PET Scan: Purpose, Preparation, Procedure, Results and Risk

A positron emission tomography, also known as a PET scan is an imaging test that uses radiation to show activity on a cellular level within the body.

Updated: June 16, 2022

A positron emission tomography, also known as a PET scan is an imaging test that uses radiation to show activity on a cellular level within the body. It not only shows the features of an organ, but also gives a picture of how it is functioning. A PET scan can produce multidimensional, color images of the functioning of the human body when combined with a CT or MRI scan.
A PET scan is used to diagnose certain health conditions, to plan treatment, to find out how an existing condition is developing, and to see how effective a treatment is.
The scan is not painful, but patients should not consume any food for at least 4 to 6 hours before a scan. They should drink plenty of water.
In a PET scan, a machine detects radiation that is emitted by a radiotracer. The scan uses a special dye that has radioactive tracers. A radiotracer consists of radioactive material that is tagged to a natural chemical, such as glucose. Depending on the part of the body is being examined, these tracers are either swallowed, inhaled, or injected into a vein in your arm.Your organs and tissues then absorb the tracer which help your doctor to see how well your organs and tissues are working when highlighted under a PET scanner. The tracer will collect in areas of higher chemical activity. As a higher level of chemical activity is commonly associated with disease, these activity of tracers will be helpful in diagnosing the disease.These areas of disease will show up as bright spots on the PET scan. Active areas are bright on a PET scan which are known as hot spots.
The areas will be less bright where cells need less energy and are called as cold spots. As cancer cells are very active in using glucose, compared with normal cells, a radiotracer made with glucose will light up areas of cancer.
Fluorodeoxyglucose (FDG) is an example of a glucose-based radiotracer. Here, radioactive fluoride molecules are tagged to glucose to make a radiotracer. Oxygen can be also be used instead of glucose.

Purpose of PET scan:

Usually a PET scan is done to inspect your blood flow, your oxygen intake, or the metabolism of your organs and tissues. It is most commonly used in cancer treatment, neurology, and cardiology. It show problems at the cellular level, providing the best view of complex systemic diseases.
PET scans are most commonly used to detect:

  • cancer
  • heart problems
  • brain disorders, including problems with the central nervous system (CNS)


As cancer cells have a higher metabolic rate than noncancerous cells, it will show up as bright spots on PET scans. PET scans are not only useful in detecting cancer, but also helpful for seeing if the cancer has spread or if a cancer treatment is working.
It is also done to check for a cancer recurrence. It is possible for noncancerous conditions to look like cancer on a scan. Therefore it is very important that these scans should be read carefully by your doctor for a proper diagnosis. It is also common for solid tumors to fail to appear on PET scans. A PET scan can give an idea of how well chemotherapy is working.

Heart problems:

PET scans can detect the areas of decreased blood flow in the heart. During the scan the healthy heart tissue will take in more of the tracer than unhealthy tissue or tissue that has decreased blood flow. So, different colors and degrees of brightness on the scan will indicate different levels of tissue function.

Brain disorders:

The main fuel of the brain is glucose. During PET scans, the tracers will be attached to glucose there by detecting which areas of the brain are utilizing glucose at the highest rates. Looking in to the scan, the doctor will able to know how the brain is working, and check for any abnormalities.
PET scans are used to help diagnose and manage many central nervous system (CNS) disorders, including:

Alzheimer's disease:

By measuring the uptake of sugar in specific parts of the brain, it can help diagnose Alzheimer's disease. Brain cells that are affected by Alzheimer's tend to use glucose more slowly than normal cells.


PET can reveal the part of the brain which is affected by epilepsy. This can help doctors decide on the most suitable treatment. It can also be useful if surgery is necessary.


The affected area of the brain can be detected and hence the cause of depression.

Head trauma: 

The affected area of the brain can be detected by PET scan in case of serious injuries.

Parkinson's disease:

As the affected area of the brain can be detected by PET scan, clumps of specific substances within brain cells can be detected which are the microscopic markers of Parkinson's disease.
A CT or MRI scan can assess the size and shape of body organs and tissue. But how these work cannot be assessed by these procedures. A PET scan can show how an organ works, but without a CT or MRI image, it can be difficult to pinpoint the exact location of activity within the body. A PET scan combined with a CT or MRI scan, can show how a part of the body is working.

