What Is a FDG-PET Scan?

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  • Written By: Jillian O Keeffe
  • Edited By: PJP Schroeder
  • Last Modified Date: 28 September 2019
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A FDG-PET scan is a medical imaging procedure. It involves an injection of radioactive tracer liquid inside the body that congregates at tumors and other sites where the cells divide more quickly than usual. The acronym FDG-PET scan stands for fluorodeoxyglucose (FDG)-positron emission tomography (PET), where the FDG is the radioactive liquid and the PET is the scanning machinery.

Positron emission tomography scanners detect photons of light energy and build three-dimensional images from the photons they receive. Scientists can use this ability to detect photons for medical diagnostics if they know where the photons come from and what they represent. A normal person under the scanner does not emit enough photons for the scanner to build an image that shows up tumors or other medical conditions. Therefore, the patient under investigation must receive a photon energy source in an injection.

The injection contains a form of sugar that is radioactive. This sugar is the FDG part of the scan. Cells in the human body use glucose, a sugar, as the primary energy source to drive all reactions and growth. FDG is a glucose molecule with a radioactive fluorine atom attached. It is radioactive, but it is not strong enough to pose a significant health risk.


Typically, a patient undergoing a FDG-PET scan gets an injection of the FDG into a vein. The radioactive sugar moves throughout the blood system and travels to places that need it most. Tumor cells grow out of control, and so they require more glucose energy than most other cells. Areas of the body, such as the brain in suspected Alzheimer's Disease cases, that do not exhibit a normal level of growth and glucose uptake show a reduced radioactive glucose uptake.

After a period of about an hour, the doctor then asks the patient to lie down on the scanning table. By this time, the radioactive glucose will have made its way into the cells that want it most. Radioactive molecules, like FDG glucose, are inherently unstable and break down by emitting particles that contain energy.

As the FDG breaks down, it emits a particle called a positron, which then splits up into two photons. It is the photons produced by the FDG that the PET scanner detects. As the FDG congregates in highly active cells like tumors, most of the photons come from those areas.

When about an hour on the scanning table has passed, the patient can get up. The scanning machine then forms a complete FDG-PET scan image from the information provided by the photons. A doctor interprets the areas of low or heavy shading from the photon activity as part of his or her diagnosis. Tumors, heart disease, and diseases of the brain are all illness that a FDG-PET scanner can help identify.


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