November 11, 2008

Cardiac MRI
By Dan Harvey
Radiology Today
Vol. 9 No. 22 P. 14

Cardiac MRI offers physicians comprehensive data related to cardiovascular function and disease. Because it’s noninvasive and doesn’t expose patients to ionizing radiation, it entails less risk than more widely applied diagnostic procedures.

Further, cardiac MRI fosters rapid analysis and increased accuracy. In certain patient subsets, the modality offers a favorable alternative to competing modalities such as stress tests, electrocardiography, cardiac CT, and SPECT imaging.

However, despite the modality’s potential advantages and the growing body of recent research that underscores its value and greater potential, its application has been hampered by practical considerations such as cost and training, as well as perceptions about how it fits into the overall scheme of cardiac imaging.

Current Technology and Usage
A flexible tool, cardiac MRI has been deployed for a variety of specific purposes, such as the detection and management of congenital heart disease and cardiac masses, the assessment of valvular and ventricular function, myocardial perfusion, and the development of patient treatment protocols.

Presently, most cardiac MRI exams are accomplished on 1.5T systems. But facilities with the economic, technical, and staff resources are looking toward 3T systems. “The main advantages of 3T MRI scanners are most evident in the areas of myocardial perfusion and delayed enhancement imaging,” says Ricardo C. Cury, MD, medical director of cardiac MRI and CT at the Baptist Cardiac and Vascular Institute in Miami.

“Cardiac MRI also adds value in the areas of viability and coronary MRA [MR angiography],” adds Scott D. Flamm, MD, who heads the cardiovascular imaging division at the Cleveland Clinic’s Imaging Institute in Ohio, a site that routinely deploys cardiac MRI for patient care.

Meanwhile, applications with the mainstay 1.5T systems have been well established and bolstered by systemic advancements that include more sophisticated coil design (ie, up to 32 coil elements for cardiac imaging) that provides clinicians increased capabilities in parallel acquisition techniques, as well as a positive trade-off between signal-to-noise ratio spatial resolution and image acquisition, says Cury, who is also a consulting radiologist at Massachusetts General Hospital and an assistant professor of radiology at Harvard Medical School.

Such recent technical developments in the cardiac MRI field, adds Flamm, have enabled users to probe deeper into an increasing number of potential clinical applications. In the meantime, cardiac MRI is a complex tool that provides versatility even within a tightly focused target area. For instance, Flamm says the Cleveland Clinic can direct cardiac MRI advantages toward a wide spectrum of cardiovascular issues. “These include both acquired and congenital cardiac disease,” he says. “In the acquired-disease area, we use it for ischemic and viability evaluation and valvular disease for both qualitative and quantitative assessments.”

In addition, the Cleveland Clinic deploys cardiac MRI extensively for infiltrative cardiomyopathy, particularly hypertrophic cardiomyopathy. “We also use it for pericardial disease, constrictive pericarditis, cardiac masses, thoracic aortic disease, pulmonary vein assessment, and arteritis,” says Flamm.

The menu of applications is just as varied at Thomas Jefferson University Hospital in Philadelphia. “We have been using cardiac MRI to look at cardiac masses, valvular function, and aortic diseases,” says Christopher Roth, MD, a cardiac MRI specialist and an assistant professor of radiology in Jefferson’s division of body MRI. “Cardiac MRI provides us with information not available from other modalities.”

Problem-Solving Role
Still, despite the technological developments that propel it forward, the modality is generally perceived and utilized as an imaging tool of last resort. “Typically, the way things happen, patients first undergo echocardiograms, nuclear imaging tests, and angiograms before they might even get to cardiac MRI,” says Cury, describing the general workup progression. “Once those exams are completed, and if questions still remain, then physicians typically turn to cardiac MRI to provide the elusive answers to the remaining difficult diagnostic questions.”

“It has been cast in the designated problem-solver role in the realm of cardiac imaging,” adds Roth.

But Cury believes that cardiac MRI can and should be positioned closer to the front end of the diagnostic process—a placement that would, in turn, heighten the level of cardiac patient care. “What I’d like to see happen is that cardiac MRI moves into the forefront of cardiac imaging so that it isn’t the last resource for difficult cases,” he says.

Further, he has ventured into a venue where this can happen, at least for a specific patient populations.

Emergency Department Tool
Cury and his colleagues at Massachusetts General Hospital and Harvard Medical School recently conducted a study that demonstrated the viability of cardiac MRI as a noninvasive test option that could accurately assess patients who presented in the emergency department with acute chest pain.

Their results, published in the August 19 issue of Circulation, suggest that cardiac MRI could be more effective for triaging chest pain patients compared with traditional risk-assessment techniques. Among current clinical methods are detailed assessments based on electrocardiograms (ECGs) and cardiac enzyme measurements. But these aren’t always effective techniques, as their results may compel physicians to admit patients who really don’t require hospitalization.

Specifically, the study shows that cardiac MRI, combined with T2-weighted imaging and left ventricular wall thickness analysis, provided a high level of diagnostic accuracy in detecting patients with acute coronary syndrome (ACS) and excluding those without.

