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Background and Purpose-In ischemic stroke, diffusion-weighted (DW) and perfusion-weighted (PW) magnet resonance imaging (MRI) is used to define the mismatch as the therapeutic target. With positron emission tomography (PET), we characterized the metabolic patterns of tissue compartments identified by MRI and compared the volumes of mismatch to those of PET-defined penumbra. Methods-In 6 acute (median, 5.2 hours) and 7 chronic (median, 10 days) stroke patients in whom a mismatch was defined by PW/DW MRI, PET was performed (median, 120-minute delay). Cerebral blood flow (CBF), oxygen metabolism (CMRO 2 ), and oxygen extraction fraction (OEF) was determined in the areas of DWI lesion, mismatch, and oligemia. Then, the mismatch volume was compared with the volume of penumbra. Results-DWI lesions showed impaired tissue integrity (low CMRO 2 and low OEF). Mismatch areas were viable (normal CMRO 2 ) but showed largely varying OEF. Oligemic areas had metabolic patterns comparable to normal tissue. A mismatch volume was found in all 13 patients. However, only 8 of 13 had a corresponding penumbra volume that covered only a part of the mismatch. Conclusion-Our comparative PET/MRI study confirmed the current pathophysiological hypothesis for the DWI lesion and for the oligemic areas. However, the mismatch area did not reliably detect elevated OEF and overestimated the penumbra defined by PET.

Background and Purpose-In acute ischemic stroke, the hypoperfused but viable tissue is the main therapeutic target. In clinical routine, time-to-peak (TTP) maps are frequently used to estimate the hemodynamic compromise and to calculate the mismatch volume. We evaluated the accuracy of TTP maps to identify penumbral flow by comparison with positron emission tomography (PET). Methods-Magnetic resonance imaging (MRI) and PET were performed in 11 patients with acute ischemic stroke (median 8 hours after stroke onset, 60 minutes between MRI and PET imaging). The volumes defined by increasing TTP thresholds (relative TTP delay of Ͼ2, Ͼ4, Ͼ6, Ͼ8, and Ͼ10 seconds) were compared with the volume of hypoperfusion (Ͻ20 mL/100 g per min) assessed by 15 O-water PET. In a volumetric analysis, each threshold's sensitivity, specificity, and predictive values were calculated. Results-The median hypoperfusion volume was 34.5 cm 3 . Low TTP thresholds included large parts of the hypoperfused but also large parts of normoperfused tissue (median sensitivity/specificity: 93%/60% for TTP Ͼ2) and vice versa (50%/91% for TTP Ͼ10). TTP Ͼ4 seconds best identifies hypoperfusion (84%/77%). The positive predictive values increased with the size of hypoperfusion. Conclusion-This first comparison of quantitative PET-CBF with TTP maps in acute ischemic human stroke indicates thatthe TTP threshold is crucial to reliably identify the tissue at risk; TTP Ͼ4 seconds best identifies penumbral flow; and TTP maps overestimate the extent of true hemodynamic compromise depending on the size of ischemia. Only if methodological restrictions are kept in mind, relative TTP maps are suitable to estimate the mismatch volume. (Stroke.