The small iron-independent component R2', as compared with that of R2, is consistent with the hypothesis that R2' has higher iron-related specificity.

An MR tissue signature model of ischemic histopathology using ADCw and T2 can now be tested for its potential to predict reversible and identify irreversible cellular damage in human ischemic brain regions.

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Background and Purpose-Using newly developed computerized image analysis, we studied the heterogeneity of apparent diffusion coefficient of water (ADC w ) values in human ischemic stroke within 10 hours of onset. Methods-Echo-planar trace diffusion-weighted images from 9 patients with focal cortical ischemic stroke were obtained within 10 hours of symptom onset. An Iterative Self-Organizing Data Analysis (ISODATA) clustering algorithm was implemented to segment different tissue types with a series of DW images. ADC w maps were calculated from 4 DW images on a pixel-by-pixel basis. The segmented zones within the lesion were characterized as low, pseudonormal, or high, expressed as a ratio of the meanϮSD of ADC w of contralateral noninvolved tissue. Results-The average ADC w in the ischemic stroke region within 10 hours of onset was significantly depressed compared with homologous contralateral tissue (626.6Ϯ76.8 versus 842.9Ϯ60.4ϫ10 Ϫ6 mm 2 /s; PϽ0.0001). Nevertheless, ISODATA segmentation yielded multiple zones within the stroke region that were characterized as low, pseudonormal, and high. The mean proportion of low:pseudonormal:high was 72%:20%:8%. Conclusions-Despite low average ADC w , computer-assisted segmentation of DW MRI detected heterogeneous zones within ischemic lesions corresponding to low, pseudonormal, and high ADC w not visible to the human eye. This supports acute elevation of ADC w in human ischemic stroke and, accordingly, different temporal rates of tissue evolution toward infarction. (Stroke. 1998;29:1778-1782.)