Positron Emission Tomography (PET) – Nuclear Medicine Imaging Technology

Positron emission tomography (PET) is another nuclear medicine imaging method that has several advantages over SPECT. PET uses positron-emitting radionuclides that result in the emission of hillar pairs of 511 keV annihilation photons. Coincidence detection of annihilation photons avoids the need for collimation and makes PET much more efficient than SPECT for detecting radioactivity. Even more important, positron-emitting radionuclides exist for oxygen, carbon, nitrogen, and fluorine, allowing 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 enough half-life that it can be (and is) provided regionally, and there is not a populated area of ​​the United States where it is not available. 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 annihilation photons resulting from positron emission are approximately collinear. Events are only counted if two opposing detectors detect them simultaneously. The sensitive volume defined by the coincidence detectors is called reply line (LOR). Two individual detection systems with an additional matching module are used. Each individual system will generate a logic pulse when it detects an event that falls in the selected energy window. If the two logic pulses overlap in time in the match module, a match event is logged. PET systems use a large number (>10,000) of detectors arranged as multiple rings to form a cylinder. Since any detector can be coincident with other detectors on the cylinder, the resulting LORs provide enough sampling to collect the projection information required for tomography.

The intrinsic detection efficiency of an individual 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 the intrinsic efficiency for some of the detectors is greater than 0.8. Match detection requires that both listeners register an event. Since the interactions at the two detectors are independent, the intrinsic efficiency of the coincidence depends on the product of the intrinsic efficiency at each detector. As a result, the coincidence detection efficiency is always less than that of a single detector, and that difference is magnified for low-efficiency detectors. Due to the need for high intrinsic efficiency, scintillators are virtually the only materials currently used as detectors in PET imaging systems.

A match event is logged when there is an overlap of the individual logic outputs in the match modules. The time width of the overlap depends on the scintillation characteristics of the detectors. For current PET scanners, that width is between 6 and 12 ns. Although this is a very short time compared to most human activities, it is quite a long time compared to the distances traveled by photons traveling at the speed of light. Light travels at approximately 30 cm/ns, so a duration of 6 ns corresponds to a distance uncertainty of about 90 cm, which is the approximate diameter of the detector ring. As a result, the differential source distance between detectors has no observable effect on the timing of coincidence events in conventional PET systems.

The arrival time of the annihilation photons is truly simultaneous only when the source is located precisely midway between the two opposing coincidence detectors. If the source is offset from the midpoint, there will be a corresponding arrival time interval, since one photon’s annihilation will have a shorter distance to travel than the other. As discussed above, this time differential is too small to be useful in PET systems of conventional design. However, several of the scintillators used in PET scanners (eg, LSO, LYSO) are capable of a faster response than the 6-12 ns timing discussed above. With proper electronics, the coincidence time window has been reduced to 600 ps for these detectors, leading to a source location uncertainty of 9 cm. Even with that reduction, time-of-flight localization cannot be used to directly generate tomographic images, but can be used to regionally constrain backprojection operation to areas where sources are roughly located. In current implementations, the inclusion of time-of-flight information reduces noise in the reconstructed images by a factor of 2. Time-of-flight PET scanners were commercially available for a short time in the 1980s. These systems used detectors BaF2 which are very fast, but unfortunately have very low detection efficiency. As a result, these devices did not compete well with conventional BGO-based PET scanners. In 2006, a LYSO detector-based time-of-flight machine was reintroduced and is now commercially available.

The only criteria for logging a match event is the overlap of output pulses in the match module. True matches occur when a source is in the LOR defined by two detectors. The events detected in the two source match detectors that are not on the response line may happen by chance. As the count rate at each of the individual detectors increases, the probability of false matches from uncorrelated events increases. These events are called random Prayed accidental coincidences. The random match rate (R) is directly proportional to the width of the coincidence time window

R = 2t yes1yestwo

It is easy to see that while the true match event rate is linear with the source activity, the random match rate increases proportionally to the square of the activity. Therefore, at high count rates, the random match rate may exceed the actual match rate. Random matches provide false information and should be removed from the acquired data prior to image reconstruction. It is also obvious that the random match rate can be reduced with a smaller match time window. That requires detectors with fast response time like LSO, LYOS and GSO.

For airborne sources, it is only possible to get a true match event when the source is in the defined volume between the two match detectors. However, if the sources are distributed in some material, such as human tissue, it is possible that one or both of the annihilation photons will scatter off detectors that do not span the LOR of the source. Like the random match event, this provides false information that requires correction. The number of scattered events can be reduced by energy discrimination, but this does not completely eliminate them and additional scatter correction techniques are required for PET imaging.

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