PET Scan (Positron Emission Tomography)

A PET Scan (or Positron Emission Tomography) is a non-invasive, diagnostic examination that finds information about the activity of different parts of the body. Those parts of the body that are the most active need energy, and the energy that it uses is sugar (also called glucose). A PET scan uses a specially created substance that the body thinks is sugar, and takes up into the cells. This substance is called a ‘tracer’, and it is almost exactly like sugar, but has a small radioactive part attached to it. The images are based on the detection of radiation from the emission of positrons (positively charged electrons) from this radioactive tracer. The subsequent images created are used to evaluate a variety of diseases, with the most common use being whole body imaging of cancer.

How Does a PET Scan Work?

PET scans work to identify the cellular changes that occur in the body during disease. During a PET scan, the patient is given a substance called a tracer, typically a chemical found in the body (oxygen, glucose, nitrogen, fluorine) which has been tagged with a radioactive atom that breaks down quickly to release positrons. The most common tracer has a complicated name, but is mostly known as FDG (which stands for 2-[18F]fluoro-2-deoxy-D-glucose). FDG is very similar to glucose (sugar). Once in the body FDG travels to the area of the body that is using a lot of sugar and breaks down. In breaking down, it releases a positron. This combines with an electron from the patient sending out radioactive waves. These waves can be detected by the PET Scanner that converts the waves into electrical signals that can be analysed by a computer. The computer can then create images of the targeted tissue’s function, in either a colour code, or in black and white. Different colours or degrees of brightness on a PET image represent different levels of tissue or organ function. In the PET image to the right, darker areas represent those that are using more sugar, and so have more of the radioactive substance in them. For example, because healthy tissue uses glucose for energy, it accumulates some of the tagged glucose, which will show up on the PET images. However, cancerous tissue, which uses more glucose than normal tissue because it is growing much faster, will accumulate more of the substance and appear brighter than normal tissue on the PET images. Therefore PET scans are commonly used to detect cancer, as the scan will detect biochemical changes in the body associated with cancer as well as the extent of spread and the possible effective treatment. PET scans are frequently used to determine blood flow to the heart tissue in association with coronary heart disease and myocardial infarctions. Functions of the brain may also be examined in association with patients with memory loss, suspected or known brain tumours or seizure disorders.

During a PET Scan

PET scans are generally done in an outpatient setting and preparation is usually set by your doctor. The scan does require the patient to lie still for a period of time up to two hours, so if you feel you may have a problem with this, please talk to your doctor. Women who suspect they may be pregnant or are pregnant need to notify their doctor as this is a radioactive procedure and could have effects on an unborn foetus. The PET scanner is a donut-ring shaped apparatus with an attached table that moves into the scanning ring. Initial scans may be done before you are taken into a special injection room where the tracer is administered, usually intravenously. It will then take 30-90 minutes for the substance to travel through your body and accumulate. During this time you may be asked to rest quietly to allow the substance to travel through the body. You will then be asked to lie still on the scanner table and it will move in through the scanner ring. During the scan, which can take from thirty minutes to two hours, you must lay very still to allow for clarity of the images. After the scan there are no restrictions to daily activity, but it is recommended you drink plenty of water to flush your system of the tracer.

Benefits of a PET Scan

PET scans are beneficial as they demonstrate the biochemical changes in the body, whereas a CT or MRI scan identifies anatomical changes. A PET Scan therefore helps to identify problems at the level of their activity and function, which might change long before any changes in body structure (such as a tumour) become apparent. This allows for earlier diagnosis and more effective treatment of diseases such as cancer, and also more detailed imaging of other conditions. There is little risk involved with the intravenous administration of the radioactive “tracer” as the tracer has a short decay time of only a few hours and is quickly removed from the body.

Risks of a PET Scan

The risks associated with a PET scan are very small, and are due to reactions to the radioactive “tracer” injected. The level of tracer given to a patient is very tiny, and the chances of a reaction occurring are incredibly low.

Limitations of a PET Scan

• PET scans can sometimes show up areas of high activity which may be mistaken for cancers. Inflammatory conditions like rheumatoid arthritis or tuberculosis absorb a lot of the tracer, and so can cause confusing results.
• A PET scan is less accurate in certain situations:
  1. Slow-growing, less active tumors may not absorb much tracer.
  2. Small tumors (less than 7mm) may not be detectable.
  3. High levels of blood sugar can cause the cells to absorb this normal sugar rather than the radioactive, injected kind. Patients are usually fasted for 4 hours before a PET scan, and blood sugar levels measured to lower the chances of this happening.
• The radioactive substance has a very short decay and therefore appointments must run on schedule.
• PET scans are a very expensive form of imaging, and are not readily available. They often accompany other scans such as CT and MRI in order to be diagnostically effective.

Results of a PET Scan

After a PET scan, the doctor will receive an image of the patient displaying the different amounts of activity present in different parts of the body. An example of this is given to the right, where areas of high activity in the brain are white, and low activity are blue. If there is a cancer somewhere in the body, this area will appear more active and so PET scans are often used to see if a cancer has spread to other parts of the body or to see how advanced a cancer is. A PET scan is not as accurate for very small cancers however, so very early scans may not show anything.

References

1. Positron Emission Tomography: Assessment Report. Medicare Services Advisory Committee, 2000. Commonwealth of Australia.
2. Soben P, Udelson KE. ‘Thoracic Assessment of myocardial viability by nuclear imaging in coronary heart disease’ [online], UpToDate, 2006. Available at URL: http://www.uptodate.com (last accessed 13/07/06)
3. Stark P. ‘Thoracic positron emission tomography’ [online], UpToDate, 2006. Available at URL: http://www.uptodate.com (last accessed 13/07/06)