SIDEBAR 2.9Introduction to the Physics of PET

A radiotracer that is labeled with a positron-emitting radionuclide, such as fluorine-18, is injected into the patient undergoing a PET scan. The radionuclides then decay, emitting positrons. The resulting positrons subsequently annihilate on contact with electrons within the body (Figure 1). Each annihilation event produces two photons traveling in opposite directions that are detected by the detectors surrounding the patient. If this detection occurs within a certain time, it is considered to have come from the same annihilation event and is “coincident” (Figure 2).

FIGURE 1. Positron emission and annihilation.

FIGURE 1

Positron emission and annihilation.

FIGURE 2. Schematic of coincidence event detection.

FIGURE 2

Schematic of coincidence event detection.

In PET, there are four types of coincidence events: true, scattered, random, and multiple. Figure 3 illustrates the first three. A coincidence event is assigned to a line of response (LOR). In this way, positional information is gained from the radiation that is detected.

FIGURE 3. Types of coincidences.

FIGURE 3

Types of coincidences.

Time-of-flight means that for each annihilation event the precise time that each of the coincident photons is detected is noted and the difference in arrival time is calculated. Since the closer photon will arrive at the detector first, calculating the difference in arrival time helps determine the location of the annihilation event between the two detectors. Theoretically, perfect time-of-flight information would eliminate the need to reconstruct images. However, even the addition of imperfect time-of-flight information reduces noise and improves the image by approximating the location of the annihilation event. This improvement in image quality is particularly useful in large patients (Badawi 1999, Karp 2006).

From: 2, Nuclear Medicine

Cover of Advancing Nuclear Medicine Through Innovation
Advancing Nuclear Medicine Through Innovation.
National Research Council (US) and Institute of Medicine (US) Committee on State of the Science of Nuclear Medicine.
Washington (DC): National Academies Press (US); 2007.
Copyright © 2007, National Academy of Sciences.

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