# Positron emission tomography
**Positron emission tomography** (PET) is a nuclear medicine imaging technique that produces three-dimensional pictures of biochemical processes inside the living body by injecting a biologically active molecule labeled with a positron-emitting radioactive atom and detecting the pairs of gamma-ray photons released when those positrons annihilate with electrons in tissue.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) Because biochemical changes generally occur before anatomical changes in disease, PET can reveal the early onset of disease before structural imaging tests such as CT or MRI.[2](https://journals.physiology.org/doi/full/10.1152/physrev.00049.2010) • [3](https://www.radiologyinfo.org/en/info/pet)
| Key fact | Detail |
|---|---|
| Physical basis | Injected positron emitters annihilate with electrons, producing two coincident 511 keV gamma photons in opposite directions, which ring detectors register to localize the tracer[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) |
| Dominant tracer | F-18 fluorodeoxyglucose (FDG), a glucose analog taken up by metabolically active cells, is the most common radiotracer and the standard agent for detecting cancer and its metastases[3](https://www.radiologyinfo.org/en/info/pet) • [1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) |
| Hybrid imaging | Combined PET/CT scanners perform almost all PET scans today and serve as an important staging tool for many cancers, although its use as a routine first-line imaging tool is limited by high cost and availability[3](https://www.radiologyinfo.org/en/info/pet) • [1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) • [16](https://pmc.ncbi.nlm.nih.gov/articles/PMC11914360/) |
| Performance gains | Spatial resolution improved by a factor of 10 and sensitivity by a factor of 40 from the early 1970s designs to modern high-performance scanners; a whole-body clinical scan now takes under 10 minutes[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5374360/) |
| Resolution limit | Lesions smaller than 3 to 5 mm are hard to detect with PET scanners[5](https://academic.oup.com/edrv/advance-article-pdf/doi/10.1210/endrev/bnae001/55444615/bnae001.pdf) |
| PET versus SPECT | PET has higher sensitivity and an extensive tracer list, but higher financial costs and short-lived isotopes that require a dedicated cyclotron environment[5](https://academic.oup.com/edrv/advance-article-pdf/doi/10.1210/endrev/bnae001/55444615/bnae001.pdf) |
| Typical procedure | The radiotracer takes about 30 to 60 minutes to distribute and be absorbed; the PET acquisition itself takes 20 to 30 minutes, with total scanning time usually about 30 minutes[3](https://www.radiologyinfo.org/en/info/pet) |
## How it works
PET uses radiopharmaceuticals, molecules linked to a small amount of radioactive material, injected intravenously. The radiolabels used in PET decay by emitting positrons, particles with the same mass as electrons but oppositely charged. When a positron reacts with an electron in the body, the two particles annihilate, producing two coincident gamma-ray photons that shoot off in opposite directions. The photon energies are identical regardless of which positron-emitting radionuclide is used, and the detectors in the scanner ring measure these photon pairs to build images of internal functional molecular processes.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine)
Areas of greater tracer intensity, called "hot spots," show high chemical or metabolic activity; less intense "cold spots" indicate lower activity. The most common radiotracer, F-18 fluorodeoxyglucose (FDG), is a molecule similar to glucose. Cancer cells are more metabolically active and may absorb glucose at a higher rate, a difference visible on PET scans.[3](https://www.radiologyinfo.org/en/info/pet) FDG is only one of many radiotracers in use or in development; others target specific receptors, metabolic pathways, or disease processes such as the amyloid and oxidative-stress markers studied in neurodegenerative disease.[3](https://www.radiologyinfo.org/en/info/pet) • [1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine)
## Clinical uses
The major purpose of PET scans is to detect cancer and monitor its progression, response to treatment, and metastatic spread. Doctors also perform PET and PET/CT to assess tissue metabolism and viability, evaluate the effects of a heart attack on areas of the heart, identify heart muscle that would benefit from revascularization, and evaluate brain abnormalities such as tumors, memory disorders, and seizures.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) • [3](https://www.radiologyinfo.org/en/info/pet) In cardiology, the match/mismatch principle using N-13 ammonia for blood flow and FDG for glucose metabolism was developed and validated to determine cardiac tissue viability, which for several years was the focus of clinical PET.[6](https://www.sciencedirect.com/science/article/pii/S1095039700000510)
