Positron Emission Tomography (PET)

Positron Emission Tomography (PET)

Test Overview

Positron emission tomography (PET) is a test
that uses a special type of camera and a
tracer (radioactive substance) to look at organs in the
body. The tracer usually is a special form of a substance (such as glucose) that collects in cells that are using a lot of energy, such as cancer cells.

During the test, the tracer liquid is put into a vein
(intravenous, or
IV) in your arm. The tracer moves through your body,
where much of it collects in the specific organ or tissue. The tracer gives off
tiny positively charged particles (positrons). The camera records the positrons
and turns the recording into pictures on a computer.

PET scan
do not show as much detail as
computed tomography (CT) scans or
magnetic resonance imaging (MRI) because the pictures
show only the location of the tracer. The PET picture may be matched with those
from a CT scan to get more detailed information about where the tracer is

A PET scan is often used to evaluate cancer, check blood
flow, or see how organs are working.

Why It Is Done

positron emission tomography (PET) scan is done

  • Study the brain's blood flow and
    metabolic activity. A PET scan can help a doctor find
    nervous system problems, such as
    Parkinson's disease,
    multiple sclerosis,
    transient ischemic attack (TIA),
    amyotrophic lateral sclerosis (ALS),
    Huntington's disease,
    stroke, and
  • Find changes in the brain
    that may cause
  • Evaluate the extent of some cancers, especially
    lymphoma or cancers of the head and neck, brain, lung, colon,
    or prostate. In its early stages, cancer may show up more clearly on a PET scan
    than on a CT scan or an MRI.
  • Determine whether a growth in an organ or in tissue is likely to be cancer, such as a growth in lung tissue.
  • See how advanced a cancer is and
    whether it has spread to another area of the body (metastasized). It is often
    necessary to do both CT and PET scans to evaluate cancer.
  • Help a
    doctor choose the best treatment for cancer or to see how well treatment is working. PET scans may also be done to see
    whether surgery can be done to remove a tumor.
  • Help diagnose Alzheimer's disease when the symptoms are not clear or when a person has dementia symptoms at a young age (usually younger than 65).footnote 1 This is called amyloid imaging.
  • Find poor blood flow
    to the heart, which may mean
    coronary artery disease.
  • Find damaged
    heart tissue, especially after a
    heart attack.
  • Help choose the best
    treatment, such as
    coronary artery bypass graft surgery, for a person
    with heart disease.

How To Prepare

  • Before you have a PET scan, tell your doctor if:
    • You have
      diabetes. If you take
      medicine to control diabetes, you may need to take less
      than your normal dose. Talk with your doctor about how much
      medicine you should take.
    • You take any medicines, supplements, or herbal remedies. You may need to
      stop taking some medicines or change your dose before this
    • You are or might be
    • You are breastfeeding. The radioactive tracer used in this test can get into your breast milk. Do not breastfeed your baby for 2 days after this test. During this time, you can give your baby breast milk you stored before the test, or you can give formula. Discard the breast milk you pump for 2 days after the test.
    • You have a fear of enclosed spaces.
  • Do not smoke or drink caffeine or alcohol for 24
    hours before this test.
  • Do not eat or drink (except water) for at least
    6 hours before this test.

You may be asked to sign a consent form.

Talk to your doctor about any concerns you have regarding the need for the
test, its risks, how it will be done or what the results mean. To help you
understand the importance of this test, fill out the
medical test information form ( What is a PDF document? ).

How It Is Done

A positron emission tomography (PET)
scan is done in a hospital nuclear medicine department or at a special PET
center by a
radiologist or
nuclear medicine specialist and a technologist. You
will lie on a table that is hooked to a large scanner, camera, and

During the test

radioactive tracer is usually given in a vein
(IV). You may need to wait 30 to 60 minutes for the tracer to move through your body. During this time, you may need to avoid moving and talking.

