MRI measurements of water diffusion: impact of region of interest selection on ischemic quantification

OBJECTIVE: To investigate the effect of ADC heterogeneity on region of interest (ROI) measurement of isotropic and anisotropic water diffusion in acute (< 12 h) cerebral infarctions. METHODS AND MATERIALS: Full diffusion tensor images were retrospectively analyzed in 32 patients with acute cerebral infarction. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were measured in ischemic lesions and in the corresponding contralateral, normal appearing brain by using four ROIs for each patient. The 2 x 2 pixel square ROIs were placed in the center, the lateral rim and the medial rim of the infarction. In addition, the whole volume of the infarction was measured using a free hand method. Each ROI value obtained from the ischemic lesion was normalized using contralateral normal ROI values. RESULTS: The localization of the ROIs in relation to the ischemic lesion significantly affected ADC measurement (P < 0.01, using Friedman test), but not FA measurement (P = 0.25). Significant differences were found between ADC values of the center of the infarction versus whole volume (P < 0.01), and medial rim versus whole volume of infarction (P < 0.001) with variation of relative ADC values up to 11%. The differences of absolute ADC for these groups were 22 and 23%, respectively. The lowest ADC was found in the center, followed by medial rim, lateral rim and whole volume of infarction. CONCLUSION: ADC quantification may provide variable results depending on ROI method. The ADC and FA values, obtained from the center of infarction tend to be lower compared to the periphery. The researchers who try to compare studies or work on ischemic quantification should be aware of these differences and effects.