Diffusion-weighted magnetic resonance imaging (DWI) is a technique based on diffusion of water molecules in tissues

Diffusion-weighted magnetic resonance imaging (DWI) is a technique based on diffusion of water molecules in tissues with clinical applications to a wide array of pathological conditions. In vitro measurements of diffusion coefficients for liquids based on their nuclear magnetic resonance signal using the pulse field gradient method were originally described in the 1960s [1] and improved in the early 1970s [2]. These were followed by in vivo measurements of molecular diffusion of water and applications to various pathologic states in human and animal studies, including cerebral ischemia and neoplasms in the mid-1980s and early 1990s [3–14]. However, because of significant technical requirements of DWI, there has only been widespread clinical application of this technique in the last decade. Currently, DWI is the most reliable method for detection of early and small ischemic infarcts in the brain. However, applications of DWI extend far beyond the work-up of acute ischemia. DWI is increasingly used for the evaluation of a large variety of neoplastic and nonneoplastic conditions affecting the brain, head, and neck. This chapter begins with a description of the basic mechanisms underlying DWI changes on magnetic resonance imaging (MRI) scans, focusing on the extensive work using experimental models of brain ischemia. This is followed by a discussion of currently established and experimental clinical applications of DWI, starting with the use of DWI for imaging acute ischemic stroke. The physics of DWI is reviewed in a separate chapter and is not discussed here. Basic Mechanisms Underlying Diffusion Changes in Pathologic States