Positron Emission Tomography (PET) scans are crucial diagnostic tools that allow physicians to visualize the body's tissues and organs at a cellular level. By detecting radiation from the emission of positrons, PET imaging can reveal how well your tissues and organs are functioning. While PET scans cannot definitively confirm cancer, they play a vital role in diagnosis and monitoring the effectiveness of cancer treatment. Modern PET scans often include a Computed Tomography (CT) scan or, in some cases, Magnetic Resonance Imaging (MRI), resulting in PET-CT or PET-MRI scans, respectively. To ensure the accuracy and clarity of these images, proper preparation, including dietary adjustments, is essential.
Understanding PET/CT Scans
A PET/CT scan combines the strengths of two imaging techniques: PET and CT. CT imaging uses special X-ray equipment to produce multiple images of the inside of the body, providing detailed anatomical information. PET scanning, on the other hand, utilizes radiotracers - molecules linked to small amounts of radioactive material - to reveal metabolic activity. By combining information about the body’s anatomy and metabolic function, a PET/CT scan provides a more detailed picture of cancerous tissues than either test does alone. Aside from detection, a PET/CT scan can also determine whether the disease has spread to other parts of the body.
The process begins with the administration of a radiotracer, typically through intravenous access or a port. The radiotracer accumulates in areas of the body that are subject to examination, including cancerous tumors and areas of inflammation. The radiotracer’s emissions are detected by a PET/CT scanner, which produces images and provides molecular information.
Why Diet Matters Before a PET Scan
The most common type of PET scan uses fluorodeoxyglucose (FDG), a radioactive sugar, as a radiotracer to examine sugar metabolism. Because the radioisotope used in PET imaging, fluorine 18 fluorodeoxyglucose or 18F-FDG, is a glucose transporter, it will go to any metabolically active areas in the body. If glucose levels are elevated from food or drink consumed prior to the test, the level of insulin will increase. When insulin levels increase, our muscles tend to “suck up” glucose to use in our daily activities. Therefore, adhering to specific dietary guidelines before a PET scan is crucial for optimal imaging. These guidelines primarily aim to control blood sugar levels and minimize interference with the radiotracer's uptake. For ideal imaging, it is requested that PET scan patients have nothing to eat or drink for a specified period prior to their scan and have a normal fasting glucose level. If the fasting glucose levels are too high or too low, it may be necessary to reschedule the patient for another day to ensure that optimal images will be obtained.
Specific Dietary Guidelines
Here's a detailed breakdown of the dietary restrictions you'll likely need to follow:
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Fasting Period: Do not eat or drink anything, except water, for at least 4-6 hours before the exam. This helps ensure a normal fasting glucose level.
Permitted Items: Plain, unflavored, non-carbonated water is usually allowed.
Prohibited Items: Avoid sugary drinks, coffee, tea, juice, and any food items during the fasting period. Also avoid cough drops or gum.
Last Meal: Your last meal before the fasting period should include foods high in protein and plenty of water.
Additional Preparations
Besides dietary restrictions, several other factors can influence the outcome of a PET scan:
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Medications: Take your usual medications except diabetic medications, unless instructed otherwise. If you are diabetic, do not take diabetic medications for 12 hours before your scan. Any non-diabetic medications may be taken the day of your scan.
Physical Activity: It’s also important that you don’t exercise for 24 hours before your PET scan.
Clothing: If possible, do not wear clothing with belts, zippers, snaps or metal of any kind to your appointment. Try to stay warm on the day of your scan.
Hydration: It is important that you stay well hydrated. You should drink plenty of fluids after the test is finished.
Prior Scans: If you have had a prior PET/CT and or CT scan at another facility, you must bring a copy of that disc or discs.
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Medical Conditions: Please notify your physician if you are claustrophobic or pregnant.
The PET Scan Procedure: What to Expect
On the day of your appointment, plan to be in the PET/CT department for approximately 2 to 3 hours. Here's a general overview of the process:
Confirmation: The medication dose for your PET scan will not be ordered unless you confirm your appointment the day before. Someone will call you to confirm.
Arrival and Initial Assessment: The team will test your blood glucose level. We are looking for your blood glucose level to be between 70mg/dL and 200mg/dL. Nursing staff will place a small needle into a vein, usually in your arm or the back of your hand, to fit an intravenous line (a thin plastic tube) through which the liquid radioactive material is injected. If needed Port-A-Cath can be accessed.
Radiotracer Injection: When you arrive for your PET scan, your care team will start an IV line, and a radiotracer will be injected into your body. If the radiotracer is FDG and you are diabetic, your care team will check your blood sugar before starting.
Waiting Period: You will be asked to rest quietly on a recliner, avoiding movement or talking for approximately one hour. After the injection you may be given one cup of an oral barium contrast to drink. You will rest quietly in a reclining chair for 75 minutes. During this time, you may read, listen to music with headphones or read while the tracer goes through your bloodstream.
Bladder Emptying: You will be asked to empty your bladder right before the PET scan because having a full bladder can affect the quality of the images.
Scanning: Once you’re ready to start the scan, you’ll be asked to lie on your back on a padded table. The table will move into a large, tunnel-shaped scanner where the PET scan is performed. The CT scan is done first and takes less than 2 minutes. It’s very important to remain still during the test.
Post-Scan: The intravenous line will be removed before you leave. After your PET scan, a very small amount of the radiotracer will remain in your body. So, be sure to drink plenty of water to help flush it out of your system.
Results: A specialist will review your PET scan and share the results with your doctor.
Important Precautions After the Scan
After your test, you will be considered “radioactive” due to the injection you receive. We advise patients to stay away from children and pregnant women for 6 to 24 hours following the time of injection.
Potential Side Effects and What to Do
Commonly, contrast is injected into a vein to better define the images throughout the body. If the radiologist believes this is helpful, a small intravenous (IV) line is placed in an arm vein. Through this line, the contrast and the isotope will be injected. If you feel any symptoms such as nasal congestion, itchy eyes, hives, rashes, sneezing, restlessness, tremors, pain, nausea, vomiting, dizziness, please notify the staff immediately. The staff is prepared to handle these situations. If you feel any of these symptoms after you have left the department, please contact your doctor or go to the nearest emergency room.