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 like, 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.

Fast facts on PET scans

Here are some key points about PET scans. More detail is in the main article.

  • 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.

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PET scans demonstrate the physical state and function of organs.

In a PET scan, a machine detects radiation that is emitted by a radiotracer.

A radiotracer consists of radioactive material that is tagged 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 tagged to glucose to make a radiotracer. FDG is the radiotracer most commonly used today.

Instead of glucose, oxygen can be used.

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PET 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

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PET images show the active cells as bright or 'hot' spots.

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 out of 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 based on 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.