Conversion map from quantitative parameter mapping to myelin water fraction: comparison with R 1 ·R 2 * and myelin water fraction in white matter.

OBJECTIVE: 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 (MWFQPM ).

MATERIALS AND METHODS: 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 MWFQPM map. MWF atlas template was generated using converted to MWF from R1 ·R2 * per WM region.

RESULTS: 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).

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