Temporal relationship between apparent diffusion coefficient and absolute measurements of cerebral blood flow in acute stroke patients.

BACKGROUND AND PURPOSE Diffusion-weighted imaging (DWI) has been established as a marker of acute ischemic brain injury. We sought to determine the relationship between cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) and to explore whether the elapsed time between MRI acquisition and symptom onset alters this relation in acute stroke patients. METHODS Sixteen acute stroke patients were studied with DWI and perfusion-weighted imaging, from which ADC and CBF were calculated. ADC values were normalized (nADC) to the contralateral, nonischemic hemisphere and then correlated pixel by pixel with CBF within a region of interest defined by abnormal transit time. To explore potential temporal effects on the relationship between CBF and nADC, patients were divided into 2 groups based on the duration between symptom onset and MR imaging for data analysis: group A, 2 to 4 hours (n=8), and group B, 4.5 to 6.5 hours (n=8). RESULTS nADC was plotted against CBF for each pixel in all 16 subjects, and a composite relationship was derived. After a gradual decline, an abrupt drop in nADC occurred below a CBF threshold value of 21 mL x min(-1) x 100 g(-1). When subjects were divided into early and late imaging groups, the group of patients imaged earlier (group A) had a lower threshold (15 mL x min(-1) x 100 g(-1)) than the group imaged later (group B, 24 mL x min(-1) x 100 g(-1)). CONCLUSIONS Our results demonstrate that a relationship between nADC and CBF exists in the ischemic brain and that ADC values alone may provide useful information in predicting perfusion status. However, this relationship may change with elapsing time between stroke onset and imaging.