Time course of an ischemic infarction. Images demonstrate the evolution of an ischemic infarction involving the left cerebellar hemisphere and left middle cerebellar peduncle. Both transverse DW images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time msec/echo time msec, 6,000/108; matrix, 256 × 128; field of view, 400 × 200 mm; section thickness, 6 mm with 1-mm gap) and transverse ADC maps are displayed. The patient underwent MR imaging 6 hours after the onset of acute neurologic symptoms. At 6 hours, the lesion (arrows) is hyperintense on the DW images and hypointense on the corresponding ADC map. The lesion becomes progressively more hyperintense on DW images, reaching its maximum hyperintensity at the 58-hour time point, when it also reaches its maximum hypointensity on ADC maps. At 7 days, there is ongoing resolution of the lesion on both DW images and ADC maps. By 134 days, there is subtle hypointensity on the DW image and hyperintensity on the ADC images.
The time course does not always conform to the aforementioned outline. With early reperfusion, pseudonormalization (return to baseline of the ADC reduction associated with acute ischemic stroke) may occur at a much earlier time—as early as 1–2 days in humans given intravenous recombinant tissue plasminogen activator less than 3 hours after stroke onset (40). Furthermore, Nagesh et al (41) demonstrated that although the mean ADC of an ischemic lesion is depressed within 10 hours, different zones within an ischemic region may demonstrate low, pseudonormal, or elevated ADCs, suggesting different temporal rates of tissue evolution toward infarction. Despite these variations, tissue characterized by an initial reduction in ADC nearly always undergoes infarction in humans.
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