MR imaging of anisotropically restricted diffusion of water in the nervous system: technical, anatomic, and pathologic considerations.

The use of MR imaging to image anisotropically restricted diffusion (ARD) of water in the nervous system is described. The theoretical basis for the use of the pulsed gradient spin echo sequences is outlined, including an estimate of the range of cell dimensions that can be studied with this technique. The importance of restricted diffusion across myelinated white matter fibre tracts is emphasised and the capacity of MR imaging to demonstrate fibre pathways as a function of their direction is illustrated. Technical developments that have been implemented include 256 x 256 spatial resolution, a wider range of diffusion times Td, and an increased range of diffusion sensitivity parameters b. Effects of these are illustrated together with the use of gradient moment nulling methods, oblique sensitisation, and a smaller set of gradient coils that enable shorter values of echo time to be used with the same value of b. The anatomical basis for ARD imaging is analysed, and association, commissural, and projection fibre tracts are demonstrated in different planes. The published literature on variations of the apparent diffusion coefficient from normal is reviewed and examples where diffusion weighted images revealed information that was not necessarily apparent with conventional sequences are illustrated. These include cases of multiple sclerosis, chronic head injury, progressive multifocal leucoencephalopathy, cerebrovascular disease, astrocytoma, and probable metastases to the brain. Imaging of ARD affords a fascinating conjunction between the microscopic movement of water, the properties of myelinated white matter fibres, gross anatomy of the brain, and changes of the diffusion of water in disease.