PET scan: purpose and results

What a PET Scan Can Catch Before Other Tests Can

PET scan: purpose and results

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- 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.

Illustration by Emily Roberts, Verywell

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 the function, rather than the structure, of living cells. 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.

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 transporter molecules in cells. Because cancer cells multiply rapidly and do not undergo programmed cell death 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)
  • 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 dotatate, 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.

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.

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 lihood 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:

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

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 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- 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.

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.

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 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.

You can have a PET scan if you have diabetes but need to ensure that your blood glucose levels are


PET scan: purpose and results

PET scan: purpose and results

A PET (positron emission tomography) scan is a medical imaging technique that provides useful information about the function of organs and tissues within the body. It involves the administration of small amounts of radioactive pharmaceuticals, also known as ‘tracers’. Tracers release invisible energy that can be detected by a special camera to produce 3-dimensional images.

Un plain X-rays and other medical imaging techniques, which only provide information on the structure of the body, PET scans can be used to monitor small functional changes over time and can even assess your response to treatment. This is useful in certain diseases, such as cancer, where functional changes may be occurring before any structural changes are evident.

Why is it done?

PET scans can provide detailed information about the function of your organs in one image.

Your doctor may order a PET scan to assess blood flow around your body, to differentiate healthy tissue from damaged tissue and to assess the metabolic function of tissues.

This is especially useful in assessing cancer, heart disease and brain disease where changes may show up on a PET scan before they appear on other types of imaging (e.g. CT or MRI).

How does it work?

PET scans involve the use of tracers, called radiopharmaceuticals, that are introduced into the body intravenously (by injection into a vein), orally (by mouth) or by inhalation.

As tracers break down, they release small positively charged particles known as ‘positrons’. Positrons interact with negatively charged electrons within the body and result in the production of 2 photons.

A PET scanner can detect these photon emissions to create images.

There are various types of tracers that can be used depending on the reason the procedure is being done. These include:

  • 18-fluorodeoxyglucose (FDG): This is the most commonly used tracer and is derived from glucose, a simple sugar. It is taken up into tissues in different concentrations depending on the metabolic rate of that tissue. It is most commonly used to assess cancer, but can also be used to assess brain diseases and heart diseases. Cancer cells have high metabolic activity and show up as ‘hot spots’ on the PET scan.
  • Gallium-68 PSMA: This is most commonly used for prostate cancer staging or restaging.
  • Gallium-68 Dotatate: This is most commonly used for neuroendocrine or carcinoid tumour staging or restaging.

Tracers accumulate in areas with high metabolic activity. These areas are known as ‘hot spots’, and appear brighter than the surrounding tissue on the PET scan images.

This can provide doctors with vital information about processes that are occurring within the body. For example, healthy heart vessels will take up more tracer than unhealthy areas.

So, if certain vessels of the heart are not taking up the tracer as well as other vessels, this may indicate reduced blood flow and raise suspicion of coronary artery disease.

Who should have a PET scan?

A PET scan can be used in the assessment of cancers, brain disease and heart disease.

PET scans for cancer

PET scans can be used in the early detection of cancer, monitoring of disease progression, assessing response to treatment and in detecting metastasis (spread of cancer to other areas of the body).

Radioactive glucose molecules (FDG) are the most common tracers used in the assessment of cancer. Cancer cells appear brighter than normal tissue on the images as they break down glucose at a higher rate. It is also useful in distinguishing between benign (non-cancerous) and malignant (cancerous) tumours.

Several types of cancers can be assessed using PET scans, such as:

  • Brain cancer
  • Head and neck cancer
  • Thyroid cancer
  • Colorectal cancer
  • Pancreatic cancer
  • Breast cancer
  • Prostate cancer
  • Lymphoma
  • Melanoma.

PET scans for heart disease

Doctors can use PET scans to diagnose coronary artery disease (CAD) and to identify areas of reduced blood flow due to narrowed arteries. A PET scan may also identify areas of the heart muscle that have been damaged following a heart attack.

