In the past 30 years, nuclear cardiology—the diagnostic specialty that helps identify and measure the extent of coronary artery disease—has evolved to the point where it has become the standard of care.
In the past 5 years, the specialty has enjoyed robust growth—between 10% and 15% each year, according to Gary Heller, MD, PhD, director of the Nuclear Cardio-logy Laboratory at Hartford Hospital (Hartford, Conn). To put that rate in perspective, he compares it to stress echocardiology, which has been growing only 1% to 2%. “This represents a substantial difference,” he says.
Indeed, nuclear cardiology has become the most important tool in evaluating patients for coronary artery disease. “That’s why there are 9 million–plus procedures performed each year,” says Jeff Kao, general manager of global nuclear medicine for GE Medical Systems (GEMS of Waukesha, Wis).
Siemens Medical Solutions’ c.cam (left), a dedicated, small-footprint cardiac gamma camera system, is designed to enhance imaging accuracy and efficiency (right) as well as improve patient comfort—all in an in-office setting.
Nuclear cardiology uses mildly radioactive pharmaceuticals and gamma cameras to image blood flow to the heart for detecting the presence and extent of coronary artery disease. Most often performed with a stress test (typically treadmill exercise), it can help determine if more testing is necessary. A nuclear cardiology exam also can determine an individual’s risk for a future heart attack as well as measure the extent of damage from a previous attack.
The earliest procedures in this field were developed in the late 1960s. By the 1970s, after radiopharmaceuticals became more effective, the specialty began attracting much more interest. In the ensuing years, continuing technological development, coupled with better radiotracers, has further increased the modality’s effectiveness and versatility—as well as the number of nuclear cardiologists.
“Its roots were as a diagnostic technique, and it developed into a test for risk stratification, whereby we can determine which patients are at the highest risk for a subsequent problem and then gear the therapy accordingly,” says Robert C. Hendel, MD, president of the American Society of Nuclear Cardiology (ASNC of Bethesda, Md) and director of the nuclear cardiology section of cardiology at Rush University Medical Center (Chicago).
As Hendel indicates, nuclear cardiology also helps physicians determine patient management decisions and treatment strategies for ischemic heart disease and congestive heart failure. Other potential applications loom on the horizon, he remarks.
Modes of Application
Richard A. Goldstein, MD, MBA, FACC, has been practicing nuclear cardiology since 1980, when he was director of nuclear cardiology at the University of Texas–Houston. Currently, he is associate director of the Heart Institute at Albert Einstein Medical Center (Philadel-phia). Throughout his career, Goldstein has seen a lot of changes.
“I was in [the field] when it was just planar imaging,” he recalls. “There was one camera that you would put on a particular view for 10 minutes, and then you would move it to the next view for 10 minutes, and then the third view for 10 minutes—rather than have it go around in an orbit of 180?.”
Obviously, planar imaging had limitations, but recent advancements have overcome its drawbacks. The specialty took a quantum leap when single photon emission computed tomography (SPECT), a nuclear imaging technique, was integrated with myocardial perfusion imaging (MPI).
Essentially, SPECT involves obtaining a set of images around the chest following the injection of a radiotracer. SPECT makes it easier to identify defects both at rest and during stress; MPI, a proven diagnostic tool for detecting coronary artery disease, reveals blood flow.
During a SPECT MPI procedure, two sets of images are obtained for comparison. The first is acquired after a period of rest and the second after a period of stress. During the first part of the procedure, the technologist injects the patient with a radiotracer—usually thallium 201 (Tl 201) or technetium 99m (Tc 99m) sestamibi—and then positions the patient on his back between a set of gamma cameras. These cameras rotate around the patient’s chest, taking multiple images that a computer reconstructs as a picture of the heart. The cameras also follow the path of the radiotracer.
During the second part, a technologist places electrodes on the patient’s chest to monitor heart rhythm by electrocardiography. Stress is induced by exercise, but it also can be induced with a drug injection, usually dipyridamole, for patients who can’t exercise. After the stress, a radiotracer is again injected to acquire a second set of images, with the cameras again following the path of the radiotracer. Injured areas won’t take up the radioactivity while uninjured areas will, which helps spot infarction and ischemia. The two sets of images are then compared to determine the regions of ischemia.
The advancement of SPECT MPI enabled the development of gated SPECT, which, so far, is regarded as the most significant development in the specialty. It provides information about both blood flow and function in a single study, and it offers improved specificity by reducing the number of attenuation artifacts, thus enhancing the diagnostic and prognostic capabilities of SPECT. Accep-tance was rapid and enthusiastic. Less than 10 years ago, the technique was unknown; today, gated SPECT is used in almost all of the hospitals in the United States.
“State-of-the-art nuclear cardiology means that you are doing gated SPECT,” comments Hartford Hospital’s Heller. “I’d estimate that 95% of labs are using it. Essentially, if you’re going to perform a stress perfusion study, gated SPECT is what you do, because you can measure both ventricular perfusion and function.”
