Single photon emission computed tomography (SPECT) is a noninvasive technique used to create very clear, three-dimensional pictures of a major organ (e.g., the heart) by measuring blood flow through the organ. This test is very useful to help diagnose coronary artery disease.

SPECT scans use radionuclide imaging – a technique that involves the injection of very small amount of a radioactive substance called a tracer. Energy from the tracer in the body is detected by a gamma ray camera, which takes pictures that are reconstructed by a computer. A tracer is not a dye (contrast medium).

People may experience some slight discomfort from the needle used to insert an intravenous (I.V.) line in the crook of their arm, which is necessary to give the patient the tracer. Otherwise, these tests are painless.

Although the tracer is flushed harmlessly from the bodies of most individuals in about 48 hours, there are some people (e.g., pregnant or breastfeeding women) who should not have a radionuclide test. Patients are encouraged to discuss with their physician any concerns they may have about radiation exposure.

Cardiac SPECT (single photon computed tomography) scans are noninvasive imaging tests that can provide very accurate, three-dimensional pictures of the heart’s structures and function. This is accomplished through the use of a radionuclide tracer that is injected into the patient’s blood and travels to the heart. As it is travels through the heart, the tracer emits energy that is visible to a special camera called a gamma camera. During a SPECT test, the gamma camera rotates around the patient, taking pictures of the tracer’s progress as it is absorbed by the heart muscle. If absorbed unevenly or too slowly, a physician may suspect heart disease and order further tests or make a diagnosis.

Today, many cardiac patients undergo a SPECT scan as part of their diagnostic work-up for detection and evaluation of heart disease. Cardiac SPECT imaging is a valuable tool for:

SPECT scans are increasingly used to monitor patients following bypass surgery, in which a section of a blood vessel from another part of the body is taken and used to create a detour around a clogged coronary artery. Studies have shown that SPECT scans can be an effective tool in determining if patients remain at a high risk for heart attack or other major cardiac event. They can be used, for example, to assess patients with left bundle branch block who have had a heart attack. SPECT scans have also been used to predict potential heart problems for patients with diabetes who are unable to exercise and are at high risk for heart attack. Because SPECT scans are generally effective tools in predicting heart attack risk, they are also used in hospital emergency departments for evaluation of patients who arrive with complaints of chest pain.

Cardiac SPECT scans are particularly important in the diagnosis of heart disease in women because other tests such as the electrocardiogram (EKG) and the exercise stress test and planar scans could fail to detect heart disease in women accurately. This is because women’s thicker breast tissue can make it difficult to obtain clear images using older traditional planar imaging or diagnostic techniques. A SPECT scan gives improved image quality. SPECT scans are also being used in the diagnosis and evaluation of tumors in the breast and other areas, including the brain.

There are a number of important similarities and differences between SPECT and some other imaging techniques:

• SPECT, the standard magnetic resonance imaging (MRI) and the traditional computed tomography (CT) scan all produce an image of an organ’s anatomy. However, only SPECT provides a picture of how an organ is functioning by measuring its ability to absorb the tracer, which allows physicians to measure how quickly the heart muscle takes up blood, rather than an image of its anatomy. This information is often used to detect and monitor coronary artery disease.
  
  
• Both SPECT and a positron emission tomography (PET) are forms of nuclear imaging. However, the SPECT test differs from the PET test in that the tracers used in SPECT testing stay in the bloodstream, whereas the sensitive tracers used in PET are absorbed into the tissues themselves. In PET testing, this is accomplished by attaching radionuclide tracers to molecules such as glucose that travel into cells, thus allowing physicians to gauge the metabolic function of the organ by measuring glucose uptake patterns, for example. While PET testing may yield more detailed information, it is more expensive and requires use of radioactive particles that have an extremely short half-life and must be produced relatively near to the PET scanner. SPECT scanners, by contrast, use more stable radionuclide tracers and are more common and less expensive.
  
  
• Both SPECT and a coronary angiogram can help detect arterial stenosis (narrowing or blockage of an artery). However, SPECT is often used as an initial assessment to avoid a coronary angiogram because SPECT is noninvasive and relatively inexpensive.

There are certain conditions that could interfere with the clarity of SPECT images, making a patient ineligible for a SPECT scan. These conditions include viral infection of the heart (myocarditis) or recent lung infection. Pregnant or breastfeeding women should also avoid the SPECT scan because of its use of a radioactive tracer.

During a SPECT scan, the patient is injected with a small amount of radioactive tracer. This tracer remains in the bloodstream and can be tracked as blood travels through an organ (e.g., perfuses the organ). As the tracer travels, gamma cameras (which are rotating around the patient’s body) take pictures to monitor its progress. The cameras record the distribution, position and rate of flow of the tracer through the heart. Computer graphics are used to create three-dimensional images of blood flow through the heart that can be projected onto a computer monitor for the physician to study.