The PET scan can distinguish between healthy heart muscle and damaged heart muscle. It can help determine whether there is damaged heart muscle that may be saved if blood flow can be restored.

Doctors can use this information to determine what treatment options are best for you, such as coronary artery bypass grafting surgery (CABG) or angioplasty.

PET scans for brain disease

PET scans can be used to assess brain diseases such as cancer, seizures and Alzheimer’s disease.

They can provide useful information on blood flow and oxygenation of the brain and highlight areas that aren’t working properly.

The brain uses glucose as its main source of fuel, and the PET scan will show up areas of the brain that are using the most radioactive glucose tracer and therefore are the most active.

What happens during a PET scan?

Before the procedure, the nuclear medicine scientist will have a chat with you about the procedure and determine if you have any underlying medical conditions that are important to know about before the procedure, such as diabetes. People with diabetes may need to follow special instructions.

The tracer will either be swallowed, injected or inhaled. You may need to wait 30-60 minutes for the tracer to spread around your body before you undergo the scan.

You will be required to wear a hospital gown and will be asked to lie still on a padded bed which is slid into the PET scanner. The scanner will proceed to take numerous images of your body. During this time, the scanner may produce noises such as clicks and buzzing sounds.

It is important that you do not move once inside the scanner otherwise the images produced may be blurry.

In most cases, doctors will combine computed tomography (CT) scan with the PET scan, known as ‘hybrid imaging’. The CT scan will be done first and usually takes around 10 minutes to complete. The CT scan can create detailed images of the structure of the body, while the PET scan provides information on the function of the body.

How long does a PET scan take?

The overall time you will spend in the nuclear medicine department or facility may be 2 to 3 hours. But you will only be required to lie on the scanner bed from anywhere between 10 to 40 minutes.

Some people may start to feel claustrophobic within the machine. If this happens, tell the doctor or nurse and they may give you medication to help you relax.

If you know you are claustrophobic, make sure you discuss this with your doctor before beginning the scan.

What happens after the scan?

The intravenous line will be removed and you should be able to go home.

  • Continue with normal daily activities: After you have gone home, you can continue with your normal activities. The tracer should not make you feel sick or drowsy.
  • Avoid contact with pregnant or breastfeeding women: You may release a small amount of radiation 6-12 hours after the procedure while the tracer is being removed by your body, so it is advised to avoid contact with pregnant or breastfeeding women.
  • Drink plenty of water: FDG is removed from your body via the kidneys and is present in urine. You should drink plenty of water following the procedure to quickly eliminate the tracer from your system. In some cases, the doctor may prescribe a diuretic tablet to help empty your bladder.

How do I prepare for a PET scan?

Your doctor will tell you exactly what you need to do before the PET scan.

Tell your doctor if:

  • You have an underlying medical condition, such as diabetes
  • You are taking any medications, including over the counter medications, vitamins or supplements
  • You have any allergies
  • You are pregnant or breast-feeding
  • You have had something to eat or drink (other than water) in the last 4-6 hours
  • You are claustrophobic.

Generally, you will be required to:

  • Avoid strenuous activity for 24 hours before the procedure
  • Avoid high carbohydrate and high sugar meals for 24 hours before the procedure. This includes alcohol, bread, rice, cereals, soft drinks and fruit juice
  • Fast for 4-6 hours prior to the procedure (including chewing gum or lollies), although plain water is allowed
  • Avoid talking for 20 minutes before the procedure
  • Remove all jewellery, watches and body piercings. You should not be wearing any metal during the scan, including any on bra straps.

Does a PET scan hurt?

If the tracer is introduced into the body intravenously, you may feel some discomfort from the injection process. The tracer itself is completely painless and you will not feel discomfort during the procedure.

Is a PET scan safe?

A PET scan is generally a safe procedure. Tracers introduced into the body are short-lived and are quickly eliminated. In the time taken for the tracer to be eliminated, you may emit a small amount of radiation. As a result, it is advised you avoid contact with pregnant or breastfeeding women.

