Clinical Utility of CMRO2 Obtained with MRI in Determining Ischemic Brain Tissue at Risk
It has been suggested that the relationship between oxygen delivery and oxygen demand defines brain tissue at risk during cerebral ischemia. We have recently demonstrated that a quantitative estimate of cerebral blood oxygen saturation (and thereby oxygen extraction fraction, OEF) can be obtained using MRI in vivo. When combined with MR estimated cerebral blood flow (CBF), a quantitative measure of MR-CMRO2 can be calculated. In this study, we sought to explore the potential clinical utility of MR-CMRO2 in determining brain tissue at risk during cerebral ischemia. Seven patients with acute ischemic strokes were imaged at 4.5±.9 hrs (tp1) and 6 of the patients were imaged 3–5 days after stroke onset (tp2) with diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI). In addition, a 2D multiecho gradient/spin echo sequence was employed to acquire images which were subsequently used to estimate MR-OEF. Singular value decomposition was used to obtain a quantitative estimate of CBF to calculate MR-CMRO2 (= MR-OEF x MR-CBF). At tp1, 5/7 patients showed a DWI-PWI mismatch (PWI>DWI) while 2/7 patients exhibited match. At tp2, only 1 patient showed PWI<DWI, and the remaining patients had matched defects. In addition, 4/6 patients had DWI lesion volumes similar to tp1 abnormal PWI volumes, while 2/6 had smaller lesion volumes at tp2. Regional measurements of MR-CMRO2 were obtained in the regions where PWI and DWI abnormalities coincided (“core”), regions where PWI were abnormal (“core+penumbra”), and the corresponding regions on the contralateral hemisphere. In patients who demonstrated mismatched PWI-DWI defects at tp1, CMRO2 in the “core” regions was 35.3±3.4% of the contralateral region. In contrast, CMRO2 in the “core+penumbra” was 48.9±5.4% of the contralateral region, which was significantly different from “core” CMRO2 (p<0.003). Furthermore, the two patients with matched DWI-PWI at tp1 had CMRO2 values 16.6±14.1% of the contralateral regions. Analysis of tp2 images revealed that all “core” regions became infarcted. These results suggest that brain regions with CMRO2 above 35.3% of the contralateral hemisphere may represent salvageable tissue.