Spectral lineshape reflects microscopic structure and ordering

Introduction: The cellular structure of living tissues is much finer than the achievable resolution of MRI images. Water molecules diffusing through the tissue explore its microstructure, but the observed signal, the sum of all spin contributions, is effectively averaged over macroscopic voxels, so that most of the subvoxel information is lost. The relevant question is which of the structural parameters survive this averaging and can be quantified from the NMR signal. In this work we show for the first time that the NMR spectral lineshape is sensitive to the degree of structural order in tissues. That is, the lineshape distinguishes between an ordered and a disordered arrangement of cells with otherwise identical NMR parameters (Fig. A). Our analytical results agree well with Monte Carlo simulations of transverse relaxation in synthetic three-dimensional media.