Remarkable advances in technology now make it possible to examine how the brain looks, works and gets its blood supply. These tests can outline the affected part of the brain and help define the problem created by stroke. Most of these tests are safe, painless and can be done as an outpatient. However, in many cases these tests are ordered when a patient is hospitalized with a stroke. A doctor must decide on a case-by-case basis whether such tests will be useful, and if so, which ones to use. The following tests are described in this section:

What imaging tests are done on the brain?

• Computerized axial tomographic scan (CT or CAT scan) — Uses X-rays to generate an image of the brain. Doctors use CT to determine whether a stroke has occurred and, if so, what kind. (Ischemic strokes are caused by a clot that blocks an artery. Hemorrhagic strokes result from a ruptured blood vessel in the brain causing bleeding into brain tissue.) CT scanning takes from 5 to 10 minutes to complete (mostly less than 5 minutes). The test causes no discomfort.
  
• Magnetic resonance imaging scanning (MRI) — The stroke patient is placed into the MRI scanner. This scanner has a magnetic field in which the head is subjected to bursts of energy of a known magnetic frequency. The response of the brain cells to these bursts of energy is detected as signals that ultimately generate an image of the brain. MRI can give very accurate images of the brain. These are used to determine the presence, location and size of aneurysms and arteriovenous malformations, which are potential sources for hemorrhagic stroke. This test is performed in 40 minutes to one hour, and causes no discomfort.
  
• Radionuclide angiography — Radioactive compounds are injected into a vein in the arm; the bloodstream then carries them toward the head. As the radioactive compound circulates in the bloodstream, it constantly emits bursts of radiation. Once the radioactive compound reaches the brain, these bursts of radiation are detected and used to form an image of the brain. This imaging procedure can show areas where the brain has been deprived of blood flow and is damaged.

What tests show the brain's electrical activity?

• Electroencephalogram (EEG) — Small metal disks (electrodes) are placed at strategic locations on a person's scalp. The electrodes can detect the electrical activity in the form of impulses that are then transcribed to paper. By observing such impulse characteristics as intensity (the size of the impulse), duration (the width of the impulse), frequency (how often impulses occur during a given time) and location (what region of the brain produces these impulses), an EEG can provide valuable information about underlying problems in the brain. Some people who have strokes are prone to seizures, and this test will help doctors determine if seizures are present and if treatment with medications is needed.
  
• Evoked response test — A diagnostic procedure that provides a measurement of the brain's ability to process and react to different sensory stimuli. A doctor evokes a visual response by flashing a light or checkerboard pattern in front of a patient. For auditory evoked responses, a doctor produces a sound in one of the patient's ears. For bodily evoked responses, one of the nerves in an arm or leg is electrically stimulated. The responses from these sensory stimuli can indicate abnormal areas of the brain.

What tests show blood flow?

• Doppler ultrasound test — Uses high-frequency sound waves to detect blockages in the carotid artery. A Doppler probe or instrument capable of generating ultrasound waves is placed on the neck very near to the carotid artery. Ultrasound waves from the probe travel through the neck and bounce off the moving blood cells. The reflected sound wave, now returning to the probe at a different frequency, is then detected by the same probe. The change in frequency of the sound waves relates to the speed of the blood cells and thus the blood flow. This test takes an hour or more, and causes no discomfort.
  
• Carotid phonoangiography — A sensitive microphone is placed on the neck, very close to the carotid artery, to record sounds. Ordinarily, in a normal artery, blood flows in a smooth and controlled manner. However, the presence of blockages, such as those caused by atherosclerosis, causes the blood flow to become turbulent. This turbulent blood flow can create a sound, called a bruit (BROO'e), that can be detected and registered by the microphone. The presence of a bruit may indicate a blockage in the carotid artery and is cause for more tests.
  
• Digital subtraction angiography (DSA) — Gives an image of the brain's major blood vessels. A thin plastic tube (a catheter) is inserted into a major artery of the leg and advanced through the body's major vessels until it reaches the brain's blood vessels. A contrast dye is injected through the catheter and allowed to circulate in the bloodstream. At that point, an X-ray machine quickly takes a series of pictures of the head and neck. The images track the movement of the contrast dye as it moves through the brain's blood vessels. This imaging technique lets the doctor identify and localize the source of a blocked blood vessel that caused the stroke. Some people may feel a warm sensation as the contrast medium is injected into the blood vessels.

For stroke information, call the American Stroke Association at 1-888-4-STROKE.

Related AHA and ASA publications:

• Stroke: Are You at Risk? (also in Spanish on ASA Web site)
• Warning Signs of Stroke (Quick and Easy Reading) (also in Spanish on ASA Web site)
• Understanding Stroke (also in Spanish)
• "What Are Stroke and TIA?" and "What Are the Warning Signs of Stroke and TIA?" in Let's Talk About Stroke kit (also in Answers By Heart Spanish kit)

Related AHA Scientific Statements:
Angiography/Angioplasty
Imaging
Stroke