Positron Emission Tomography (PET) - Nuclear Medicine Imaging Technology

 Positron emission tomography (PET) is uncharacteristic gate to nuclear medicine imaging that has several advantages on peak of SPECT. PET uses positron-emitting radionuclides that consequences in the emission of collinear pairs of 511-keV annihilation photons. The coincidence detection of the annihilation photons obviates the dependence for collimation and makes PET in the disaffect-off more efficient than SPECT for detecting radioactivity. Even more importantly, there are positron-emitting radionuclides for oxygen, carbon, nitrogen, and fluorine, which allows a broad range of molecules to be labeled as systematic agents. Many of these radionuclides have hasty half-lives and require an on the subject of-site cyclotron. However, 18F has a adequately long half-energy 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 understandable from radionuclide generators that pay for the radionuclides on the order of demand despite their rushed half-lives.

Coincidence detection provides spatial utter without the dependence for gain collimation by taking advantage of the fact that the annihilation photons resulting from positron emission are on colinear. Events are by yourself counted if they are simultaneously detected by two opposed detectors. The sore volume defined by the coincidence detectors is called a parentage of reply (LOR). Two single detection systems are used previously one more coincidence module. Each individual system will generate a logic pulse following than they detect an business that falls in the fixed computer graphics window. If the two logic pulses overlap in period at the coincidence module, a coincidence matter is recorded. PET systems use a large number (>10,000) of detectors arranged as complex rings to form a cylinder. Since any one detector can produce an effect coincidence past new detectors in the cylinder, the resulting LORs meet the expense of enough sampling to mass the projection guidance required for tomography.

The intrinsic detection efficiency for a singles detector depends going as regards for the atomic number, density, and thickness of the detector. Ideally, the intrinsic detection efficiency should be 1, but at 511 keV that is hard to achieve, although intrinsic efficiency for some of the detectors is greater than 0.8. Coincidence detection requires that both detectors register an issue. Since the interactions at the two detectors are independent, the coincidence intrinsic efficiency depends in imitation of reference to the product of the intrinsic efficiency at each detector. As a consequences, coincidence detection efficiency is always less than that for a single detector, and that difference gets magnified for low-efficiency detectors. Because of the dependence for high intrinsic efficiency, scintillators are just about the without help materials currently used as detectors in PET imaging systems.

A coincidence issue is recorded behind than there is an overlap of the singles logic outputs at the coincidence modules. The mature width of the overlap depends on the subject of the scintillation characteristics of the detectors. For current PET scanners, that width ranges from 6 to 12 ns. Although that is a very short epoch compared to most human happenings, it is fairly long compared to distances covered by photons traveling at the speed of well-ventilated. Light travels gone hint to 30 cm/ns so that a 6 ns duration corresponds to a estrange uncertainty of not quite 90 cm, which is the approximate detector arena diameter. As a result, the differential estrange of the source along amid detectors has no observable effect regarding the timing of the coincidence activities in permissible PET systems.

The start grow antiquated of the annihilation photons is in fact simultaneous single-handedly once the source is located precisely midway together in the middle of the two opposed coincidence detectors. If the source is displaced from the midpoint, there will be a corresponding beginning period interval since one annihilation photon will have a shorter set against to travel than the supplementary. As discussed above, this period differential is too little to be useful in conventionally expected PET systems. However, several of the scintillators used in PET tomographs (e.g., LSO, LYSO) are capable of faster response than the 6 to 12 ns timing discussed above. With capture electronics, the coincidence timing window has been reduced to 600 ps for these detectors, regulating a source localization uncertainty of 9 cm. Even considering that reduction, period-of-flight localization cannot be used to directly generate tomographic images, but it can be used to regionally restrict the backprojection operation to areas where the sources are more or less located. In current implementations, the combination of epoch-of-flight recommendation reduces noise in the reconstructed images by a factor of 2. Time-of-flight PET tomographs were actually commercially animate for a rushed era in the 1980s. These systems used BaF2 detectors which are utterly hasty, but unfortunately have utterly low detection efficiency. As a outcome, these devices did not compete competently behind the okay PET tomographs based upon BGO. In 2006, a period-of-flight robot based upon LYSO detectors was reintroduced and is now commercially believe to be not guilty.

The unaided criterion for recording a coincidence issue is the overlap of output pulses at the coincidence module. True coincidences occur when a source lies upon the LOR defined by two detectors. It is attainable that behavior detected at the two coincidence detectors from sources not upon the lineage of tribute could happen by unintended. As the adding together going on together rate at each of the singles detectors increases, the likelihood of untrue coincidences happening from uncorrelated happenings increases. These behavior are called random or accidental coincidences. The random coincidence rate (R) is directly proportional to the width of the coincidence time window (t) and the product of the singles rate at the two detectors (S1 and S2):

R = 2t S1S2For more info SVCI 6154A.

It is easy to complete to to see that though the real coincidence business rate is linear taking into consideration the source brawl, the random coincidence rate increases proportional to the square of the row. Thus, at high adjoin rates, the random coincidence rate can exceed the real coincidence rate. The random coincidences have enough maintenance disloyal recommend and mannerism to be removed from the acquired data prior to image reconstruction. It is along with obvious that random coincidence rate can be edited considering a smaller coincidence time window. That requires detectors gone a fast response time taking into consideration LSO, LYOS, and GSO.

For sources in melody, it is isolated doable to obtain a genuine coincidence event once the source lies in the defining volume in the company of the two coincidence detectors. However, if the sources are distributed in some material, bearing in mind human tissue, it is practicable for one or both of the annihilation photons to be scattered into detectors that don't encompass the LOR of the source. Like the random coincidence event, this provides disloyal want that requires correction. The number of scattered undertakings can be condensed by animatronics discrimination, but this does not eliminate it all and supplementary scatter correction techniques are required for PET imaging.

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