Estimation of the Onset Time of Cerebral Ischemia Using T1ρ and T2 MRI in Rats

Background and Purpose-Time of ischemia onset is the most critical factor for patient selection for available drug treatment strategies. The purpose of this study was to evaluate the abilities of the absolute longitudinal rotating frame (T-1 rho) and transverse (T-2) MR relaxation times to estimate the onset time of ischemia in rats. Methods-Permanent middle cerebral artery occlusion in rats was used to induce focal cerebral ischemia and animals were imaged with multiparametric MRI at several time points up to 7 hours postischemia. Ischemic parenchyma was defined as tissue with apparent diffusion coefficient of water <70% from that in the contralateral nonischemic brain. Results-The difference in the absolute T-1 rho and T-2 between ischemic and contralateral nonischemic striatum increased linearly within the first 6 hours of middle cerebral artery occlusion. The slopes for T-1 rho and T-2 fits for both tissue types were similar; however, the time offsets were significantly longer for both MR parameters in the cortex than in the striatum. Conclusions-T-1 rho and T-2 MRI provide estimates for the onset time of cerebral ischemia requiring regional calibration curves from ischemic brain. Assuming that patients with suspected ischemic stroke are scanned by MRI within this timeframe, these MRI techniques may constitute unbiased tools for stroke onset time evaluation potentially aiding the decision-making for drug treatment strategies. (Stroke. 2010;41:2335-2340.)