Positron emission tomography detectors

Like the monoamine hypothesis of depression, such a simple hypothesis was appeaUng but, perhaps predictably, a Uttle too simple to be true. Further research using a technique known as positron emission tomography (PET) showed the relationship between dopamine and schizophrenia is more complex. PET detects radioactive emissions of certain isotopes these isotopes are incorporated into a molecule and injected into a patient. The machine measures the radioactivity with detectors positioned aroimd the body. PET lets researchers study the distribution of certain molecules in Uving tissue since, imUke autoradiography, the tissue is not sliced and treated chemically. The amoimt of radioactivity must be small, however, to avoid harming the human subjects. 

The production of short-lived positron emitters has been described in section 12.2. By interaction with electrons, the positrons are annihilated and two y-ray photons of 511 keV each are emitted simultaneously in opposite directions. By measuring these photons by means of a suitable arrangement of detectors, exact localization of the radionuclides in the body is possible. This is the basis of positron emission tomography (PET), which has found broad application in nuclear medicine. The most frequently used positron emitters are listed in Table 19.2. They are preferably produced by small cyclotrons in the hospitals or nearby. 

In positron emission tomography (PET) the two SllkeV y-ray photons emitted simultaneously in opposite directions are registered by y-ray detectors, indicating that the positron decay must have occurred somewhere along the line between these two detectors. The same holds for other events of positron decay, and the radionuclide can be localized at the intersection of these lines. 

Since the publication of this book in 2005, the growth of positron emission tomography (PET) and PET/CT modality has been phenomenal with the discovery of efficient detectors and the addition of sophisticated scanners and powerful software. Because of the vast advancement and changes in technology, and also with great appreciation of the success of the first edition, we thought a second edition of the book would be appropriate. 

Positron emission tomography (PET) uses a device, in which several detectors are positioned circularly around the object. When a positron is emitted it almost immediately annihilates with an electron. As a result of the annihilation two y-quanta are ejected in opposite directions. As two detectors detect simultaneously these 511keV y-quanta the occurrence is with a certain probability due to a positron emission at the fine connecting the two detectors. As several occurrences at different angles are recorded an image of the object can be constructed. 

Figure 10.2 Positron emission tomography (PET) scan shows normal brain activity during sleep. A radioactive isotope that emits positrons is made into a chemical compound that is absorbed by active areas of the brain. The emitted positrons collide with nearby electrons and produce gamma rays that pass through the skull to detectors surrounding the patient s head. A computer uses the detector data to construct the image.

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