Advances in clinical neuroimaging: functional MR imaging techniques.

Abstract

U INTRODUCTION Initial efforts at developing useful magnetic resonance (MR) neuroimaging techniques were hampered by the limitations of conventional methods ofMR imaging. Ti and T2, the image contrast parameters used in conventional imaging, are not always sensitive or specific in characterizing tissue or distinguishing healthy from pathologic states. To address these limitations, developments were made that allow water diffusion (1-5), temperature (6-8), and motion (9) to be measured. These functional imaging techniques have significandy advanced neuroradiology by extending the anatomic capabilities of conventional MR imaging methods to the acquisition of tissue function maps of high spatial and temporal resolution. Hemodynamic imaging is perhaps the most clinically significant of these functional MR imaging techniques. Progress in imaging tissue blood flow, blood volume, and blood oxygenation has been spurred by two parallel developments. First, high-speed gradientecho (10) and echo-planar (1 1) imaging have now been implemented on commercially available equipment, resulting in a greatly increased speed of MR data collection. Second, a greater understanding now exists concerning high-magnetic-susceptibility exogenous and endogenous MR imaging contrast agents. Because of the intimate link between tissue function, metabolism, and blood supply (12), functional MR imaging oftissue hemodynamics can supply specific and sensitive information on healthy and pathologic states. By combining the use of magnetic-susceptibility contrast material and high-speed imaging, we can measure cerebral blood volume (CBV) in tumors and stroke, as well as estimate changes in cerebral blood flow (CBF) and blood oxygenation to visualize human brain activity in real time.