Positron Emission Tomography is an imaging technique which maps the distribution of beta-positive emitters throughout the body. The positrons (positive electrons) emitted are identified by the fact that, once they have lost their energy (their range does not exceed a few millimeters), they annihilate with an electron to yield two gamma photons each of 511 keV of energy and emitted back to back. Both gamma reach simultaneously a pair of opposing detectors placed on either side of the annihilation location. Electronic circuits associating these pairs of detectors are designed to identify the annihilation photons.
The two photons are considered relevant if their energies are around 511 keV and their detection times differ by less than one ten-billionth of a second. The collimation is ‘electronic’ as there is no need for lead collimators such as the ones which are used in gamma cameras (although some 2D imaging methods do require such lead collimators).
The complex mathematical algorithm is due to Godfrey Hounsfield and Allan Cormack who received the 1979 Physiology or Medicine Nobel Prize for their theory of "Computed Tomography" Combining data obtained from different angles allows for the construction of a series of tomographic cross-sections of the patient’s body. These planar projections can then be used to three-dimensional image of where the radioactive tracers are located in the patient.
Unlike the case of gammas camera, various types of PET camera are available. The scintillating crystal can be made of sodium iodide NaI (Tl), Bismuth germanate (BGO), lutetium oxyorthosilicate (LSO) or even gadolinium oxyorthosilicate (GSO).
It is also possible to use a two-headed gamma camera to detect coincidence events, by increasing the thickness of the NaI (T) crystals and adding an electronic system capable of registering such coincidence events. This technique is known as Emission Tomography with Coincidence Detection (ETCD).
Cameras designed for scans involving positron emitters have all an annular (ring-like) structure. Scans with cameras of this kind are generally faster and offer much higher-resolution images than those with ETCD cameras.
Finally, the most recent PET scanners can have a camera attached to them (PET/TDM cameras), resulting in a hybrid apparatus which takes anatomic images from the scanner and superimposes them on the functional images from the PET module.
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