Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, regional chemical composition, and absorption. Different tracers are used for various imaging purposes, depending on the target process within the body. For example, ¹⁸
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-FDG is commonly used to detect cancer, NaF¹⁸
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is widely used for detecting bone formation, and oxygen-15 is sometimes used to measure blood flow.

PET is a common imaging technique, a medical scintillography technique used in nuclear medicine. A radiopharmaceutical — a radioisotope attached to a drug — is injected into the body as a tracer. When the radiopharmaceutical undergoes beta plus decay, a positron is emitted, and when the positron collides with an ordinary electron, the two particles annihilate and gamma rays are emitted. These gamma rays are detected by gamma cameras to form a three-dimensional image, in a similar way that an X-ray image is captured.

PET scanners can incorporate a CT scanner and are known as PET-CT scanners. PET scan images can be reconstructed using a CT scan performed using one scanner during the same session.

One of the disadvantages of a PET scanner is its high initial cost and ongoing operating costs.