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About PET Scans

What are PET Scans?

Positron emission tomography, or PET scanning, is a type of nuclear medicine imaging exam that shows the structure and function of organs and tissues.

Imaging exams are routinely used to provide information on the shape and size of parts of the anatomy. This information is used to diagnose illness, and to make decisions about treatment.

What’s the Difference Between a PET Scan and a CT Scan or an MRI?

Traditional imaging (such as Ultrasound, X-ray, CT or MRI) may detect disease based on physical changes in the person’s anatomy.

PET scanning reveals changes in biochemical processes (e.g., sugar metabolism) that may occur before any changes in anatomy are visible.

The advantage of PET scanning is that it may show more extensive disease than the CT or MRI. This information could change how the illness is managed (for example, whether a patient is offered aggressive, potentially curative treatment as opposed to palliative treatment).

How Does a PET Scanner Work?

The most common radiopharmaceutical used in PET scanning is FDG (18-Fluorodeoxyglucose), which has a small amount of radioactive material, known as a radiopharmaceutical, combined with sugar. Cancer cells often use more sugar than normal cells or scar tissue, so the FDG becomes concentrated in areas of the body that have cancer.

PET scanning uses a small amount of radioactive material that is usually injected into a person’s bloodstream, or sometimes swallowed by mouth or inhaled as a gas.

The PET scanner works with a computer to create two- and three-dimensional images of the structure and function of organs and tissues. This technology is useful to determine the extent of some cancers, which may help define the most appropriate treatment.

To improve accuracy, most PET scanners are integrated with a CT scanner (PET/CT). This way, information about metabolic changes (through a PET scan) is obtained at the same time as information about the anatomy (through a CT scan), and the results can be combined. This improves the physician’s ability to determine exactly where in the body the metabolic changes are taking place.

Balancing Risks and Benefits

As for any diagnostic test, the use of PET must be based on evidence. The test has risks as well as benefits.

There is a small amount of radiation exposure with a PET or PET/CT scan. Although the level is low, this means that PET scanning should only be performed when the results of the scan may benefit the patient.

The test can show false results if the individual has a chemical imbalance (e.g., diabetes, inflammation or infection), or the metabolic activity of the cancer has been suppressed by chemotherapy. It is also important to recognize that a PET scan can be falsely negative if the tumour cells are not metabolically very active, as is the case with some cancers, or if the number of cancer cells is so small that their metabolic activity is below the level of detection of the PET scan.