Three-Dimensional Efficient Myelin-Weighted Imaging Utilizing Direct Visualization of Short Transverse Relaxation Time Component (ViSTa)

Abstract

Measuring myelin concentration in the brain has important implications in basic science and clinical practice. In MRI, myelin water imaging (MWI) has been suggested as a surrogate biomarker that provides high sensitivity and specificity for myelin. However, multi-exponential fitting is ill-conditioned, and it is sensitive to noise and artifacts, particularly in vivo. To overcome the ill-conditioned fitting problem, the two-dimensional ViSTa myelin-weighted imaging technique was proposed, and it provides a substantially improved myelin-weighted image. However, it is based on a two-dimensional single-slice acquisition scheme, and it is a limitation. In this study, a whole brain-covered 3D ViSTa sequence, based on a 3D segmented echo planar imaging (EPI) sequence with a pair of slice selective inversion RF pulses, was proposed. To investigate the 3D ViSTa myelin weighted image, the distribution of myelin content in the white matter of the brain was measured using both conventional MWI and ViSTa MWI. The proposed 3D ViSTa method achieves a whole brain-covered (FOV = 240 × 240 × 128 mm³) myelin water-weighted image in less than 8 min (1.5 × 1.5 × 4 mm³) and does not require heavy post-processing. Pseudo-quantification (apparent MWF) can be provided by normalizing the ViSTa image with a PD-weighted image. The voxel-wise correlation between the conventional MWI and the 3D ViSTa yielded a mean correlation coefficient of 0.74 ± 0.03 (mean ± standard deviation of the five subjects), demonstrating a high spatial similarity in myelin-weighted contrast between the two maps. The proposed 3D ViSTa with pseudo-quantification may be useful in clinical applications when absolute quantification is not necessary.