Recent advent in diffusion-weighted imaging

Full text: Diffusion weighted imaging (DWI) and diffusion-tensor imaging (DTI) has now become one of the essential research/clinical tools in analyzing the brain in both normal and pathological states. Water diffusion preferentially diffuses in a direction parallel to the axon's longitudinal axis but is relatively restricted in the perpendicular axis. This phenomenon can be represented mathematically by the so called diffusion ellipsoid, or tensor. The diffusivity along the principal axis, λ1 is also called the longitudinal, axial, or parallel diffusivity. The tensors of cerebral white matter can be reconstructed to track three-dimensional macroscopic fiber orientation in the brain. The translation of the longest axis of the tensor (v1) into neural trajectories can be achieved by various algorithms, among which the FACT algorithm will be one of the most frequently used. The tractography technique allows in vivo localization of neuronal fiber tracts, which was not previously possible. As a clinical tool, this technique primarily targets the intracranial space occupying lesions, i.e. brain tumors and vascular malformations. This has been also utilized for other conditions, such as stroke imaging and in assessing degenerative diseases. It is important to notice that DTI is prone to partial volume averaging especially at the areas with crossing fibers. Thus, when comparing results from different institutes, it will be ideal to standardize the voxel size to avoid this effect. The most important limitation of tractography will be that it has not yet been fully validated. Attempts to validate this technique have been made in the past, but are limited to comparisons of the tractographic images and known neuroanatomy. One has to interpret the results with cautions when applying this technique clinically. In addition to these above mentioned relatively established techniques, the presentation will cover a couple of new topics that utilize the DWI, which are the q-space imaging and temperature measurements. Q-space imaging is a novel MR technique that enables the assessment of ultrastructural changes of white matter. This is a technique that uses much higher b-values (e.g. 10,000 s/mm2) and the method is also known as dynamic MR microscopy. It was originally designed to measure the compartment size of porous material filled with water. This technique with high b values enables measurement of the mean displacement (MD) of the most slowly diffusing water molecules. The second topic will be the temperature measurement. There are a few different ways of assessing brain temperature in vivo, but most of them, including MR spectroscopy, require extended length of imaging time. Our method is thought to be one of the most clinically applicable one, which uses a simple postprocessing of the DWI. Although only applicable to the non-restricted water, e.g., cerebrospinal fluid (CSF), it is thought to be potentially useful for assessing the patients' brain conditions