Gated SPECT is similar to standard SPECT, except that 8 to 16 images are acquired at each stop. Images are acquired in both a standard data set (used to evaluate perfusion) and a gated SPECT data set (used to assess function). Introduction of technetium-labeled radiopharmaceuticals and dual-detector SPECT systems offered a significant improvement in image quality. “It is probably the most commonly used approach and is the standard at most centers,” Goldstein reports. “Cardiologists like it because it gives them more information.”
The SPECT component, Goldstein says, gives details about blood flow at both rest and stress. The gated part provides information about function, specifically injection fraction (blood flow into the heart) and regional wall motion.
F. David Rollo MD, PhD, chief medical officer for Philips Nuclear Medicine (Andover, Mass), explains that gated SPECT essentially creates an unsophisticated motion picture. “You’re using an EKG to gate the images being collected so that with each segment of the EKG, a different picture will be made that represents the state of the position of the heart at that particular part of the EKG cycle,” he explains. “At the end, you put all of these images together, and they allow you to look at the heart motion that has been synchronized with the EKG.”
Rollo likens the result to a cartoon flip book, where you flip through a series of still pictures and observe the appearance of movement. By looking at the motion of the heart wall this way, he says, you can tell if a patient has suffered a myocardial infarction, for example, because the damaged area won’t move.
The CardioMD from Philips Medical Systems is an in-office nuclear camera system featuring the fixed 90-degree Forte gamma camera and Pegasys workstation.
In the Office
The other major trend in nuclear cardiology today is the movement of the specialty into the freestanding clinic and private-office settings. “This is where almost all of the growth in the past 5 years has occurred,” reports the ASNC’s Hendel.
In fact, Rollo points out that nearly 54% of all nuclear medicine procedures are nuclear cardiology, nearly 70% of which are performed in the outpatient setting. And Heller says that several reasons are behind this trend. Most importantly, cardiologists find it very useful for their patients.
“The cardiologists were the first ones to take [nuclear cardiology] out of the hospital and bring it to the office,” reports Walter Gaman, MD, senior managing partner of Healthcare Associates (Irving, Texas). “Right now, in our area, we are the only such practice to do it, but I think it’s only a matter of time before others do it as well.”
Cardiology, Rollo points out, is unique in that when a patient is referred, the cardiologist takes complete responsibility. It’s a different situation than one involving an oncologist, for example, who would use another source of diagnostic information and refer a patient to a radiologist for a diagnostic study. The radiologist would send the information back to the oncologist, and then the oncologist would decide what to do next.
Conversely for the heart patient, the cardiologist takes responsibility for the patient’s diagnosis and treatment. If not, then a cardiologist who wants a diagnostic study—specifically nuclear cardiology—would have to send the patient to the hospital. The cardiologist would also have to go to the hospital to do the exercise testing. “This is inconvenient and inefficient, because the cardiologist could have been back at the office performing a cardiac catheterization,” Rollo comments.
Further, when the study is interpreted, the cardiologist would have to be present to determine the significance of the disease revealed by the images. On top of that is the matter of scheduling: The cardiologist might have to wait 2 or 3 days before it’s convenient to both his schedule and to the hospital radiologist’s schedule. That could be crucial time for a heart patient. “In the 3 days of waiting for the procedure to be performed, the patient could die from a heart attack,” Rollo says. “So cardiologists took it upon themselves to create their own specialty of nuclear cardiology.”
Overall, it presents a win-win situation for the physician and the patients. For the patient, it provides comprehensive care in one setting. For the physician, it’s economically advantageous. “It has favorable reimbursement,” Heller says. “The physicians feel that it not only provides a value but they can get paid for doing it.”
Appropriately, cardiology training programs now offer nuclear cardiology, resulting in an increase in physicians versed in the modality.
Interpreting Exams
As part of this trend, more cardiologists are doing their own interpretations. To Goldstein, this step seems only natural. “In the private sector, if cardiologists have a technique that provides them with the answers they need, and they also get to bill for it, they’re much less likely to send a similar type of analysis to a radiologist,” he points out.
Rollo says this trend is significant but not yet universal; it’s a matter of training and certification.
Currently in his office, family physician Gaman says he and his colleagues don’t feel qualified to interpret nuclear cardiology. “Our scans are interpreted by radiologists, but most cardiologists are doing nuclear cardiology in their offices and interpreting themselves,” he says.
But Hendel doesn’t feel anyone needs to be, or should be, excluded from the process. Rather, he feels a cooperative and collaborative relationship can be developed to the benefit of all. “All specialties bring particular strengths to the table,” he emphasizes. “The ideal arrangement is cooperation among the specialties, each bringing something from his background that is unique and potentially beneficial to the overall procedures for patient care.”
Product Development
As nuclear cardiology activity in the office setting increases, so does appropriate product development. Manufacturers are expanding their product lines to help facilitate the performance of nuclear cardiology in outpatient settings. Recent innovations include small-footprint cameras dedicated to nuclear cardiology. “They also cost less, thus a private nu-clear cardiology group can afford to do nuclear cardiology,” Hendel says.