The study included 62 patients who came into the emergency setting with acute chest pain, as well as negative cardiac biomarkers, and without ECG changes that would indicate acute ischemia. All subjects were imaged on a 1.5T scanning system equipped with an eighth-element, phased-array cardiac coil. Following contrast injections, the patients were scanned within the parameters of a specially developed protocol, which consisted of four steps. “Those steps included T2-weighted imaging, first-pass perfusion, delayed enhancement MRI to look for areas of myocardial necrosis, and cine wall motion, which evaluates left ventricular function,” explains Cury.

Employing this new protocol, the researchers were able to accomplish 30-minute workups that exhibited accuracy levels quite high for detection of patients with ACS. Also, the addition of the T2-weighted MRI sequence enabled physicians to see edema in the myocardium, a primary sign of acute injury to the heart. Further, the protocol allowed them to differentiate between patients with acute vs. old myocardial infarction (MI).

“This was important because some of these patients that come into the emergency setting may have a prior history of MI, or they may even have silent MI. The T2-weighted imaging enabled us to make that differentiation,” says Cury. “Other modalities (such as nuclear imaging, cardiac CT, and ECG) aren’t reliable when it comes to distinguishing between the acute, or new, and the chronic, or old, infarct.”

This differentiation helps physicians increase predictive capabilities when determining which patients are at high risk for ACS and which ones could be sent home—an important consideration, since many patients entering an emergency department who complain of severe chest pain face the risk of ACS despite the fact that initial biomarkers (cardiac enzymes), cardiac injury, and ECG changes may not indicate that a serious problem exists.

“Differentiation was one of two most significant findings,” says Cury. “The second involved the ability to detect myocardial edema and necrosis before the rise in critical biomarkers [eg, cardiac enzymes], which made the detection quite timely in terms of patient care.”

In the large picture, study percentages were positive. “Overall, diagnostic accuracy was 93%, while sensitivity was 85% and specificity exceeded 90%,” says Cury.

Specifically, researchers wrote that their four-step cardiac MRI protocol “increased specificity, positive predictive value, and overall accuracy from 84% to 96%, 55% to 85%, and 84% to 93%, respectively, compared with the conventional [cardiac MRI] protocol (cine, perfusion, and delayed-enhancement magnetic resonance imaging).”

In their paper, the researchers concluded, “[Our study] indicates that a new [cardiac MRI] protocol improves the detection of patients with acute coronary syndrome in the emergency department and adds significant value over clinical assessment and traditional cardiac risk factors.”

Expanding on this conclusion, Cury says, “Overall, I believe that cardiac MRI fits in well with the emergency department, particularly with those patients that have intermediate risk stratification and even patients with a potential intermediate-to-high risk for ACS that have a normal ECG and negative cardiac enzyme. Cardiac MRI would serve this population well.”

He adds that, for the low-risk population, cardiac CT would likely be the better test, as it can be accomplished more rapidly than cardiac MRI and has a very high negative predictive value. “As such, you could rule out patients with coronary stenosis and plaque and discharge these patients early,” he says. “However, cardiac CT may present some issues for patients who have had stents, as well as calcifications and prior infarcts, for example. For such patients, I believe cardiac MRI would work better, and that would be one application that would bring cardiac MRI closer to the forefront of imaging and evaluation.”

Barriers to Application
Still, despite ongoing cardiac MRI development, the modality hasn’t experienced widespread application.

“It’s one thing to do a study that demonstrates significant findings, but deploying the technology on a routine basis is another issue,” says Flamm. “It’s just not that simple.

“When it comes to choosing a modality for diagnosing cardiovascular disease, the question doesn’t always involve the best tool to make the diagnosis,” he adds. “Often, it comes down to what is the simplest and most accessible tool.”

And cardiac MRI is by no means simple. As Flamm indicates, while it represents an extremely powerful tool, cardiac MRI requires substantial training and expertise that’s far greater than what’s needed for more readily accessible modalities such as echocardiography or cardiac CT. “As a result, there are fewer experienced users, and that has limited cardiac MRI’s broader application,” he points out. “Few sites can implement this on a routine basis.”

“Right now, it is mostly employed in larger hospitals and large academic centers,” says Roth. “It hasn’t yet filtered down into community settings, and technological complexity is the foremost reason.”

In general, MRI is always more complicated than the alternatives, he says, and that is especially the case with cardiac MRI due to substantial hurdles that include cardiac pulsation and breathing motion artifacts, among other issues that slow down the developmental curve, as far as usage.

Beyond the intimidating technical aspects and the required training for both radiologists and technologists, economic factors stand in the way. “Simply stated, usage involves certain financial realities,” says Flamm. “Some competing modalities are reimbursed to a greater degree. Also, these modalities may take less time to achieve their ends. So, when you look at the balance between time involvement and payment, you see that it’s not currently favorable to cardiac MRI compared to other techniques such as cardiac CT.”

Economics also involves equipment costs. Ideally, users need to purchase a high-field magnet (at least a 1.5T system), which is expensive, as Roth points out. “You’re also facing other hardware constraints, as well as implementation of the software required to perform the studies,” he adds. “On top of all of that, you need the necessary postprocessing software. Looking at this from both the intellectual, or training, aspect and the required capital investment, it’s easy to see why cardiac MRI really hasn’t quite taken off yet.”

Moreover, available alternatives can effectively accomplish what users require. “Other modalities have proven to be quite accurate, and they can provide the kind of information needed in the clinical circumstances where they’re applied,” says Roth.

— Dan Harvey is a freelance writer based in Wilmington, Del., and a frequent contributor to Radiology Today.