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

Abstract 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 ofT 1 D$$ {T}_{1D} $$ 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 theT 1 D ( myelin ) $$ {T}_{1D}\left(\mathrm{myelin}\right) $$ fit parameter in the fasciculus gracilis white matter region of the three‐week post‐injury cord.T 1 D ( myelin ) $$ {T}_{1D}\left(\mathrm{myelin}\right) $$ andT 1 D$$ {T}_{1D} $$ (non‐myelin) were estimated to be approx. 8 and 1.5 ms, respectively. Conclusion The drop inT 1 D ( myelin ) $$ {T}_{1D}\left(\mathrm{myelin}\right) $$ in the three‐week post‐injury cords suggests thatT 1 D ( myelin ) $$ {T}_{1D}\left(\mathrm{myelin}\right) $$ could potentially distinguish between functional myelin and myelin debris; however, more studies are needed to confirm this.