What Is a PET Scan?

What to expect when undergoing this test

Positron emission tomography (PET) is a type of imaging technology used to evaluate how your tissues and organs work at the cellular level. It involves the injection of a short-acting radioactive substance, known as a radiotracer, which is absorbed by biologically active cells. You are then placed in a tunnel-like device that is able to detect and translate the emitted radiation into three-dimensional images. By identifying abnormalities in the metabolism of a cell, a PET scan can diagnose and assess the severity of a wide range of diseases, including cancer, heart disease, and disorders of the brain.

what to expect during a PET scan
Illustration by Emily Roberts, Verywell

Purpose of Test

Positron emission tomography has a broad range of diagnostic applications but is typically ordered if your doctor either suspects cancer or that a cancer may have spread. It is routinely used to assess the status of your heart prior to bypass surgery, especially if other imaging tests are inconclusive. It is also commonly ordered if early Alzheimer's disease is suspected or to evaluate the brain prior to surgery to treat refractory seizures.

Beyond these indications, a PET scan is also commonly used to stage cancer, to evaluate the extent of damage following a heart attack or stroke, and to monitor your response to cardiovascular, neurological, or cancer treatments.

PET differs from CT and MRI in that it examines active, living cells and evaluates how they function. By contrast, CT and MRI are used to detect damage caused by a disease. In essence, PET looks at how your body responds to a disease, while computed tomography (CT) and magnetic resonance imaging (MRI) look at the damage caused by one.

Among its many functions, PET can measure blood flow, oxygen intake, how your body uses glucose (sugar), and the speed by which a cell replicates. By identifying abnormalities in cellular metabolism, a PET scan can detect the early onset of a disease well before other imaging tests.

Types

PET can be used to diagnose different conditions depending on the type of radiotracer used. The most common tracer, known as fluorodeoxyglucose (FDG), is used in 90 percent of PET scans, the procedure of which is commonly referred to as FDG-PET.

When injected into the bloodstream, FDG is taken up by glucose molecules in cells. Because cancer cells multiply rapidly and do not undergo programmed cell death like normal cells, they will absorb far more FDG in the course of metabolizing sugar.

FDG can also be used to highlight areas of low metabolic activity caused by the obstruction of blood flow. Similarly, FDG-PET can spot changes in oxygen and glucose levels in the brain consistent with disease, impairment, and psychiatric illness.

Other types of radiotracers highlight cellular abnormalities not detected by FDG. These include:

  • 11C-metomidate used to detect adrenocortical tumors (those occurring in hormone-producing cells of the adrenal cortex)
  • Fluorine-18 used to identify blood clots
  • Fluorodeoxysorbital (FDS) used to diagnose bacterial infections
  • Fluorodopa used to detect neuroendocrine tumors (those occurring in hormone-producing cells of the nervous system)
  • Gallium-68, also used to detect neuroendocrine tumors
  • Nitrogen-13 and oxygen-15 used to detect impaired blood flow

There are well over 40 different radiotracers used for PET scanning purposes with more being developed every day.

Conditions Diagnosed

PET is primarily used to diagnose cancer, cardiovascular disease, and neurologic disorders.

For cancer, PET is especially useful as it can scan the entire body and pinpoint both a primary tumor and areas of metastasis (where the cancer has spread). With that being said, not all cancers can be detected by PET.

Those that can include:

For cardiovascular disease, a PET scan can reveal areas of decreased blood flow to the heart, brain, or lungs. By viewing the effects of circulatory impairment, your doctor can make the most appropriate treatment choice, including angioplasty or cardiac bypass surgery.

PET can also help predict the likelihood of a heart attack or stroke by detecting and measuring the hardening of arteries (atherosclerosis).

Among the cardiovascular conditions the test can diagnose:

For neurologic disorders, a PET scan can be used to measure brain activity in relation to areas of high and low radioactivity. Since the brain requires large amounts of glucose and oxygen to function, any shortages can easily be detected on a scan.

Among the neurologic disorders a PET can help diagnose:

In addition, PET can be used to detect bacterial infections, most specifically enterobacterial types associated with endocarditis, septic arthritis, osteomyelitis, and central nervous system infections.

Combination Scanning

When constructing a diagnosis, there is an advantage to looking at both the cause and consequence of a disease. It is for this reason that PET is frequently combined with CT or MRI, an approach referred to as either special views or co-registration. Doing so provides the doctor with both anatomic (physical) and metabolic (biochemical) information.

