Positron Emission Tomography with Computed Tomography (PET /CT)

What Is PET/CT?

Positron emission tomography (PET) examinations belong to the field of nuclear medicine. In nuclear medicine, radioactive substances are used to make changes in tissue activity within the body detectable. In a PET, metabolic processes within the body are imaged.

PET/CT (positron emission tomography/computed tomography) displays metabolically altered tissue on a three-dimensional CT image, which allows the examined processes to be localized as precisely as possible. In this case, tomography means cross-sectional imaging: the body is displayed slice by slice as if cut into wafer-thin pieces.

Making Metabolic Processes Visible

In nuclear medicine, so-called carriers, or tracers, are coupled with radioactive substances (radionuclides) and administered to the body. The radiopharmaceuticals are absorbed within the body and metabolized in the tissue. During metabolism, positrons are released from the radioactive substances; the tissue can detect the reactions.

Depending on the activity of the tissue under investigation, increased or decreased metabolism of the tracer occurs here. For example, in malignant changes, such as metastases, or inflammations, there is an increase in metabolism in the tissue. For instance, in "cold nodules" of the thyroid gland, the activity of the tissue is reduced compared to the surrounding area ( thyroid scintigraphy).

In the PET/CT examination, the radioactive substances released by the tissue due to metabolic activity are collected using appropriate equipment (gamma camera) and displayed coupled with the three-dimensional sectional image of the body (from the computed tomography). Through proper conversion, tissue with increased or decreased metabolic activity is imaged on the PET/CT and can thus be precisely localized.

PET/CT is used, among others, to visualize metastases in oncology (tumor medicine) or to assess the vitality of the heart muscle in the field of cardiology.

Indications: When Is PET/CT Performed?

PET/CT is primarily used to visualize tumors or metastases (tumor metastases) in oncology, in the field of neurology (neurology) for the diagnosis of dementia and epilepsy, or to assess the vitality of the heart muscle in the field of cardiology.

In the field of oncology, PET is mainly used for the evaluation of

• Pulmonary nodules (exclusion of lung cancer, bronchial carcinoma),
• Colon cancer (colorectal carcinoma)
• Melanoma
• Breast cancer
• Head and neck tumors
• Bone metastases
• Prostate cancer
• Pancreatic cancer
• Esophageal cancer
• Lymph gland cancer
• Thyroid cancer
• Ovarian cancer

Procedure and Duration of a Positron Emission Tomography with a Computed Tomography (PET/CT) Examination

At the beginning of the examination, the patient is administered the tracer interspersed radioactive substance. Usually, this is done intravenously, directly into the bloodstream by inhalation, through the respiratory tract. The dose of the radiopharmaceutical is adapted to the patient's weight and the examination device. The tracer used and the coupled radionuclide depends on the type of tissue examined in each case.

For example, 18F-choline is used in prostate cancer examinations. Since tumor cells usually have increased energy consumption and therefore metabolize more glucose (dextrose) than the surrounding tissue, 18F-fluorodeoxyglucose (FDG)-PET is regularly performed to search for hidden tumor cells. FDG-PET is the most commonly used form of positron emission tomography.

Following injection/inhalation, the tracer must accumulate in the body. This uptake phase, during which the radioactive substances are absorbed into the relevant tissue, takes about 50 to 75 minutes. The tracer decays relatively quickly in the tissue so that the radionuclide is released and decays. The released energy can be recorded using a gamma camera. The temporal and spatial distribution of the detected radiation allows the calculation of available images of the individual tissue sections.

Following the imaging phase, a CT (computed tomography) scan is usually started first to be directly calculated with the subsequent PET scan. Usually, the CT for PET/CT is performed as a so-called low-dose CT scan, associated with lower radiation exposure for the patient.

After the scan, the images are appropriately computed and evaluated. In PET/CT, the PET and CT scan images are coupled together to produce a standard image for the examiner. Within this image, it is possible to switch between the different sectional planes and views and scroll through them with a mouse click. When evaluating the images, it is essential to distinguish between physiological and pathological uptake of the tracer. For example, FDG (fluorodeoxyglucose, glucose) can accumulate physiologically in addition to pathological tumor tissue due to wound healing processes, muscle tension, or tremors. Increased accumulation is indicated by bright dots or spots on PET/CT imaging. This is because the tumor cells "light up" in the image. Nuclear medicine physicians usually evaluate the examination within one day.

The examination time alone on the device is 20-40 minutes. Therefore, the total time patients should plan for is 2-3 hours.

Costs: How Expensive Is a PET/CT?

Positron emission tomography is one of the most expensive imaging procedures in medicine today. Whole-body PET-CT costs (as of 2008) about 1123€ in Germany: in the context of an inpatient stay, the statutory health insurance in Germany usually covers the costs of this examination.

PET/CT: Advantages and Disadvantages

Since PET uses radioactive substances and computed tomography uses X-rays, the indication for this examination must be made with caution. For children, in particular, it is advisable to use the low-radiation PET-MRI if possible. Unfortunately, however, this form of imaging is only available in a few centers throughout Germany.

Since foreign drugs (tracers or contrast agents for CT) are administered during PET-CT, allergic reactions up to and including anaphylactic shock with a cardiovascular arrest can occur as a side effect at any time. Discuss this with our specialists if you have an increased risk of allergy or have already had corresponding experience.

Increased accumulation of radionuclides does not always indicate tumorous tissue. For example, the image can be altered by inflammation in the tissue. Also, not all tumors show increased cell activity, so PET-CT examination is not suitable for every type of tumor.

PET is not used for morphology but functional imaging of the tissues. Here, cross-sectional images of the body coupled with metabolic processes are imaged. PET/CT allows a very accurate assessment of tumor size, tumor activity, and possible metastases. This information is often essential for further therapy planning. However, since the radiation exposure from the combined PET/CT examination is higher than PET or CT alone, it is necessary to weigh with our specialists who form of examination is right for you.

Studies have shown that the accuracy of PET/CT findings has increased significantly compared to the individual preparation and reporting of PET and CT.

Which Doctors and Clinics Are Specialized in Positron Emission Tomography?

In general, nuclear medicine physicians are specialists in positron emission tomography testing.

We can help you find an expert for your condition. All doctors and clinics listed have been reviewed by us for their outstanding specialization in PET-CT and are awaiting your inquiry or request for treatment.

Sources:

• AMBOSS – Nuklearmedizin und Strahlenschutz
• www.krebsinformationsdienst.de/untersuchung/pet-technik.php
• de.wikipedia.org/wiki/Positronen-Emissions-Tomographie