What is Diffusion-Weighted MR (Magnetic Resonance) Imaging?

Diffusion-weighted MR (magnetic resonance) imaging has steadily evolved from a basic research tool to a clinical tool. Diffusion is a physical property of molecules referring to their ability to move randomly in relation to their thermal energy. Molecular motion is referred to as Brownian motion and it is a random translational movement that occurs at the microscopic level. It is measured in terms of the diffusion coefficient which, in general, increases in more dilute solutions and has a directional component. Since diffusion is a reflection of very small-scale motion, diffusion imaging must be very sensitive to motion. Hardware and technical advances have enabled the detection of this very small-scale motion. It represents a major advance in the evolution of pulse sequences that can make subtle abnormalities more obvious and can provide different characterization of tissues and their pathologic processes.

Diffusion-weighted imaging has made its greatest impact on the imaging evaluation of ischemic stroke. The diffusion coefficient of brain decreases within minutes of onset of ischemia. Ischemic diffusional changes occur much earlier than conventional T2 MR signal changes. While standard MR sequences may detect ischemic infarction as early as three hours after ictus, diffusion imaging detects ischemia within minutes. The pathologic basis for diffusion changes in ischemic brain is still subject to some controversy, but it is felt to be related to the formation of cytotoxic edema with the influx of sodium and water into the cell and subsequent reduction of extracellular volume. These fluid shifts are secondary to ischemia related dysfunction of the cell membrane sodium-ATP dependent pump. The resultant restricted or reduced diffusion is detected as an area of increased signal on a diffusion-weighted MR image. It appears that there is a tight coupling between ATP levels and ATP-dependent pump function, and it is likely that there is a correspondingly narrow range of ischemic conditions under which diffusion-weighted signal changes are reversible. Therefore, abnormalities seen on diffusion images secondary to ischemia are almost always areas of irreversible infarction.

Until the FDA approval of alteplase for IV (intravenous) fibrinolytic treatment of acute cerebrovascular thrombosis, there was little, if any, active management of acute, nonhemorrhagic stroke. Clinical outcomes from alteplase, as well as other acute stroke therapies could be improved with better patient selection. For example, fibrinolytic therapy of patients who do not have vascular occlusive disease is not likely to help. Likewise, patients with already completed major strokes might not benefit from any therapy. More advanced imaging techniques such as diffusion imaging is likely to improve triage of acute stroke patients into appropriate treatment, or non-treatment groups.