The Radiotracer and Its Role
Positron Emission Tomography (PET) scans are medical imaging tests that help doctors visualize the function of organs and tissues within the body. Unlike X-rays or CT scans that show anatomical structures, PET scans reveal metabolic activity, which can indicate disease at an early stage. This unique capability stems from the use of a special type of radioactive material known as a radiotracer.
The most commonly utilized radiotracer in PET imaging is Fluorodeoxyglucose, abbreviated as FDG. FDG is essentially a sugar molecule tagged with a small amount of a radioactive isotope, fluorine-18. When injected into the bloodstream, cells absorb FDG similar to how they absorb regular glucose for energy. Cells with higher metabolic activity, such as cancer cells or areas of infection, tend to absorb more of this FDG, making them “light up” on the PET scan images. The scanner then detects the positrons emitted by the decaying fluorine-18, creating detailed images of these active areas.
How Your Body Clears Radioactivity
After a PET scan, the radioactivity within your body diminishes through two primary mechanisms: biological elimination and radioactive decay. Your body naturally processes and expels the radiotracer, primarily through the urinary system. Drinking plenty of fluids following the scan helps to accelerate this excretion process.
Concurrently, the radioactive material itself undergoes a natural process called radioactive decay. This means the unstable radioactive atoms transform into more stable, non-radioactive forms over time. A key concept here is “half-life,” which is the specific time it takes for half of the radioactive material to decay. For the common FDG radiotracer, the half-life of fluorine-18 is approximately 110 minutes, or just under two hours. Most of the radioactivity from FDG is typically gone from the body within 6 to 12 hours, reaching negligible levels within 24 hours.
Minimizing Exposure to Others
While the amount of radioactivity after a PET scan is temporary and low, some precautionary measures are recommended to minimize potential exposure to others. Staying well-hydrated and frequent urination helps flush the radiotracer out of your system more quickly. It is also advisable to practice good hygiene, such as washing hands thoroughly after using the toilet.
For a period of approximately 6 to 12 hours after the scan, it is generally recommended to limit prolonged close contact with pregnant women, infants, and young children. This includes avoiding close physical proximity like lengthy cuddling or holding. If close contact cannot be completely avoided, maintaining some distance is suggested. In some cases, sleeping in a separate bed from these sensitive groups for the first night may be advised.
Putting Radioactivity into Perspective
The radiation dose from a PET scan, while involving a radioactive substance, is generally considered small and the medical benefits often outweigh the minimal risks. The amount of radiation exposure from a PET scan, especially a PET/CT combination, can range from 7 to 25 millisieverts (mSv). The average person is exposed to about 3 mSv of natural background radiation annually from sources like cosmic rays, soil, and radon gas.
A PET/CT scan’s radiation dose is roughly equivalent to about 8 years of average background radiation exposure. For comparison, a standard chest X-ray exposes a person to about 0.1 mSv, equivalent to about 10 days of natural background radiation. The doses used in PET scans are carefully controlled and fall within safe limits.