Answer:
A PET scan works in the following way: a radioactive source (called tracer) is ingested by the patient. This radioactive source emits positrons (beta source), which annihilate with electrons in the body, producing a pair of gamma rays that is able to penetrate through the skin and being detected outside by a gamma ray detector.
This system can be used to produce a tridimensional image of cancers. In fact, the concentration of the tracer is larger where there is an abnormal metabolic activity, which is a sign of the presence of cancer cells. The detector placed outside the body of the patient is able (by using the direction of both the gamma photons emitted in the annihilation) to track back the point from which the gamma rays have originated, and therefore, to "see" where the metabolic activity is higher, indicating the presence of cancer cells.
If infrared ratiation was emitted instead of gamma rays, the PET would not work. In fact, the infrared photons have less energy than the gamma rays and would not penetrate through the skin: as a result, the outside the detector would not be able to detect any photon, so the PET would not work.