In rare circumstances, some people may have a severe allergic reaction to the tracer. It is not possible to predict which people will have an allergic reaction unless they have previously had a reaction to a tracer. All PET scan centres will be prepared to deal with an allergic reaction.


The amount of radiation you will be exposed to during a PET scan is equivalent to 3 years of natural background radiation from the environment. However, with repeated scans, the radiation dose can accumulate. Your doctor will ensure the benefits of the procedure outweigh the negative effects so that you are not exposed to unnecessary radiation.

Can you have a PET scan if you are pregnant or breastfeeding?

Please inform your doctor if you are pregnant or breastfeeding. PET scans are generally not recommended for pregnant or breastfeeding mothers.

There is a small risk that your unborn baby will be exposed to radiation during the procedure. Studies are unclear on the exact amount of radiation your baby will be exposed to; however, it is generally low. Your doctor will only advise a PET scan if the benefits to the mother outweigh the harm to the baby. Otherwise, your doctor will consider alternative forms of imaging.

The tracer accumulates in the bladder after the procedure. The bladder is in close proximity to the uterus and is a primary source of radiation to the fetus. If you are pregnant and undergoing a PET scan, ensure you drink plenty of water after the procedure to eliminate the tracer from your bladder. This will minimise radiation exposure to your baby.

If you are breastfeeding, it is recommended to avoid breastfeeding for 6-12 hours after the procedure as the tracer may be present in your breast milk. The exact time frame depends on the procedure and which tracer is being used. To feed your baby during this time, you can express breast milk prior to the procedure or alternatively use formula.

It is also advised to avoid close contact with your baby for 6-12 hours following the procedure.

Do I get the results on the same day?

No, PET scan results are not available straight away. It takes time for the computer to process the information to produce images. Once images have been produced, they will be interpreted by a qualified radiologist who will then send a report to your referring doctor. You will need to make a follow-up appointment with your doctor to receive the results.

What can I expect from the results?

It is difficult to say what you can expect from the results, as it depends on what the doctors are looking for. Generally, doctors will be looking for areas where there is increased uptake of the tracer (‘hot spots’) or areas where the tracer isn’t reaching as well as it should. The doctors may use this information to determine what the next best step should be.

There are certain areas of the body that take up the tracer at a higher rate than other areas, including the brain and the bladder. As a result, these areas normally appear brighter on scan images in all individuals.


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American College of Radiology Imaging Network. About PET scans. Updated February 2007. Accessed October 28, 2019. 4. Society of Nuclear Medicine and Molecular Imaging. Fact sheet: what is PET?

aspx?ItemNumber=5649. Published 2016. Accessed October 28, 2019. 5. Kisby G, Shetty A. Positron emission tomography. Radiopaedia. Updated August 2019. Accessed October 28, 2019. 6. Takalkar AM, Khandelwel A, Lokitz W, Lilien DL, Stabin MG.

18F-FDG PET in pregnancy and fetal radiation dose estimates. J Nucl Med. 2011; 52(7): 1034-1040. 7. Miele E, Spinelli GP, Tomao F, et al. Positron emission tomography (PET) radiotracers in oncology – utility of 18F-fluoru-deoxy-glucose (FDG)-PET in the management of patients with non-small-cell lung cancer (NSCLC).

J Exp Clin Cancer Res. 2008; 27(1): 52. 8. Inside Radiology. PET Scan. For health professionals. Modified March 2018.

9. Diagnostic Imaging Pathways. Information for consumers – Positron Emission Tomography (PET). Reviewed July 2017.



PET scans: Uses, risks, and procedure

PET scan: purpose and results

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

It is most commonly used in cancer treatment, neurology, and cardiology.

Combined with a CT or MRI scan, a PET scan can produce multidimensional, color images of the inside workings of the human body.

It shows not only what an organ looks , but how it is functioning.

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.

  • PET scans are often used to diagnose a condition or to track how it is developing.
  • Used alongside a CT or MRI scan, it can show how a part of the body is working.
  • PET scans are often used to investigate epilepsy, Alzheimer’s disease, cancer, and heart disease
  • A 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.

A radiotracer consists of radioactive material that is to a natural chemical, such as glucose.