Modern PET scanners are now available with integrated CT scanners (PET-CT) which can create two sets of precisely matched images. There are also integrated PET-MRI scanners that are currently only used for head and brain imaging.

Risks and Contraindications

A PET scan is painless and poses few risks. The scanner itself does not emit radiation, and the amount of radiotracer used for the imaging is so small as to not require the use of standard radiation precautions.

Since the radiotracer is essentially glucose with a radioactive isotope attached, the drug half-life is extremely short. Some of the agents have a half-life as short as two minutes (such as oxygen-15), while others may be active for up to two hours (such as with FDG). In most cases, the drug will be in and out of your system within a day.

While the injection itself may cause localized pain and swelling, allergic reactions are rare, and there are no outright contraindications to the procedure, including pregnancy.

The only other concern—and, in some ways, the most significant—is the risk of claustrophobia. If being placed inside the tube-like device makes you nervous, let your doctor know in advance. In extreme cases, the doctor may prescribe a mild sedative, such as low-dose Valium (diazepam) or Ativan (lorazepam), to help reduce anxiety.

PET and Obesity

A PET scan may not be possible if you are obese and unable to fit into the scanning chamber (gantry). The scanning table has a maximum weight capacity of 425 to 450 pounds, while the gantry has a diameter of only 27.5 inches (70 centimeters). Image quality may be degraded if weight and size limits are exceeded.

Moreover, the radiotracer dose may not be adequate enough to achieve a quality image in those with increased body mass. While upping the dose may help, it cannot be raised beyond a certain point due to potential harm.

An increased body mass can also cause a more diffuse scattering of radiation, further reducing image quality. Newer multidetector scanners are able to overcome some of these concerns, while efforts are being made to develop PET systems with a 35-inch (95-centimeter) gantry.

PET-CT Precautions

If undergoing a combination PET-CT scan, the iodine-based contrast dye used for the CT component can cause side effects, including nausea, vomiting, headache, itching, flushing, and mild rash. In rare cases, a serious, all-body allergic reaction known as anaphylaxis may occur.

It is important to advise your doctor if you have an iodine allergy, a seafood allergy, or have had a bad reaction in the past to a contrast dye used for a CT or X-ray study.

In general, CT scans are not recommended during pregnancy unless the benefits of the scan clearly outweigh the potential risks.

PET and Diabetes

You can have a PET scan if you have diabetes but need to ensure that your blood glucose levels are between 4 and 10 millimoles (mmol) before testing. If your glucose levels are high, the radiotracer will not be taken up efficiently in cells. If your insulin is high, it will cause an increased uptake of the radiotracer and throw off the results.

If your blood sugar is uncontrolled, you need to advise your doctor in advance so that special dietary or pharmaceutical measures may be used to achieve control.

Before the Test

Preparation for a PET scan can vary slightly based on the aims of the procedure. The main goal is to restrict the intake of carbohydrates and sugar to ensure your blood glucose levels are normal and that the radiotracer will be evenly distributed throughout the body.

Timing

PET scans generally take around an hour and a half to perform from start to finish, including waiting time. However, there are variations that can take several hours.

You will want to arrive at least 30 minutes in advance so that you can settle in comfortably without rushing. It is important to arrive on time so that you can receive the radiotracer an hour before the actual scan. Late arrivals can throw off an entire day's schedule and may leave the staff with no other option but to reschedule you. 

As you will need to stop eating prior to the test, most scans are scheduled for the morning.

Location

PET scans are performed on an outpatient basis, most commonly in the nuclear medicine imaging unit of a hospital or in a dedicated facility. The room itself is called either the scanning room or procedure room.

The PET scanner is a large machine with a doughnut-shaped hole in the center, similar to a CT or MRI unit. Within the scanner are a series of ring-shaped sensors that detect subtle radiation emissions from your body.

The signals are translated into digital images into a separate control room. The procedure will be monitored the entire time by a skilled technologist who will communicate with you through a two-way speaker.

What to Wear

Depending on the part of the body being examined, you may be asked to undress partially or fully. While you may be provided a secure locker to store your belongings, it is best to leave any valuables at home.

