Interpretation of inhomogeneous magnetization transfer in myelin water using a four-pool model with dipolar reservoirs.

Purpose: To confirm ihMT's specificity to myelin, an ihMT presaturation module was combined with a Poon-Henkelman multi-echo spin-echo readout to separate the ihMT signal in myelin water from intra-/extra-cellular water. This study explored the relationship between two quantitative myelin imaging techniques and measured the ihMT signal of myelin water.

Methods: Six rats were injured; three were sacrificed three weeks post-injury, and three were sacrificed eight weeks post-injury, and three healthy control rats were also sacrificed. The nine formalin-fixed rat spinal cords were imaged using a Poon-Henkelman multi-echo spin-echo readout with an ihMT prepulse at different strengths of $T_{1D}$ filtering at 7T.

Results: The proposed model was able to characterize the ihMT decay signal in myelin water and intra-/extra-cellular water pool. From this proposed four-pool model with dipolar order reservoirs, we see a drop in the $T_{1D}\left(\text{myelin}\right)$ fit parameter in the fasciculus gracilis white matter region of the three-week post-injury cord. $T_{1D}\left(\text{myelin}\right)$ and $T_{1D}$ (non-myelin) were estimated to be approx. 8 and 1.5 ms, respectively.

Conclusion: The drop in $T_{1D}\left(\text{myelin}\right)$ in the three-week post-injury cords suggests that $T_{1D}\left(\text{myelin}\right)$ could potentially distinguish between functional myelin and myelin debris; however, more studies are needed to confirm this.