Courtesy of Intermountain Medical Imaging, Boise, Idaho.

A lung scan is a test that uses a radioactive tracer to look at the lungs. Areas of the lung that retain too much air show up as bright or "hot" spots on the pictures. Areas that are not receiving enough air show up as dark or "cold" spots.

Figure 1 shows normal blood flow in healthy lungs. Figure 2 shows dark areas where blood clots (pulmonary emboli) are preventing normal blood flow through the lungs.