The PET scanner, which is shaped like a doughnut, moves
around you. The scanned pictures are sent to a computer screen so your doctor
can see them. Many scans are done to make a series of pictures. It is very
important to lie still while each scan is being done. At some medical centers,
a CT scan will be done at the same time.

For a PET scan of the
brain, you will lie on a bed. You may be asked to read, name letters, or tell a
story, depending on whether speech, reasoning, or memory is being tested.
During the scan, you may be given earplugs and a blindfold (if you do not need
to read during the test) to wear for your comfort.

If you are
having a PET scan of your heart, electrodes for an
electrocardiogram (EKG, ECG) will be put on your

During the test, you will be alone in the scanner room. The
technologist will watch you through a window and you will be able talk to him
or her through a two-way intercom at all times.

The test takes 1
to 3 hours.

After the test

After the test, drink lots of fluids for the next 24
hours to help flush the tracer out of your body.

How It Feels

You will not feel pain during the test.
The table you lie on may be hard and the room may be cool. It may be difficult
to lie still during the test.

You may feel a quick sting or pinch
when the IV is put in your arm. The tracer is unlikely to cause any side effects. If you don't feel well during or after the test, tell the person who is doing the test.

You may feel nervous inside the PET


There is always a slight chance of damage to
cells or tissue from radiation, including the low levels of radiation used for
this test. But the chance of damage is usually very low compared with the
benefits of the test.

Most of the tracer will be flushed from your
body within 6 to 24 hours.
Allergic reactions to the tracer are very rare.

In rare cases, some soreness or swelling may develop at the IV site where
the radioactive tracer was put in. Apply a moist, warm compress to your


Positron emission tomography (PET) is a
test that uses a special type of camera and a
tracer (radioactive substance) to look at organs in the

radiologist may discuss preliminary results of the PET
scan with you right after the test. Complete results are usually available in 1
to 2 days.

Positron emission tomography (PET)

Blood flow is normal and organs are working
well. The flow and pattern of the tracer shows normal distribution in the

Abnormal: Heart:
  • Decreased blood flow and increased
    metabolism may show that the blood vessels are
    narrowed or blocked. This may mean
    coronary artery disease (CAD) is
  • Decreased blood flow and glucose metabolism may mean that
    heart tissue is scarred and damaged, such as from a
    heart attack.
  • Areas of increased glucose metabolism or
    lower oxygen use and blood flow may mean you have
  • Decreased oxygen use and blood
    flow may mean a
    stroke has occurred.
  • Decreased glucose
    metabolism may mean a form of
    dementia. Dementia may be caused by Parkinson's disease,
    Huntington's disease, or mental illness, such as
  • Patterns of blood flow and
    oxygen use that are not normal may mean a brain tumor is present.
  • A special test (called amyloid imaging) may show signs of Alzheimer's disease.
Tumor detection:

Areas of increased glucose metabolism may mean
a tumor is present.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Being pregnant. A PET scan is not usually done
    during pregnancy because the radiation could harm the unborn baby (fetus).
  • Using caffeine, tobacco, or
    alcohol in the past 24 hours.
  • Not being able to lie still for the
  • Using
  • Taking medicines, such as
    insulin, that change your
  • Having recently had surgery, a
    chemotherapy, or
    radiation therapy.

What To Think About

  • A
    CT scan and PET scan are often done at the same



  1. Johnson KA, et al. (2013). Appropriate use criteria for amyloid PET: A report of the Amyloid Imaging Task Force, the Society of Nuclear Medicine and Molecular Imaging, and the Alzheimer's Association. Alzheimer's and Dementia, 9(1): e1–e16.

Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.


ByHealthwise StaffPrimary Medical Reviewer Adam Husney, MD - Family Medicine E. Gregory Thompson, MD - Internal Medicine Martin J. Gabica, MD - Family Medicine Kathleen Romito, MD - Family Medicine Specialist Medical Reviewer Howard Schaff, MD - Diagnostic Radiology

Current as ofNovember 29, 2017

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