How Accurate Is a Myocardial Perfusion Scan?

A mycocardial perfusion scan is an imaging study that shows how well blood flows through the heart muscle. The sensitivity, specificity and accuracy of cardiac imaging with single-photon emission computed tomography (SPECT) is between 44 percent and 70 percent depending on the blood vessels involved.

In the United States, one person dies every 36 seconds due to cardiovascular diseases. This fact itself underlines the importance of screening the at risk (vulnerable) population for heart diseases.

A myocardial perfusion scan (radionuclide myocardial perfusion imaging [rMPI] or nuclear stress test) is an imaging technique to study how well the blood flows through the heart muscle. It provides important information about blood flow to the heart muscle at rest and work. A negative scan means there is no underlying hinderance to the blood flow to the heart muscle, whereas a positive scan suggests that the heart muscle is not getting adequate blood supply. A positive test may prompt further surgical management of heart vessel disease by classifying the patient as medium or high risk.

The two techniques for the myocardial perfusion scan are single-photon emission computed tomography (SPECT) and positron emission tomography (PET). The main difference between SPECT and PET scans is the type of radioactive material (radiotracers) used.

A myocardial perfusion scan does not show the actual obstruction in the blood vessels, but depending on the location of the involved muscle, it can indirectly tell your doctor which arteries are affected. These tests can also show a previously missed silent heart attack.

The sensitivity, specificity and accuracy of cardiac imaging with single-photon emission computed tomography (SPECT) is between 44 percent and 70 percent depending on the blood vessels involved. This means that the nuclear stress test or myocardial perfusion scan may miss three out of every 10 patients with severe disease.

In patients with a high risk of heart disease, computed tomography (CT) angiograms are 91 percent to 96 percent accurate and, therefore, are a better first-line test for diagnosis of heart disease than a cardiac perfusion scan.

A myocardial perfusion scan is performed as follows

• A myocardial perfusion scan is performed in a resting state and by stressing the heart. The entire procedure can take up to four hours.
• The entire procedure is done under the supervision of a doctor.
• An intravenous (IV) line will be started in your hand or arm and you will be connected to an electrocardiograph (ECG) machine with wires. A blood pressure cuff will be placed on your arm. A tracer (radioactive substance) will be injected into the IV line in your arm. It takes 10 to 60 minutes for the tracer to be absorbed by the body cells. The tracer travels through the bloodstream to the healthy heart muscle. Because only the healthy heart muscle cells can take up this substance, these areas will be eventually highlighted by the tracer. An exercise scan is conducted first. The rest scan is conducted about three hours later.
• The stress study: The heart is subjected to “stress” from exercise by making you run on a treadmill or ride a stationary bicycle at increasing speeds. This activity is continued until you achieve the target or desired heart rate. If you are unable to walk or ride, the heart can be stressed by taking a certain medicine that increases your heart rate or dilates blood vessels mimicking the exercise pattern. You will continue running until the doctor instructs otherwise. You will then be placed into the heart scanner so that a special camera captures your heart images during the stress. If you have any symptoms such as dizziness, chest pain, extreme shortness of breath or severe fatigue at any point during the procedure, let the healthcare provider or technologist know.
• The second set of scans will be taken three to six hours after the first set. During this time, you will not be allowed to eat anything, smoke or drink coffee. In some cases, your doctor may decide to have you return on another day for the second set of scans.
• Rest scan: The second set of scans will be similar to the first set, except that you will not be asked to exercise or given any medicine. You will be given an IV tracer and you will be made to lie on the table while the scanner takes pictures of your heart.
• Once the second set of scans has been completed, the IV line will be removed and you will be allowed to leave.
• Both the resting and stress scan reports are examined and compared. The highlighted area will tell the doctor about the extent to which the heart muscles are well perfused. The scan will also reveal the areas where blood flow is compromised. Your doctor will explain the results and decide on a further course of action.