If you are undergoing a PET-CT or PET-MRI scan, be aware that metal objects can interfere with the imaging. As such, avoid wearing clothes with snaps, zippers, buckles, or rivets. You should also leave jewelry, piercings, hairpins, or non-essential dental appliances at home. 

A pacemaker or artificial joint will not affect a PET-CT. Similarly, many modern pacemakers and implants are MRI-safe (also known as MRI-conditional).

Food and Drink

You will need to begin a restricted low-carbohydrate, no-sugar diet 24 hours before the scan to ensure that your blood glucose remains within normal limits. 

You won't necessarily be restricted in the amount you eat but will have to avoid high glycemic index (GI) foods, which raise your blood sugar. You will mainly consume proteins (such as meat, nuts, or tofu) and non-starchy vegetables.

Foods to avoid include:

  • Bread and cereals
  • Caffeine
  • Candy, including chewing gum, cough drops, and mints
  • Dairy, including milk, yogurt, and cheese
  • Fruit and fruit juices
  • Rice and pasta
  • Sweetened beverages

Six hours before the scan, you need to stop eating altogether. You will still be able to drink water and take most of your medications as prescribed. If a medication requires food, speak with your doctor; you will likely need to delay the dose until after the test is complete.

Four hours before the scan, you will need to stop taking insulin or any oral medications used to control diabetes. Your doctor will likely offer additional dietary instructions based on your glucose control.

Other Restrictions

In addition to food, you need to avoid strenuous exercise 24 hours in advance of the test. This includes any activity that significantly increases your heart rate. Doing so can affect your body’s insulin response and cause a drop in blood sugar (hypoglycemia).

What to Bring

In addition to your ID and health insurance cards, you may want to bring a magazine or audiobook with you since you will be resting for an hour after the radiotracer injection. If your child is having the PET scan, bring some toys or a storybook to keep the child calmly entertained. Avoid action video games, which can get the child overexcited and affect blood glucose levels.

If you have diabetes, be sure to bring your glucose monitor. While the lab will test your blood in advance of the scan, you may want to check yourself after since you will not have eaten for some time.

If the procedure makes you nervous, bring headphones and some calming music to listen to while you are being scanned. You can also check to see if the scanning room is equipped with audio music selections; many are.

Bring along someone to drive you home if you anticipate having to take a sedative for the procedure; you can consider having someone "on call" if you're unsure.

Cost and Health Insurance

A PET scan is costly, sometimes prohibitively so. Depending on where you live and the facility you use, a conventional PET scan may cost anywhere from $1,000 to $2,000. For a whole-body PET-CT scan, the price can jump well above $6,000.

It is no surprise, therefore, that a PET scan requires insurance pre-authorization. While many plans will grant approval for the diagnosis and staging of lung cancer, esophageal cancer, colorectal cancer, lymphoma, melanoma, and head and neck cancers, others will deny coverage for post-treatment scans and certain heart and brain investigations.

Even if they do, the co-pay or co-insurance costs alone can make the procedure unaffordable. Even if you have reached your out-of-pocket maximum, there is still no guarantee your insurance will grant approval. It is important, therefore, to understand the terms of your policy and how they specifically apply to the use of PET scans.

If you are turned down by your insurer, ask for the reason in writing. You can then take the letter to your state insurance consumer protection office and ask for help. Your doctor should also intervene and provide additional motivation as to why the test is essential.

If you are uninsured, shop around for the best price and speak with the facility about monthly payment options. Some facilities may offer a cash discount of 20 percent or more if you pay upfront. Don't be afraid to negotiate if it makes the difference between getting a vital test and not.

Other Considerations

If you are breastfeeding, you may want to pump breast milk ahead of time and keep it on hand until the radiotracer is fully cleared from your body. You can ask the doctor or technologist when to start breastfeeding again.

While the radiotracer in your breast milk is unlikely to do any harm to the baby, there is still not enough long-term research to conclude that it is entirely safe.

During the Test

To produce the most accurate PET results, you need to follow the pre-test instructions exactly. If you are unable to do so for any reason, let the medical team know when you arrive. In some cases, you may still be able to have the test. In others, you may need to reschedule.

The test will be conducted by a nuclear medicine technologist. A nurse may also be on hand.

Pre-Test

On the day of the test, after signing in and confirming your insurance information, you may be asked to sign a liability form stating that you are aware of the purpose and risks of the procedure. Be sure to advise the technologist or nurse if you are pregnant, have diabetes, have a known drug allergy, or are claustrophobic.

