Identification of Collaterally Perfused Areas following Focal Cerebral Ischemia in the Rat by Comparison of Gradient Echo and Diffusion-Weighted MRI

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Abstract
Diffusion-weighted (DW) and gradient echo (GE) magnetic resonance images were acquired before and after occlusion of the middle cerebral artery (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, decreased following ischemia within a region that extended beyond the core area observed with DW imaging. This hypointensity is attributed to increases in blood volume and/or oxygen extraction fraction, which result from a decrease in perfusion pressure in the collaterally perfused area. The evolution of the GE imaging signal intensity from different regions was studied for 3.5 h following the occlusion. In the core area, the GE imaging signal returned towards baseline values after ∼1–2 h, while it remained stable in the surrounding area. This feature may reflect a decrease in hematocrit due to microcirculatory defect and/or a decrease in the oxygen extraction fraction due to ongoing infarction of the tissue and may indicate that tissue recovery is severely compromised. The combined use of DW and GE imaging offers great promise for the noninvasive identification of specific pathological events with high spatial resolution.