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.
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