A thrombolytic agent, recombinant tissue plasminogen activator (rt-PA), was recently approved in Japan for use on patients within 3 hrs of the onset of cerebral infarction. In order to salvage cerebral tissue after an ischemic insult, it is crucial to detect the ischemic lesion before it becomes irreversible and to detect the core and penumbra areas of the lesion for guidance in selecting the suitable therapy. In this symposium we discuss the detection of ischemic lesions using plain CT, perfusion CT, and MRI. In the section on plain CT, we present a typical case with early CT signs. In the section on perfusion CT, we report on the feasibility and limitation of the technique for the diagnosis of acute cerebral infarction. In the section on MRI, we study the usefulness of DWI for the early and highly reliable detection of ischemic stroke.