This radiotracer is injected into the body, where it travels to cells that use glucose for energy.

The more energy a group of cells needs, the more the radiotracer will build up in that location. This will show up on images that are reconstructed by a computer.

The cells, or activity, will show up as “hot spots” or “cold spots.”

Active areas are bright on a PET scan. They are known as “hot spots.”

Where cells need less energy, the areas will be less bright. These are “cold spots.”

Compared with normal cells, cancer cells are very active in using glucose, so a radiotracer made with glucose will light up areas of cancer.

A radiologist will examine the image produced on the computer, and report the findings to a doctor.

An example of a glucose-based radiotracer is fluorodeoxyglucose (FDG). In FDG, radioactive fluoride molecules are to glucose to make a radiotracer. FDG is the radiotracer most commonly used today.

Instead of glucose, oxygen can be used.

Share on PinterestPET scans are an important part of the diagnostic process in cancer and epilepsy, and can directly inform the next stage of treatment.

PET scans are often used with CT or MRI scans to help make a diagnosis or to get more data about a health condition and the progress of any treatment.

While an MRI or CT scan shows how part of the body looks, a PET scan can reveal how it is functioning.

PET scans are commonly used to investigate a number of conditions.

Epilepsy: It can reveal which part of the brain the epilepsy is affecting.

This can help doctors decide on the most suitable treatment, and it can be useful if surgery is necessary.

Alzheimer’s disease: PET scans can help diagnose Alzheimer’s disease by measuring the uptake of sugar in specific parts of the brain.

Brain cells that are affected by Alzheimer’s tend to use glucose more slowly than normal cells.

Cancer: PET scans can reveal the presence and stage of a cancer, show whether and where it has spread, and help doctors decide on treatment.

A PET scan can give an idea of how well chemotherapy is working, and it can detect a recurring tumor sooner than other techniques.

Heart disease: A PET scan can help detect which parts of the heart have been damaged or scarred, and it can help identify circulation problems in the working of the heart.

This information can help plan treatment options for heart disease.

Medical research: Researchers can learn vital information by using PET scans, especially about the workings of the brain.

Differences between PET, CT, and MRI scans

A CT or MRI scan can assess the size and shape of body organs and tissue, but they cannot assess how these work.

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.

Combined a PET scan with a CT scan can give a more well-rounded picture of the patient’s situation

A PET scan is normally an outpatient procedure.

Normally, the patient should not consume any food for at least 4 to 6 hours before the scan, but they should drink plenty of water. They may have to avoid caffeine for at least 24 hours before the scan.

First, the doctor will inject a small amount of radiotracer into a vein. The tracer can also be breathed in as a gas, taken by mouth, or injected directly into an organ.

Depending on which the organ is involved, it may take from 30 to 90 minutes for the radiotracer to reach the targeted part of the body.

Meanwhile, the patient will normally be asked to stay still and not talk. Some patients may be given medication to relax.

The patient will probably need to wear a gown, and they may have to remove jewelry.

When the patient is ready, they will be taken to a special room scan for the scan. They will lie down on a cushioned examination table.

The table slides into a large hole so that the patient is surrounded by the machine.

The patient will have to stay as still as possible. They may be able to listen to music.

During the scan, the machine takes images.

Depending on which part of the body is being scanned, this should take about 30 minutes.

It is not painful. If the patient feels unwell, they can press a buzzer to alert the staff.

A qualified practitioner will watch the patient during the scan.

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 to flush the radioactive drugs their system more quickly. The radiotracers should have left the body completely within 3 to 4 hours.

There is a risk of radiation exposure.

For most people, the benefits of having a PET scan outweigh the risks.

However, as a PET involves radioactive material, it is not suitable for everyone.

Normally, a pregnant woman should not have a PET scan, as the radioactive material may affect the fetus or the infant.

If a woman is breastfeeding, she should follow directions for pumping and discarding breast milk, and ask the doctor when it is safe to resuming breastfeeding the test performed.

Any woman who is pregnant or breastfeeding should tell her doctor straight away before having a PET scan.

