On my work experience recently in radiology I unfortunately didn’t get the chance to look at one of these, which was in the realm of the mysterious nuclear medicine nearby. I did however get to see the final images from this type of scan combined with a CT scan and wanted to research PET further.
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| How it works |
PET scans are where particle physics meets clinical medicine. The procedure involves injecting a radioactive isotope into the patient inside a scanner. The radioisotope decays emitting a positron, the antimatter alternative of an electron, same in mass but positively charged. The positron travels through typically 1mm of tissue during which it loses speed enough to collide with an electron. The two particles annihilate each other and two gamma rays are emitted around 180o from each other. The scanner detects gamma rays and stores information about it if two rays are detected roughly 180o from each other. All other rays are ignored. By getting lots of gamma ray pairs and by tracing back the rays to their source a three-dimensional picture of the body showing where the radioactive isotope has gone. PET scans are usually combined with a CT scan to make a PET-CT scan so that the two can be registered together and the anomalies of can be corrected by the other.
The radioactive isotopes used in PET are typically normal biological molecules such as glucose with a few radioactive atoms such as carbon-11. The isotope needs to be such that it has a long enough half-life to be made off site and imported and short enough so that the patient is not in the scanner for weeks to get enough data. A common tracer is fludeoxyglucose (18F) or FDG, which is essentially a glucose molecule with the hydroxyl group on the second carbon replaced with an atom of fluorine-18.
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| A FDG molecule |
The advantage of FDG is that like normal glucose it is quickly take in up by cells that are respiring quickly such as brain, kidney and more importantly cancer cell. Because the hydroxyl group on the second carbon in the ring (that FDG lacks) is involved part way through the process of breaking down glucose, the FGD molecule is only half way converted to water and carbon dioxide. The FGD-phosphate that results when the process is stopped at this point remains in the cell due to its ionic charge until the fluorine-18 decays leading to the positron emission that is then detected. The decay helpfully leaves non-radioactive products that are metabolised normally.
Scans of cancerous tumours are 90% of all PET scans. In the area they are especially useful for tracing the spread of cancer cells. Cancers spread by metastasis. This is when the primary tumour formed by a mutation grows into the path of a lymph node or blood vessel. Cells from the tumour are spread through the lymph or blood system and end up elsewhere in the body. By doing a PET scan using FDG on people with cancers that commonly metastasize it is possible to assess how much the cancer has spread which is a significant factor in the treatment and prognosis of the disease.
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| The result of a PET-CT scan to assess the spread of a lung cancer tumour |
There are however a few problems with a PET scan. Due to the quick decay of the tracers the hospital in which it is situated must have a very speedy transport link to the location the tracer is made so the isotope doesn’t decay on route. Few hospitals can afford the costs involved. The scan also gives the patient a significant dose of radiation that could cause more harm than good if used unwisely. A typical PET and CT combination gives a combined dose of 23-26mSv of radiation compared with 0.2mSv for a chest x-ray or 7mSv for a normal CT.
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