Positron Emission Tomography (PET) is one of the two most used methods in medical imaging, the other being functional Magnetic Resonance Imaging (fMRI).

PET is based on the detection of radioactive tracers, or more precisely positron transmitting radiopharmaceutical compounds: Fluorodeoxyglucose F18 (FDG), which is a modified sugar tagged with Fluoride 18 and has a half-life of 110 minutes, Carbon 11 (20 seconds), or Oxygen 15 (123 seconds). These radio-elements, which have a short life span, are incorporated into molecules which are then injected into the organism in order to study the physiological or pathological phenomena whose secrets we want to pierce. Thus radioactive water (tagged with Oxygen 15) is a tracer used to measure blood flow; other molecules are used to evaluate glucose consumption, neurotransmission or the synthesis of proteins.

But how does the detection of the tracers work? The radioactive transformation is accompanied by the emission of a positron. After travelling around a millimetre this is annihilated when it encounters an electron. This annihilation reaction causes the emission of two photons at 180° to each other. These photons are harvested by the crown of detectors which surround the patient. This data, after being processed by a computer, allow one to reconstruct and quantify the distribution of the tracer in the organ studied. From that one can build up a three-dimensional image of how it is functioning.