity; Spirals; Trajectory optimization; universal pulses; Workflow; high-field MRI; local excitation; parallel transmit; pTx; 9.4 T; gradient optimization; universal pulses
• ISSN: 0952-3480; 1099-1492; 1099-1492
• DOI: 10.1002/nbm.4728

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 B0/B1+ map data from 18 subjects measured at 9.4 T are available as open source (https://github.com/ole1965/workflow_OTUP.git). Transmit k‐space trajectories (stack of spirals and SPINS) were optimized to best match different excitation targets for pTx RF universal and subject specific tailored pulse design. The workflow produced appropriate transmit k‐space trajectories for each test target. Based on the optimized transmit trajectories it was possible to create target specific universal and tailored pulses with similar performances in simulation and in vivo at 9.4 T.