GRAPE-based slab-selective universal parallel transmit excitation pulses for whole-brain MRSI at 7 Tesla

Motivation: Whole-brain MRSI at ultra-high field is hampered by excitation field inhomogeneities which cause flip angle variations and, consequently, reduced SNR, especially in the cerebellum. Goal(s): To improve the flip angle homogeneity and, consequently, increase the SNR of whole-brain ultra-high field MRSI by utilizing universal pTx-pulses. Approach: Frequency-selective universal pTx-pulses(UP) were designed for homogeneous excitation with the GRAPE approach and implemented in a pulse-acquire-based MRSI sequence with concentric-ring readout at 7T. The spectral SNR was estimated with LCModel and compared to CP excitation. Results: GRAPE-UP excitation achieves a mean SNR increase of 23% in the cerebellum and slight SNR increase over the whole brain. Impact: This work shows the feasibility of GRAPE-based frequency-selective universal parallel transmit excitation pulses for whole-brain MRSI. These reduce spatial flip angle variations, and the resulting SNR loss, which is one of the main difficulties in whole-brain MRSI at ultra-high field.