Cardiac CT and MRI: Where Do We Stand?

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As cardiologists have done in the past with new technologies, they will have to find ways to use CT and MRI complementarily, rather than in a competitive manner.

Many tests in one

While cardiac CT often receives headlines, cardiac MRI has maintained a strong reputation for its ability to answer numerous questions for cardiovascular patients. However, many physicians utilize MRI only as a problem-solving tool. Using MRI to solve indeterminate tests is at least better than sending these patients to the cath lab or surgery, says Sunil K. Ram, MD, a radiologist with Scottsdale Medical Imaging, Scottsdale, Ariz., and a clinical assistant professor of diagnostic radiology at the University of Florida and Shands Hospital, Gainesville, Fla.

Ram conducts outreach education for referring primary care physicians and cardiologists. He has found his cardiac MRI referrals reflect an even balance between both groups. “Each has learned to use the technique effectively,” he says.

He also would like to see physicians use MRI as a first-line tool. Rather than have a stress echo or nuclear test, patients can undergo a stress cardiac MRI, which holds a number of benefits, Ram says. “Compared to a nuclear stress test, MRI is much faster—40 minutes versus four hours—and does not expose the patient to radiation. MRI is much more accurate than both nuclear and echo stress, and a cardiac stress MRI provides more information than perfusion, including viability, function and morphology, at a much higher resolution than either nuclear or echo,” he says.

Validation studies
Cardiac MRI is a well-validated tool. For the most part, however, cardiologists have never felt comfortable with MRI and that is partly why the modality has not gained widespread adoption. Cardiac CT is a different story.

The widespread use of cardiac CT, primarily for coronary CT angiography (CTA), has outpaced the clinical literature. Cardiologists like the ability of CTA to quickly rule out coronary artery disease (CAD). As with MRI, CTA also provides a plethora of functional data, which many cardiologists insist on including in the report. Some imagers, most notably U. Joseph Schoepf, MD, director of CT research and development at the Medical University of South Carolina in Charleston, S.C., have called it unethical not to include functional data with a CTA report.

Nevertheless, there are still unanswered questions about whether CTA improves patient survival or is cost effective.

“The whole area of cost effectiveness is suspicious to clinicians because it is a theoretical look at how things happen,” says Udo Hoffmann, MD, MPH, director of the Cardiac MR CT PET Program at Massachusetts General Hospital and an associate professor of radiology at Harvard Medical School in Boston. “People need to recognize that cost effectiveness is one additional data point to quantify the value of an imaging method and the analysis is only as good as the assumptions going into it.”

The best data come from randomized controlled trials, Hoffmann says, and PROMISE and ROMICAT 2—which recently received funding—are intended to fill in the outcomes and cost-effectiveness gaps.

The randomized controlled PROMISE (Prospective Multicenter Imaging Study for Evaluation of Chest Pain) trial will compare CTA with standard-of-care functional stress tests (echo, nuclear or treadmill) in 10,000 low- to intermediate-risk chest pain patients, who will be followed for up to four years. While the composite primary endpoint is a reduction of death, MI, major peri-procedural complications and hospitalization for unstable angina, the secondary endpoints include medical costs, resource utilization and cost effectiveness. “This is a new kind of study for CTA,” Hoffmann says. “Past studies have focused on accuracy and specificity. We’re moving beyond that. We want to know how, if at all, this test will improve outcomes and if it is cost effective in the long term.”

Hoffmann and other investigators will calculate hospital costs by modeling different rates. “We want to know the value of the test from a societal perspective, not from the perspective of stakeholders such as patients, physicians or hospitals, which can vary,” he says. “By using modeling in this way, we can arrive at costs that are generalizable from situation to situation. That is almost as good as it gets.”

ROMICAT 2 (Rule Out Myocardial Infarction using Computer Assisted Tomography) builds on the success of the initial ROMICAT trial, an observational study that looked at 368 patients and found CTA could have ruled out CAD in more than half of the patients (J Am Coll Cardiol 2009;53[18]:1642-50). ROMICAT 2, which intends to enroll 1,000 acute chest pain patients at seven sites, will randomize patients to either CTA or standard triage.

Like PROMISE, ROMICAT 2 will collect data about institutions, readers and equipment, which will help “determine if any of those factors have an effect on economic or health outcomes,” Hoffmann says.
Proven prognostics

While practitioners and payors wait for definitive data on cardiac CT, cardiac MRI has no such problem. “Cardiovascular MR has matured as a clinical tool, having passed through validation, standardization and clinical roll-out phases,” wrote Andrew S. Flett, MBBS, and colleagues from University College London Hospitals NHS Trust (Circ Cardiovasc Imaging 2009;2;243-250). “[Cardiac] MR now aids the prediction of clinical outcomes with a growing prognostic evidence base,” they concluded after a systematic review of 32 outcome studies with 8,855 patients.

Flett et al determined that cardiac MRI has proven value in these areas:
  • Ischemic heart disease, including acute coronary syndrome, infarct size, microvascular obstruction, peri-infarct zone, chronic CAD, silent MI detection, revascularization, ischemic cardiomyopathy and resynchronization therapy;
  • Stress MR, including dobutamine wall motion assessment, mixed stress (dobutamine and adenosine) and vasodilator perfusion;
  • Cardiomyopathy, including dilated and hypertrophic cardiomyopathy, thalassemia, arrhythmogenic right ventricular cardiomyopathy, myocarditis and amyloidosis; and
  • Other conditions, including pulmonary arterial hypertension, congenital heart disease and plaque characterization and stroke prediction.

A typical exam for Ram and colleagues will almost always include an assessment of morphology, ventricular function, valves, perfusion and viability. Arteries are imaged in special cases such as congenital anomalies. “If you compare the cost of cardiac MRI, which is roughly the same as a nuclear medicine stress test, it appears to be extremely cost effective,” he says.

Nevertheless, a few things must happen before cardiologists routinely order cardiac MRI. “They must have access to high-quality cardiac MRI services, either at a university program or a subspecialty practice,” Ram says, “and they cannot feel threatened by the modality, fearing that it may result in fewer procedures in their own offices.”