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

Abstract T 1 D , the relaxation time of dipolar order, is sensitive to slow motional processes. Thus T 1 D 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 T 1 D model associated with a modified ihMT sequence was previously proposed for in vivo evaluation of T 1 D with MRI. However, experiments have suggested that myelinated tissues exhibit multiple T 1 D components probably due to a heterogeneous molecular mobility. A bi-component T 1 D 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 T 1 D s of about 500 μ s and 10 ms. The proposed model seems to further explore the complexity of myelin organization compared to the previously reported mono-component T 1 D model.