IDENTIFICATION OF COLLATERALLY PERFUSED AREAS FOLLOWING FOCAL CEREBRAL-ISCHEMIA IN THE RAT BY COMPARISON OF GRADIENT-ECHO AND DIFFUSION-WEIGHTED MRI

Diffusion-weighted (DW) and gradient echo (GE) magnetic resonance imag es were acquired before and after occlusion of the middle cerebral art ery (MCA) in the rat. Upon occlusion, an increase in DW imaging signal intensity was observed in a core area within the MCA territory, most likely reflecting cytotoxic edema. The signal from GE images, which is sensitive to changes in the absolute amount of deoxyhemoglobin, decre ased following ischemia within a region that extended beyond the core area observed with DW imaging. This hypointensity is attributed to inc reases in blood volume and/or oxygen extraction fraction, which result from a decrease in perfusion pressure in the collaterally perfused ar ea. The evolution of the GE imaging signal intensity from different re gions was studied for 3.5 h following the occlusion. In the core area, the GE imaging signal returned towards baseline values after similar to 1-2 h, while it remained stable in the surrounding area. This featu re may reflect a decrease in hematocrit due to microcirculatory defect and/or a decrease in the oxygen extraction fraction due to ongoing in farction of the tissue and may indicate that tissue recovery is severe ly compromised. The combined use of DW and GE imaging offers great pro mise for the noninvasive identification of specific pathological event s with high spatial resolution.