<prompt> A decrease in cerebral blood flow to zero causes death of brain tissue within4 -- 10 minutes20 -- 30 minutes50 -- 60 minutes80 -- 90 minutes  <response>: A 4-10 minutes Decrease in cerebral blood flow to zero causes death of brain tissue within 4-10 minutes; Values 16-18 mL100 g tissue per minute cause infarction within an hour; and Values 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 transient symptoms, and the clinical syndrome is called a TIA. Tissue surrounding the core region of infarction is ischemic but reversibly dysfunctional and is referred to as the ischemic penumbra. The penumbra may be imaged by using perfusion-diffusion imaging with MRI or CT 