A Positron Emission Tomography (PET) scan is an advanced diagnostic imaging procedure that shows how organs and tissues function at a cellular level. Unlike X-rays or CT scans, which show structure, a PET scan highlights metabolic activity using a small amount of radioactive material, called a radiotracer. This radiotracer, often fluorodeoxyglucose (FDG), is injected into the bloodstream and travels to areas with high energy consumption. The PET scanner detects the energy emitted by the tracer, creating detailed, three-dimensional images that reveal which tissues are more active.
Medical Conditions That Require a PET Scan
Physicians order a PET scan to gain functional insight into a disease process, especially when metabolic changes occur before structural ones. Applications concentrate across three major areas: oncology, neurology, and cardiology. Cancer cells, for example, have a significantly higher metabolic rate than normal cells, causing them to rapidly absorb the FDG radiotracer.
In oncology, the PET scan is used for initial cancer staging and determining if the disease has spread (metastasis). It is also effective for monitoring treatment response, revealing if a tumor is shrinking and has reduced metabolic activity following therapy. The scan can also help detect cancer recurrence by identifying small areas of renewed cellular growth after treatment is complete.
Neurologically, the test evaluates brain disorders that affect glucose metabolism. It helps differentiate Alzheimer’s disease from other forms of dementia by showing characteristic patterns of reduced glucose uptake in specific brain regions. PET scans can also pinpoint the focus of seizure activity in epilepsy patients or assess the extent of damage to brain tissue following a stroke.
In cardiology, the scan evaluates blood flow and assesses the viability of heart muscle tissue, particularly after a heart attack. Using specific tracers, doctors determine if a non-functioning area of the heart is stunned and potentially recoverable or permanently scarred. This information guides decisions about whether procedures to restore blood flow, such as angioplasty or bypass surgery, will be beneficial.
Patient Preparation Requirements
Accurate PET scan results rely on the patient following strict preparation requirements before the appointment. Since the most common radiotracer, FDG, is a sugar analog, patients must fast from all food and sugary liquids for a minimum of four to six hours prior to the exam. This fasting ensures the body’s natural glucose levels are low, preventing competition with the injected radiotracer and allowing efficient uptake by targeted cells.
Patients should drink plain water for hydration but must avoid all other beverages, chewing gum, mints, or hard candies, as these stimulate metabolic activity. Patients must also avoid strenuous exercise or repetitive physical activity for a full 24 hours before the scan. Physical exertion causes the radiotracer to accumulate in active muscle tissue, which can obscure the areas the doctor needs to evaluate.
Preparation for diabetic patients is more specific and requires careful coordination to ensure blood sugar control. High blood glucose levels, typically above 175 mg/dL to 200 mg/dL, compromise image quality because the excess sugar competes with the FDG for cellular uptake. Diabetic patients receive customized instructions on medication timing and may need to adjust insulin doses or diet to meet the required blood glucose range on the day of the scan.
The Physical Scanning Process
The total time spent at the facility for a PET scan appointment typically ranges from two to three hours. Upon arrival, a technologist confirms the patient’s preparation and inserts an intravenous (IV) line. A blood glucose test is performed immediately to verify that the sugar level is within the acceptable range for a quality scan.
The small dose of the FDG radiotracer is injected through the IV line. Following the injection, the patient enters an uptake period, usually lasting between 45 and 90 minutes. During this time, the patient must rest quietly in a dimly lit room, remaining still to allow the tracer to circulate and accumulate in the targeted tissues. Limiting movement and talking is important because muscle activity can draw the tracer away, potentially creating misleading scan results.
When the uptake period is complete, the patient moves to the scanner, which resembles a large, doughnut-shaped machine. The patient lies flat on a narrow table that slides slowly through the opening. The actual scanning time is relatively short, typically lasting between 20 and 45 minutes, and the patient must remain completely motionless. The process is painless, though some patients may feel minor discomfort from lying still for an extended time.
Conditions That Prevent Receiving a PET Scan
While PET scans are safe, certain physical or medical conditions can prevent the procedure. The most significant contraindication is confirmed or suspected pregnancy due to radiation exposure from the radiotracer. Healthcare providers often require a pregnancy test for women of childbearing age before administering the radioactive material.
Breastfeeding mothers are advised to pump and discard breast milk for a specific period after the scan, often several hours, until the radioactivity level is safe for the infant. Uncontrolled blood glucose is another common reason for rescheduling, as high sugar levels prevent the FDG tracer from being adequately absorbed. If a diabetic patient’s blood sugar is above the facility’s limit on the day of the exam, the scan is postponed until the level is controlled.
The physical inability to remain still for the required duration of the uptake and scanning process can also be a barrier. Movement during the scan severely blurs the images, making them unusable for diagnosis. In rare cases, severe renal impairment might be a consideration, depending on the specific radiotracer used, since the kidneys eliminate the material from the body.