Many medical imaging techniques focus on anatomical structures, but positron emission tomography (PET) is different: It produces pictures that highlight what cells are doing. A patient is injected with a radioactive tracer, and the cells that absorb it most readily glow bright.

Here the tracer is radioactive glucose. Because cancer cells grow and divide rapidly, they use a lot of energy, sucking up glucose and giving themselves away; the red coloring denotes disease in the patient's liver and shoulder area. The brain and heart (the C-shaped blotch is the heart's muscular wall, called the myocardium) are also big energy users, so they show up too. Combining PET scans with CT scans sharpens anatomy in the image. Image 1 shows PET alone; 2 shows CT alone; and 3 shows PET fused with CT, making it possible to see more exactly where the problem lies.

Like MRI equipment, PET instruments take data in multiple planes. Only one slice appears in the images here, but combining all the slices produces a 3-D picture.