The imaging in PET is all indirect. Like CT, MRI, and SPECT, PET relies on computerized reconstruction procedures to produce tomographic images. [15] It is performed by means of detecting positron-emission by use of tomography. [9] Two ways in which radionuclides decay that will reduce excess positive charge on the nucleus include the neutralization of a positive charge with the negative charge of an electron or the emission of a positron from the nucleus. The positron will then combine with an electron from the surroundings and annihilate. Upon annihilation both the positron and the electron are then converted to electromagnetic radiation. This electromagnetic radiation is in the form of two high-energy photons which are emitted 180 degrees away from each other. It is this annihilation radiation that can be detected externally and is used to measure both the quantity and the location of the positron emitter. [3]
Simultaneous detection of two of these photons by detectors on opposite sides
of an object places the site of the annihilation on or about a line connecting
the centers of the two detectors. At this point mapping the distribution of
annihilations by computer is allowed. [14] If the annihilation originates
outside the volume between the two detectors, only one of the photons can be
detected, and since the detection of a single photon does not satisfy the
coincidence condition, the event is rejected. [3] Simultaneous detection
provides a precise field of view with uniform sensitivity. This occurs because
wherever the disintegration takes place between the two detectors, the photons
must in sum have traveled the full interdetector distance in order that the
event be recorded. [5]