Evaluation of Normal-Appearing White Matter in Multiple Sclerosis Using Direct Visualization of Short Transverse Relaxation Time Component (ViSTa) Myelin Water Imaging and Gradient Echo and Spin Echo (GRASE) Myelin Water Imaging

Background: In multiple sclerosis (MS), not only lesions but also normal MRI-appearing white matter (NAWM) may undergo demyelination.

Purpose: To demonstrate the detection of NAWM demyelination using direct visualization of short transverse relaxation time component myelin water imaging (ViSTa-MWI) and to compare the results with those of conventional gradient echo and spin echo (GRASE)-MWI.

Study type: Control/cohort.

Population: Twenty-five MS patients and 18 healthy controls (HC).

Field strength/sequence: 3T/ViSTa and GRASE-MWI.

Assessment: Using ViSTa and GRASE-MWI, myelin water fraction (MWF) of NAWM or normal WM was compared between MS (all patients or early-stage MS patients) and HC. The comparison was performed for a global WM mask and five regional WM masks.

Statistical tests: A general linear model was applied for the comparison. A statistical power and a minimum sample size for the significant difference were obtained. Spearman's correlation coefficient was calculated between MWF and clinical measures and between ViSTa-MWF and GRASE-MWF for the global WM mask.

Results: MWFs of ViSTa were significantly lower in the MS patients than those in the HC in all masks (P < 0.001). GRASE-MWI results revealed reduced MWFs only in global WM, genu, and optic radiation. ViSTa-MWI had higher statistical powers than that of GRASE-MWI (power: ViSTa = 99.2 ± 1.6% and GRASE = 75.5 ± 31.0%; sample size: ViSTa = 18 ± 9 and GRASE = 78 ± 75). In early-stage MS, MWFs of ViSTa were significantly lower than those of the HC in all masks except for centrum semiovale; however, MWFs of GRASE MWI were significantly lower only in optic radiation. Disease duration was correlated with both MWIs (ViSTa; r = -0.437 and GRASE; r = -0.445). ViSTa and GRASE MWFs were significantly correlated in the HC (r = 0.664) and MS (r = 0.768).

Data conclusion: ViSTa-MWI may detect a reduction of MWF in NAWM of MS.

Level of evidence: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1091-1098.