Probing neural tissues at small scales: Recent progress of oscillating gradient spin echo (OGSE) neuroimaging in humans

Abstract

The detection sensitivity of diffusion MRI (dMRI) is dependent on diffusion times. A shorter diffusion time can increase the sensitivity to smaller length scales. However, the conventional dMRI uses the pulse gradient spin echo (PGSE) sequence that probes relatively long diffusion times only. To overcome this, the oscillating gradient spin echo (OGSE) sequence has been developed to probe much shorter diffusion times with hardware limitations on preclinical and clinical MRI systems. The OGSE sequence has been previously used on preclinical animal MRI systems. Recently, several studies have translated the OGSE sequence to humans on clinical MRI systems and achieved new information that is invisible using conventional PGSE sequence. This paper provides an overview of the recent progress of the OGSE neuroimaging in humans, including the technical improvements in the translation of the OGSE sequence to human imaging and various applications in different neurological disorders and stroke. Some possible future directions of the OGSE sequence are also discussed.

Keywords: Diffusion; Diffusion dispersion; Diffusion time; MRI; Microstructure; Oscillating gradient; Size.

Fig. 1

Comparison between the cosine and trapezoid-cosine (a) gradient waveforms and (b) corresponding encoding spectra ${\left|F\left(\omega \right)\right|}^2$. The figure is reprinted with permission from (Van et al., 2014).

Fig. 2

Representative diffusion time dependence of DTI metrics: MD (mean diffusivity), PD (parallel diffusivity), RD (radial diffusivity), and FA (fractional anisotropy) in three white matter regions and three gray matter regions of a healthy subject. GCC: genu, BCC: body, and SCC: splenium of the corpus callosum. TL: temporal lobes. PF: posterior fossa, and ICG: insula/cingulate gyri. The figure is reprinted with permission from (Tan et al., 2020).

Fig. 3

Representative images of raw dMRI, PGSE MD, and OGSE MD maps at multiple time points post-onset of ischemic stroke. The figure is reprinted with permission from (Baron et al., 2015).

Fig. 4

Comparison of RD maps in healthy control (top) and MS patient (bottom). From left to right, the anatomical image, RD map acquired with an effective diffusion time of 66.7 ms, and RD map acquired with an effective diffusion time of 8.77 ms are shown. A larger difference is observed between the healthy control and MS patient with the shorter diffusion times achievable with OGSE. The figure is reprinted with permission from (By et al., 2019).