Correlation of the average water diffusion constant with cerebral blood flow and ischemic damage after transient middle cerebral artery occlusion in cats

Magnetic resonance water diffusion imaging can detect early ischemic changes in stroke. Using a middle cerebral artery occlusion model, we examined which range of values of the orientation-independent diffusion quantity D(av) = 1/3Trace(D⇆) = 1/3(D(xx) + D(yy) + D(zz)) is an early noninvasive indicator of reduced cerebral perfusion and focal brain injury. Cats underwent either a 30-min occlusion followed by 3.5 h reperfusion (n = 7) or a 60-rain occlusion followed by 4-h reperfusion (n = 6). Repeated measurements of CBF were made with radiolabeled microspheres, and acute focal injury was measured with triphenyltetrazolium chloride (TTC) staining. During occlusion, the decrease in D(av) correlated with CBF for caudate [30=min occlusion (n = 13): p < 0.0001; 60-min occlusion (n = 6): p < 0.02] and for cortex [30-min occlusion (n = 12): p < 0.0001; 60-min occlusion (n = 5): p< 0.04]. Variable caudate and hemispheric injury levels were found among cats in both groups. The area of tissue injury demarcated by TTC began to correlate with the area of reduced D(av) by 30 min of occlusion (p < 0.02), and this correlation improved (p < 0.0001) at 1, 1.5, and 2.0 h after the onset of occlusion. The time necessary to reach a one-to-one correspondence between the percent of hemisphere injured and the percent of hemispheric area with D(av) < 0.65 x 10^-9 m²/s was 2 h after occlusion. Thus, the absolute value of D(av) is a good indicator of the risk of tissue injury, whereas the combination of D(av) and the length of time of D(av) reduction is an excellent predictor of acute focal tissue injury demarcated by TTC staining.