20.5 Positron Emission Tomography
Medical imaging methods that use ionizing radiation rely on how radiation interacts with matter: as photons pass through tissue they may be absorbed or scattered, and the pattern of energy deposition and attenuation is what ultimately produces measurable signals at a detector. In nuclear medicine, the radiation originates inside the patient from an administered radiopharmaceutical, and the detected signal reflects physiological function rather than only anatomy. A key functional technique is Single Photon Emission Computed Tomography (SPECT), in which a gamma-emitting tracer is introduced into the body and a rotating gamma camera records many projection views; these projections are then reconstructed to form cross sectional images of tracer distribution, providing three-dimensional information about perfusion and organ function. Positron Emission Tomography (PET) extends this idea using positron-emitting tracers: after emission, the positron travels a short distance in tissue and annihilates with an electron, producing two gamma photons emitted in nearly opposite directions; detecting these photons in coincidence allows the system to localize the event and reconstruct a quantitative map of tracer uptake. Consequently, PET is a non-invasive technique that images metabolic and biochemical activity at the cellular/molecular level, making it highly valuable for early cancer detection and staging, evaluating brain function, and identifying cardiac abnormalities before clear anatomical changes appear (Fig. 20.10).
(Fig. 20.10: Image of PET-scanner)