<prompt> Cerebral Ischemia occurs when cerebral blood flow is less than10 ml100gminute20 ml100gminute40 ml100gminute50 ml100gminute  <response>: Acute occlusion of an intracranial vessel causes a reduction in blood flow to the brain region it suppliesCerebral blood flow to zero causes death of brain tissue within 4-10 minutes16-18 mL100 g tissue per minute cause infarction within an hour; 20 mL100 g tissue per minute cause ischemia without infarction unless prolonged for several hours or days.If blood flow is restored prior to a significant amount of cell death, the patient may experience only TIA.Tissue surrounding the core region of infarction is ischemic but reversibly dysfunctional and is referred to as the ischemic penumbra. The ischemic penumbra will eventually infarct if no change in flow occurs.Ref: Harrisons 18e p3271 