In the past, when nuclear cardiology was performed in hospitals, procedures were performed with cameras designed to do all types of nuclear medicine procedures—such as scans of the bones, brain, liver, and kidneys. These systems were large and heavy, requiring a lot of software. Newer systems feature fixed, dual-head cameras with relatively small detectors, and they’re designed specifically for nuclear cardiology. “They’re not really acceptable for general nuclear work, but they’re fine for cardiology,” Goldstein explains. “They don’t take up a lot of room in an office. At Einstein, we use a small camera made by Siemens.”
Siemens Medical Solutions (Malvern, Pa) is one of the top three vendors in this market niche. At the ASNC meeting last September, the company introduced its small-footprint c.cam, a reclining dedicated cardiac gamma camera system that features myocardial viability and perfusion capabilities as well as integrated software for analysis of ejection fraction and wall motion. The product was designed to provide a fast, easy, and cost-effective way to create an in-office nuclear cardiology department or to expand existing services.
The Millennium MyoSIGHT from GEMS offers one of the smallest footprints in the nuclear cardiology arena, requiring just 110 square feet of floor space.
Despite the growth in outpatient nuclear cardiology and its lucrative market, not all procedures are moving out of the hospital. “The hospitals do provide full-service capacity, and they’re enjoying the growth as well,” reports GEMS’ Kao.
Rollo concurs, adding, “The other part of this trend is that many hospitals now have a nuclear cardiology department, and they perform the nuclear cardiology procedures in that department rather than in radiology.”
Still, the biggest current challenge for manufacturers, Kao indicates, is designing equipment that is ideally suited for the private office but that doesn’t compromise image quality and patient care. GEMS has weighed in with its own Millennium MyoSIGHT, which, according to the company, has the smallest footprint in the industry. This dual-detector, variable-angle system performs all nuclear cardiac procedures, including 180? and 360? SPECT. Like other systems of its type, the MyoSIGHT is designed to optimize patient comfort and throughput, and it fits into the office setting, as it requires only 110 square feet of floor space. In addition, the newest generation of the system has a fast functional imaging workstation, called Xeleris, as well as a workflow system that fully automates exams.
Rollo reports that over the past few quarters, Philips has had a more than 70% market share for office-setting equipment. Its CardioMD system is an in-office nuclear camera dedicated to cardiac applications. It is a fixed 90? camera specifically designed to meet the unique requirements of an office-based practice. The system’s Forte gamma camera has an open gantry design and features that streamline workflow. Its Pegasys workstation was designed to provide excellent image quality.
Attenuation Correction
Along with their other capabilities, most of the new systems can offer attenuation correction—an innovation that has an enormous impact on the field of nuclear cardiology. The attenuation, or disappearance, of photons in SPECT imaging degrades the quality of scans and negatively impacts image accuracy.
Attenuation results from variations of tissue covering the heart. Thick tissue can absorb the photons emitted from the heart. When this happens, it appears like a decrease in activity, which is what indicates ischemic heart disease. Problems arise when imaging female patients with large breasts, male bodybuilders who have thickened pectoral muscles, or patients who are obese. These types of patients’ scans result in attenuation artifacts. In a large-breasted woman, for example, images could show a decreased activity in the interior myocardium.
“It’s not necessarily that she actually has decreased blood flow there,” Goldstein says. “But because the counts have to go from her heart through her chest to the detector, there will appear to be a defect.” Clearly, many unnecessary procedures can result. Upon observing an attenuation artifact, a physician would feel obligated to send the patient to the cardiac catheterization lab, just to make sure she doesn’t have heart disease.
But attenuation correction has increased physicians’ diagnostic confidence. “Atten-uation correction provides more than 90% accuracy,” Rollo says. “Without it, the accuracy is somewhere in the 80s.”
However, attenuation correction hasn’t caught on yet like gated SPECT has. But, as Heller reveals, that’s only because it’s an area that’s still relatively new, and some early missteps were made. “There were some algorithms that didn’t work quite as well as the ones currently available, so people kind of got soured on it,” he says. “But I use it and find it extremely useful and valuable. It works very well with the right system. I think it is something that is really going to take off in the next 3 or 4 years. Right now, we’re only at the start of that curve, not at the end of it.”
Future Directions
A complementary area of research that is generating a great deal of interest is radiopharmaceuticals. Heller feels that current research in this area will take nuclear cardiology toward molecular imaging, which, he thinks, can become very important in terms of targeting various diseases’ states.
Hendel agrees. “Nuclear cardiology could become a tool for getting at the molecular basis of cardiovascular disease,” he says. “We are seeing innovations in receptor imaging, and we’re also seeking molecular probes for detection of myocardial necrosis—or programmed cell death, also known as apoptosis. These are only a few examples of a burgeoning field.”
In the meantime, nuclear cardiology as a field will continue to grow, thanks to its increasingly high degree of accuracy and effectiveness. Moreover, it is a cost-effective modality that, as Kao points out, is helping with the nation’s medical costs at a critical time. “We all understand the expense of having catheterization and interventional procedures,” he says. “The ability to do noninvasive procedures and understanding what is happening in the heart is not only good for the patient but for the cost of medicine in general.”
Dan Harvey is a contributing writer for Medical Imaging.
