Preparing for a PET scan starts about 24 hours before your appointment and involves a few straightforward steps: adjusting your diet, skipping exercise, and choosing the right clothes. Most of the prep is designed to keep your blood sugar stable, because the tracer injected during the scan is a sugar-based compound that lights up on the images. If your blood sugar is too high or your muscles are actively burning fuel, the tracer goes to the wrong places and the images come out blurry or misleading.
Fasting and Diet Before the Scan
You need to fast for at least 6 hours before your PET scan. During that fasting window, plain water is the only thing you can have. No coffee, no flavored water, no gum, no candy. Anything containing sugar or artificial sweeteners can raise your blood sugar or trigger an insulin response, both of which interfere with how the tracer distributes through your body.
Aim to drink at least half a quart of plain water before the scan, starting at least an hour before your appointment. Staying hydrated helps the tracer circulate and also makes it easier for the technologist to find a vein for the injection. If you have kidney or heart problems and your doctor has told you to limit fluids, follow those instructions instead.
Skip Exercise for 24 Hours
Avoid any strenuous physical activity for 24 hours before your scan. That means no running, jogging, cycling, or heavy lifting. Even a long vigorous walk counts. When your muscles work hard, they pull in more of the sugar-based tracer, which creates bright spots on the scan that have nothing to do with what your doctor is looking for. Resting the day before keeps the tracer available for the tissues your doctor actually needs to see.
What to Wear
Dress in warm, comfortable layers with no metal. Zippers, snaps, buckles, underwire bras, and jewelry all interfere with the scanning equipment and will need to come off. If you show up wearing them, the facility will ask you to change into scrubs, which is fine but adds a small hassle. Sweatpants, a pullover, and a sports bra or bralette are ideal. Layering matters because scan rooms are kept cold to protect the equipment.
Managing Diabetes on Scan Day
If you have diabetes, your scan requires a bit of extra planning. Most facilities will not proceed if your blood sugar is above 200 mg/dL, because elevated glucose competes with the tracer and degrades image quality.
If you manage your diabetes with oral medications like metformin, you can generally take them as prescribed. However, metformin increases tracer uptake in the bowel, which can create distracting signals on the images. Some facilities ask you to stop metformin for a day or two before the scan to reduce this effect. Ask your scheduling team about their specific protocol.
Insulin timing is more precise. Rapid-acting insulin should be taken no closer than 4 hours before the scan. Short-acting insulin needs a 6-hour gap. Intermediate or long-acting insulin should not be used on the day of the scan at all. Your care team will help you adjust your schedule, and late morning appointments tend to work best for people on oral diabetes medications since you can eat a normal breakfast and then begin your fast.
What Happens at the Appointment
Plan on the whole visit taking about two hours, even though the actual scan is a fraction of that. Here’s the general flow:
- Check-in and blood sugar test. A technologist will check your blood glucose with a finger stick. If it’s within range, you move forward.
- Tracer injection. You’ll get a small IV injection of the radioactive tracer. It doesn’t hurt more than a standard blood draw.
- Quiet rest period. After the injection, you sit or lie still in a dimly lit room for up to 60 minutes while the tracer circulates and settles into your tissues. No phone scrolling, no reading, no talking. Even small muscle movements can pull the tracer into your hands or jaw and muddy the images. This is usually the hardest part for people, so bring a blanket or ask for one.
- The scan itself. You lie on a table that slides slowly through the scanner. The machine is open on both ends and wider than an MRI, so it feels less confining. You’ll need to stay still, and the scan typically takes 20 to 30 minutes.
If You Get Anxious in Enclosed Spaces
PET scanners are not as tight as MRI machines, but they can still trigger claustrophobia. If you know enclosed spaces make you anxious, call your facility ahead of time. Many centers can prescribe a mild oral sedative for you to take before the appointment. If you do take a sedative, you’ll need someone to drive you home afterward, so arrange that in advance.
After the Scan
The tracer is mildly radioactive, but it breaks down quickly in your body. For 6 hours after the scan, avoid close contact with pregnant women and small children. Drinking extra water after the scan helps flush the remaining tracer through your kidneys faster. You can eat normally as soon as the scan is over, and most people go right back to their regular day.