Nuclear Cardiology: Myocardial Perfusion Imaging
Heart disease is the foremost cause of death in the western world. The major hurdle in the treatment of heart disease is the early and accurate diagnosis of the heart disease. Nuclear cardiology has played a crucial role in setting up techniques for the early diagnosis of heart disease and assessment of coronary artery disease.
In coronary artery disease, there is a narrowing of the blood vessels supplying the heart due to deposition of fat within the arterial walls. This phenomenon is called atherosclerosis and causes clogging of the arteries that result in impairment of the blood flow to the heart.
Nuclear cardiology studies employ non-invasive techniques for assessing myocardial blood flow, the heart’s pumping function, and also to identify the size and location of the damaged heart tissue.
Myocardial perfusion imaging (MPI) is the most widely used nuclear cardiology technique that deals with the assessment of blood flow to the heart (the myocardium). In this procedure, a radioactive tracer is injected into the bloodstream and heart function is analyzed by the doctor using a scanner called a SPECT (single photon emission computed tomography) camera.
The MPI procedure involves two sets of images by the SPECT camera; one with the patient at rest and other under stress that induces the heart to work harder. There are two ways to induce stress on the heart, which include:
- Exercising on a treadmill is the usual way of inducing stress on the heart.
- Drug-induced stress, generally used for the physically disabled patients. The drugs employed are dipyridamole, adenosine, regadenoson, and dobutamine.
In case of significant blockage of the coronary artery, the heart muscle may not receive enough blood supply during exercise or drug-induced stress. A decrease in the blood flow is detected by the images of MPI.
The basic steps involved in myocardial perfusion imaging are as follows:
- The patient is placed on a bed and the radioactive material is injected into the arm vein that will travel to the heart muscle.
- After 45 minutes the patient is again placed on the bed with both arms placed overhead and the patient is asked to remain motionless. A series of images are taken with the specially designed SPECT camera.
- The leads of an electrocardiogram are then placed on the chest of the patient to monitor the rhythm of the heart.
- The patient is then exposed to stress test either through exercise or drug induced; depending on the patient’s condition.
- The patient’s maximum level of exercise is assessed and a second injection of radioactive tracer is administered.
- The possibility of adverse symptoms such as breathing problems, chest pain, arm pain, palpitations, headache, difficulty in walking or any other discomfort, after the test, are discussed with the patient.
- After 45 minutes, a second set of images are taken in the same pattern and with the same instrument.
- Finally, the doctor interprets the images and prepares the report.
Other additional nuclear cardiology procedures that are used include radionuclide angiography or ventriculography and positron-emission tomography (PET). These are non-invasive procedures and also help in the assessment of the following parameters:
- Heart’s pumping ability
- Tissue damaged by previous heart attack
- Functioning of the heart valves
- Integrity of the cardiac chambers
The basic instructions to a patient undergoing MPI are as follows:
- Patient should avoid intake of food before the test, as they have to perform exercise on the treadmill.
- Take the prescribed medications with water before the test.
- The patient should wear loose-fitting clothing and comfortable shoes for the treadmill procedure.
- Avoid caffeinated foods and drink such as coffee, tea, colas, and chocolate, at least 12 hours before the test.