Positron Emission Tomography (PET Imaging)
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Positron Emission Tomography (PET) equipment
What is Positron Emission Tomography?
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.
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What are some 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 infarction.
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.
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How should I prepare for the procedure?
PET is usually done on an outpatient basis. Your doctor will give you detailed
instructions on how to prepare for your examination. 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.
Note: Diabetic patients should ask for any specific diet guidelines to control
glucose levels during the day of the test.
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What does the equipment look like?
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.
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How does the procedure work?
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.
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How is the procedure performed?
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.
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What will I 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.
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Who interprets the results and how do I get them?
Patients undergo PET because their referring physician has recommended it.
A radiologist who has specialized training in PET will interpret the images
and forward a report to your referring physician. It usually takes one
to three days to interpret, report, and deliver the results. In order to
facilitate interpretation, you may be asked to bring any outside examinations
with you, such as recent CT (CAT) scans or MRI scans.
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What are the benefits vs. risks?
* 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.
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What are the 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. It is important to be on time for the appointment and to receive
the radioactive substance at the scheduled time. PET must be done by a
radiologist who has specialized in nuclear medicine and has substantial
experience with PET. Most large medical centers now have PET services available
to their patients. Medicare and insurance companies cover many of the applications
of PET, and coverage continues to increase.
Finally, 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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