Preparation of PET scan:

Inform your doctor about any prescription, over-the-counter (OTC), or supplemental medications you are taking. You may be asked to refrain from strenuous physical activity, such as exercise, one or two days preceding the test.
You will be asked to stick to a low-carbohydrate, no-sugar diet a day before the test. Certain food and beverages such as cereal, pasta, bread, rice, milk and yogurt, whether dairy or nondairy, fruit and fruit juices, alcohol, caffeinated beverages and candy should be avoided before one day. However you can eat meat, tofu, nuts, and non starchy vegetables.
Hours before the test don't eat or drink anything if you are receiving anesthesia for the procedure. You can drink only a few sips of water if you need to take any medications.
You need to still refrain from eating anything for six hours before your scan, if you are not receiving anesthesia. Also avoid chewing gum or sucking on hard candy, cough drops, or mints.
Just before the procedure, you will be asked to change into a hospital gown and to remove any metal objects as these can interfere with the scan results. These items include jewelry, glasses, and dentures. However, medical devices such as pacemakers and artificial hips won't affect your results.
You should also inform your doctor about any medical conditions you have. As the test may be unsafe for fetus, inform your doctor if you are pregnant or believe you could be pregnant. As you won't be able to breastfeed for 24 hours after the test, you may need to pump and store your breast milk 24 hours prior to the procedure if you are breastfeeding. You will get special instructions for test preparation if you have diabetes. This is because fasting could affect your blood sugar levels. You will be told to take your normal dose of insulin and eat a light meal four hours before your scan.

Procedure PET scan:

A PET scan is normally an outpatient procedure. You will get tracers either through a vein in your arm, through a solution you drink, or in a gas you inhale before the scan. As it takes time to absorb the tracers in your body, you will have to wait about an hour or more before the scan begins. It will depend on the area of the body being scanned for the absorption of traces completely. You should limit any movement, relax, and try to stay warm while waiting for the scan. You should avoid television, music, and reading if you are undergoing a brain scan.
During the scan you have to lie on a narrow table attached to a PET machine and the table glides slowly into the machine so that the scan can be conducted. You may be asked to hold your breath for several seconds and need to lie still during the scan. You will hear buzzing and clicking noises during the test. During the scan, the machine takes images. Depending on which part of the body is being scanned, this should take about 30 to 45 minutes.
It is not painful. If the patient feels unwell, they can press a buzzer to alert the staff. The entire testing procedure typically takes about 2 hours. Most patients can go home as soon as the scan is finished. Patients should consume plenty of liquid in order to flush the radioactive drugs out of their system more quickly. Generally, all tracers leave your body after two days. Because radioactive material will remain in your body for about 12 hours, you should limit your contact with both pregnant women and infants during this time.


A trained specialist called radiologist will interpret the PET images and share the information with your doctor. The results are usually ready for your doctor within two business days. Your doctor will discuss the results with you at your follow up appointment.


The PET scan involves radioactive tracers. But the exposure to harmful radiation is minimal. The radiation levels are too low to affect normal processes in your body.
The risks of the test are minimal compared to the benefits. The results are much more beneficial in diagnosing serious medical conditions. The tracer is essentially glucose with the radioactive component attached. This makes it very easy for your body to eliminate the tracers, even if you have a history of kidney disease or diabetes.
There is a small possibility of an allergic reaction to the tracer. Therefore, people who are allergic to iodine, aspartame, or saccharin should alert their doctor.
Usually a tracer made up of diluted barium sweetened with saccharin can be used for people who can't have an iodine tracer.
People who are most likely to have an allergic reaction to the iodine tracer if they have

  • a history of allergic reactions to PET scans
  • allergies
  • kidney disease
  • heart disease
  • dehydration
  • the blood cell disorders sickle cell anemia, polycythemia vera, and multiple myeloma
  • asthma
  • a drug regimen that includes beta-blockers, nonsteroidal anti-inflammatory drugs (NSAIDs), or interleukin-2 (IL-2)

As radiation isn't considered safe for developing fetuses, you shouldn't get a PET scan if you are pregnant or think you may be pregnant. Any woman who is pregnant or breastfeeding should tell her doctor straight away before having a PET scan. The women should follow directions for pumping and discarding breast milk, if she is breastfeeding. Also ask the doctor when it is safe to resuming breastfeeding based on the test performed.
As the radioactivity poses a small risk, a patient may be advised to stay away from pregnant women, infants, and young children for a few hours following a PET scan.
Other risks of the test include discomfort if you're claustrophobic or uncomfortable with needles.The injection may also lead to symptoms such as bleeding, bruising, or swelling.

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