A Positron Emission Tomography scan or PET Scan is a radiological procedure used for finding particular types of tissues in the body and studying its metabolism. It is often combined with a CT Scan or MRI.
The patient is prepared by swallowing a tracer - a special type of radioactive isotope that both binds to living tissue and will emit a positron (a positively charged electron) when it decays. The tracer is added to a chemical that will bind to the tissue under investigation.
When the chemical with the tracer binds to the organ under investigation, the patient is put in the scanner, which can detect gamma ray photons. When a positron is emitted by the radioactive element, it immediately undergoes mutual annihilation with an electron and emits two gamma ray photons that travel in exactly opposite directions. Although gamma rays are found in normal background radiation, the computer attached to the scanner will only record pairs of photons which reach the scanner at almost precisely the same moment and in a single line leading from the patient's organ. The computer calculates from the delay in arrival where in the body the positron annihilated itself, producing a 3D scan of the organ in question. Modern machines can even take a "motion picture" in time of activity in the organ from a series of 3D scans.
It's primary use is in oncology as tracers can be designed which will take up only cancerous cells, therefore allowing a scan to show tumors and even metastasis. It is also used in neurology to track brain activity as the tracers can be attached to a form of glucose, showing where it is used in the brain.
Although it is not invasive, the tracer will expose the patient to radioactivity, although not as much radioactivity as a CT Scan.
PET scanning is used in many disciplines.
- Oncology: The tracer used in oncological scanning is called fluorine-18 flurodeoxyglucose, or FDG-PET, and is a glucose analog that is taken up by glucose-using cells and undergoes phosphorylation by hexokinase, an enzyme whose mitochondrial form is at much higher levels in rapidly-growing malignant tumors. Once the FDG enters the cell, it stays there until it decays as after phosphorylation, it cannot exit the cell therefore causing intense radiolabeling of areas with high glucose uptake such as the brain and most cancers. This process is used for diagnosing, staging and monitoring of cancers. Scans such as these make up 90% of all PET scans in current practise.
- Neurology: In neurology, PET scans are used to measure the flow of blood to the different parts of the brain using the tracer oxygen-15, though this is a difficult process. It is used to do things like differentiate between Alzheimer's disease and other dementing processes in addition to diagnose Alzheimer's early. It can also be used to find seizure focuses.
- Cardiology: PET imaging is not used very widely because of the cost involved, but it has been used for studying atherosclerosis and vascular disease. Recently, there has been research suggesting that PET scans of atherosclerosis can detect whether patients are at risk for strokes.
- Psychiatry: Again, PET imaging is not used widely because of the cost, but there are radioligands that have been used to bind to dopamine, serotonin and opiate receptors amongst others to compare the state of these receptors between healthy controls and people with psychiatric conditions such as schizophrenia and mood disorders.