A PET radiopharmaceutical has also been approved by the FDA to aid in the accurate diagnosis of Alzheimer's disease, which previously could be diagnosed with accuracy only after a patient's death.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine)
PET also anchors radiotheranostics, in which a small diagnostic dose of a radiopharmaceutical is imaged first and, if uptake is appropriate, a larger therapeutic quantity of the paired agent is given. In July 2026 the FDA approved lutetium Lu 177 vipivotide tetraxetan (Pluvicto) combined with androgen receptor pathway inhibitor therapy for PSMA-positive metastatic prostate cancer, with patients selected using gallium Ga 68 gozetotide or another approved [PSMA PET](https://www.edgepedia.org/psma-pet) product based on PSMA expression in tumors.[7](https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-lutetium-lu-177-vipivotide-tetraxetan-androgen-receptor-pathway-inhibitor-therapy)
## Hybrid scanners: PET/CT and PET/MRI
Combined PET/CT scanners acquire both exams of the same body regions in one examination, fusing PET's functional information with CT's anatomic detail; fused scans show tumors more clearly and are often used to diagnose and monitor cancerous growths and their response to therapy.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) PET/CT is seen by many as the test of choice in imaging patients with lymphoma, both for staging and for assessing response to therapy through changes in FDG uptake, and FDG PET/CT is well established in multiple myeloma.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5623147/)
PET/MRI systems combine simultaneously acquired metabolic information with MRI's high soft-tissue contrast and functional data. They are more costly than PET/CT systems because of the inherent cost of MRI technology and the research and development required for integrated PET/MRI.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5623147/) A significant advantage is lower patient radiation exposure, since the CT component is replaced by MRI and X-ray exposure is avoided, which makes PET/MRI particularly recommended for pediatric and adolescent patients requiring one or more studies.[9](https://pubmed.ncbi.nlm.nih.gov/27349326/) • [10](https://www.ncbi.nlm.nih.gov/books/NBK576053/pdf/Bookshelf_NBK576053.pdf) PET/MRI remains an emerging technology that is not available everywhere, and some interpreters may be unfamiliar with its artifacts or uncomfortable reading the diagnostic MRI portion of the exam.[3](https://www.radiologyinfo.org/en/info/pet) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5623147/)
## History
The medical imaging possibilities of positron-emitting substances were first realized in the early 1950s, when physicist [Gordon Brownell](https://www.edgepedia.org/gordon-l-brownell) and neurosurgeon William Sweet at [Massachusetts General Hospital](https://www.edgepedia.org/massachusetts-general-hospital) developed the first large-scale human positron imaging device, a simple scanner using two opposed detectors with coincidence localization for brain tumors.[11](https://www.ncbi.nlm.nih.gov/books/NBK232487/pdf/Bookshelf_NBK232487.pdf) • [12](https://pmc.ncbi.nlm.nih.gov/articles/PMC3653214/) The development of cyclotrons that could produce short-lived positron-emitting radionuclides, advocated for medical use by researchers at Washington University and later Massachusetts General Hospital, set the stage for PET; smaller, self-shielding negative-ion cyclotrons with lower installation and operating costs later made the technology accessible to more researchers.[13](https://www.bruker.com/en/products-and-solutions/preclinical-imaging/nmi/pet-mr/_jcr_content/root/sections/more_information/sectionpar/linklist_93057888_co/contentpar-1/calltoaction.download-asset.pdf/links/item11/T190826-pet-imaging-past-present-future-app-note.pdf)
