What Is a PET Scan?

How and Why a PET Scan Is Done and Possible Side Effects

Updated on April 11, 2024

Medically reviewed by Lindsay Cook, PharmD

A positron emission tomography (PET) scan provides detailed three-dimensional images of your internal body organs and tissue, which helps diagnose a wide range of conditions.

Some people worry about the side effects of a PET scan because a short-acting radioactive substance is injected into your body to produce the images. However, there are very few risks associated with a PET scan.

This article describes what PET scans are used for, including the diseases they can help diagnose, and how the procedure is performed. It also explains the possible risks and what the PET test results mean.

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

Purpose of Test

Positron emission tomography (PET) is an imaging test that detects changes in metabolism and your body's biochemistry. The patterns of these changes can help diagnose different diseases.

PET scans use a radioactive drug called a radiotracer that is injected into your bloodstream. The tracer can help show if metabolic activity in tissue and organs is typical (normal) or atypical (abnormal) based on how much of the tracer is taken up by cells.

A PET scan can evaluate metabolic functions like blood flow, oxygen intake, how your body uses glucose (blood sugar), and the speed by which cells replicate.

Based on atypical changes, a PET scan can sometimes detect disease before other imaging tests, like computed tomography (CT) or magnetic resonance imaging (MRI), can.

PET is primarily used for the following purposes:

Cancer

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

Those that can include:

Cardiovascular Disease

For cardiovascular disease, a PET scan can reveal areas of decreased blood flow to the heart, brain, or lungs. By viewing adverse changes to your circulation, a healthcare provider can make the most appropriate treatment choices (such as deciding between angioplasty or cardiac bypass surgery).

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

Cardiovascular conditions PET can diagnose include:

Neurological Disorders

A PET scan can 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.

Neurologic disorders a PET can help diagnose include:

Other Applications

In addition, PET can detect bacterial infections, most specifically enterobacterial types associated with endocarditis, septic arthritis, and osteomyelitis.

Combination Scanning

When diagnosing a disease, there is an advantage to looking at both the cause and consequence of a disease. For this reason, a PET is frequently combined with CT or MRI to provide the healthcare provider with both anatomical (physical) and metabolic (biochemical) information.

Modern PET scanners are now available with integrated CT scanners (PET-CT) or MRI scanners (PET-MRI) which create two sets of precisely matched images.

How PET Scans Are Done

PET scans are performed on an outpatient basis in the nuclear medicine imaging unit of a hospital or a dedicated facility. The test is performed by a nuclear medicine technologist. A nurse may also be on hand.

Prepare for the Scan

In preparation for the scan, you will need to eat a low-carbohydrate, no-sugar diet 24 hours beforehand to keep your blood sugar within normal limits. As a general rule, you would avoid high glycemic index (GI) foods like bread, dairy, pasta, and juices that raise your blood sugar.

You need to avoid strenuous exercise 24 hours before the test, which can alter your normal insulin response and cause a drop in your blood sugar (hypoglycemia).

Six hours before the scan, you will need to stop eating altogether. Four hours beforehand, insulin or any other diabetes medications would also need to be stopped.

PET Scan Procedure

The PET scanner is a large machine with a doughnut-shaped hole in the center, similar to a CT or MRI unit. The procedure itself is relatively standard and performed as follows:

Once you have arrived and changed into a hospital gown, an intravenous (IV) line will be inserted into a vein in your arm to deliver the radioactive tracer, most commonly fluorodeoxyglucose (FDG).
You will need to relax in a quiet, reclined state for 60 minutes until the radioactive agent has fully circulated.
After 60 minutes, you will be led to the scanning room and positioned on the bed at the opening of the unit. 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.

The PET scan can take anywhere from 20 to 45 minutes to complete, depending on the purpose and scope of the test. Some variations of the test can take longer, particularly if a PET-CT or PET-MRI is performed.

Side Effects of PET Scan

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.

Risks and Contraindications

The side effects of PET scans are relatively minor and mainly involve pain and swelling at the injection site, Allergic reactions are rare. There are no outright contraindications to the procedure.

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 healthcare provider know. If needed, you may be given a mild sedative such as low-dose Valium (diazepam) or Ativan (lorazepam).

Obesity

A PET scan may not be possible if you are obese and unable to fit into the scanning chamber. Most scanning tables have a maximum weight capacity of 450 pounds and a chamber 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 to achieve a quality image if you are extremely overweight. While upping the dose may help, it cannot be raised beyond a certain point due to potential harm.

Newer multidetector scanners can overcome some of these concerns. A few manufacturers are also offering units with chambers as large as 30.5 inches (78 centimeters).

Pregnancy

PET can be performed during pregnancy. The tracer will not harm the fetus. If you are nursing, your healthcare provider may recommend that you avoid breastfeeding for 24 hours just to be safe. You can prepare by pumping enough breastmilk beforehand or using bottled formula.

On the other hand, a PET-CT scan may not be appropriate if you are pregnant. In general, CT scans are not recommended in people with pregnancy unless the benefits clearly outweigh the potential risks.

Diabetes

You can have a PET scan if you have diabetes but need to ensure that your blood glucose levels are under 200 mg/dL before scanning. If your glucose levels are high, the radiotracer will not be taken up efficiently in cells.

On the other hand, 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 healthcare provider in advance so that special dietary or pharmaceutical measures can be taken to achieve control.

Results of a PET Scan

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

The image may highlight "hot spots" where excessive amounts of tracers have accumulated; these are areas of high cellular metabolism. While this may be suggestive of cancer, there may be other explanations as well. Expert interpretation by an oncologist and lab pathologist is needed.

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

Summary

A PET scan is an imaging test that can help detect diseases based on changes in your body's metabolism. It involves a radioactive tracer that is injected into your bloodstream. The scanner measures metabolic activity based on how much or little of the tracer is taken up by cells.

PET scans can be used to detect different types of cancers, cardiovascular diseases, and neurological disorders. Side effects are few, and there are no absolute contraindications to the scan other than obesity due to the size and weight limitations of the units.

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