Sensitivity of Magnetic Resonance Diffusion-Weighted Imaging and Regional Relationship Between the Apparent Diffusion Coefficient and Cerebral Blood Flow in Rat Focal Cerebral Ischemia

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Abstract
Magnetic resonance (MR) diffusion-weighted imaging (DWI), a noninvasive procedure, may play an important role in detecting and accurately localizing the extent of evolving infarction within the period immediately following stroke. We evaluated the sensitivity and specificity of DWI in detecting ischemia and compared a quantitative measure derived from the DWI, the apparent diffusion coefficient (ADC), with autoradiographic cerebral blood flow (CBF) in an experimental model of focal cerebral ischemia in rats. MR imaging data were obtained with a General Electric 4.7-T horizontal bore magnet CSI II system with self-shielded gradients. DWI was acquired within 41 +/- 6 minutes (mean +/- SD) after onset of ischemia and repeated at 169 +/- 14 minutes, followed by CBF determination at 237 +/- 21 minutes. DWI, ADC, and CBF images from each animal were then compared. The sensitivities for detecting an abnormality at 1 and 3 hours for DWI were significantly different, and the sensitivity of 3-hour DWI did not differ from the CBF sensitivity of 99%. A mean +/- SD ADC threshold of 460 +/- 95 microns 2/s was defined as 45% higher than the low ADC in the ischemic core compared with the contralateral ADC. Subthreshold ADC area and ischemic area were significantly correlated (r2 = .69, P < .05). In 19 of 48 regions of interest classified as ischemic (< 35 mL.100 g-1.min-1) from both the 3-hour ADC and CBF images, 3-hour ADC correlated significantly with CBF (r2 = .27, n = 19, P < .05), whereas in the nonischemic regions ADC was inversely correlated with CBF. Several ischemic regions showed a sharp drop in ADC to 37% (P < .001, n = 5) compared with all other regions (n = 43) from 1 to 3 hours. Because of the change in the sensitivity of detecting ischemia with DWI, the difference in correlation of CBF with ADC between ischemic and nonischemic cortex, and the presence of several regions in which ADC dropped to 37% from 1 to 3 hours, our data suggest that ADC values potentially can be used to monitor evolving infarction.