Positron emission tomography Guide, Meaning , Facts, Information and Description
Positron emission tomography (PET) is a nuclear medicine medical imaging technique where radioactive short-lived 'tracer' isotopes which emit a positron, chemically combined with metabolically active molecules, are injected into a living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (usually a sugar) becomes concentrated in tissues of interest, then the subject is placed in the imaging scanner. The short-lived isotope decays, emitting a positron. After travelling less than one millimeter the positron annihilates with an electron, producing a pair of gamma ray photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes. The technique depends on simultaneous or "coincidental" detection of the pair of gamma photons: photons which do not come in pairs (within a few nanoseconds) are ignored. By measuring where the gamma rays end up, their origin in the body can be plotted, allowing the chemical uptake or activity of certain part of the body to be determined. The scanner uses the pair-detection events to map the density of the isotope in the body, in the form of slice images separated by about 5mm. The resulting map shows the tissues in which the molecular probe has become concentrated, and is read by a nuclear medicine physician or radiologist, to interpret the result in terms of the patient's diagnosis and treatment. PET scans are increasingly read alongside CT scans, the combination giving both anatomical and metabolic information (what the structure is, and what it's doing). PET is used heavily in clinical oncology (medical imaging of tumours and search for metastases) and in human brain and heart research.PET scanning is invasive, in that radioactive material is injected into the subject. However the total dose of radiation is small, usually around 7 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest X-Ray, up to 8 mSv for a CT scan of the chest, 2-6 mSv per annum for aircrew, and 7.8 mSv per annum background exposure in Cornwall (Data from UK National Radiation Protection Board).
Alternative methods of scanning include computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) and single photon emission computed tomography (SPECT). The spatial and temporal resolution of images developed with these techniques, from better to worse, are generally CT, MRI, PET, SPECT.
However, while other imaging scans such as CT and MRI, isolate organic anatomical changes in the body, PET scanners are capable of detecting areas of molecular biology detail (even prior to anatomical change) via the use of radiolabelled molecular probes that have different rates of uptake depending on the type of tissue involved. The changing of regional blood flow in various anatomical structures (as a measure of the injected positron emitter) can be visualized and relatively quantified with a PET scan.
Radionuclides used in PET scanning are typically isotopes with short half lives such as Carbon-11, Nitrogen-13, Oxygen-15, and Fluorine-18 (half-lives of 20 min, 10 min, 2 min, and 110 min respectively). Due to their short half lives, the isotopes must be produced in a cyclotron at or near the site of the PET scanner. These isotopes are incorporated into compounds normally used by the body such as glucose, water or ammonia and then injected into the body to trace where they become distributed.
PET as a technique for scientific investigation is limited by the need for clearance by ethics committees to inject radioactive material into participants, and also by the fact that it is not advisable to subject any one participant to too many scans. Furthermore, due to the high costs of cyclotrons needed to produce the short-lived radioisotopes for PET scanning, few hospitals and universities are capable of performing PET scans.
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2 PET scans safety 3 PET history and current deployment 4 See also |
PET is a valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.
A radioactive isotope of an element is injected into the bloodstream. The amount has to be carefully calculated to avoid an overdose which could have fatal consequences. Radiation can cause cells to mutate and change.
Edward J. Hoffman and Michael Phelps developed the first human PET scanner in 1973 at Washington University in St. Louis. See also history of brain imaging.
PET scanning is a capital-intensive and very specialised technique which is limited to facilities close to a cyclotron for 18-F FDG, and in the same building as the cyclotron for shorter halflife isotopes. Although developed in the 1970s, PET scanning was limited to research until the US Medicare system announced reimbursement for certain specific conditions, such as the staging of particular cancers. Oncology reimbursement now drives the majority of PET installations, which are concentrated in major US cities and particularly in the retirement states such as Florida. Such capital-intensive medical techniques are taken up more slowly in Europe; the cities of Houston and Dallas between them, have more PET scanners than the whole of Europe (fewer than twenty as of 2004). Insurance reimbursement for a PET scan is typically in the area of $1500 which is divided between the operator of the scanner and the interpreting physician.
The most significant industry trend in PET today is the combination of PET and CT scanners into a single unit, providing registered images of the patient in both modalities. This is a significant aid in the interpretation of PET data, since anatomical structures are not clear in the PET image. However, although a CT scan is taken in just a few seconds (minimising the effects of patient movement, breathing, heartbeat and bowel action), the PET scan takes around half an hour, so the registration of the two is not precise.Applications
PET scans safety
PET history and current deployment