10 - Localization of stroke syndromes using diffusion-weighted MR imaging (DWI)

Summary

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.