MRI assessment of multiple dipolar relaxation time (T1D) components in biological tissues interpreted with a generalized inhomogeneous magnetization transfer (ihMT) model.

T1D, the relaxation time of dipolar order, is sensitive to slow motional processes. Thus T1D is a probe for membrane dynamics and organization that could be used to characterize myelin, the lipid-rich membrane of axonal fibers. A mono-component T1D model associated with a modified ihMT sequence was previously proposed for in vivo evaluation of T1D with MRI. However, experiments have suggested that myelinated tissues exhibit multiple T1D components probably due to a heterogeneous molecular mobility. A bi-component T1D model is proposed and implemented. ihMT images of ex-vivo, fixed rat spinal cord were acquired with multiple frequency alternation rate. Fits to data yielded two T1Ds of about 500 μs and 10 ms. The proposed model seems to further explore the complexity of myelin organization compared to the previously reported mono-component T1D model.