Introduction

DWI is an MR imaging technique in which microscopic water motion is responsible for the contrast within the image. DWI has assumed the role of a valuable imaging technique because it provides information that is not available on standard T1-and T2-weighted MR images. By showing hyperacute brain ischemia within minutes after stroke onset, diffusion-weighted imaging has gained importance in the assessment of stroke, whereas CT or T2-weighted MR images become positive only after several, usually 5 or 6 hours after stroke onset. In a rodent model, sensitivity of diffusion-weighted imaging in the detection of acute infarction has amounted to 60% within 50 minutes and 100% within 2 hours after symptomatology onset.

Clinical representations of DWI results

Diffusion of water molecules alters conventional T1- and T2-weighted MR imaging, because it induces a signal dephasing and a signal loss. On the other hand, through adequate MR sequences, this signal loss can be turned into a specific information, which constitutes the basis of DWI. Enhancement of the DWI signal is afforded by adding a bipolar gradient, which consists of two sequential pulses superimposed to the classical 90°-180° spin echo sequence. The first gradient pulse is applied between the 90° and the 180° pulses. Motion during and after this gradient pulse induces dephasing of the transverse magnetization of the static and mobile molecules.