Nuclear Cardiology
William A. Crafford, Jr., M.D., F.A.C.C.

Although significant inroads into diagnosis and therapy have evolved over the last 25-30 years, heart disease still remains the number one cause of death and disability in the Western world and claims the life of 40 percent of the 2.4 million Americans who die each year. Tremendous insights into causes, therapeutic interventions, and life style alterations have significantly reduced the risk of heart disease and enabled more effective management. The mainstay of treatment is still early diagnosis and intervention before severe heart damage occurs. With this goal in mind, nuclear cardiology has evolved for the noninvasive early diagnosis of coronary artery disease. It significantly embellishes the standard treadmill stress test which until the end to mid late 70’s was the mainstay in the diagnosis of coronary artery disease. Administration of radioactive isotope tracers with the use of special sensors called gamma cameras provide actual visualization of the heart muscle supplied by normal and diseased heart arteries.

From the late 1970’s to the early 1990’s, the first and most widely used radioactive tracer was Thallium 201. This radioactive element was similar to potassium, which is found dissolved in the blood throughout the human circulation. Thallium emits a single package of energy called a photon, and follows the coronary artery blood supply to the heart. It is taken up by the heart muscle supplied by those arteries in proportion to the blood supply received by that muscle. Diagnostic studies evolved in which Thallium was administered at rest and defined the distribution of blood supply to the resting heart muscle. The patients were then exercised with injection of Thallium prior to the end of exercise. This then defined the distribution of blood supply under stress.  The resulting rest and stress heart images were then compared to each other. Areas that fail to show Thallium uptake at rest and stress indicate “dead tissue”, and therefore previous heart attack. However, areas that only indicate decreased Thallium heart muscle uptake at stress were found to represent blocked heart arteries that needed further attention such as heart bypass surgery or balloon dilating catheter procedures to avoid impending heart attacks. These diagnostic studies required the patient to be able to ambulate and therefore were not helpful in those more debilitated individuals.

In the early to mid 1980’s it was found that venous administration of stimulatory agents such as Dobutamine, dipyridamole or adenosine could be used to simulate exercise in those individuals who could not walk or pedal a bicycle. This was combined with Thallium administration as well and enabled physicians to predict the risk for heart disease in disabled, nonambulatory patients. Combined with evolution in computer image reconstruction software, this allowed for the successful clearance of these debilitated patients for risky surgical procedures, and again their possible need for catheter-based interventions or heart surgery to prevent permanent heart muscle injury.

In the late 80’s and early 90’s Thallium was replaced by a new radioactive isotope called Technetium-99m attached to the carrier agent Cardiolite® (sestamibi). This agent has higher energy, stays around less time, and provides clearer images. It is not subject to being “washed-out” or redeposited in the heart muscle in a short time as was the problem with Thallium. Again, rest and stress images provide more accurate delineation of patients at risk for future heart attacks and those that have had significant damage from preceding heart events. This new single photon isotope also allows for assessment of heart function previously not available with Thallium. The “squeezing” ability of the heart in its different walls can now also be accurately analyzed.

These “noninvasive” tests allow for the less expensive but accurate diagnosis of blocked cardiac arteries without subjecting patients to unnecessary and potentially riskier invasive arterial insertion of catheters and injection of dye. Further computer enhancement with the ability to divide the heart into small slices called tomograms further improved accuracy. These studies today remain important diagnostic tools in the noninvasive prediction of the severity of cardiac artery disease. These Technetium-99m agents themselves do not pose an allergic risk and are dissipated in 6-8 hours. Current studies have shown that these radioactive tracer procedures pickup approximately 92%-95% of obstructive coronary artery disease if present. However, there is some problem with over diagnosing heart artery disease that is not really present in about 10%-15% of cases.

Technetium-99m radioactive tracers are also used to label red blood cells and follow their motion through the heart cycle to evaluate heart function. This is often used to assess patients risk for heart cancer chemotherapeutic agents, which may potentially be toxic to the heart muscle. This also is helpful in assessing the patient’s recovery from heart surgery, balloon catheter directed procedures, as well as preceding heart attacks.

Newer nuclear techniques also involve the use of “PET” imaging, which uses very short-lived radioactive tracers that emit “positrons” instead of single photons. There agents are tagged with chemical that interact with the basic nutritional and functional pathways of diseased and healthy heart muscle. “PET” imaging techniques are very definitive for assessment of heart damage and recoverability, as well as assessing cardiac muscle blood supply at rest or with stress. Accuracy seems to be even better when compared to present nuclear techniques. Availability of “PET” technology is planned to be available in Wilmington for the summer of 2003.

Newer radioactive isotopes and imaging techniques are being developed every day helping us to more accurately define the basic chemical function or dysfunction of the heart and circulatory system. Further inroads with these techniques are also being made in the diagnosis and management of cancer, as well as brain function. The future is yet to reveal what further diagnostic and potentially therapeutic benefits are still to evolve from this expanding technology.