MRI :: Effectiveness of noninvasive whole-heart MRI

Researchers have long sought to find a reliable way to see inside the human heart without having to insert tubes, inject dyes or expose patients to potentially dangerous radiation. One possible solution is the whole-heart MRI, which provides 3-D images of the heart and its blood vessels without the risk of an invasive procedure.

New research published in the Nov. 21, 2006 edition of the Journal of the American College of Cardiology continues to advance the understanding of how whole-heart coronary MRI can be used to detect heart disease.

One drawback of MRIs of the heart is that clear images can be difficult to obtain. The heart’s constant beating coupled with a patient’s constant breathing can blur images, rendering them unusable.

Typically, images are collected during the fraction of a second that the heart rests between beats, a period called diastole. For this new study, however, researchers at Mie University Hospital and Matsusaka Central Hospital in Japan sought to determine whether image data should be collected at other times, too.

Under the leadership of Hajime Sakuma, MD, PhD, researchers at the two hospitals evaluated patients to determine whether for some people ? based on their heart rates and other factors ? better images could be obtained while their hearts were actively contracting to pump blood, a period called systole.

“Suppression of motion blurring is critically important to obtain diagnostic coronary MR angiography,” said Dr. Sakuma, professor and vice chairman of Mie University Hospital. “In previous studies, image data was acquired during mid-diastole, as the heart rested. However, we found the optimal timing of data acquisition varies in each patient.”

For the study, researchers used whole-heart MRIs to evaluate 131 patients suspected of having coronary artery stenosis, a narrowing of the arteries caused by fatty plaques. In 48 patients, especially those with faster heart rates, the optimal time to collect data occurred while the heart was contracting rather than resting. Overall, useable images were collected in 113 patients, or 86 percent of the study participants.

“This customized approach in each subject substantially reduced motion blurring of the coronary artery and improved the detection of coronary arterial stenoses compared with previous studies,” Dr. Sakuma said.

The research team also analyzed the accuracy of the resulting coronary MRI diagnoses when compared with traditional X-ray coronary angiography. The X-ray procedures involve inserting a catheter into the heart and releasing a contrast dye to create X-ray images of the coronary arteries. Although they are considered the “gold standard” for diagnosing heart disease, these procedures can cause serious complications in some patients, including heart attack and death. In addition, many patients with kidney problems cannot tolerate the contrast dye needed to obtain useable X-rays.

Overall, Dr. Sakuma and his colleagues found that for coronary arteries of at least 2 millimeters in diameter, whole-heart coronary MRIs resulted in an accurate diagnosis of “significant narrowing” of the blood vessels in 87 percent of patients. Significant narrowing was defined as a reduction of at least half the normal diameter of the lumen, which is the blood-carrying channel inside the artery. When researchers studied isolated segments of arteries, their ability to accurately diagnose significant disease using whole-heart MRIs increased to 94 percent.

The researchers concluded that whole-heart MRIs were able to detect significant narrowing of the arteries with “moderate sensitivity and high specificity,” in 82 percent and 90 percent, respectively, of study participants. This means the tests produced only a small number of false-positive results, but a more moderate level of false-negative findings. (False-positive results incorrectly conclude that patients have a particular condition when they actually are free of the disease, while false-negative results incorrectly indicate that patients do not have a disease when evidence of it actually exists.)

Overall, Dr. Sakuma said, the research demonstrates the potential of whole-heart MRIs in a broader group of patients and validates the need for larger, multicenter trials, some of which already have begun.

“While more research and refinement are needed before coronary MR angiography is ready for widespread clinical use, the technology is ideally suited for screening coronary artery disease and is worth the effort for further development,” Dr. Sakuma concluded. “Coronary MR angiography is completely noninvasive and does not expose the patients to any radiation. In addition, coronary MR angiography does not require administration of contrast medium, which is important in patients with impaired renal function.”

Warren J. Manning, MD, section chief of noninvasive cardiac imaging at Beth Israel Deaconess Medical Center in Boston, did not participate in the research, but agreed coronary MR angiography deserves further study. Dr. Manning wrote an editorial that will accompany publication of the new research.

“Cardiac MRI provides a comprehensive assessment of the heart, allowing us to look at the beating heart, valve function and areas of prior heart attack to determine whether these areas still have viable tissue,” he said.

Dr. Manning also said coronary MRI is one of two promising imaging techniques now being advanced and studied to assess the coronary arteries. The other is coronary computed tomographic angiography ? a rapid CT scan of the heart that provides a series of cross-sectional images or “slices.” So far, the CT technique has proven to be easier for patients to comply with and for healthcare professionals to use, he said.

“A coronary CT requires patients to hold their breath for only 15 to 20 seconds, and it is completed in just a few minutes,” said Dr. Manning, also a professor of medicine and a professor of radiology at Harvard Medical School. “Its drawbacks include that it exposes patients to a significant amount of radiation ? up to twice as much as a traditional cardiac catheterization and coronary angiography ? and it requires patients to be injected with a dye solution.”

Coronary MRIs, on the other hand, can take up to 15 minutes or more to complete, during which patients must lie as motionless as possible, he said. Although the MRI doesn’t involve radiation or contrast dye ? making it easier to repeat ? it also doesn’t produce usable images as often as the CT scan. When images are obtained, however, both tests appear to be similar in their ability to diagnose disease, Dr. Manning said.

But, he cautions, both techniques have been studied only in patients who are very likely to have coronary artery disease, which is much different than the typical patient.

“The use of both technologies in patients who have atypical chest pain or simply have multiple risk factors for coronary artery disease is unproven,” he said. “More research is needed to address these issues. In the end, it’s not just whether we can get pictures. We have to understand what they mean and their impact on our patient’s care.”

Also in this issue of JACC

Researchers at Mayo Clinic in Rochester, Minn., sought to determine whether they could better detect early signs of coronary artery disease by comparing and contrasting the results of two types of diagnostic tests: electron-beam computed tomography and exercise echocardiography.

Electron-beam computed tomography (CT) scans the heart for signs of potentially dangerous calcium buildup in the coronary arteries. Exercise echocardiography uses ultrasound technology to create live images of the heart, allowing physicians to analyze the muscle wall to ensure that the heart is receiving enough oxygen-rich blood to remain healthy.

For their study, Mayo Clinic researchers retrospectively reviewed the records of 556 patients who had undergone both tests within a short time frame. The researchers determined that evidence of calcium buildup had “statistically significant but limited correlation” with heart-wall information gleaned from exercise echocardiography. Still, they said, the tests “offer complementary information” that may benefit patients at risk for heart disease.

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