Inferring Microstructural Information of White Matter from Diffusion MRI

This chapter describes the utility of diffusion MRI for extraction of microstructural information of neuronal tissue, specifically the white matter. Neuronal tissues in general, and white matter in particular, are complex tissue structures composed of several cellular compartments. Diffusion MRI has the potential to infer features of these compartments as it actually measures the mean displacement of water molecules rather than their diffusion coefficient. Thus, assuming that the displacement of water molecules is affected by tissue microstructure, diffusion MRI should become sensitive to structural parameters of the tissue. The signal decay in diffusion MRI experiments is complicated and only part of it is acquired and analyzed by conventional diffusion imaging (diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI)). When acquiring the entire signal decay, it is apparent that it is composed of several diffusing components (free, hindered, and restricted diffusion), allowing compartment-specific information to be retrieved, using advanced acquisition and analysis frameworks. The analysis frameworks (e.g. q-space imaging and the composite hindered and restricted model of diffusion (CHARMED)) follow the q-space theory and relate signal properties to displacement characteristics. It is suggested that restricted diffusion is apparent in neuronal tissues with water molecule diffusion within the axons as the main contributor to it. Thus restricted diffusion as measured by MRI enables the extraction of quantitative, morphologically related parameters such as the axon density and axon diameter distributions. The different analysis routines and their biophysical meaning along with selected applications are presented and discussed.