Effect of echo time and T-weighting on GRASE-based Tw/Tw ratio measurements at 3T.

Tissue contrast can be enhanced by dividing T-weighted (Tw) images by T-weighted (Tw) images to map the so-called Tw/Tw ratio, which has become an increasingly popular technique for quantifying brain tissue changes associated with neurodevelopment, aging, and a variety of neurodegenerative diseases. However, although it is self-evident that Tw/Tw ratios increase with the amount of T-weighting in the Tw image - which is determined by the echo time (TE), all else being equal - longer TEs also reduce the signal-to-noise ratio (SNR) of the Tw images, and it is not clear how these SNR characteristics affect the reliability of Tw/Tw measurements. Therefore, the current study systematically investigated how different amounts of T-weighting affected Tw/Tw measurements in order to determine whether there is an optimal amount of T-weighting. Tw images were acquired from 10 neurologically healthy adults using a 3D turbo field echo (TFE) sequence, and a series of T-weighted images were extracted from a multi-echo 3D combined gradient- and spin-echo (GRASE) sequence. Analyses of 12 anatomically defined brain regions revealed that both the mean and standard deviation of the Tw/Tw measurements increased exponentially with TE of the Tw images, and that Tw images with TE ≈ 120-160 ms yielded the most consistent/reproducible contrast between white matter ROIs and the whole-brain Tw/Tw signal. Furthermore, comparisons between Tw/Tw measurements and multi-component T-relaxation myelin water fractions (MWFs) in the same brain regions revealed that Tw images with TE ≥ 160 ms drastically reduced the degree of correlation between Tw/Tw measurements and MWF. Overall, these findings suggest that: 1) there is a substantial trade-off between increased Tw/Tw contrast (based on longer TEs for the Tw images) and the reliability of quantitative Tw/Tw signals; and 2) the optimal TE for Tw GRASE scans is between 120 ms and 160 ms for calculating Tw/Tw ratios.