Following a PET scan, a patient may be advised to stay away from pregnant women, infants, and young children for a few hours, as the radioactivity poses a small risk.

Very rarely, an individual may have an allergic reaction to the tracer.


Positron Emission Tomography and Computed Tomography (PET-CT) Scans

PET scan: purpose and results

A positron emission tomography scan is known as a PET scan. PET scan is a type of test that may be used in cancer treatment. It can be done along with a CT scan. If so, doctors call it a PET-CT scan. But you might also just hear it called a PET scan.

For some types of cancer, a PET-CT scan is a way to help find cancer and learn its stage. Stage is a way to describe where the cancer is and if it has spread.

Doctors also learn information about the stage if and how the cancer is affecting your body's functions. Knowing the stage of cancer helps you and your doctor choose the best treatment.

It also helps your doctor predict your chance of recovery.

How is a PET-CT scan different than a CT scan?

A CT scan shows detailed pictures of the organs and tissues inside your body. A PET scan can find abnormal activity and it can be more sensitive than other imaging tests. It may also show changes to your body sooner. Doctors use PET-CT scans to provide more information about the cancer.

In addition to learning your cancer stage, a PET-CT scan can help the doctor:

  • Find the right place for a biopsy.
  • Learn if your cancer treatment is working.
  • Check for new cancer growth after treatment has ended.
  • Plan radiation therapy.

How does a PET-CT scan work?

Before your PET-CT scan, you will get an injection of a small amount of a radioactive sugar called fluorodeoxyglucose-18. This substance is sometimes called FGD-18, radioactive glucose, or a tracer.

The cells in your body absorb sugar. Areas that use more energy pick up more of the sugar. Cancer cells tend to use more energy than healthy cells.

The PET scan shows where the radioactive tracer is in your body.

The CT scan takes x-rays of your body from different angles. You might get a shot of dye before the x-rays. This helps some of the details show up better. Finally, a computer combines the PET and CT images. Your doctor gets a detailed 3-D result that shows anything abnormal, including tumors.

Are PET-CT scans safe?

PET-CT scans do carry a risk of radiation. This type of scan uses some radiation from x-rays, the substance used in the PET scan, or both. Scanning a smaller body area means less radiation. So does a CT without the dye that helps show details.

The benefits of these tests are usually greater than the risks. During these tests, you will be exposed to small amounts of radiation. This low dose of radiation has not been shown to cause harm. For children or for other people who need multiple PET scans, CT scans, and x-rays, there may be a small potential increased risk of cancer in the future.

Doctors can use lower dose scans or limit the areas that need to be scanned. Make sure all of your doctors know how many imaging scans you have had, including the number and type.

This information can help them decide which scans to use in the future to help reduce your risk.

If you are concerned about your radiation exposure, talk with your doctor, including asking whether you can have another type of test that uses less radiation instead.

Who will do my PET-CT scan?

A technician who specializes in doing these types of scans will do your PET-CT scan. After the test is done, a doctor who specializes in looking at the test results will look at your scan. This person is a nuclear medicine specialist or a radiologist. They will decide what the results mean.

PET-CT scans can be done at a hospital or a center that does imaging tests.

Getting ready for a PET-CT scan

When you schedule a PET-CT scan, the staff will tell you how to get ready. Be sure to follow their instructions carefully to avoid affecting your scan results. Talk with the staff about the following topics, and ask questions about any information that is unclear or concerning to you.

What to eat. You may be told to drink only clear liquids after midnight the night before the scan. Depending on what part of your body will be scanned, you may need to stop all eating and drinking 4 hours before the scan. For some scans, you can eat and drink normally.

Your medications and health history. Ask whether you should take your usual medications or supplements on the day of the test.

Also, let them know if you have diabetes or other medical conditions. In particular, diabetes can alter your test results and the radioactive tracer can impact your blood sugar.

If you are breastfeeding or could be pregnant, tell the staff. These scans can put the baby at risk.

Allergies. Let the staff know about any drug or food allergies you have, including any allergic reactions to iodine you may have had in the past.

