Assessment of Optimal T1/T2-Weighted Combinations for Myelin Sensitivity: Effects of Echo Time and Exponents

The aim of this study is to improve the traditional T1-weighted (T1w) over T2-weighted (T2w) ratio as a proxy for myelin by investigating the optimal T2w TE and combination of exponent-weighted T1w and T2w images ( ${\mathrm{T1w}}^{x_1}/{\mathrm{T2w}}^{x_2}$ ).

T1w and T2w Gradient And Spin Echo (GRASE) data were acquired from 14 volunteers, 6 of whom had a repeated GRASE scan. ${\mathrm{T1w}}^{x_1}/{\mathrm{T2w}}^{x_2}$ ratios were computed for combinations of $x_1$ and $x_2$ ranging 0-5 in steps of 0.1 and T2w GRASE TE ranging 10-160 ms. Ratios were correlated with myelin-water fraction (MWF) maps, as a reference MRI myelin biomarker. Analyses were performed on white matter (WM) and deep gray matter (dGM). Reliability was evaluated for six subjects.

The optimized ratio with TE $=10\text{ms}$ , $x_1=2.3$ and $x_2=1.1$ significantly increased correlation with MWF compared to the traditional $\mathrm{T1w}/\mathrm{T2w}\left(80\text{ms}\right)$ ratio (paired $t$ -test on Fisher- $z$ values, optimized $\rho =0.55$ , traditional $\rho =0.38$ , $p<0.001$ ) while preserving excellent test-retest reliability (ICC(3,1) = 0.95 for both).

The ratio associated with myelin can be improved using optimized parameters without compromising reliability. Specifically, low T2w TE can significantly increase the correlation with MWF. T1w over proton density-weighted (T1w/PDw) metric can be a viable alternative to $\mathrm{T1w}/\mathrm{T2w}$ , more strongly associated with MWF.