Conversion map from quantitative parameter mapping to myelin water fraction: comparison with R₁·R₂^* and myelin water fraction in white matter

To clarify the relationship between myelin water fraction (MWF) and R₁⋅R₂^* and to develop a method to calculate MWF directly from parameters derived from QPM, i.e., MWF converted from QPM (MWF_QPM).

Subjects were 12 healthy volunteers. On a 3 T MR scanner, dataset was acquired using spoiled gradient-echo sequence for QPM. MWF and R₁⋅R₂^* maps were derived from the multi-gradient-echo (mGRE) dataset. Volume-of-interest (VOI) analysis using the JHU-white matter (WM) atlas was performed. All the data in the 48 WM regions measured VOI were plotted, and quadratic polynomial approximations of each region were derived from the relationship between R₁·R₂^* and the two-pool model-MWF. The R₁·R₂^* map was converted to MWF_QPM map. MWF atlas template was generated using converted to MWF from R₁·R₂^* per WM region.

The mean MWF and R₁·R₂^* values for the 48 WM regions were 11.96 ± 6.63%, and 19.94 ± 4.59 s⁻², respectively. A non-linear relationship in 48 regions of the WM between MWF and R₁·R₂^* values was observed by quadratic polynomial approximation (R² ≥ 0.963, P < 0.0001).

MWF_QPM map improved image quality compared to the mGRE-MWF map. Myelin water atlas template derived from MWF_QPM may be generated with combined multiple WM regions.