Can MRI diagnose LV dysfunction, blockages better than ultrasound?
Researchers refine an alternative for patients restricted from stress echo
Researchers at Wake Forest University Baptist Medical Center in Winston-Salem, NC, report the MRI has proven to be just as accurate as cardiac catheterization and angiography for purposes of diagnosing blockages in vessels leading to the heart and for the evaluation of patients with CHF due to left ventricular (LV) systolic dysfunction. The new successful use of magnetic resonance imaging (MRI) provides images that are more useable than those produced by ultrasound in a dobutamine stress test.1
Greg Hundley, MD, assistant professor of cardiology and radiology at Wake Forest and lead study author, says fast cine MRI has substantial clinical and research applications. "Clinically, one can diagnose inducible ischemia or assess for the presence of contractile reserve in patients with LV systolic dysfunction at rest. For research purposes, the technique can be combined with other MRI assessments of the vasculature to gain a more comprehensive assessment of the pathophysiology of heart failure."
With fast cine MRI, the investigators viewed the LV wall as it beat and were able to capture movement almost as it was contracting and relaxing — close to "real time." The research team is now working to measure flow with MRI in the right coronary artery, another common site of blockages.
Craig Hamilton, PhD, assistant professor of radiology and Hundley’s colleague, says that they do several thousand stress tests using dobutamine and ultrasound at their hospital each year.
"But some of those patients get unusable pictures either because they are obese or if they have lung disease that disrupts the ultrasound pictures," says Hamilton. "Our goal was to track those patients with MRI instead." About a fifth of the overall stress test population cannot produce usable images, he says. MRI has been well accepted for brain scans for the past 10 years, but not as well accepted in cardiac work because the heart moves so much compared to the brain.
Randolph Martin, MD, associate dean of clinical development and director of noninvasive cardiology at Emory University in Atlanta says stress testing is designed to determine whether the cause of a patient’s CHF is coronary artery disease, and if so, whether the patient may benefit from revascularization.
"Coupling MRI to a stress test is a new application," he says. "It will have benefit for CHF patients, but I’m not sure MRI will supplant echo for CHF diagnosis. It’s echo that now has a key role in diagnosing the extent of the heart failure and its etiology." MRI is good way to image the heart, he says, especially for patients you can’t echo. "But the number of those noncandidates has decreased dramatically with the advent of improved instrumentation and improved contrast agents."
The physicians at Emory use ultrasound for both exercise and pharmacologic stress testing, but are beginning to look at MRI in the context of stress testing as a research protocol. "MRI has advantages," explains Martin. "First, it’s operator-independent; you just have to get the patient into the magnet. Second, it gives two- and three-dimensional views of the heart and the great vessels. And third, there’s no radiation involved."
But MRIs cannot be performed on everyone. "If you have metal clips in your body, or pacemakers, or artificial valves, MRI’s out of the question. And it is those patients who typically have need for stress tests."
But, he says, no test has a 100% performance rate. "Echoes can’t be done on those who are obese or have lung disease. But contrast agents are being used extensively now to turn technically difficult echoes into acceptable images, so that makes a difference."
Martin points out three more disadvantages of cardiac MRI:
- It’s not a portable, bedside procedure.
- "EKG can’t be used to monitor ischemia because MRI distorts ischemia. That’s a limitation."
- You have to gate, or pace with an electronic signal, to use MRI, and patients with irregular rhythm or atrial fibrillation cannot be MRI’d.
Hamilton’s team shot several different views of the heart while the patient was in the scanner. "To get cardiac MRI readily accepted," he says, "we decided to pattern our method after ultrasound methodology so the physician reading the result would receive an image that looked familiar. Our aim was to provide pictures similar to those they get with ultrasound, but of patients who cannot do the ultrasound test. We took views that cut through the heart three different ways, both lengthwise and crosswise, so the result was six views focused on the wall of the left ventricle."
Whether ultrasound or existing cardiac MRI methods are used, the physician can only visually assess the images to discover how the muscle is thickening and thinning as it beats. "The physician visually assesses if certain parts of the heart muscle are not thickening and thinning properly," Hamilton says. "But that assessment will vary depending on who looks at the picture. Ideally, we would like a quantitative measure so we can objectively measure the thickness of the wall as the heart beats. Then we can create a mathematical plot showing muscle action. That’s the idea behind HARP [HARmonic-Phase]." (See related article on HARP technology, at right.)
The Wake Forest researchers’ study demonstrated that MRI not only handled those patients who get a poor ultrasound stress test, but also it compared well with angiography in diagnostic capability. "We took the most difficult of the stress test patients and successfully performed a test on them utilizing MRI," says Hamilton. "A number of these patients went on to receive caths after we gave them the MRI tests. Their caths unequivocally identified stenosis and other heart problems, and the diagnoses reached with MRI matched well with that reached with angiography findings." About 40 patients had both MRI scanning and catheterization, and the MRI was just as effective as catheterization at diagnosing significant blockages in the distal left main and the left anterior descending arteries. Among the approximately 100 patients who had a negative MRI stress test, 97% were free of heart disease within the first year of testing.
The researchers achieved two breakthroughs in MRI imaging to develop their new test. First, they created computer software to control the scanner and analyze data that made it possible — for the first time — to measure blood flow in the small vessels leading to the heart. Second, they used this imaging technique to measure flow while the heart is stressed and beating close to capacity because that is the ideal time to assess whether a blockage requires treatment. When patients take dobutamine to temporarily increase their heart rate for the test, blood flow should also increase. If the flow isn’t increasing to meet the heart’s demands, bypass surgery or angioplasty may be indicated.
The researchers write that the new MRI test cannot currently replace catheterization as a road map to performing angioplasty or bypass surgery, but they believe it holds promise.
References
1. Hundley WG, Hamilton CA, Thomas MS, et al. Utility of fast cine magnetic resonance imaging and display for the detection of myocardial ischemia in patients not well suited for second harmonic stress echocardiography. Circulation 1999; 100:1,697-1,702.
2. Pohost GM, Biederman RWW. The role of cardiac MRI stress testing: Make a better mouse trap’ Editorial. Circulation 1999; 100:1,676-1,679.
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