Positron emission tomography (PET) scans are used to see metabolic activity in the tissue of the body. To do this, multiple images of the body or part of the body are created to show the scanned area layer by layer in thin "slices." The computer uses these slices to make a three-dimensional (3D) image.
A weakly radioactive substance called a tracer is used to allow doctors to see the metabolic processes. The tracer is typically injected into the body. It is used at a dose that is considered safe for humans. "Radioactive" means that the chemical substance decays (breaks down) on its own and gives off radiation (energy). By measuring this radiation, the PET scanner can follow exactly where the substance moves inside the body.
What are PET scans used for?
The special thing about PET is that it shows how active the metabolism in certain body tissues is. Your metabolism is the sum of chemical reactions that keep your body alive. PET makes use of a trick: A weakly radioactive substance – usually fluorine – is bound to glucose (sugar) molecules to form a compound. This chemical compound (fluorodeoxyglucose or FDG) is absorbed by the cells from the blood, just like normal sugar, and then used to produce energy. The radiation that is released when this radioactive substance breaks down can be detected by the PET scanner. The cells and tissue that use a lot of energy (and have a high rate of metabolism) are then particularly easy to see. This is especially true of brain cells, heart muscle cells, and inflamed tissue, but also of tumor tissue.
Tissue that has a high rate of metabolism and uses a lot of sugar can be seen as especially dark areas on black-and-white PET images, or as especially bright areas on color images.
How can you prepare for a PET scan?
Your doctor will give you detailed information about preparing for a PET scan beforehand. It's important to know that you aren't allowed to eat anything or drink anything with sugar in it for several hours before having a PET scan with FDG. This could be difficult if you have diabetes, for instance. So it's a good idea to let the doctor know whether you have any other medical conditions or are using medication, or whether you are pregnant or breastfeeding. The doctor should also be told about any possible allergies or past allergic reactions. This is because the substance that is injected may lead to allergic reactions in very rare cases.
It is important to lie down and keep still for a while after having the injection so that the weakly radioactive substance can spread evenly through your body. The scan itself usually takes about half an hour. You shouldn't move during the scan either so that the images it creates are as sharp as possible. During this time you will be lying inside a narrow tube that makes noises. This can be uncomfortable, and it causes some people to panic. If you tend to feel anxious in tight spaces, you can talk to the doctors beforehand.
What is the difference between PET and other imaging techniques?
Other medical imaging techniques like x-ray, magnetic resonance imaging (MRI) or computed tomography (CT) scans show the body's tissues, organs and bones. PET, on the other hand, shows how active a particular tissue is. In other words, it measures the metabolic activity.
Another difference is that, unlike in an x-ray or ultrasound scan, the radiation isn't produced outside of the body by a device and then sent through the body. Instead, radioactive substances are introduced into the body and become part of the body’s metabolic processes. These substances then give off radiation from inside the body. In this way, a PET scan is similar to a procedure called scintigraphy, which also uses radioactive tracers that are put into the body.
What is PET computed tomography (PET/CT)?
Nowadays, PET is very often combined with computed tomography (CT) in one scanner. The combination of the two makes it possible to get a clearer picture of exactly where the areas of more active tissue are.
PET scans are very accurate. Even changes of only a few millimeters can be seen, such as those affecting tumor tissue.
PET is especially suited for examining organs or types of tissue that use a lot of energy and can clearly be differentiated from nearby tissue or other organs because of their fast metabolism.
In ultrasound, CT and MRI, cancerous lymph nodes can't be seen until they have become enlarged. PET can find them sooner because of their abnormally high metabolic activity.
If someone has had chemotherapy or radiation therapy for cancer, PET allows doctors to see exactly whether remaining enlarged lymph nodes or abnormal areas of tissue are only “dead” scar tissue or whether there are still areas of active tumor cells.
And it can measure how much higher the metabolism is in a certain area. This could be important for monitoring the progression of disease.
PET scans are complicated and expensive: It is difficult to make the radioactive substance. And you need access to a laboratory that can work with radioactive material.
The laboratory and the PET scanner also have to be located close together, because the radioactive substances produced for the PET scan break down within one day. For this reason, there are only a few places in Germany where you can have a PET scan.
It only makes sense to have a PET scan if the patient benefits from it because it leads to better treatment outcomes compared to other kinds of scans. Because PET scans are so sensitive, there is a risk that they might discover abnormalities that would never have caused any health problems. That would lead to further treatments that you don't actually need.
Radiation exposure: Are PET scans dangerous?
The level of radiation exposure during a PET scan is limited. It is similar to that of other imaging techniques that use x-rays, like computed tomography. A single PET scan gives off about the same level of radiation that one person is exposed to in one year through natural radiation in the environment.
PET uses substances that break down very quickly in the body, and soon leave the body through the kidneys. This happens even faster if you drink a lot of fluids. The half-life of the most commonly used radioactive tracer is just under two hours. In other words: After about two hours only half of the radioactivity is left, after four hours one quarter is left, after eight hours one sixteenth is left, and so on. Still, PET scans should only be done if really needed. This is also true for other types of examinations that use x-rays.
Andreae S, Avelini P, Berg M et al. Lexikon der Krankheiten und Untersuchungen. Stuttgart: Thieme; 2008.
Kasper DL, Fauci AS, Hauser SL et al. Harrison's Principles of Internal Medicine. New York: McGraw-Hill; 2015.
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