While the pre-scan procedures can vary based on the condition being diagnosed, they more or less follow similar steps:

  • After signing in, you will be taken to a changing room and asked to remove some or all of your clothing. A hospital gown will be provided to change into.
  • Once you have changed, you will be led to a sterile intravenous (IV) room where a nurse or technologist will record your height and weight and take a small blood sample to test your glucose levels.
  • If your glucose levels are OK, you will be approved to proceed and positioned on the examining table. A flexible IV catheter will be inserted into a vein in your arm or hand.
  • The radiotracer will then be injected through the IV line. You may feel a cool sensation moving up your arm when this happens, but there will generally be no other side effects. (For some procedures, an oral or inhaled radiotracer may be used in place of an injection.)
  • You will then need to relax in a quiet, reclined state for 60 minutes until the radioactive agent has fully circulated. If you are having a brain scan, you will need to minimize stimulation both before and during the test. This means avoiding TV, reading, music, etc.

    Throughout the Test

    After 60 minutes, you will be led to the scanning room and positioned on the scanning bed at the opening of the machine. Once positioned, the technologist will glide the bed into the PET chamber via remote control.

    You will then need to remain still while the scan is performed. There may be times when the technologist will ask you to hold your breath or adjust your position. During the scan, you will hear whirring and clicking sounds.

    If a PET-CT scan is performed, the CT scan will be performed first. The CT scan only takes around two minutes. The PET scan will follow and can take anywhere from 20 to 45 minutes, depending on the purpose and scope of the test.

    Some variations of the test can take longer. For example, some cardiac investigations may include a PET scan before and after exercise. Others may require additional radiotracers and drugs to be delivered during the procedure. In cases like these, a scan can take anywhere from two to three hours to complete.

    Post-Test

    Once the scanning is done, you will be asked to wait until the technologist is able to double-check the images. In some cases, you may be asked to return for additional scans. Don’t let this worry you. More often than not, the technologist simply needs a clearer image or to adjust your position to get a better visualization of the area of interest.

    Once the technologist has approved the scans, you can change back into your clothes. If you have diabetes, you will want to check your blood sugar levels. If the reading is high, let the medical staff know.

    After the Test

    Most people are able to drive themselves home after a PET scan. The only exception is if you took a Valium or Ativan in advance of the procedure. If so, you will need to be driven.

    You will not be radioactive to anyone who touches, kisses, or stands close to you. There is no recovery time, and you can return to your normal diet and routine unless your doctor tells you otherwise.

    If you experience unusual symptoms, such as nausea, headache, fever, vomiting, or rash, call your doctor immediately.

    Interpreting the Results

    The PET images will usually be sent to your doctor within 48 hours, along with a report detailing the normal and abnormal findings.

    The image will highlight "hot spots" where excessive amounts of radioactive isotopes have accumulated; these are areas of high cellular metabolism. While this may be suggestive of cancer, the spots are difficult to decipher and there may other explanations. Your doctor may require multiple tests to come to a definitive diagnosis.

    By contrast, areas with less radioactive accumulation are known as "cold spots." This indicates areas of low metabolic activity, often as a result of reduced blood flow or possibly tissue necrosis (tissue death). 

    Follow-Up

    PET scans are as useful for tracking the progression of a disease as they are for diagnosing it in the first place. They are especially helpful in assessing your response to cancer treatment as the tumors begin to shrink and go into remission.

    PET can also be used to evaluate the damage caused to the heart after a heart attack or the brain after a stroke. Doing so provides the doctor with a blueprint of functional tissue and can help predict your long-term outcome (prognosis).

    A Word From Verywell

    A PET scan is a sophisticated tool that helps us look beyond the damage caused by a disease to the way in which our body responds to it. By combining it with CT or MRI technology, doctors are given a more precise portrait of how advanced, aggressive, or likely a disease may be.

    While expensive, a PET scan is far less costly and invasive than exploratory surgery. As such, it is important to advocate for yourself if your doctor recommends the test, but your insurance company turns you down.

    In some cases, this may mean changing doctors, especially if the one you’re with is not advocating for you or is not a specialist in the field of medicine you need. Oftentimes, by moving to a larger dedicated practice—one that treats a lot of people with the same condition as yours—you will have the support, expertise, and systems needed to motivate for treatment.

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