MRI measurements of water diffusion and cerebral perfusion: their relationship in a rat model of focal cerebral ischemia.

The aim of this study was to examine the quantitative relationship between changes in apparent diffusion coefficient (ADC) and transverse relaxivity (delta R2*) measurements of relative perfusion deficits within the gradients of a focal ischemic insult. Sixty minutes after permanent occlusion of the middle cerebral artery, rats (n = 7) were subjected to spin echo diffusion-weighted scans followed by fast low-angle shot (FLASH) perfusion-sensitive scans. Diffusion-weighted images showed clear ischemic lesions in the affected basal ganglia and cortex. Ischemic deficits were demonstrated as a decrease in first-pass transit of injected boluses of gadodiamide. ADC maps were generated and regions of interest (ROIs) were obtained to span the range of ADC reductions from the lesion center or core to the periphery or penumbra. Corresponding ROIs from the bolus injection images were used to calculate perfusion indexes relative to contralateral levels as ratios of delta R2* integrals and ratios of delta R2* peak values. In all animals, the degree of ADC reductions was related to the degree of delta R2* perfusion deficits, ranging from severe ischemia in the core of the lesion to intermediate and moderate changes toward the lesion periphery. In the ischemic periphery, ADC reductions were linearly correlated with delta R2* peak ratios. However, no significant correlation was found between ADC reductions and delta R2* integral ratios. These data suggest that magnetic resonance measurements of ADC and delta R2* peak ratios can be used to quantitatively assess the variable gradients in focal ischemia, including potentiallyn critical areas at risk in the ischemic periphery.