Myelin Measurement: Comparison Between Simultaneous Tissue Relaxometry, Magnetization Transfer Saturation Index, and...

Magnetization transfer (MT) imaging has been widely used for estimating myelin content in the brain. Recently, two other approaches, namely simultaneous tissue relaxometry of R₁ and R₂ relaxation rates and proton density (SyMRI) and the ratio of T₁-weighted to T₂-weighted images (T₁w/T₂w 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 T₁w/T₂w ratio, such evidence is limited. In 20 healthy adults, we examined the correlation between these three methods, using MT saturation index (MT_sat) 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_sat showed strong correlation in the WM (r = 0.72), only weak correlation was found between T₁w/T₂w and SyMRI (r = 0.45) or MT_sat (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_sat indicates that both methods are similarly suited to measure myelin in the WM, whereas T₁w/T₂w ratio may be less optimal.