Diffusion Weighted Fast Spin Echo With RF-Encoded Slabs for Portable Low-Field MRI Systems

To develop a robust diffusion weighted (DW) acquisition with clinically feasible scan times for portable MRI systems.

A linear reconstruction model including: tailored RF-encoding schemes, motion-induced phases, and systematic phase errors was developed for slab diffusion weighted fast spin echo (FSE) data acquired with quadratic phase increment. Systematic phases imparted by diffusion gradients were obtained using a calibration phantom. Trace-weighted DW-FSE was compared to DW-EPI in vivo on a portable 110 mT H-based system. A high-NEX DW-FSE acquisition was performed in healthy volunteers followed by Monte Carlo retrospective undersampling to evaluate expected image quality and mean diffusivity variation in a DWI protocol with clinically feasible scan times.

CPMG calibration and polynomials fitted to systematic phase errors enabled accurate estimation of motion-induced phases. Accounting of different phase sources reduced ghosting artifacts and improved SNR efficiency by enabling RF-encoding. RF-encoded slab DW-FSE obtained in vivo trace-weighted images without distortion artifacts and with voxel sizes 5× smaller than DW-EPI. The retrospectively undersampled acquisition showed that 12 cm slice coverage with voxel size 2.5 × 2.5 × 5 mm³ could be achieved in 12 min scan time. The mean diffusivity standard deviation between Monte Carlo trials was 0.2 × 10^-3 mm²/s.

Diffusion weighted FSE is a robust, distortionless approach for high SNR efficiency DWI on portable MRI systems. Multiband quadratic phase increment DW-FSE leverages the low SAR and higher peak B₁ provided by low field strengths and is less dependent on performant hardware compared to DW-EPI.