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Positron emission tomography

Positron Emission Tomography (PET) is a medical imaging technique similar to nuclear medicine where radioactive 'tracer' isotopes which emit a positron are injected into a living subject. After travelling less than one millimeter the positron annihilates with an electron, producing a pair of gamma ray photons in opposite directions. The technique depends on simultaneous or "coincidental" detection of this pair of 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. It 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/patient. 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 are Single Photon Emission Computed Tomography (SPECT), Computed Tomography (CT), Magnetic Resonance Imaging (MRI) and functional Magnetic Resonance Imaging (fMRI). The spatial and temporal resolution of images developed using PET may not be as good as with some of the other techniques.

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 radioisotopes 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 subjects, and also by the fact that it is not advisable to subject any one subject 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.

Applications of PET scanning

PET is a valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.

  1. Oncology: PET scanning with the tracer (18F)fluorodeoxyglucose (FDG, FDG-PET) is widely used in clinical oncology. This tracer mimics glucose and is avidly taken up and retained by most tumours. As a result FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers. However because individual scans are more expensive than conventional imaging with CT and MRI, expansion of FDG-PET in cost-constrained health services will depend on proper Health Technology Assessment.
  2. Neurology: PET scanning of the brain is based on an assumption that areas of high radioactivity are associated with brain activity. What is actually measured indirectly is the flow of blood to different parts of the brain, which is generally believed to be correlated, and usually measured using the tracer (15O)oxygen.
  3. Cardiology: In clinical cardiology FDG-PET can identify so-called "hibernating myocardium", but its cost-effectiveness in this role versus SPECT is unclear.
  4. Psychology: To measure neurological activity in amnesiac patients, as in Shaheen Lakhan's review on source amnesia.