The available tests fall into three main categories: imaging, blood flow tests and electrical tests. Imaging is used to create a picture of the brain. It provides crucial information about the cause of a stroke, and the location and extent of the injury. There are two techniques commonly used: computed tomography (CT or CAT Scans) and magnetic resonance imaging (MRI). Electrical tests show the brain’s electrical activity. Electroencephalograms (EEGs) simply measure brain’s electrical activity whereas an evoked response test measures how the brain handles sensory information related to hearing, body sensation or vision. Blood flow tests usually use ultrasound technology to determine the amount of blood flow through the blood vessel. Another method is angiography, where first a special dye is injected into the blood vessels, followed by X-ray photography.