T1D, the relaxation time of the dipolar order, is a probe for membrane dynamics and organization that could be used to assess myelin integrity. A single-component T1D model associated with a modified ihMT sequence had been proposed for in vivo evaluation of T1D with MRI. However, experiments and simulations revealed that myelinated tissues exhibit multiple T1D components. A bi-component T1D model is proposed and validated. Fits in a rat spinal cord yield two T1Ds of about 10 ms and 400 μs. The results suggest that myelin has a dynamically heterogeneous organization.