Doctors usually use scans, such as computed tomography (CT) scans or positron emission tomography (PET) scans, to see whether lung cancer has spread to mediastinal lymph nodes. A computer linked to an x-ray machine creates CT scans. These scans give a series of detailed pictures of body structures. The PET scan is a relatively new test that involves injecting a radioactive substance into a vein. Cancer cells take up and trap the substance more than do normal cells. The PET scan gives pictures of areas of the body that are “lit up” because cells in that area are rapidly trapping the radioactive substance. There are many studies that look at the ability of CT and PET scans to detect the spread of lung cancer to mediastinal lymph nodes.