A Positron Emission Tomography (PET) scan is a medical imaging procedure that provides functional information about the body’s tissues and organs. Unlike X-rays or CT scans, which show structure, a PET scan illustrates biological activity by utilizing a small amount of radioactive material, known as a radiotracer. This substance is introduced into the body, and the subsequent images help physicians diagnose or monitor various conditions. Patient concerns about residual radioactivity and the potential for exposure to others are common after the procedure. The duration of radioactivity is determined by the tracer’s properties and the body’s natural clearance mechanisms.
What Makes You Radioactive
The temporary radioactivity after a PET scan comes from the injected radiopharmaceutical, which is specifically engineered to be short-lived. In most PET scans, this substance is Fludeoxyglucose F 18, or F-18 FDG. This molecule is a modified glucose analog containing the radioactive isotope Fluorine-18 (F-18). The tracer mimics natural glucose and is rapidly taken up by cells with high metabolic rates, such as cancer cells, which appear as bright spots on the scan.
The total dose of the radiotracer is very small. Once inside the body, the F-18 isotope emits positrons that interact with electrons in the surrounding tissue. This interaction produces gamma rays that the PET scanner detects to create the images. The material is designed to decay quickly, ensuring minimal exposure to the patient.
The Short Half-Life of the Tracer
The radioactivity diminishes rapidly due to the short half-life of the F-18 isotope. Half-life describes the time required for half of the radioactive atoms in a sample to decay into a stable, non-radioactive form. The physical half-life of Fluorine-18 is approximately 110 minutes, or just under two hours. This means the amount of radioactive material remaining in the body is cut in half every 110 minutes.
The decay process continues exponentially, quickly reducing the total activity over time. After five half-lives, roughly nine hours, only about 3% of the initial radioactive material remains. Consequently, the radioactivity is considered negligible for practical purposes within 10 to 20 hours after the injection.
This rapid physical decay is the main mechanism by which the residual radiation disappears from the body. By the day following the PET scan, the amount of F-18 remaining has decayed to an essentially undetectable level. The short half-life is a deliberate feature of the tracer, minimizing the duration of radiation exposure.
How Your Body Clears the Material
While physical half-life accounts for the majority of the reduction, the body also actively clears the material physiologically, further speeding up the process. A significant portion of the F-18 FDG radiotracer that is not taken up by cells remains circulating in the bloodstream. This un-metabolized tracer is predominantly eliminated through the renal system.
The kidneys quickly filter the circulating tracer, which is then excreted through urination. Approximately 20% of the total F-18 activity is cleared renally within the first two hours after injection. This biological clearance helps reduce the overall radiation burden. Patients are encouraged to drink plenty of fluids after the scan, as increased hydration promotes frequent urination and flushes the remaining tracer out more rapidly.
Patient Safety Guidelines After the Scan
Because a small amount of radioactivity is present immediately following the procedure, specific safety guidelines are recommended for the first few hours. These precautions minimize radiation exposure to sensitive individuals, such as pregnant women and young children. Most institutions recommend limiting close or prolonged contact with these groups for about six to twelve hours after the scan. This includes avoiding holding a baby, sitting next to a small child for an extended period, or sleeping in the same bed as a pregnant partner during this initial window.
Maintaining good hygiene is also important to reduce exposure from bodily fluids containing the tracer. Patients should use a toilet and flush twice after each use to ensure residual material is fully cleared. Thorough handwashing after using the restroom is advised to prevent accidental transfer of the tracer. By the following morning, the radioactivity level will have decayed significantly, and these restrictions can generally be lifted.