Positron emission tomography: Encyclopedia II - Positron emission tomography - Description

Positron emission tomography - Description

A short-lived radioactive tracer isotope which decays by emitting a positron, chemically incorporated into a metabolically active molecule, is injected into the 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 up to a few millimeters the positron annihilates with an electron, producing a pair of annihilation photons (similar to gamma rays) 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 coincident detection of the pair of photons: photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored. By measuring where the annihilation photons end up, their origin in the body can be plotted, allowing the chemical uptake or activity of certain parts 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 5 mm. 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 or MRI scans, the combination giving both anatomic and metabolic information (what the structure is, and what it is doing). PET is used heavily in clinical oncology (medical imaging of tumours and the search for metastases) and in human brain and heart research.

Alternative methods of scanning include x-ray computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI), ultrasound and single photon emission computed tomography (SPECT).

However, while other imaging scans such as CT and MRI, isolate organic anatomic changes in the body, PET scanners are capable of detecting areas of molecular biology detail (even prior to anatomic 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 anatomic 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 ¹¹C (~20 min), ¹³N (~10 min), ¹⁵O (~2 min), and ¹⁸F (~110 min). Due to their short half lives, the radionuclides must be produced in a cyclotron at or near the site of the PET scanner. These radionuclides 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. Such labelled compounds are known as radiotracers.

PET as a technique for scientific investigation in humans 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. In neurological research, this limitation can be partly overcome by the use of short-lived radionuclides that result in a lower radiation dose. PET also has an expanding role in the assessment of response to therapy, and in particular cancer therapy (e.g. Young et al. 1999).

PET is also used in pre-clinical studies using animals, where it allows repeated investigations into the same subjects. This is particularly valuable in cancer research, as it results in an increase in the statistical quality of the data (subjects can act as their own control) and very substantially reduces the numbers of animals required for a given study. A further limitation arises from the high costs of cyclotrons needed to produce the short-lived radionuclides for PET scanning (for example ¹⁸F). Few hospitals and universities are capable of maintaining such systems, and most clinical PET is supported by third-party suppliers of radio-tracers which can supply many sites simultaneously. This limitation restricts clinical PET primarily to the use of tracers labelled with ¹⁸F, which has a half life of 110 minutes and can be transported a reasonable distance before use, or to ⁸²Rubidium, which can be created in a portable generator and is used for myocardial perfusion studies.

Adapted from the Wikipedia article "Description", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki