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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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