A computed tomography scan (CT scan) of the head directs a series of X-rays through the brain that are analyzed by a computer to produce detailed pictures of the brain. A CT scan is commonly the first test done on people who are thought to have had a stroke, although MRI also may be done. The equipment to do a CT scan is available at most hospitals, and the test can be done quickly.

• If a CT scan of the head is done 24 to 48 hours after a stroke, it can often help identify the site and the extent of the brain injury from the stroke.
• A CT scan may not always identify small areas of bleeding around the brain (subarachnoid hemorrhage) or bleeding that occurred several days before the scan.
• A CT scan of the head may be normal during the first 48 to 72 hours after a stroke caused by a clot (ischemic stroke). If an ischemic stroke is suspected, another CT scan is recommended 72 hours after the stroke.
• A CT scan cannot show the difference between a stroke that was caused by a clot that forms within an artery (thrombus) and a clot that has traveled through the blood (embolus).
• A CT scan cannot always detect the damage caused by a stroke in the lower rear part of the brain (cerebellum) and the part of the brain that connects with the spinal cord (brain stem). In these cases, an MRI may be needed to determine the location of brain damage caused by a stroke.