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Introduction
 Positron
emission tomography, also called PET imaging or a PET
scan, is a diagnostic examination that involves the
acquisition of physiologic images based on the detection
of positrons. Positrons are tiny particles emitted from
a radioactive substance administered to the patient.
The subsequent views of the human body developed by this
technique are used to evaluate a variety of diseases.
Common
uses of the Procedure
PET scans are used most often to detect cancer
and to examine the effects of cancer therapy by
characterizing biochemical changes in the cancer.
These scans are performed on the whole body. PET
scans of the heart can be used to determine blood flow
to the heart muscle and help evaluate signs of coronary
artery disease. PET scans of the heart can also be
used to determine if areas of the heart that show
decreased function are alive rather than scarred due to
a prior heart attack, called a myocardial infraction.
Combined with a myocardial perfusion study, PET scans
differentiate nonfunctioning heart muscle from heart
muscle that would benefit from a procedure, such as
angioplasty or coronary artery bypass surgery, which
would reestablish adequate blood flow and improve heart
function. PET scans of the brain are used to
evaluate patients who have memory disorders of an
undetermined cause, who have suspected or proven brain
tumors or who have seizure disorders that are not
responsive to medical therapy and, therefore, are
candidates for surgery.
Preparing
for the Procedure
You should wear comfortable, loose-fitting
clothes. You should not eat for four hours before the
scan. You will be encouraged to drink water.
Your doctor will instruct you regarding the use of
medications before the test. Diabetic
patients should ask for any specific diet guidelines to
control glucose levels during the day of the test.
About
the Equipment
You will be taken to an examination room that
houses the PET scanner, which has a hole in the middle
and looks like a large doughnut. Within this
machine are multiple rings of detectors that record the
emission of energy from the radioactive substance in
your body and permit an image of your body to be
obtained. While lying on a cushioned examination
table, you will be moved into the hole of the machine.
The images are displayed on the monitor of a nearby
computer, which is similar in appearance to the personal
computer you may have in your home.
The
Procedure
 Before
the examination begins, a radioactive substance is
produced in a machine called a cyclotron and attached,
or tagged, to a natural body compound, most commonly
glucose, but sometimes water or ammonia. Once this
substance is administered to the patient, the
radioactivity localizes in the appropriate areas of the
body and is detected by the PET scanner.
Different
colors or degrees of brightness on a PET image represent
different levels of tissue or organ function. For
example, because healthy tissue uses glucose for energy,
it accumulates some of the tagged glucose, which will
show up on the PET images. However, cancerous tissue,
which uses more glucose than normal tissue, will absorb
more of the substance and appear brighter than normal
tissue on the PET images.
A nurse
or technologist will take you into a special PET
examination room. You will lie down on an
examination table and be given the radioactive substance
as an intravenous injection (although, in some cases, it
will be given through an existing intravenous line or
inhaled as a gas). It will then take approximately
30 to 60 minutes for the substance to travel through
your body and be absorbed by the tissue under study.
During this time, you will be asked to rest quietly in a
partially darkened room and to avoid significant
movement or talking, which may alter the localization of
the administered substance. After that time,
scanning begins. This takes an additional 30 to 45
minutes.
Some
patients, specifically those with heart disease, may
undergo a stress test in which PET scans are obtained
while they are at rest, and again after undergoing the
administration of a pharmaceutical to alter the blood
flow to the heart.
Usually,
there are no restrictions on daily routine after the
test, although you should drink plenty of fluids to
flush the radioactive substance from your body.
What
You Will Experience During the Procedure
The administration of the radioactive substance
will feel like a slight pinprick if given by intravenous
injection. You will then be made as comfortable as
possible on the examination table before you are
positioned in the PET scanner for the test. You
will be asked to remain still for the duration of the
examination. Patients who are claustrophobic may
feel some anxiety while positioned in the scanner.
Also, some patients find it uncomfortable to hold one
position for more than a few minutes. You will not
feel anything related to the radioactivity of the
substance in your body.
Benefits
and Risks of PET Scans
 Because
PET allows study of body function, it can help
physicians detect alterations in biochemical processes
that suggest disease before changes in anatomy are
apparent on other imaging tests such as CT or MRI scans.
Because the radioactivity
is very short-lived, your radiation exposure is
extremely low. The substance amount is so small that it
does not affect the normal processes of the body.
The
radioactive substance may expose radiation to the fetus
of patients who are pregnant or the infants of women who
are breast-feeding. The risk to the fetus or
infant should be considered related to the potential
information gain from the result of the PET examination.
If you are pregnant you should inform the PET imaging
staff before the examination is performed.
Limitations
of Positron Emission Tomography
PET can give false results if a patient's
chemical balances are not normal. Specifically,
test results of diabetic patients or patients who have
eaten within several hours prior to the examination can
be adversely affected because of blood sugar or blood
insulin levels.
Also,
because the radioactive substance decays quickly and is
effective for a short period of time, it must be
produced in a laboratory near the PET scanner. The
value of a PET scan is enhanced when it is part of a
larger diagnostic work-up. This often entails comparison
of the PET scan with other imaging studies such as CT or
MRI.
How should I prepare
for a PET and PET/CT scan?
You may be asked to
wear a gown during the exam or you may be allowed to
wear your own clothing.
Women should always
inform their physician or technologist if there is any
possibility that they are pregnant or if they are
breastfeeding their baby.
You should inform your
physician and the technologist performing your exam of
any medications you are taking, including vitamins and
herbal supplements. You should also inform them if you
have any allergies and about recent illnesses or other
medical conditions.
You will receive
specific instructions based on the type of PET scan
you are undergoing. Diabetic patients will receive
special instructions to prepare for this exam.
If you are
breastfeeding at the time of the exam, you should ask
your radiologist or the doctor ordering the exam how
to proceed. It may help to pump breast milk ahead of
time and keep it on hand for use after the PET
radiopharmaceutical and CT contrast material are no
longer in your body.
Metal objects
including jewelry, eyeglasses, dentures and hairpins
may affect the CT images and should be left at home or
removed prior to your exam. You may also be asked to
remove hearing aids and removable dental work.
Generally, you will be
asked not to eat anything for several hours before a
whole body PET/CT scan since eating may alter the
distribution of the PET tracer in your body and can
lead to a suboptimal scan. This could require the scan
to be repeated on another day, so following
instructions regarding eating is very important. You
should not drink any liquids containing sugars or
calories for several hours before the scan. Instead,
you are encouraged to drink water. If you are
diabetic, you may be given special instructions. You
should inform your physician of any medications you
are taking and if you have any allergies, especially
to contrast materials, iodine, or seafood.
You will be asked and
checked for any conditions that you may have that may
increase the risk of using intravenous contrast
material.
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