Cardiac Nuclear Medicine
What is Cardiac Nuclear Medicine?
Nuclear medicine is a healthcare specialty involving the use of radioactive
compounds to perform diagnostic imaging examinations that can lead to the
effective treatment of many diseases. Although nuclear medicine is often
considered an independent discipline, it is closely related to radiology
in that radiation is used to develop images of human anatomy.
Cardiac nuclear medicine refers to these diagnostic tests that are used
to examine the anatomy and function of the heart.
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What are some common uses of the procedure?
Cardiac nuclear medicine tests are indicated for individuals with unexplained
chest pain or chest pain brought on by exercise (called angina) to permit
the early detection of heart disease.
The most common cardiac nuclear medicine procedure, called myocardial perfusion
scanning, enables the visualization of blood-flow patterns to the heart
walls. The test is important for evaluating the presence and extent of
suspected or known coronary artery disease (blockages) as well as the results
of previous injury to the heart from a heart attack, called a myocardial
infarction. It also can be done to evaluate the results of bypass surgery
or other percutaneous revascularization procedures designed to restore
the blood supply to the heart.
Heart-wall movement and overall heart function can be evaluated with cardiac
gating, a technique that synchronizes the images of the heart with different
parts of the cardiac cycle (contracting or relaxing) as determined by an
electrocardiogram (ECG) , which records the electrical currents that activate
the heart muscle and cause it to pump.
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How should I prepare for the procedure?
You should avoid caffeine (coffee, tea, etc.) and smoking for 48 hours before
the examination. You should not eat or drink anything after midnight before
the procedure, but continue taking medications with small amounts of water
unless your physician says otherwise. Wear comfortable, rubber-soled walking
shoes and loose-fitting clothes for your procedure. Tell the technologist
and supervising physician if you have asthma or a chronic lung disease
or have problems with your knees, hips or keeping your balance, which may
limit your ability to perform the exercise needed for this procedure.
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What does the equipment look like?
The imaging equipment, called a gamma camera or scintillation camera, consists
of specialized detectors enclosed within a metal housing. The detector
portion of the camera can be changed to a variety of positions to obtain
images of the body from different directions. A nearby computer console,
possibly in another room, is used to develop the images of the heart.
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How does the procedure work?
Coronary arteries are best evaluated by determining the changes in blood
flow to the heart due to exercising. Consequently, you will undergo a stress
test—most commonly through physical exercise—to make your heart
work harder than normal. Then you will be given a radioactive compound,
called a radiopharmaceutical or tracer. This compound will collect in parts
of your heart with good blood flow and will give off gamma rays. The gamma
camera detects the rays. Subsequently, a computer following a set of complicated
mathematical formulas will construct images of the heart based on the detected
gamma rays.
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How is the procedure performed?
For the stress part of the examination, you will exercise by either walking
on a treadmill or pedaling a stationary bicycle for a few minutes. While
you exercise, the electrical activity of your heart will be monitored by
electrocardiography (ECG), and your blood pressure will be measured frequently.
Before you stop exercising, you will be given the radiopharmaceutical through
a line leading into a vein in your arm. The compound is given when the
blood flow to the heart is at its peak caused by your exercising. This
provides the best opportunity to identify regions of the heart that are
not receiving adequate blood flow.
One minute later, you will stop exercising. Approximately one half-hour
later, as you lay on an examining table, the compound will have collected
in your heart. The gamma camera will then be used to obtain images. The
gamma camera likely will move slowly and automatically in an arc over the
front of your chest after it is positioned initially by the technologist.
The images obtained after exercise must usually be compared with images
of your heart obtained after injection of the same radiopharmaceutical
while you are resting. This may be performed before or after the exercise
part of the examination, depending on the protocol used. Comparison of
the exercise and resting images is done to determine whether coronary blood
flow has changed once you have rested, and to evaluate for coronary artery
disease.
If you are unable to use a treadmill or bicycle, you will not exercise but
be given a drug that will cause your heart to work as hard as if you had
exercised. You will then be given the radiopharmaceutical.
Immediately after the procedure, a diagnostic radiologist with specialized
training in nuclear medicine will check the quality of the images to ensure
that an optimal diagnostic study has been performed.
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What will I experience during the procedure?
You may experience some minor discomfort from the intravenous injection
of the radiopharmaceutical.
You will be asked to exercise until you are either too tired to continue
or short of breath, or if you experience chest pain, leg pain, or other
discomfort that causes you to want to stop.
If you are given a medication to increase blood flow because you are unable
to exercise, the medication may induce a brief period of feeling anxious,
dizzy, nauseous, shaky or short of breath. In rare instances, if the side
effects of the medication are severe or make you too uncomfortable, other
drugs can be given to stop the effects.
Most patients can resume regular activities immediately after the procedure.
The radioactivity in your body will decrease due to the natural process
of radioactive decay. In addition, radioactivity will decrease because
the radiopharmaceutical passes out of the body in the urine or stool.
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Who interprets the results and how do I get them?
Generally, patients undergo a nuclear medicine examination because a referring
physician has recommended it. A physician who has specialized training
in nuclear medicine will interpret the images. It usually takes one to
three days to interpret, report, and deliver the information to your referring
physician. Your referring physician will then inform you of the results.
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What are the benefits vs. risks?
Benefits
- The functional information regarding blood flow to the heart and
the pumping function of the heart is well demonstrated. This information
may be used to determine what treatment or additional testing, if any,
is needed.
- Computers are involved in the generation of the images, so measurements
or quantification of function as well as the determination of abnormalities
are possible.
- Because the procedure is generally performed according to standardized
protocols, the type of examination done at one hospital is likely
to be similar to that provided at other hospitals, making the information
easy to understand or to transfer to all doctors who may be involved
in your care.
Risks
- If you have coronary artery disease, it is possible that you could
experience chest pain, or angina, when stress by exercise or by being
given a drug is applied to your heart. However, your test will be carried
out under the supervision of a specialist trained to monitor you and
your heart by using information being provided by the electrocardiogram,
by your heart rhythm, and by your blood pressure. If necessary, medication
can be given for your chest pain. You will be monitored long enough
to ensure that you are at your baseline; that is, the condition you
were in when you came for the test.
- The use of a radioactive substance will result in exposure to a small
amount of radiation to the heart and to the body. However, the amount
of radioactivity administered is the smallest possible to provide adequate
images. Cardiac nuclear medicine procedures have been done for more
than three decades, and no long-term adverse effects have been reported
from such low-dose studies.
- Allergic reactions to radiopharmaceuticals can occur, but are extremely
rare.
- As with all radiologic procedures, it is important that you inform
your physician and the technologist if you are pregnant. In general,
exposure to radiation during pregnancy should be kept to a minimum.
Depending on the nature of your medical problem, the cardiac nuclear
medicine procedure may be postponed until after your pregnancy.
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What are the limitations of Cardiac Nuclear Medicine?
Compared to most radiology studies, cardiac nuclear medicine procedures
are time-consuming. They involve either exercise or the administration
of a drug to increase blood flow to the heart, obtaining gamma-camera images
(usually two sets of images separated by a few hours), and then computer
manipulation of the information. Depending on the exact procedure performed,
the myocardial perfusion scan takes between two and five hours. Occasionally,
a patient may be asked to return to the nuclear medicine department the
next day. An outpatient may be allowed to leave the hospital between the
two sets of images. An inpatient will usually return to his or her hospital
room between the imaging sessions.
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