The Physics of PET/CT scanners

Positron emission tomography (PET) offers a number of unique advantages compared to other imaging modalities. PET measures the two annihilation photons that are produced back-to-back after positron emission from a radionuclide tagged tracer molecule, which is chosen to mark a specific function in the body on a biochemistry level (Fig. 1). Hence PET provides molecular imaging of biological function instead of anatomy. The detection of both annihilation photons in coincidence yields increased sensitivity over all other forms of medical imaging. The time-coincident imaging of the two high-energy annihilation photons (discussed below) allows accurate attenuation correction from either a dedicated transmission scan or from CT information. This allows extraction of accurate information from PET images. Only minute amounts of imaging substrate need to be injected (tracer principle) because of the high sensitivity of PET. In addition, positron emitting isotopes that are used in medical imaging (C-11, N-13, O-15, F-18, etc.) are relatively short-lived, which enables optimal use of imaging photons while keeping patient radiation dose low. Furthermore, many of these isotopes can be incorporated into biological substrates (glucose, H2O, NH3, CO2, O2, etc.) and pharmaceuticals, without altering their biological activity.