Treatments and diagnostic tests

What happens during a PET scan?

Positron emission tomography (PET) is an examination that creates several cross-sectional images of the body or of a particular part of the body. These images are displayed layer by layer in thin slices. PET uses a radioactive substance that is not considered harmful for humans at the dosage given in this examination. The most commonly used substance is called fluorodeoxyglucose (FDG). The average radiation exposure is about the same as the exposure from a computed tomography of the chest.

Radioactive means that the chemical decays without outside influence. In the process of decaying, it gives off radioactivity. This activity is harnessed to produce PET images. A PET scanner can precisely trace this substance’s path through the body and measure the radiation emitted by the body. These results are then used to create the different cross-sectional images. A computer can produce a spatial (3-dimensional, 3-D) image from these cross-sections.

What is a PET scan good for?

The special thing about PET: it shows how active the metabolism in certain tissues is. PET makes use of a trick: a radioactively marked substance – fluorine is often used – is bound to glucose, which is absorbed by the body’s cells from the blood just like common glucose, and then used to produce energy. This makes it possible to see in images especially those cells or tissues that consume a lot of energy and have a high rate of metabolism. This is especially true of brain cells, cardiac muscle cells, and inflamed tissue, but also of tumor tissue. Tissue that has a high rate of metabolism and a high consumption of sugar appears as especially dark spots on black-and-white PET images, and on color images as especially bright spots.

What is the difference between PET and other imaging techniques?

In other medical imaging techniques like conventional x-ray, or magnetic resonance tomography (MRT) or computed tomography (CT), tissues, organs and bones themselves are depicted. PET, on the other hand, shows how active particular tissue is. In other words: it is metabolic activity that is being measured.

A second difference: In contrast to what happens in x-ray or ultrasound, radiation is not produced outside of the body by a device and then sent through the body. Instead, radioactive substances are introduced that become part of the body’s metabolic processes. These substances then emit radiation themselves from the body.

In this respect a PET scan resembles scintigraphy, which also uses marked substances that are put into the body. The so-called SPECT is a scintigraphy method that also creates cross-sectional images. In comparison to SPECT, PET is much more exact and accurate.

What is PET computed tomography?

Nowadays, PET is very frequently combined with computed tomography in one device. PET provides information on the metabolic activity going on in the tissue being examined. At the same time, computed tomography is performed to get a more exact anatomical mapping of the findings than PET alone.

There are also already devices that combine PET scanner and magnetic resonance tomography (MRT) (PET/MRT).

What is special about PET?

  • A PET scan is very exact. It can often show changes in, for example, tumor tissue, even if they are only a few millimeters in size.
  • It is especially suited for examining organs or types of tissue and changes in tissue that consume a lot of energy and can clearly be differentiated from neighboring organs or tissue by virtue of their fast metabolism.
  • By the same token, lymph nodes that have been affected as a result of cancerous disease can only be detected by imaging techniques like ultrasound, CT or MRT if they are enlarged. PET would be able to detect the increase in sugar metabolism caused by malignant changes also in lymph nodes that have not yet become enlarged.
  • PET is also used after treating enlarged lymph nodes or remaining tumor tissue with chemotherapy or radiation therapy to know exactly whether it is only “dead” scar tissue or whether there are still active tumor cells there.
  • Another use of PET: it can also measure how much higher metabolism is in a certain region. This could be important in monitoring the progress of disease.
  • A PET scan is complicated and expensive. Producing the radioactive substance is complicated. Also, a laboratory must be available where it is possible to work with radioactive material.
  • The laboratory and the PET scanner also have to be located close together, because the radioactive substances produced for PET only keep for a limited time and will have decayed after one day at the latest. For this reason, there are only a limited number of locations in Germany where it is possible to do a PET scan.

The critical issue is whether the possible advantages of PET in comparison with other examination techniques resulted in improved treatment and treatment outcome. This would mean that the examination also has a benefit for the patient.

Radiation exposure: Is a PET scan dangerous?

The level of radiation exposure during a PET scan is limited and is similar to other x-ray procedures like computed tomography. A single PET has about the level of radiation that one person is exposed to in one year through natural radiation in the environment.

PET uses substances that decay very rapidly in the body, and are quickly eliminated from the body by the kidneys. This happens even faster if you drink a lot. The half-life of the most commonly used radioactively marked glucose analogue FDG is just under 2 hours. This means that after about 110 minutes, only half of the original radioactivity is left, and after 4 hours only one-fourth, and so on.

Nevertheless, a PET scan should only be performed when it is really needed. This applies to other types of x-ray exams as well.