Selective RF excitation designs enabled by time‐varying spatially non‐linear ΔB0 fields with applications in fetal MRI. Issue 5 (21st December 2021)

• Record Type: Journal Article
• Authors: Zhang, Molin; Arango, Nicolas; Stockmann, Jason P.; White, Jacob; Adalsteinsson, Elfar
• Is Part Of: Magnetic resonance in medicine. Volume 87:Issue 5(2022)
• Page Start: 2161
• Page End: 2177
• Publication Date: 2021-12-21
• Subjects: auto‐differentiation -- fetal MRI -- selective RF excitation -- time‐varying shim array fields; Nuclear magnetic resonance -- Periodicals; Electron paramagnetic resonance -- Periodicals; 616.07548
• DOI: 10.1002/mrm.29114
• Languages: English
• ISSNs: 0740-3194

Abstract

Abstract: Purpose: To demonstrate, through numerical simulations, novel designs of spatially selective radiofrequency (RF) excitations of the fetal brain by both a restricted 2D slice and 3D inner‐volume selection. These designs exploit a single‐channel RF pulse, conventional gradient fields, and the spatially non‐linear Δ B 0 fields of a multi‐coil shim array, using an auto‐differentiation optimization algorithm. Methods: The design algorithm jointly optimizes the RF pulse and the time‐varying Δ B 0 fields, which is produced by a 64‐channel multi‐coil Δ B 0 body array to augment the RF and the linear gradient fields, using an auto‐differentiation approach. Two design targets were specified, one a 4‐mm thick slice with a limited in‐slice extent in one dimension ("restricted slice"), and the other a 3D inner‐volume selection encompassing the fetal brain ("inner volume"). The RF duration was limited to 2 ms for the restricted slice excitation and 6 ms for the inner‐volume excitation. Results: Excitation profiles were achieved for both the restricted slice excitation task (one‐minus‐minimum magnitude, 8%) within the region of interest (ROI) and (maximum‐minus‐zero magnitude, 8%) in the suppressed regions and the fetal brain volume excitation task (13% and 9%, respectively). Conclusions: The proposed joint design of RF and time‐varying, spatially non‐linear Δ B 0 fields achieves the target excitation profiles with short RF pulse durations and demonstrates the potential to enhance fetal MRI with multi‐channel body shim arrays.