Scan: A type of test that makes detailed pictures of areas inside the body. A scan may also refer to the picture that gets made during the test. Scans may be used to help diagnose disease, plan treatment, or find out how well treatment is working. There are many different types of scans, including computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, and nuclear medicine scans (such as bone scans and liver scans). CT scans are done with an x-ray machine linked to a computer. MRI scans are done with radio waves and a powerful magnet linked to a computer. Nuclear medicine scans are done with small amounts of radioactive substances that are injected into the body and a special machine that detects the radioactive substance.