The current invention provides a method of identifying a ischemic lesion. The method includes loading perfusion imaging data into an electronic memory element and deriving perfusion maps from the perfusion imaging data, where the perfusion Performs necessary image pre-processing maps include a cerebral blood volume (CBV) map and an arterial delay time (DT) map, which utilize arterial delay and dispersion effects. Ischemic pixels are determined from the perfusion imaging taking into account arterial delay and dispersion effects data, where the DT is greater than a predetermined first threshold value and the CBV is below a second threshold value and the infarct portion of the ischemic lesion is determined, where DT is greater than a predetermined third threshold value and/or the CBV is below a forth threshold value. A cluster analysis is applied to all of the determined ischemic lesion and infarct pixels and the penumbra is then determined, where mismatch regions between the ischemic lesion and the infarct core define the penumbra.