Another Shade of Gold

Currently, the gold standard for identifying stenosis in coronary arteries involves making an incision in the patient's groin and snaking a thin tube through a main artery to the heart. But the reign of cardiac catheterizations as the first tool employed in identifying coronary artery disease is drawing to a close.

Cardiac caths (aka coronary angiograms) aren't perfect, as they are limited by their ability to see only the lumen of the heart. That's where coronary CT angiograms (CTA) come in. Coronary CTA can visualize the anatomy of the heart, including blood-vessel walls, which helps physicians identify soft plaque.

"Coronary CTA evaluates for narrowing of the coronary arteries just like cardiac cath, and it also provides additional information about plaque sitting in the artery wall. CTA demonstrates the plaque burden as well as specific characteristics of the plaque, such as whether it is soft or calcified and whether it is smooth or ulcerated," says Ethan Halpern, MD, MS, professor of radiology and director of cardiac CT at Thomas Jefferson University (TJU of Philadelphia). "In addition, CTA demonstrates cardiac function to a much better degree than a cardiac cath does, allowing one to evaluate both left and right ventricular function and myocardium as well as to evaluate the aortic and mitral valves."

As physicians are discovering, this soft plaque is less stable and more likely to become inflamed and rupture, creating a tiny thrombosis. "And that thrombosis will be dislodged into the coronary artery, resulting in a heart attack," explains Claudio Smuclovisky, MD, director of South Florida Medical Imaging (Boca Raton, Fla).

Even more troubling is that a significant number—some guess the percentage to be as high as 50%—of people who die from a myocardial infarction would have returned a negative cath and a negative nuclear stress test, because these patients do not have a flow-limiting obstruction to the coronary arteries.

Soft plaque can be viewed easily with a coronary CTA, leading many to ponder its potential to eventually replace traditional angiograms altogether. The appropriate question, however, might not be can coronary CTA take over, but should it.

"There's been an inappropriate preoccupation with the idea of replacing [angiograms], but I think you have to consider all that the angiography environment represents now," says Richard White, MD, clinical director of the Center for Integrated Non-Invasive Cardiovascular Imaging and head of the section of cardiovascular imaging in the department of radiology at The Cleveland Clinic. "It represents high-resolution imaging down to a millimeter so that the interventionalist can decide if he or she wants to intervene—and if he or she chooses to, he or she can do so in the same environment."

White believes that the best application of CTA would be in addressing preclinical and subclinical questions, and eventually in identifying plaque patterns that could indicate more effective treatments. "I think that's the exciting part of this technology," he adds. "[The exciting part, to me, is] not in replacing the cath, which can be done safely."

Smuclovisky concurs. "CTAs shouldn't replace traditional angiograms; rather, it's going to explode the field of interventional cardiology by enhancing and helping better determine which patients need treatment."

New Technology, New Applications

Although the technology is still relatively new, the most likely course this noninvasive approach will see is in dramatically decreasing the number of unnecessary diagnostic caths. Both pricey and invasive, cardiac caths are often superfluous, with anywhere from 20% to 40% of all procedures being unnecessary, because the heart was clear of obstruction.

"We want to reduce the amount of invasive tests, because they have more risks and higher costs. A coronary CTA should always be done prior to an invasive procedure, to see if it is really needed," says Steven M. Strobbe, DO, executive physician and CEO of the Florida Institute for Advanced Diagnostic Imaging (Port Richey, Fla) and a Medical Imaging Editorial Advisory Board member. "My goal is to take that cath out of the cardiologist's hand and replace it—as much as I can—with a mouse."

For many major medical facilities, such a transition is starting to happen; and in areas where coronary CTA is in place, administrators see a noticeable difference.

"It has actually shifted the practice of our interventional cardiologists—70 percent of their cardiac caths were diagnostic, and now we're seeing that about 70 percent of their caths are interventional," Smuclovisky says. "They're using the CTA to guide their decisions in regards to which patients require intervention, and that is very significant."