Any marked change in the absorption rate from established “normal values” could be a sign of heart disease. If the heart absorbs abnormally high or low levels of the tracer element, the pictures produced will be either very bright (a “hot spot”) or very dark (a “cold spot”), respectively. Either extreme is a clear sign of abnormal function, damage or disease of the heart. Depending on which organ of the body is being imaged, physicians may use one of several different chemicals as the tracer. These chemicals are called radiopharmaceuticals, which are proteins or organic molecules with radionuclides attached. Some radiopharmaceuticals contain proteins that will collect in the heart muscle, while others are more useful for studies of other organs such as the lungs or the brain.

The two most common radiopharmaceuticals used for cardiac SPECT imaging today are thallium-201, and technetium-99m. A relative newcomer is technetium Tc99m sestamibi, which has been very successful in obtaining vivid images of the female heart. Women’s thicker breast tissue and smaller heart makes it more difficult to obtain good diagnostic images of the heart, and as a result, women’s heart problems are often misdiagnosed. Recent studies have suggested that this tracer is more effective than thallium when used in cardiac SPECT imaging in women.

Still other chemicals are being studied, including several forms of technetium-99m such as technetium-99m tetrofosmin. This chemical was shown to be roughly as effective as sestamibi. Another new chemical, technetium-99 NOET, seems to have the imaging capability of regular technetium-99 but with better redistribution qualities.

There is only one type of SPECT scan and it is a nuclear test. However, there are two types of stress tests that use SPECT scans. The first is the nuclear stress test. A nuclear stress test takes very clear, three-dimensional pictures of the heart when the person is at rest and shortly after physical stress. By measuring the rate of absorption of the radioactive tracer before and after exercise, these pictures show contrasts between light and dark spots, which can indicate areas of damage or reduced blood flow that are present before, during and after exercise.

If the person is unable to exercise, the physician may choose to do a type of nuclear stress test called the pharmacological stress test. Also known as a chemical stress test, this type of diagnostic tool is used with patients who are unable to perform physical activity or in cases where the chemical stress test would be more accurate. This might include patients who cannot walk because of arthritis or another limb infirmity, or due to the presence of asthma or emphysema that would hinder their ability to exercise. Finally, chemical stress tests may be ordered for patients who have an abnormal electrocardiogram (EKG) or pacemaker.

A chemical stress test measures the reaction of the heart under chemically induced stress to assess the blood supply and motion of the heart muscle. This test relies on the injection of another drug, in addition to the tracer, that causes the heart to react as if the person were exercising even though they are actually at rest. Drugs that may be used to produce the pharmacological stress include:

Another type of cardiac SPECT scan is the gated SPECT scan, which includes the multi-gated acquisition scan (MUGA). This type of scan uses an electrocardiogram (EKG) to time exactly when the gamma camera takes pictures of the heart’s functions and structures. In this way, physicians can obtain images of the heart when it is contracting (systole) and relaxing (diastole). The gated SPECT scan provides a much more complete picture of how a heart is functioning and dramatically improves the accuracy of the diagnosis. The images obtained with gated SPECT scans have the additional advantage of being highly reproducible and consistent.

For specific information on how to prepare for a nuclear stress test or a pharmacological stress test, and how those tests are done, see Nuclear Stress Test.

For specific information on how to prepare for a multi-gated acquisition test (MUGA), and how this test is done, see MUGA Scan.

After the test, patients can usually resume normal activities, diet and medications immediately. The physician may provide the patient with a preliminary report of the test results, although a complete report will take a few days. A follow-up visit in the physician’s office will likely be scheduled to discuss the test results, treatment options and future risks relating to any cardiac abnormality detected during the SPECT scan.

Based on the diagnosis, additional testing or treatment may be recommended. Results will aid the physician in determining whether further testing, such as cardiac catheterization, is necessary.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions regarding SPECT scans:

1. How long will this test take?
   
   
2. Is there any danger from exposure to radiation during this test?
   
   
3. Is there a limit to how many SPECT scans I should have because of exposure to radiation?
   
   
4. Will my insurance cover this test?
   
   
5. Is there anything I can do after the test to help flush the radioactive tracer from my system?
   
   
6. If this test shows problems with my heart, what is the next step?
   
   
7. If this test does not show problems with my heart, do you recommend that I get additional scans in the future?
   
   
8. Is this test noisy, or will I be in an enclosed area?
   
   
9. What is the sensitivity of this test?
   
   
10. Are there any other alternatives if I am uncomfortable with exposure to radiation?
    
    
11. Should I have a chemical or exercise SPECT stress test?