Positron emission tomography (PET) is another approach to nuclear medicine imaging that has several advantages over SPECT. PET uses positron-emitting radionuclides that result in the emission of collinear pairs of 511-keV annihilation photons. The coincidence detection of the annihilation photons obviates the need for collimation and makes PET far more efficient than SPECT for detecting radioactivity. Even more importantly, there are positron-emitting radionuclides for oxygen, carbon, nitrogen, and fluorine, which allows a wide range of molecules to be labeled as diagnostic agents. Many of these radionuclides have short half-lives and require an on-site cyclotron. However, 18F has a long long half-life that it can be (and is) regionally provided, and there is no populated area of the United States where it is unavailable. Several others such as 82Rb and 68Ga are available from radionuclide generators that provide the radionuclides on demand despite their short half-lives.
Coincidence detection provides spatial resolution without the need for lead collimation by taking advantage of the fact that the annihilation photons resulting from positron emission are approximately colinear. Events are only counted if they are simultaneously detected by two opposed detectors. The sensitive volume defined by the coincidence detectors is called a line of response (LOR). Two single detection systems are used with an additional coincidence module. Each individual system will generate a logic pulse when they detect an event that falls in the selected energy window. If the two logic pulses overlap in time at the coincidence module, a coincidence event is recorded. PET systems use a large number (> 10,000) of detectors arranged as multiple rings to form a cylinder. Since any one detector can be in coincidence with other detectors in the cylinder, the resulting LORs provide sufficient sampling to collect the projection information required for tomography.
The intrinsic detection efficiency for a singles detector depends on the atomic number, density, and thickness of the detector. Ideally, the intrinsic detection efficiency should be 1, but at 511 keV that is difficult to achieve, although intrinsic efficiency for some of the detectors is greater than 0.8. Coincidence detection requires that both detectors register an event. Since the interactions at the two detectors are independent, the consistency intrinsic efficiency depends on the product of the intrinsic efficiency at each detector. As a result, collision detection efficiency is always less than that for a single detector, and that difference gets magnified for low-efficiency detectors. Because of the need for high intrinsic efficiency, scintillators are naturally the only materials currently used as detectors in PET imaging systems.
A coincidence event is recorded when there is an overlap of the singles logic outputs at the coincidence modules. The time width of the overlap depends on the scintillation characteristics of the detectors. For current PET scanners, that width ranges from 6 to 12 ns. Although that is a very short time compared to most human activities, it is fairly long compared to distances covered by photons traveling at the speed of light. Light travels approximately 30 cm / …