B1+ inhomogeneity mitigation for diffusion weighted MRI at 7T using TR‐FOCI pulses

Abstract

AbstractPurposeThe purpose of this study is to improve the image quality of diffusion‐weighted images obtained with a single RF transmit channel 7 T MRI setup using time‐resampled frequency‐offset corrected inversion (TR‐FOCI) pulses to refocus the spins in a twice‐refocused spin‐echo readout scheme.MethodsWe replaced the conventional Shinnar‐Le Roux‐pulses in the twice refocused diffusion sequence with TR‐FOCI pulses. The slice profiles were evaluated in simulation and experimentally in phantoms. The image quality was evaluated in vivo comparing the Shinnar‐Le Roux and TR‐FOCI implementation using a b value of 0 and of 1000 s/mm2.ResultsThe b0 and diffusion‐weighted images acquired using the modified sequence improved the image quality across the whole brain. A region of interest–based analysis showed an SNR increase of 113% and 66% for the nondiffusion‐weighted (b0) and the diffusion‐weighted (b = 1000 s/mm2) images in the temporal lobes, respectively. Investigation of all slices showed that the adiabatic pulses mitigated inhomogeneity globally using a conventional single‐channel transmission setup.ConclusionThe TR‐FOCI pulse can be used in a twice‐refocused spin‐echo diffusion pulse sequence to mitigate the impact of inhomogeneity on the signal intensity across the brain at 7 T. However, further work is needed to address SAR limitations.