INFERRING MICROSTRUCTURAL FEATURES AND THE PHYSIOLOGICAL-STATE OF TISSUES FROM DIFFUSION-WEIGHTED IMAGES

We review several methods that have been developed to infer microstruc tural and physiological information about isotropic and anisotropic ti ssues from diffusion weighted images (DWIs), These include Diffusion I maging (DI), Diffusion Tensor Imaging (DTI), isotropically weighted im aging, and q-space imaging. Just as DI provides useful information abo ut molecular displacements in one dimension with which to characterize diffusion in isotropic tissues, DTI provides information about molecu lar displacements in three dimensions needed to characterize diffusion Is anisotropic tissues. DTI also furnishes scalar parameters that beh ave like quantitative histological or physiological 'stains' for diffe rent features of diffusion, These include Trace(D), which is related t o the mean diffusivity, and a family of parameters derived from the di ffusion tensor, D, which characterize different features of anisotropi c diffusion, Simple thought experiments and geometrical constructs, su ch as the diffusion ellipsoid, can be used to understand water diffusi on in isotropic add anisotropic media, and the NMR experiments used to characterize it.