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Cardiac Nuclear Medicine: Myocardial Perfusion Imaging, Radionuclide Ventriculography
Nuclear medicine is a subspecialty within the field of radiology that uses very small amounts of radioactive material to diagnose or treat disease and other abnormalities within the body. Nuclear medicine imaging procedures are noninvasive and usually painless medical tests that help physicians diagnose medical conditions. These imaging scans use radioactive materials called a radiopharmaceutical or radiotracer. Depending on the type of nuclear medicine exam you are undergoing, the radiotracer is injected into a vein, swallowed by mouth or inhaled as a gas and eventually collects in the area of your body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radiotracer absorbed by your body and to produce special pictures offering details on both the structure and function of organs and other internal body parts. In some centers, nuclear medicine images can be superimposed with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses. Cardiac nuclear medicine studies provide pictures of the structure and function of the heart.
Myocardial Perfusion Imaging: A small amount of an imaging agent {thallium or sestamibi (Cardiolite) or tetrofosmin (Myoview)}, is injected into the blood stream during rest and during exercise or chemical stress. A scanning device (gamma camera) is used to measure the uptake by the heart of the imaging material during (exercise or chemical stress) and at rest. If there is significant blockage of a coronary artery, the heart muscle may not get enough of a blood supply in the setting of exercise or during chemical stress. This decrease in blood flow will be detected by the images. Myocardial perfusion studies can thus identify areas of the heart muscle that have an inadequate blood supply as well as the areas of heart muscle that are scarred from a heart attack. In addition to the localization of the coronary artery with atherosclerosis, myocardial perfusion studies quantify the extent of the heart muscle with a limited blood flow and can also provide information about the pumping function of the heart. Thus, it is superior to routine exercise stress testing and provides the necessary information to help identify which patients are at an increased risk for a heart attack and may be candidates for invasive procedures such as coronary angiography, angioplasty and heart surgery.
Evaluation of Cardiac Function with Radionuclide Ventriculography What are some common uses of the procedure?Physicians use cardiac nuclear medicine studies to help diagnose symptoms such as:
How should I prepare?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. See the Safety page for more information about pregnancy and breastfeeding related to nuclear medicine imaging. You should inform your physician of any medications you are taking as well as vitamins and herbal supplements and if you have any allergies. Also inform your doctor about recent illnesses or other medical conditions. You should inform your 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. Jewelry and other accessories should be left at home if possible, or removed prior to the exam because they may interfere with the procedure. You should avoid caffeine (coffee, tea, etc.) and smoking for 48 hours before your examination. You should not eat or drink anything after midnight on the day of your procedure, but you may continue taking medications with small amounts of water unless your physician says otherwise. What does the equipment look like?Most nuclear medicine procedures use a gamma camera, a specialized camera encased in metal that is capable of detecting radiation and taking pictures from different angles. It may be suspended over the examination table from a tall, moveable post or it may be part of a metal arm that hangs over the table. The camera could also be located within a large, doughnut-shaped scanner similar in appearance to a computed tomography (CT) scanner. In some imaging centers, the gamma camera is located beneath the exam table and out of view. A nearby computer aids in creating the images from the data obtained by the camera or scanner. How does the procedure work?With ordinary x-ray examinations, an image is made by passing x-rays through your body from an outside source. In contrast, nuclear medicine procedures use a radioactive material called a radiotracer, which is injected into your bloodstream, swallowed by mouth or inhaled as a gas. This radioactive material accumulates in the organ or area of your body being examined, where it gives off a small amount of energy in the form of gamma rays. A gamma camera, PET scanner, or probe detect this energy and with the help of a computer create pictures offering details on both the structure and function of organs and other parts of your body. In order to evaluate the coronary arteries, heart scans are often performed immediately after patients have engaged in physical exercise (called a stress test) so that blood flow throughout the heart is maximized. These images of the heart are compared with heart images taken while the patient is at rest. Patients who are unable to exercise are given a drug that increases blood flow to the heart. How is it performed?Nuclear medicine imaging is usually performed on an outpatient basis, but is often performed on hospitalized patients as well. You will be positioned on an examination table. If necessary, a nurse or technologist will insert an intravenous (IV) line into a vein in your hand or arm. The exam will begin with a stress test, which requires you to exercise either by 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 frequently measured. When blood flow to the heart has reached its peak, you will be given the radiotracer through your IV. One minute later, you will stop exercising and you will be positioned on a moveable examination table. If you are unable to use a treadmill or bicycle, you will not exercise but you will be given a drug that will increase blood flow to the heart. Approximately one half-hour later, the imaging will begin. Once the technologist has positioned the gamma camera, it will move slowly in an arc over your chest. This same heart scan will be performed at another time, when you have not been exercising (called a resting scan). Images of your heart obtained after you exercise will be compared with images of your resting heart. Actual scanning time for each heart scan varies from 25 to 40 minutes depending on the type of scanner used. When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed. If you had an intravenous line inserted for the procedure, it will be removed. What will I experience during and after the procedure?Most nuclear medicine procedures are painless. If the radiotracer is given intravenously, you will feel a slight pin prick when the needle is inserted into your vein for the intravenous line. When the radioactive material is injected into your arm, you may feel a cold sensation moving up your arm, but there are generally no other side effects. 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. It is important that you remain still while the images are being recorded. Though nuclear imaging itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging. Unless your physician tells you otherwise, you may resume your normal activities after your nuclear medicine scan. Through the natural process of radioactive decay, the small amount of radiotracer in your body will lose its radioactivity over time. In many cases, the radioactivity will dissipate over the first 24 hours following the test and pass out of your body through your urine or stool. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly. You should also drink plenty of water to help flush the radioactive material out of your body. Who interprets the results and how do I get them?A radiologist who has specialized training in nuclear medicine will interpret the images and forward a report to your referring physician. What are the benefits vs. risks?Benefits
Risks
What are the limitations of Cardiac Nuclear Medicine?Nuclear medicine procedures can be time-consuming. It can take hours to days for the radiotracer to accumulate in the part of the body under study and imaging may take up to several hours to perform, though new equipment is available that can substantially shorten the procedure time. The resolution of structures of the body with nuclear medicine may not be as clear as with other imaging techniques, such as CT or MRI. However, the information gained from nuclear medicine is unequaled in other imaging techniques.
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