The first modern transaxial PET scanner, the ancestor of today's devices, was developed in the mid-1970s by the group of Phelps, Hoffman, and Ter-Pogossian at Washington University.[14](https://jnm.snmjournals.org/content/64/12/1998) In 1973 researchers there had built PETT III, which used annihilation coincidence detection to generate reconstructed transaxial tomographs and provided the foundations for the evolution of PET tomographs.[13](https://www.bruker.com/en/products-and-solutions/preclinical-imaging/nmi/pet-mr/_jcr_content/root/sections/more_information/sectionpar/linklist_93057888_co/contentpar-1/calltoaction.download-asset.pdf/links/item11/T190826-pet-imaging-past-present-future-app-note.pdf) FDG was synthesized in 1978, clinical PET/CT scanners emerged in 1998, and commercial simultaneous clinical PET/MR imaging followed in 2006.[13](https://www.bruker.com/en/products-and-solutions/preclinical-imaging/nmi/pet-mr/_jcr_content/root/sections/more_information/sectionpar/linklist_93057888_co/contentpar-1/calltoaction.download-asset.pdf/links/item11/T190826-pet-imaging-past-present-future-app-note.pdf) Landmark detector materials included the bismuth germanate (BGO) scintillator and lutetium oxyorthosilicate (LSO), whose adoption produced a revolutionary advance in PET scanners.[6](https://www.sciencedirect.com/science/article/pii/S1095039700000510)
## Limitations and comparison with SPECT
PET and SPECT are the two most common imaging modalities in nuclear medicine, but they differ in the radioactivity they use: SPECT cameras detect single gamma rays from radionuclides such as Tc-99m, while PET relies on positron annihilation coincidence detection.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine) PET offers higher sensitivity, easier quantitative imaging, and an extensive list of tracers, but its drawbacks include higher financial costs and the short half-lives of PET isotopes, which require a dedicated cyclotron environment nearby.[5](https://academic.oup.com/edrv/advance-article-pdf/doi/10.1210/endrev/bnae001/55444615/bnae001.pdf) • [10](https://www.ncbi.nlm.nih.gov/books/NBK576053/pdf/Bookshelf_NBK576053.pdf) PET/CT also involves ionizing radiation from both components and is susceptible to artifacts from metal objects such as dental fillings or implants, which can degrade image quality.[15](https://onlinelibrary.wiley.com/doi/abs/10.1002/imm3.70045)
The total radiation dose from most diagnostic nuclear medicine studies is no more than that from routine chest x-rays or CT exams, and the risk from cumulative low-level exposure is accepted to be quite small in contrast to the expected benefit of a medically needed study.[1](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine)
## References
1. [Nuclear Medicine, NIBIB, National Institutes of Health](https://www.nibib.nih.gov/science-education/science-topics/nuclear-medicine)
2. [A Molecular Imaging Primer: Modalities, Imaging Agents, and Applications, Physiological Reviews](https://journals.physiology.org/doi/full/10.1152/physrev.00049.2010)
3. [PET/CT - Positron Emission Tomography/Computed Tomography, RadiologyInfo.org](https://www.radiologyinfo.org/en/info/pet)
4. [History and future technical innovation in positron emission tomography, Journal of Medical Imaging](https://pmc.ncbi.nlm.nih.gov/articles/PMC5374360/)
5. [SPECT versus PET, Endocrine Reviews](https://academic.oup.com/edrv/advance-article-pdf/doi/10.1210/endrev/bnae001/55444615/bnae001.pdf)
6. [The History of Positron Emission Tomography, Molecular Imaging and Biology](https://www.sciencedirect.com/science/article/pii/S1095039700000510)
7. [FDA approves lutetium Lu 177 vipivotide tetraxetan with androgen receptor pathway inhibitor therapy for prostate cancer, FDA](https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-lutetium-lu-177-vipivotide-tetraxetan-androgen-receptor-pathway-inhibitor-therapy)
8. [PET/MRI: Where Might It Replace PET/CT?, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5623147/)
9. [PET/MRI: A luxury or a necessity?, PubMed](https://pubmed.ncbi.nlm.nih.gov/27349326/)
10. [PET-MRI radiation considerations, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK576053/pdf/Bookshelf_NBK576053.pdf)
11. [Free Executive Summary, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK232487/pdf/Bookshelf_NBK232487.pdf)
12. [The history of cerebral PET scanning, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3653214/)
13. [PET imaging: past, present, future, Bruker application note](https://www.bruker.com/en/products-and-solutions/preclinical-imaging/nmi/pet-mr/_jcr_content/root/sections/more_information/sectionpar/linklist_93057888_co/contentpar-1/calltoaction.download-asset.pdf/links/item11/T190826-pet-imaging-past-present-future-app-note.pdf)
14. [One Hundred Years of the Tracer Principle, Journal of Nuclear Medicine](https://jnm.snmjournals.org/content/64/12/1998)
15. [PET/CT and PET/MRI in Cancer Diagnosis and Management, iMetaMed](https://onlinelibrary.wiley.com/doi/abs/10.1002/imm3.70045)
16. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC11914360/)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Clinical assessment and procedures › Medical imaging and radiography*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license