Checking Out the Neighborhood

More good news: The ability to image soft plaque within artery walls isn't the only benefit gleaned from coronary CTAs. An ancillary benefit is the broader scope of information it captures.

"When a coronary CTA is performed on a patient with chest pain, in addition to evaluating the coronary arteries, we can perform a complete evaluation of the chest," says TJU's Halpern. "We're evaluating for lung disease, we're evaluating the pulmonary arteries for a pulmonary embolism—those often are causes of chest pain. So even if it isn't coronary artery disease, it still could be the right study to give you the answer for chest pain."

This comprehensive picture demands a detailed look at the entire area covered by the scan, requiring cardiologists and radiologists to work together to best care for the patient.

"It's extremely important that if a cardiologist reads the heart, a radiologist reads the rest of the chest, because, as a general rule, five percent of all patients have some other lesion in their chest," Strobbe says, noting that his standard procedure requires both specialists to review all coronary CTAs. "We have two eyes there, and our cardiologists are very happy with that; they understand the need."

Not all turf wars are conceded so readily.

"The politics are very difficult, but one thing is certain: Like it or not, cardiologists and radiologists have to work together for the program to be successful; there's just no way around it," Smuclovisky emphasizes. "I have case after case that prove it can be life-saving for patients and have found numerous extracardiac diseases, like cancer and pulmonary embolism. There must be a collaborative effort, not working in a vacuum."

A Moving Target

The heart is a muscle in motion. This simple fact complicates the application of today's powerful MRI systems in cardiac applications. A modality renowned for its ability to provide excellent, multiplanar images of soft tissue without radiation exposure, today's higher magnetic fields provide a better signal-to-noise ratio. The downside is that these improvements often are accompanied by increased artifacts.

"There are some theoretical gains, but as you're increasing the good signal to noise, you're also increasing your artifact factor, so that's a big limitation," says The Cleveland Clinic's White. "It has gotten better, but these are just the challenges introduced with the higher field strength—you can't change physics."

Physics might not budge, but MRI manufacturers are working to decrease artifacts in future systems.

"They have to really work to solve the unique technical challenges of doing the heart at 3T," says Lawrence N. Tanenbaum, MD, FACR, section chief of MRI, CT, and neuroradiology at the New Jersey Neuroscience Institute of JFK Medical Center – Edison Imaging (Edison, NJ) and assistant professor of the department of neuroscience at Seton Hall School of Graduate Medical Education (South Orange, NJ). "But these technical challenges are getting a lot of attention right now, so there is every expectation that 3T will be a big player in cardiac disease over the next 12 to 18 months."

A New Dynamic

Higher-field magnets have become commonplace; however, many physicians feel the uncharted territory of 3T MRI—as applied to the heart—warrants some caution.

According to Duane L. Hart, imaging engineering service manager of clinical engineering services at The Ohio State University Medical Center (Columbus, Ohio), "3T MRI is in its infancy in cardiac imaging, and there are significant challenges to diagnosing from the 3T." During construction of OSU's Richard M. Ross Heart Hospital, Hart oversaw design and development, as well as procurement and installation of all equipment. "All of our imaging and research for cardiac MR has been done on a 1.5T, so the question a facility has to ask is, ‘Do you want to be an adopter of technology where you have to develop what needs to be done for the patient, or do you want a technology that can be applied to your patient today?'" he asks.

White agrees that a vigilant approach is best when deciding whether or not to switch from a 1.5T to a 3T system. "We can profit from past experiences, but we have to gain a whole new set of experiences," he says. "It's not a completely lateral move, and I don't think it should be considered flippantly."

