Mapping the myelin bilayer with short-T2 MRI: Methods validation and reference data for healthy human brain

Purpose To explore the properties of short-T-2 signals in human brain, investigate the impact of various experimental procedures on these properties and evaluate the performance of three-component analysis. Methods Eight samples of non-pathological human brain tissue were subjected to different combinations of experimental procedures including D2O exchange and frozen storage. Short-T-2 imaging techniques were employed to acquire multi-TE (33-2067 mu s) data, to which a three-component complex model was fitted in two steps to recover the properties of the underlying signal components and produce amplitude maps of each component. For validation of the component amplitude maps, the samples underwent immunohistochemical myelin staining. Results The signal component representing the myelin bilayer exhibited super-exponential decay with T-2,T-min of 5.48 mu s and a chemical shift of 1.07 ppm, and its amplitude could be successfully mapped in both white and gray matter in all samples. These myelin maps corresponded well to myelin-stained tissue sections. Gray matter signals exhibited somewhat different components than white matter signals, but both tissue types were well represented by the signal model. Frozen tissue storage did not alter the signal components but influenced component amplitudes. D2O exchange was necessary to characterize the non-aqueous signal components, but component amplitude mapping could be reliably performed also in the presence of H2O signals. Conclusions The myelin mapping approach explored here produced reasonable and stable results for all samples. The extensive tissue and methodological investigations performed in this work form a basis for signal interpretation in future studies both ex vivo and in vivo.