MRI assessment of multiple dipolar relaxation time ( T 1 D ) components in biological tissues interpreted with a generalized inhomogeneous magnetization transfer (ihMT) model

Abstract : T 1D , the relaxation time of dipolar order, is sensitive to slow motional processes. Thus T 1D 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 1D model associated with a modified ihMT sequence was previously proposed for in vivo evaluation of T 1D with MRI. However, experiments have suggested that myelinated tissues exhibit multiple T 1D components probably due to a heterogeneous molecular mobility. A bi-component T 1D 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 1D s 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 T 1D model.