A positron emission tomography (PET) scan is a diagnostic imaging test that reveals how your tissues and organs are functioning by measuring metabolic activity in the body. Unlike other imaging methods that focus on anatomical structure, a PET scan provides molecular information to help detect diseases such as cancer, heart conditions, and neurological disorders. Accurate results depend entirely on strict adherence to dietary and activity restrictions beforehand. Failing to follow these guidelines can interfere with the tracer’s distribution, leading to poor image quality and requiring the procedure to be rescheduled.
Understanding Glucose and the PET Scan Tracer
The core of the dietary requirement lies in the tracer used, which is most often Fluorodeoxyglucose (FDG). FDG is a glucose analog, meaning its molecular structure is nearly identical to natural sugar, allowing it to mimic glucose and be absorbed by the body’s cells. Cells with high metabolic rates, such as cancer cells, inflammatory cells, and active areas of the brain and heart, take up glucose rapidly.
The goal of the scan is to have the FDG tracer accumulate in these metabolically active areas, which then emit signals detected by the PET scanner. Once inside a cell, FDG is chemically trapped because it cannot be fully metabolized, causing it to accumulate and appear brightly on the images. If you consume foods high in sugar or carbohydrates before the scan, your body’s natural glucose levels spike. This surge of natural glucose competes directly with the injected FDG tracer for cellular absorption, leading to poor distribution and potentially an inaccurate scan.
The Approved Pre-Scan Diet
The essential dietary strategy for the 24 hours leading up to the procedure is consuming a high-protein, low-carbohydrate diet to maintain low blood glucose levels. This minimizes circulating natural glucose, allowing the FDG tracer to be absorbed more effectively. Acceptable protein sources include unseasoned meats such as chicken, turkey, fish, and beef, as well as eggs, which provide necessary calories without spiking glucose.
To further limit carbohydrate intake, select non-starchy, low-carbohydrate vegetables:
- Spinach
- Broccoli
- Green beans
- Cauliflower
- Asparagus
Small quantities of high-fat dairy, such as butter and hard cheeses, are permitted as they contain minimal carbohydrates. Drinking plenty of plain water is also encouraged in the 24 hours prior to the scan, as proper hydration helps the kidneys flush the non-metabolized radioactive tracer out of the body.
Foods and Beverages to Strictly Avoid
Strict avoidance of all high-carbohydrate and high-sugar items must begin 12 to 24 hours before the appointment. This includes simple carbohydrates like bread, pasta, rice, cereals, and potatoes, as well as all fruits, fruit juices, processed foods, candy, cookies, and sweetened beverages like soda.
Several non-food items must also be eliminated because they can affect metabolism or muscle activity, which interferes with the tracer’s uptake. Caffeine and alcohol should be avoided for at least 24 hours, and sometimes longer, as they can alter the body’s metabolic state. Nicotine products, including tobacco, must also be avoided for 24 hours before the scan.
In addition to dietary restrictions, strenuous physical activity, such as running or heavy lifting, should be avoided for 24 to 48 hours before the procedure. Exercising causes muscles to take up more glucose, which draws the FDG tracer away from the intended target areas and can create false-positive readings on the scan.