MRI is similar to a CT scan but requires a different machine and takes longer. In it radioactive compounds are injected into a vein in the arm, and a machine similar to a Geiger counter creates a map showing their uptake into different parts of the head. The pictures, rather then showing the brain's structure, show how it functions. This test can detect blocked blood vessels and areas where the brain is damaged. A machine equipped with pens transcribes this activity onto large pieces of paper.
Evoked responses measure how the brain handles different sensory stimuli.
They can detect abnormal areas of the brain. A doctor evokes a visual response by flashing light or a checkerboard pattern in front of a patient. There are a variety of tests that measure blood flow. The Doppler ultrasound test is performed to detect blockages in the carotid artery. In it, a gel is put on the neck or eyelids, then a technologist puts a pencil-like probe into the gel and listens to blood flowing in the carotid artery.