The overall aims of this project are to develop and implement novel methods for quantitative, in vivo characterization of tissue by magnetic resonance imaging (MRI). The microscopic compartition of water in tissues reflects potentially important structural properties that may be probed by diverse MRI measurements. In particular, water diffusion and nuclear magnetic resonance (NMR) relaxation cannot be described by single components in many tissues. With some limited success, multiple component characterization of these attributes has been proposed and studied in an attempt to extract specific information about the micro-anatomical water compartments from which they are derived. The further development of efficient and effective methods for acquiring and analyzing subvoxel characteristics promises to be useful for assessing structure and pathophysiology in various tissues, particularly nerve and muscle. The studies proposed herein will provide faster, more accurate, and more informative techniques for compartmental studies using MRI. Experimental studies on model tissues will establish comprehensive and quantitative in vivo descriptions of water diffusion, longitudinal relaxation and transverse relaxation, and how they correlate to each other and the physical compartments from which they are derived. These observations will then be used to design novel MRI methods, which are more specific for depicting tissue microstructure in vivo. One example, amongst others, is the aim to develop rapid and easily implemented MRI methods of visualizing and quantifying myelin content in the brain based on detailed compartmental models of relaxation and diffusion in white matter.