In vivo multi-slice mapping of myelin water content using T2* decay

Quantitative assessment of the myelin water content in the brain can substantially improve our understanding of white matter diseases such as multiple sclerosis. In this study, in vivo myelin water content was estimated using T₂ relaxation with multi-slice acquisitions in magnetic resonance imaging (MRI). The main advantages of using T₂ relaxation are (1) a low specific absorption rate (SAR), which is especially beneficial for imaging at high field strengths, (2) a short first-echo time (~2ms) and short echo spacing (~1ms), which allows for the acquisition of multiple sampling points during the fast decay of the myelin water signal, and (3) fast multi-slice acquisitions. High-resolution and multi-slice myelin water fraction (MWF) maps were obtained in a clinically acceptable scan time at 3T. Five healthy adults were scanned with a multi-gradient-echo sequence to acquire T₂ signal decay data. Images with a dimension of 256×256 at eight slice locations were acquired in 8.5min with a signal-to-noise ratio (SNR) of 94.8 in the first-echo images. The SNR was further increased by using an anisotropic diffusion filter. Local field gradients (LFG) were estimated from the acquired multi-slice data, and the LFG-induced signal decays were corrected with a first-order approximation of LFG using the sinc function. The corrected T₂ signal decays were analyzed with a three-pool model to quantify MWF. Our results demonstrate the feasibility of in ivo multi-slice mapping of MWF using multi-compartmental analysis of the T₂ signal decay.