What to avoid. Don't do any heavy exercise running, jogging, or weightlifting 24 hours before your exam. Exercise can make your scans less accurate.

What to wear. Wear loose, comfortable clothing without metal zippers or buttons. You will need to remove any clothing that includes metal because metal can affect the scan.

This includes belts, earrings, shirts with snaps or zippers, bras, and glasses. If your clothing cannot be worn during the scan, you can wear a hospital gown.

You will be asked to remove any jewelry, so you may want to leave it at home the day of your exam.

Insurance, costs, and consent. If you are concerned about the cost of your PET-CT scan, find out what your insurance provider will cover before the scan.

Ask how much of the cost you will have to pay. Once you get to the doctor's office or hospital, the staff will ask you to sign a consent form.

This form states that you understand the benefits and risks of the scan and agree to have it.

During the PET-CT scan

A staff member will put an IV into one of your veins. This is a thin tube with a needle attached. The IV feels a pinprick. Once the IV is in place, you will get the radioactive substance for the scan. You will not feel anything from the radioactive substance.

After the substance has been injected, you must limit your movement and avoid activity, but you can sit in a chair comfortably. Moving too much can make the substance move into areas that are not being studied. This makes it harder for doctors to read the scan. The radioactive substance takes 30 to 90 minutes to reach the body parts that will be scanned.

You may also be asked to drink a contrast liquid. You might also get contrast liquid through your IV. This contrast liquid helps make the pictures clearer. Right before the test begins, you will be asked to go to the bathroom to empty your bladder.

This liquid can make your IV area feel hot or itchy. You may also have a metallic taste in your mouth. These feelings should go away in a few minutes. If you have a more serious reaction, such as trouble breathing, say something right away.

When it is time for the scan to begin, the technologist will help you position your body comfortably on a table. You will ly lie on your back, but you may need to lie on your stomach or side. This depends on what part of your body needs to be scanned.

Sometimes a PET-CT scan is used to plan radiation therapy to treat the cancer. In this case, your body position will be very important. The technologist position you with a mask or cast. These tools help you keep your body very still during the scan.

The PET-CT machine looks a large donut. When it starts, the table slides quickly through the hole in the center. This helps show if you are in the right position. Then the table slides slowly back and forth. A staff member will watch the test from a nearby room. You can talk to them and they can talk to you.

Will I be comfortable during the scan?

The staff will make you as comfortable as possible. A PET-CT scan does not hurt. But some positions might be uncomfortable or tiring. You need to lie still for the entire scan. You might also need to keep your arms above your head. The staff member might ask you to hold your breath sometimes. Motion from breathing can cause blurry pictures.

The staff member might also raise, lower, or tilt the table during the scan. This gets pictures from different angles. Ask them to tell you when the table will move.

You can expect to hear whirring or clicking sounds from the machine. Some machines are noisier than others.

Your appointment will last 1 hour to 3 hours. Once the radioactive substance gets to the right area through the IV, the scan itself usually only takes about 30 minutes. If the machine scans a large area, the test might take longer. The staff member can tell you about how long it will take.

When the scan is finished, you might need to stay on the table while a doctor looks at the images to check to make sure the images are not blurry. If they are not clear, you might need another scan.

After the PET-CT scan

You can do normal activities after the scan. This includes driving. The staff will tell you to drink several glasses of water. This helps wash the radioactive substance and dye your body.

Questions to ask your health care team

You might want to ask your health care team the following questions before a PET-CT scan.

  • Where will I have the PET-CT scan?
  • How long will it take?
  • What are the benefits and risks?
  • Will I need dye for the CT? How will I get it? What happens if I am allergic to the dye?
  • What can I eat or drink before the scan? What about taking my usual medications?What should I avoid?
  • When will I learn the test results?
  • Who will tell me the results, and how?
  • Will I need other tests

CT Scans and Cancer Risk

Preparing for Your PET-CT Scan

Expert Podcast: Coping with “Scanxiety”

More Information Positron Emission Tomography – Computed Tomography (PET/CT)


What Is a PET Scan?

