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Facilities Overview and History

Physics of Positron Imaging

Positron Emission Tomography (PET) is considered a new modality in medical imaging, though it has been in development since the mid 1970s.

A positron is a positively charged particle with the same mass as the negatively charged electron. A positron is emitted during the + (beta-plus) decay of lightweight atoms that have an excess of protons in their nucleus. Positron emitting isotopes that are useful for imaging, such as Fluorine-18, Carbon-11 and Oxygen-15, are short half-life radioisotopes with lower energy positrons. These radioisotopes are produced in cyclotrons. Positron annihilation occurs when a positron collides with an electron; the energy released by this interaction forms two highly energetic gamma photons that travel in opposite directions.

PET imaging uses these two annihilation photons to localise the tracer molecule (for example Fluorine-18 deoxyglucose, FDG) in a human body. The photons are detected by scintillation crystals coupled with photomultiplier tubes (PMTs), units which are referred to as detectors, in the PET camera. The PMTs convert light from the scintillation crystal into an electrical signal, which is then processed by the fast and complex electronics of the PET camera. The electronic circuitry recognises as valid counts only those photons arriving at the same time (ie.in coincidence) and having the same or very similar energy. This helps to eliminate scattered and other misplaced photons.

The PET camera thus requires a number of opposing detectors. The detectors are usually installed as a number of rings or partial rings. Full ring cameras have better sensitivity, allowing for shorter scanning times, but they are more expensive than those with partial ring systems. The patient moves through the detector rings, which detect and count the photons from positron annihilation. Using coincidence counts the camera can detect the presence and position of tumour cells which have taken up the PET tracer (eg FDG).

PET cameras use sophisticated image and acquisition corrections to achieve the best resolution, uniformity and image quality. The most important correction is attenuation correction, which corrects for the attenuation of photons by overlying tissues in the patient. Other corrections include those for uniformity, distortion, random photons and energy.