Test Overview
Positron emission tomography (PET) is a test
that uses a special type of camera and a
tracer (radioactive chemical) 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
pictures 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
located.
A PET scan is often used to evaluate cancer, check blood
flow, or see how organs are working.
See pictures of a
PET scanner and
PET scans of the brain.
Why It Is Done
A
positron emission tomography (PET) scan is done
to:
- Study the brain's blood flow and
metabolic activity. A PET scan can help a doctor find
nervous system problems, such as
Alzheimer's disease,
Parkinson's disease,
multiple sclerosis,
transient ischemic attack (TIA),
amyotrophic lateral sclerosis (ALS),
Huntington's disease,
stroke, and
schizophrenia.
- Find changes in the brain
that may cause
epilepsy.
- Evaluate 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.
- 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.
- 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
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.
Tell
your doctor about any medicines and herbal remedies you take. You may need to
stop taking some medicines or change your dose before this
test.
Do not smoke or drink caffeine or alcohol for 24
hours before this test.
Do not eat or drink for at least
6 hours before this test.
Tell your doctor if you are or might be
pregnant or if you are breast-feeding. If you are breast-feeding, you will need to use formula for 1 to 2 days after the PET scan so you won't pass the tracer to your baby. This generally takes 24
hours.
Tell your
doctor if you have a fear of enclosed spaces or have ever had a panic
attack.
You may be asked to sign a consent form for this test.
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 be asked to lie on a table that is hooked to a large scanner, camera, and
computer.
The
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
body.
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, 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 may make you feel warm and flushed.
Some people feel sick to their stomach or have a headache. Tell your doctor how
you are feeling.
You may feel nervous inside the PET
scanner.
Risks
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
arm.
Results
Positron emission tomography (PET) is a
test that uses a special type of camera and a
tracer (radioactive chemical) to look at organs in the
body.
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)| Normal: | Blood flow is normal and organs are working
well. The flow and pattern of the tracer shows normal distribution in the
body. |
|---|
| Abnormal: | Heart: | - Decreased blood flow and increased
glucose
metabolism may show that the blood vessels are
blocked. This may mean
coronary artery disease (CAD) is
present.
- Decreased blood flow and glucose metabolism may mean that
heart tissue is scarred and damaged, such as from a
heart attack.
|
|---|
| Brain: | See a picture of
PET scans of the brain. |
| 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
test.
- Being too anxious.
- Using
sedatives.
- Taking medicines, such as
insulin, that change your
metabolism.
- Having recently had surgery, a
biopsy,
chemotherapy, or
radiation therapy.
What To Think About
- A
CT scan and PET scan may be done at the same
time.
- Single photon emission computed tomography (SPECT) imaging is
a method that may be used to diagnose a person with
symptoms of heart disease. SPECT imaging also may be done
to see if a person with
coronary artery disease (CAD) is likely to have a
heart attack or other serious problem.1
References
Citations
- Hendel RC, et al. (2009).
ACCF/ASNC/ACR/AHA/ASE/SCCT/SCMR/SNM 2009 appropriate use criteria for cardiac
radionuclide imaging. Circulation, 119(22):
e561–e587.
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.
Credits
| By | Healthwise Staff |
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| Primary Medical Reviewer | Adam Husney, MD - Family Medicine |
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| Specialist Medical Reviewer | Howard Schaff, MD - Diagnostic Radiology |
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| Last Revised | July 28, 2011 |
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