PET scan: purpose and results

X-ray, CT, MRI, PET — when you need to get imaging done, it can start to sound an alphabet soup. They all give pictures of the inside of your body, but each type of imaging has its own strengths. A PET scan doesn’t give you just images — it also shows how your body’s working.

PET is short for “positron emission tomography.” It provides information about blood flow and how your body’s using oxygen and sugar. That can give important clues about how a disease is unfolding.

When you get a PET scan, your doctor first gives you a radioactive substance called a radiotracer (or just “tracer”). The tracer gives off radiation, which the PET scan machine picks up on. The images you get show where in your body the tracer goes. If it builds up in certain areas, that could be a sign of disease.

A PET scan can help doctors test for disease, prepare for surgery, and see how well treatments are working. You might get one for several reasons, but they’re most often used with cancer, heart disease, and brain conditions.

Your doctor may use a PET scan to:

  • Find cancer
  • See if cancer has spread
  • Check if cancer treatment is working
  • Determine if cancer came back after treatment

With heart disease, your doctor might use a PET scan to:

Your doctor may also use it to check for brain conditions, such as:

  • Alzheimer’s disease
  • Parkinson’s disease
  • Seizures
  • Stroke
  • Tumors

Doctors use different types of imaging for different reasons. Often, you start with an X-ray because it’s a quick way to get basic information. But if you need finer detail, you might then get a CT scan or MRI.

Many doctors use MRI/PET and CT/PET hybrid scanners, which combines the two tools into a single scan. This allows doctors to do either a CT or MRI scan in combination with a PET scan all at once.

A PET scan can show what’s actually happening in your cells. One reason that’s important is because early on, some diseases don’t cause changes you can see with an MRI or CT scan. But they do cause changes in how your cells are working. That means a PET scan might help your doctor find a disease that other types of imaging can’t.

First, you’ll need to tell your doctor about any of the following:

  • Allergies, especially to contrast dye, iodine, or seafood
  • Health conditions, diabetes, or any illnesses you’ve had recently
  • Medicines, herbs, and supplements you take

If you’re a woman, tell your doctor if you’re:

  • Breastfeeding — you may need to pump milk because you can’t breastfeed until the tracer is your body. Check with your doctor to see how long you should wait.
  • Pregnant or think you might be — the tracer can harm your baby, so talk to your doctor about the best options for you.

Your doctor will give you specific directions to prepare for your scan. Be sure to follow them closely. Often you’ll need to:

  • Avoid intense physical activity for 24 hours before the scan
  • Drink only water and avoid eating for several hours prior to the scan
  • Remove all piercings, jewelry, and metal objects from your body

It depends where and why you get the scan, but typically, you:

  • Change into a hospital gown
  • Go to the bathroom
  • Get the tracer — depending on the type, you’ll either swallow it, breathe it in, or get it through a needle
  • Wait 30 minutes to an hour for your body to absorb the tracer
  • Lie very still on your back while images are taken. It’s important not to move or talk during the scan, which may last up to an hour.

The PET scan machine is a big, open circle — a standing donut — with a table that moves in and it. If you have a fear of tight, closed spaces, you may get a drug to help keep you calm. You’ll hear the machine buzz and click as it takes images.

The scan itself is painless. For some people, staying still for so long is the hardest part and may cause some aches or discomfort.

After the scan, drink plenty of fluids to help flush the tracer your body. Your doctor may suggest you avoid close contact with pregnant women, kids, or babies for a few hours since you’ll be radioactive for a short time.

A PET scan shows bright areas where there’s heavy activity in your cells, which may be a sign of disease. To get a more complete picture of what’s going on, your doctor may compare your PET scan with results from other imaging you’ve had. You can get results within 24 hours, but it depends on where you have the scan done.


American Academy of Orthopaedic Surgeons: “X-rays, CT scans, and MRIs.” “Positron Emission Tomography – Computer Tomography (PET/CT).”

The University of Texas MD Anderson Cancer Center, “Getting a PET Scan? What to Expect.”

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