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Positron Emission Tomography (PET) is increasingly becoming a common method of detecting disease in medical practice. The chapter reviewed looks at the physics of positron emission and why this property of atoms is useful in medical practice. The authors begin with a rough introduction of PET and its application in medicine to detect disease. The article states that PET entails the ejection of a radionuclide from the nucleus of an atom (Cherry and Dahlbom 11). Further decay of these radionuclides results in positrons and neutrons that collide with the surrounding particles and annihilate.
Some of the medical uses mentioned in the article include labeling of biomolecules to study the normal functioning of the body, detect disease, study drug pharmacokinetics, and cancer diagnosis (Cherry and Dahlbom 13). Positron emission constitutes one of the ways that nuclei with excess protons decay. A common compound that undergoes this kind of decay is 11C which is used to label biologic compounds. The article lists some of the other radionuclides that decay by this process (Cherry and Dahlbom 14). The decay produces simultaneous protons in opposite directions with 511keV in energy calculated using Einstein’s mass-energy equivalent. Detectors are designed to note and record the annihilation of photons in PET imaging.
The imaging systems used in PET scans have several errors that are a result of nonlinearity. Positron range, which is the distance traveled by the positron before reaching the detector, varies and may result in mispositioning in these scans. The interaction between the photons emitted in PET and tissues is also discussed in the article. Also, the chapter discusses the interaction of these radionuclides with other materials such as tungsten and lead (Cherry and Dahlbom 17). The interactions were grouped into Compton scattering, the photoelectric effect, and pair production.
Imaging systems in PET include crystal rings and photomultiplier tubes. Efficiency in detecting the 511keV photons is increased through the emission of more photon pairs (Cherry and Dahlbom 20).current PET detectors utilize scintillating detectors that detect gamma rays. Light is emitted by crystals after the reaction between the gamma rays and the high-energy photons. This light is then detected and changed into an electric current. Photomultiplier tubes are also utilized in the detection of the photons after the scintillating crystals produce their light. The quantum efficacy of these PMTs is important in determining the overall strength of an emitted light. Single-channel PMTs are used in most scanners, and they can be square or round. PMTs have an advantage over the other detectors due to their high amplification.
The other type of detection system that was used in PET scanners is the block detector readout (Cherry and Dahlbom 23). A scintillator material with multiple segmented materials is applied in these detection systems that were in use in the early 21st century. The other factor in PETs that are discussed in the chapter is coincidence detection (Cherry and Dahlbom 23). In one of the simpler coincidence detection systems, a coincidence event is recorded when the 511keV photons interact with either of the detectors. Detection events are minimized by keeping the pulses as narrow as possible. In addition, the time window that is utilized determines the detection of these events. The article discusses some of the important applications of the PET scans and the errors that may occur in the decay process leading to the production of the 511keV photons.
Works Cited
Cherry, Simon R., and Magnus Dahlbom. PET Physics, Instrumentation, and Scanners. New York, NY: Springer Science Business Media, LLC, 2006. Print.
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