It's too early to determine exactly how big of a part that 3T will play in cardiac care, but the Ross Heart Hospital was designed to incorporate it, should it become a mainstay. "We feel 1.5T is a little better-proven product for cardiac imaging in our patients, but we have an eye to the future," Hart says, adding that the building was designed to support the heavier 3T magnets when the facility is ready for them—not only in cath labs, but also in operating rooms (ORs) and noninvasive diagnostic labs. "We're looking forward to a time when the technology can move into the OR for MR-validated valve surgery. Transesophageal echocardiography currently does a very good job; however, wouldn't it be wonderful before a surgeon closes to be able to do functional MR imaging on the fresh heart repair?"

A New Dimension

Echocardiography is holding its own in the world of cardiac imaging. In addition to embracing digitalization and becoming smaller and more portable, echo systems still play a major role in diagnosing coronary artery disease.

"If you need imaging for congenital heart disease, the preferred modality is cardiac ultrasound," says Achiau Ludomirsky, MD, Louis Larrick Ward professor of pediatrics and biomedical engineering, director of the pediatric cardiology division, and chairman of pediatric cardiology at St Louis Children's Hospital. He adds that beyond initial diagnosis, echocardiography also plays a big role in diagnosing cardiac lesions from as young as 18 weeks' gestation and monitoring adults with congenital heart disease. "The Doppler modalities and the introduction of 3-D reconstruction have made it even more valuable."

The advances made in 3-D- and 4-D echo just might be the most promising advance in ultrasound. "The next revolution is truly in ultrasound and the ability to move those images across modalities," Hart says. "Our cardiothoracic and vascular surgeons can view 3-D images in the OR for use as a guide during their surgical procedure."

Well established in cardiac care, echo's role will be expanding in the future, according to Ludomirsky. "The two areas in which I see a major innovation and progress in cardiac echo are tissue characterization and therapeutic ultrasound," he says. Currently, St Louis Children's Hospital is involved in studies with high-intensity focal ultrasound in which the mechanical and thermal indexes are exceeded to perform ultrasound ablation on arrhythmias. "We believe that by focusing a beam of high-intensity focal ultrasound to a specific area, at a specific volume, we'll be able to achieve the same results of the intracardiac ablation done today without having to insert a catheter."

Predicting the Future

Easy as it is to get distracted by the promise of new technology, physicians agree that the best consequence of evolving technology is improved patient care.

Not only do CTAs shed light on soft plaque, but as facilities incorporate CTAs in their diagnostic regimen, they also are improving treatment.

"It's pretty well established that higher doses of statin are more effective than lower doses, but they carry with them a higher degree of risk and toxicity," Tanenbaum says. "It would be best to have the ability to identify early artherosclerotic disease; then, you can be more aggressive with those patients and be less aggressive on those who have risk factors but don't have artherosclerosis evident."

The hope is that as more people are screened before they develop coronary artery disease, life-threatening episodes can be avoided altogether.

"We are currently involved in a long-term study looking into using cardiac CT as a prognostic indicator to see which patients will have future cardiac events," Halpern says. "We know we can use CTA to find plaque in the walls of arteries, so our question is if we can use it to predict future cardiac events."

Edward A. Gill, MD, FACC, FASE, attending physician at Harborview Medical Center and associate professor of medicine at the University of Washington Medical Center (Seattle), also is involved in work to identify patient risk, but with echocardiography.

"We're going to be using vascular probes—not looking at stenosis, but to evaluate the carotid intermedial thickness in order to risk-stratify patients based on the state of their carotid artery, because if you have disease in your carotid, it's an indicator you also could have disease in your coronary arteries," Gill says. "The hope is this will be a surrogate evaluation of true coronary disease that can help determine the treatment, including how aggressively we should try to lower their lipids through medication."

For many, the rapid progress seen in recent years bodes well for the future.

"The practice of radiology has gone through a revolution: Before, you would get an X-ray, and film would be put on a light box; today, we're able to obtain literally thousands of images in just a few seconds," Smuclovisky says. "Diagnostic radiology has changed completely, and the technology continues to catch on. The future is incredible."