Isotropic diffusion weighting in PGSE NMR: Numerical optimization of the q-MAS PGSE sequence

Abstract

Isotropic diffusion weighting is employed in diffusion NMR and MRI for rapid determination of the trace of the diffusion tensor and for showing the presence of microscopic diffusion anisotropy in a globally isotropic material. In the recently introduced q-MAS PGSE sequence, short gradient pulses define the beginning and the end of the diffusion time by quickly increasing and reducing the magnitude of the q-vector, while isotropic diffusion weighting is achieved by low-amplitude harmonically modulated gradients that make the q-vector rotate at the magic angle from an axis fixed in the lab frame. While efficient and easily implemented on microimaging systems with high-gradient capabilities, the previous version of q-MAS PGSE is too demanding for clinical MR scanners. Here, we present numerically optimized smooth gradient waveforms yielding maximum diffusion weighting for a given maximum gradient strength and echo time.