Myelin Measurement: Comparison Between Simultaneous Tissue Relaxometry, Magnetization Transfer Saturation Index, and Tw/Tw Ratio Methods.

Magnetization transfer (MT) imaging has been widely used for estimating myelin content in the brain. Recently, two other approaches, namely simultaneous tissue relaxometry of R and R relaxation rates and proton density (SyMRI) and the ratio of T-weighted to T-weighted images (Tw/Tw ratio), were also proposed as methods for measuring myelin. SyMRI and MT imaging have been reported to correlate well with actual myelin by histology. However, for Tw/Tw ratio, such evidence is limited. In 20 healthy adults, we examined the correlation between these three methods, using MT saturation index (MT) for MT imaging. After calibration, white matter (WM) to gray matter (GM) contrast was the highest for SyMRI among these three metrics. Even though SyMRI and MT showed strong correlation in the WM (r = 0.72), only weak correlation was found between Tw/Tw and SyMRI (r = 0.45) or MT (r = 0.38) (correlation coefficients significantly different from each other, with p values < 0.001). In subcortical and cortical GM, these measurements showed moderate to strong correlations to each other (r = 0.54 to 0.78). In conclusion, the high correlation between SyMRI and MT indicates that both methods are similarly suited to measure myelin in the WM, whereas Tw/Tw ratio may be less optimal.