Positron Emission Tomography
tomography (PET) combines the techniques of nuclear scanning
and biochemical analysis to assess body processes like blood
flow and metabolism in certain organs. In this test, a radioactive
tracer is tagged to a biologically active moleculesuch
as glucose, oxygen, carbon monoxide, hormones, or neurotransmittersand
introduced into the body, usually by injection. After you enter
a special scanning machine, radiation detectors record the emissions
of the radioactive material. This information is relayed to a
computer, which constructs color-coded, cross-sectional images
depicting which areas of the organ are active. PET scanning can
provide valuable information not only about the structure of
particular organs, but also about how they work.
Purpose of the Test
To assess various
body processesparticularly brain activity, but also activity
in the heart, lungs, and other tissues and organs.
neurologic illnesses, including epilepsy, Alzheimer disease,
and other types of dementia.
To help determine
heart muscle function in patients with heart disease and distinguish
between viable and dead cardiac tissue during the early stages
of a heart attack.
To detect cancerous
tumors, determine the stage of cancer, and evaluate the effectiveness
of cancer therapy.
Who Performs It
trained radiologist or technician.
is very costly (because the positron-emitting radiotracers used
for this test must first be generated by a particle accelerator,
or cyclotron), and is not routinely performed outside of major
This test should
not be performed in pregnant or breastfeeding women because of
possible risks to the fetus or infant.
may limit the accuracy of PET scans of the heart or lung.
as tranquilizers and sedatives as well as recent use of caffeine,
alcohol, or tobacco may alter the test results.
glucose is used, the presence of diabetes may affect the results.
Blood sugar levels must be monitored during testing in most patients.
Before the Test
Do not ingest
alcohol, caffeine, tobacco, sedatives, or tranquilizers for 24
hours before the procedure.
If you have
diabetes, you will be asked to take your pretest dose of insulin
at a meal 3 to 4 hours before the test.
(IV) lines may be placed in veins in your arms, one for infusing
the radioactive tracer and the other for taking a series of blood
bladder before the test.
What You Experience
You will lie
down on a table.
tracer is either injected through one of the IV lines or inhaled
in the form of a radioactive gas.
You enter into
the PET scanning machine, and the gamma rays emitted by the radiotracer
are recorded by a circular array of detectors. The resulting
images are displayed on a computer.
You must lie
very still during the procedure.
If you are
undergoing a PET scan of the brain, special cushions may be placed
against your head to hold it in place. You may be asked to perform
various cognitive activities, such as doing a mathematical calculation
or remembering a sequence of words. To minimize external stimuli,
you may be asked to wear a blindfold and earplugs.
can take from 1 to 2 hours.
Risks and Complications
tracers used in PET scans are short-lived and rapidly cleared
from the body. They are not associated with any significant risks
After the Test
You may be
advised to stand up slowly after the procedure to avoid feeling
faint or dizzy.
You are free
to leave the testing facility and resume your normal activities.
of fluids to help flush the radioactive material from your body.
Blood may collect
and clot under the skin (hematoma) at the IV needle insertion
site(s); this is harmless and will resolve on its own. For a
large hematoma that causes swelling and discomfort, apply ice
initially; after 24 hours, use warm, moist compresses to help
dissolve the clotted blood.
reviews the PET scan data for evidence of abnormalities.
If a definitive
diagnosis can be made, appropriate therapy will be initiated.
In some cases,
additional tests, such as magnetic resonance angiography, may
be needed to establish a diagnosis and determine the extent of
From The Johns
Hopkins Consumer Guide to Medical Tests. You can order
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