Positron
Emission Tomography
Positron
emission tomography, also called PET imaging or a PET scan, is a diagnostic
examination that involves the building up of 3-d images by detecting
the concentration of radiation emission at various 
points
within the body. This means that a picture of where the radioactive tracer
has accumulated allows doctors to understand how the body is functioning
and they can use that information to decide upon what treatment is suitable.
The tracer
used is a positron emitter. Positrons
are anti-matter beta-particles emitted from the radioactive substance
administered to the patient. When a positron interacts with an electron
annihilation occurs and two gamma rays are produced that fly off
in opposite directions. The PET scanner detects this pair of gamma rays
making it possible to compute where they originated from in the body -
this will be the point where the positron was emitted, as a positron won't
get far in matter after emission.
So...
the PET scanner detects gamma rays - but the tracer emits positrons!
PETscanning is non-invasive
but does involve ionizing radiation.The images of the human body
developed with this technique can be used to evaluate a variety of diseases.
Common
uses of the procedure
- Whole body PET
scans are used most often to detect cancer and to examine the effects
of cancer therapy.
- Heart PET scans
can be used to determine blood flow to the heart muscle and help evaluate
signs of coronary artery disease. They can also be used to determine
if areas of the heart that show decreased function are alive rather
than scarred as a result of a prior heart attack, called a myocardial
infarction. Combined with a myocardial perfusion study, PET scans allow
differentiation of 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.
- Brain PET scans
are used to evaluate patients who have memory disorders of an undetermined
cause, suspected or proven brain tumours or seizure disorders that are
not responsive to medical therapy and are therefore candidates for surgery.
The
equipment
A PET scanner looks
like a large doughnut - it has a hole in the middle. Within this machine
are multiple rings of detectors that record the gamma rays emitted when
a positron from the radioactive substance in your body annihilates an
electron in the patient's body. It only 'counts' gamma rays pairs. The
information collected permits a 3-d image of the body to be obtained.
While lying on a cushioned examination table, the patient is moved into
the hole of the machine. The images are displayed on the monitor of a
nearby computer, which is similar in appearance to a home personal computer.
The
Procedure
Before the examination
begins, a positron-emitting 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. Oxygen 15 is a typical radioisotope used
in PET scanning, it decays with a half-life of 2.25 minutes by the emission
of positrons. The isotope is prepared by irradiation of nitrogen by means
of cyclotron-accelerated deuterons. The cyclotron needed for this procedure
is small enough power to be on site in a large hospital complex (although
it is an expensive piece of equipment and needs expert operation!). The
doctor is interested in finding out where this natural body compound accumulates.
From that information s/he can wok out how well body organs are functioning.
Once this tagged substance is administered to the patient, the radioactivity
localizes in the appropriate areas of the body and can then be detected
by the PET scanner.Therefore the doctor gains information on the use of
that compound by the body.
The
data from the PET scanner can be coded into colour computer images. Different
colours or degrees of brightness on a PET image are made to represent
different levels of tissue or organ function because they represent the
concentration of annihilation events at that location. 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 accumulate more of the substance
and appear brighter than normal tissue on the PET images.PET can shows
blood flow in the brain by imaging trace amounts of radioisotopes at various
locations. If it indicates a decrease in blood flow to parts of the brain
this can indicate such diseases as Alzheimer's.
How
the procedure is performed
A nurse or technologist
takes the patient into a special injection room, where the radioactive
substance is administered as an intravenous injection (although in some
cases, it will be given through an existing intravenous line or inhaled
as a gas). It then takes approximately 30 to 90 minutes for the substance
to travel through the body and accumulate in the tissue under study. During
this time, the patient will be asked to rest quietly and avoid significant
movement or talking, which may alter the localization of the administered
substance. After that time, scanning begins. This may take 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 the patient
should drink plenty of fluids to flush the radioactive substance from
his/her body as quickly as possible after the procedure and for a limited
time the patient might be asked to avoid public transport or places where
there could be a pregnant woman or a young child - as they will be a source
of gamma rays for a while - until the emission drops to background level.This
will depend on the tracer used and its half life. The shorter the half
life the quicker the level will drop to 'normal'. Oxygen tagged procedures
are very quick to become benign because the half life is so short.
What
do you experience during the procedure?
The
administration of the radioactive substance feels like a slight pinprick
if given by intravenous injection. You will then be made as comfortable
as possible 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.
Graphic from www.brainviews.com/abFiles/ImgPet.htm