positron emission tomography

positron emission tomography
Name	Positron emission tomography
Caption	A modern PET-CT scanner, combining functional and anatomical imaging.
MeshID	D049268
MedlinePlus	003827

positron emission tomography is a nuclear medicine functional imaging technique that produces a three-dimensional image of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide, or tracer, which is introduced into the body on a biologically active molecule. Its primary use is in oncology, where it is a major tool for diagnosing, staging, and monitoring the treatment of cancer, but it is also widely applied in cardiology and neurology for research and clinical diagnosis.

Principles of operation

The fundamental principle relies on the detection of annihilation events. A biologically active molecule is labeled with a radionuclide that decays by positron emission, such as fluorine-18. The emitted positron travels a short distance in tissue, losing kinetic energy until it interacts with an electron, resulting in annihilation. This process converts the mass of both particles into two gamma ray photons, each with an energy of 511 keV, which are emitted in nearly opposite directions. The scanner's ring of scintillation detectors registers these coincident photon pairs, and sophisticated image reconstruction algorithms, often based on computed tomography principles, localize the source of the annihilations to construct a quantitative image of tracer concentration.

Radiotracers and radiopharmaceuticals

The most commonly used radiopharmaceutical is fluorodeoxyglucose, abbreviated FDG, which incorporates fluorine-18. FDG is an analog of glucose and is taken up by cells with high glucose metabolism, such as rapidly dividing cancer cells and active neurons. Other important tracers include florbetapir for imaging amyloid plaques in Alzheimer's disease, rubidium-82 chloride for myocardial perfusion imaging in cardiology, and gallium-68 labeled compounds for neuroendocrine tumor detection. The production of these short-lived isotopes typically requires an on-site or nearby cyclotron and a dedicated radiochemistry laboratory.

Imaging procedure and technology

A patient is injected with the radiopharmaceutical and, after a suitable uptake period, is positioned within the scanner's gantry. Modern systems are almost exclusively combined PET-CT or PET-MRI scanners, which acquire the functional data from the scanner and anatomical data from the computed tomography or magnetic resonance imaging component in a single session. This hybrid imaging allows for precise anatomical localization of functional abnormalities. The data from the coincidence events are processed using statistical methods like ordered subset expectation maximization to create cross-sectional images, which can be viewed as slices or in three-dimensional renderings.

Clinical applications

In oncology, it is indispensable for staging lymphoma, lung cancer, and colorectal cancer, assessing treatment response, and detecting recurrence. In neurology, it is used to investigate epilepsy foci, differentiate types of dementia such as Alzheimer's disease and frontotemporal dementia, and study Parkinson's disease. In cardiology, it is considered the gold standard for evaluating myocardial viability prior to coronary artery bypass graft surgery. It also plays a role in infection imaging for conditions like osteomyelitis and fever of unknown origin.

Limitations and safety considerations

Spatial resolution is limited compared to MRI or CT, and the technique is susceptible to artifacts from patient motion. The use of ionizing radiation results in a non-negligible effective dose, though this is justified by the clinical benefit. There are limitations in imaging certain cancers, such as prostate cancer and some hepatocellular carcinomas, with FDG, necessitating the development of novel tracers. Patients with poorly controlled diabetes mellitus may have altered FDG biodistribution, potentially compromising study quality. Allergic reactions to radiopharmaceuticals are extremely rare.

History and development

The conceptual foundation was laid in the early 1950s with the work of physicists such as Gordon L. Brownell at Massachusetts General Hospital. The first systems capable of producing tomographic images were developed in the mid-1970s by teams including Michael E. Phelps, Edward J. Hoffman, and Michel Ter-Pogossian at Washington University in St. Louis. The subsequent development and clinical validation of FDG by Phelps, Hoffman, and others at the University of California, Los Angeles in the late 1970s was a pivotal breakthrough. The commercial introduction of combined PET-CT scanners in 2001, pioneered by David Townsend and Ronald Nutt, revolutionized clinical practice by enabling precise fusion of functional and anatomical data.

Category:Medical imaging Category:Nuclear medicine Category:Medical physics