Defining Roles: Where Does Cardiac MR Fit in?
Cardiac MR has had limited penetration, but that might change. Recent studies have documented its efficacy, which may drive a larger role in practice. Yet, as some experts predict an expanded role, others outline an array of obstacles.

Cardiac MR has a few inherent advantages over other cardiac imaging modalities. With radiation exposure and patient safety concerns at a crescendo, cardiac MR benefits from not requiring ionizing radiation or iodinated contrast. MRI also provides flexibility in its capability to assess numerous parameters of cardiovascular anatomy and function. It can characterize tissue composition and offers unique information about the presence and extent of myocardial scar. Cardiac MR also can quantify blood flow and measure heart wall motion.

MR vs. SPECT: Researchers weigh in

What has recent research revealed about cardiac MR's role regarding patients with suspected coronary artery disease (CAD)? Two studies—the CE-MARC trial and MR-IMPACT II—detailed the accuracy of MR compared with SPECT.

CE-MARC, published Feb. 4 in The Lancet, recruited 752 patients with suspected heart disease and scheduled them for MR and SPECT, as well as an x-ray coronary angiogram used as the reference standard. Angiography identified 39 percent of the patients with CAD. Cardiac MR had a sensitivity and specificity of 86.5 percent and 83.4 percent, respectively. In comparison, SPECT delivered a sensitivity and specificity of 66.5 percent and 82.6 percent, respectively.

MR-IMPACT II supported MR's position as a safe alternative to SPECT withhigher sensitivity to detect CAD, according to results published March 4 in European Heart Journal. Unlike CE-MARC, however, MR's specificity was inferior to SPECT in this study.

Cardiac MR in practice

As research data accrue, some clinicians are considering an expanded role for cardiac MR.

"MRI has proven itself a very robust technique," says Raymond Y. Kwong, MD, MPH, director of cardiac magnetic resonance imaging at Brigham and Women's Hospital (BWH) in Boston. At BWH, MR is often used as a tie-breaker when other modalities are not clear about what caused a patient's reduced heart function as it provides a noninvasive interrogation of the myocardial physiology, says Kwong.

Cardiac MR volumes have grown six- or seven-fold at BWH over the last decade, and it now handles approximately 1,300 cases per year, primarily to assess patients with heart failure. In the past, after an echocardiogram of a patient with suspected heart failure indicated reduced heart function with an unknown cause, an angiogram, nuclear study and/or biopsy would be ordered to clarify diagnosis. Now, MRI use is increasing, sometimes as a first test, because of its noninvasiveness and comprehensive assessment of heart failure. To grow utilization, BWH advertised its cardiac MR program by sending letters to hundreds of area practitioners, and hosted local talks to educate physicians about the modality.

"The key is to provide good relationships with prompt reporting, or phone discussion, and documentation of imaging findings with the referring physicians, so they can promptly know the results and execute plans for patient management," says Kwong.

Despite the enthusiastic deployment of cardiac MR at BWH and recent studies detailing its accuracy compared with SPECT, other experts suggest the modality may face an uphill battle. Stress echo and stress nuclear tests have a firm position in the management of CAD, says Robert O. Bonow, MD, professor of cardiology at Northwestern University in Chicago. "Echocardiography is here to stay in terms of evaluating patients with structural heart disease." But he believes that MRI could find a complementary role in diagnosing aortic aneurysm, hypertrophic cardiomyopathy or other forms of cardiomyopathies.

As cardiac MR struggles to carve diagnostic turf, its utilization may be handicapped by a lack of information about patient outcomes and cost-effectiveness. "Even though MRI may give you better images and maybe even superior diagnostic capabilities, the question becomes 'How much more superior?' Does that extra ounce of superiority lead to better patient outcomes? We don't have that information right now," says Bonow.

Until those questions are answered, it will be difficult to determine where exactly MR fits relative to other modalities.

Overcoming obstacles

In addition to uncertainties about patient outcomes and cost-effectiveness, another obstacle standing in the way of wider cardiac MR utilization is its technical complexity. While it is relatively simple to manage the technical performance of a CT study, MRI has sophisticated sequencing that requires special training for technologists and physicians.

The Society of Cardiovascular Magnetic Resonance is aware of the knowledge gap, and is developing training sessions, as well as working to make cardiac MR imaging protocols simpler and faster.

Because MRI had been the exclusive domain of radiologists, the early stages of cardiac MR adoption were marred by turf wars between cardiologists and radiologists, with each specialty touting its unique knowledge base, says Scott D. Flamm, MD, section head of cardiovascular imaging at Cleveland Clinic, who adds that both specialties are needed to obtain the best result.

"There's a lot of technical expertise that radiologists possess, and there's a lot of clinical or physiologic expertise that cardiologists have," says Flamm. "In return, radiologists can be a tempering lever as gatekeepers to make sure we aren't imaging everyone for everything."

As cooperation between the specialties continues to develop, some facilities have found ways to maximize their complementary knowledge. At BWH, for example, cardiac MR studies are read by two cardiologists and one radiologist.

As turf wars die down and training becomes more accessible, the biggest opportunities for growth will come only as physicians become more comfortable with a different strategy. Many cardiologists are familiar with nuclear technology and own these systems, which contributes to the inertia, says Kwong. There are also reimbursement issues—particularly regarding patients with pacemakers or implantable cardioverter-defibrillators—that must be ironed out.

"When an established imaging modality exists and a new modality comes in second, the new modality has to face a higher level of criticism," says Kwong. "MR has proven itself to be a robust and uniquely useful technique for patient care and more physicians are using it, but [changing widespread practice patterns] takes a long time."