Background and Purpose-The delineation of the "penumbra" is of particular interest in acute stroke imaging. The "mismatch concept" applying perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) appears to be an oversimplification of the underlying electrophysiological tissue status. An additional parameter reflecting the metabolic state of the threatened brain tissue would improve our ability to describe the penumbra. One candidate is deoxyhemoglobin (deoxy-Hb) as an indicator of the oxygen extraction fraction that can be visualized by T2*-based blood oxygen level-dependent (BOLD) imaging. Methods-We analyzed data from 32 patients with acute stroke in the territory of the middle cerebral artery. MRI included fluid-attenuated inversion recovery, DWI, PWI, time-of-flight angiography, and quantitative T2 and T2* (qT2, qT2*) imaging. Follow-up was performed on day 1 and days 5 to 8. We calculated 1/T2Јϭ1/qT2*Ϫ1/qT2. Changes of T2Ј, representing the deoxy-Hb effect, were analyzed by 3D regions of interest (ROIs): apparent diffusion coefficient lesion day 0 (L0), time-to-peak-lesion day 0 (T0), final infarct size days 5 to 8 (F5-8), lesion growth (LG; F5-8ϪL0), and surviving tissue (ST; T0ϪF5-8). Results-We observed a clear decrease of T2Ј in the infarcted hemisphere compared with the unaffected control ROIs. The mean value showed the most pronounced loss of T2Ј signal intensity in L0 (Ϫ15.7%), followed by LG (Ϫ10.5%) and ST (Ϫ8.0%). Conclusions-The

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