# PET/CT explained: what the tracer does and how to prepare *One exam shows where something is and how active it is. The tracer, the exams we do at Central, the 6-hour fasting rule, and what the visit looks like.* October 5, 2026 ยท Desert Imaging If your doctor has ordered a PET/CT, the name alone can be a lot. Two acronyms, a radioactive tracer, and instructions about not eating. Here is what the exam is, why the preparation matters, and what the visit to our Central center looks like from the moment you arrive. ## Two scans in one machine PET/CT joins two scans in a single machine. CT, computed tomography, takes detailed X-ray pictures that show exactly where things are: the size and position of an organ, a lymph node, a spot in the lung. PET, positron emission tomography, uses a small amount of a radioactive tracer to show how active the cells are in those same places. Together they can point to disease that may not show clearly on either scan alone. For most exams the tracer is a form of sugar called FDG, given through a vein. Cells that are working hard take up more sugar than cells at rest, and cancer cells tend to use more than healthy cells, so they light up on the PET pictures. The CT pictures taken in the same position tell the radiologist precisely where that activity sits. Doctors use PET/CT to diagnose cancer, to find how far it has spread, to check whether a treatment is working, and to look again after treatment ends. The exam also has uses beyond cancer, which is where the other tracers come in. ## The exams we do At Central we do: - **Whole body**, the most common exam, for staging and follow-up. - **Brain**, for questions about the brain's activity. - **Limited area**, when your doctor wants one region looked at closely. - **PSMA PET**, prostate specific membrane antigen, for prostate cancer. - **DOTATATE PET** with gallium-68, for neuroendocrine tumors. - **Cardiac PET/CT** with ammonia, to measure blood flow to the heart muscle at rest and under stress from a medication. It is a stress test done with PET, useful when a treadmill test is hard or an earlier stress test was unclear. Which one you are having is written on your order. PET/CT needs a doctor's order and, with most insurance plans, approval from your insurance before the visit. Your doctor's office usually starts that process. ## Why you cannot eat before The FDG tracer is sugar. If you have eaten, your muscles and organs are busy taking up the sugar from your meal, and the tracer spreads everywhere instead of concentrating where the radiologist needs to see it. Blood sugar has to be below a set level for the pictures to read correctly. So the rule is nothing to eat for 6 hours before your appointment, water only. For a morning appointment that means nothing after midnight. For an afternoon appointment you may have a light breakfast that is low in carbohydrates and sugar, but no coffee and no fruit. No gum, mints, cough drops or cough syrup either, even the sugar-free kind. If you have diabetes, check with your doctor about your medication ahead of time, because the timing matters for both your blood sugar and the pictures. Bring any prescription medicine with you so you can take it after the scan. ## What the visit looks like Arrive an hour before your scan time. During that hour the staff registers you, reviews your medications, and places a small IV in your arm. The tracer goes in through the IV, and then you rest quietly while it spreads through your body. That waiting is part of the exam, not a delay. The scan itself takes about 30 minutes. You lie still on a padded table that moves slowly through a wide, open ring, with most of your body outside it at any moment. The whole visit is about an hour. You can eat and drink right after. Drinking plenty of water for the rest of the day helps clear the tracer. The amount of tracer is small, and the cardiac tracer is not a dye or contrast. Tell us before the exam if you are or could be pregnant, or if you are breastfeeding. ## Where and who PET/CT is at our Central center, 7812 Gateway Blvd. East, Suite 120, El Paso, TX 79915, on an 80-slice scanner. A board-certified radiologist here in El Paso reads the exam. The report goes to the doctor who ordered it and appears in your patient portal, and your doctor goes over what it means for you and what comes next.