New MRI Technique Produces Better Artery Assessment

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Non-radioactive elastin-specific MRI dye offers advantages over x-ray and CT angiography as well as intravascular ultrasound.

A new, non-invasive magnetic resonance imaging (MRI) technique can quantify coronary artery disease in patients before they have symptoms, researchers say.

The radiation-free elastin-specific contrast dye allows identification and quantification of plaque buildup on artery walls. (Photo: courtesy by Dr. Rene Botnar, chair of cardiovascular imaging at Kings College, London, the researchers used a contrast dye specific for the protein elastin and injected it into mice with atherosclerosis. In experiments conducted by Dr. Marcus Makowski, the researchers were able to produce images with high spatial resolution allowing an accurate assessment of the quantity of plaque buildup on artery walls.

The technique has advantages over current invasive methods of coronary ultrasound and CT angiography, Botnar told The Hub in an e-mail interview. Their study appeared online Feb. 20, ahead of publication in the March Nature Medicine.

“This approach has the advantage of potentially imaging coronary plaque burden, which only can be done with intravascular ultrasound,” Botnar said, “which is invasive and thus associated with a certain risk for the patient. Alternatively, multi slice CT allows imaging coronary calcification but calcified plaques are usually old and more stable plaques. In addition, calcification affects the lumen scan and does not allow the diagnosis of those segments as a stenosis may be hidden behind the calcium signal.”

In the two-year study funded by the British Heart Foundation, Makowski investigated the build up of plaque in mice arteries. The imaging works by using a non-invasive, radiation-free contrast agent called ESMA for elastin-specific magnetic resonance contrast agent. Developed in partnership with Lantheus Medical Imaging, ESMA binds to elastin, which is a key component of connective tissues, and plays a specific role in arteries permitting wave transmission to help blood flow with each heartbeat. It is particularly abundant in large elastic blood vessels.

Changes in elastin content and the high abundance of elastin during plaque development, in combination with the imaging properties of ESMA, allow the researchers to not only identify where plaque buildup occurs in the arteries, but to measure the buildup.

If successful in developing ESMA for human use it would provide a non-invasive alternative to current methods of assessing coronary artery disease. The gold standard is x-ray angiogram, which involves threading a catheter through blood vessels and injecting contrast dye for imaging with x-rays. The drawbacks include exposure to radiation, reactions to the dye, and the small risk of puncturing a vessel. One limitation is that it cannot show the walls of the arteries.

While intravascular ultrasound images do show the artery walls and can reveal cholesterol and fat deposits, like x-ray angiogram, however, it involves threading a catheter through blood vessels. CT angiography, like x-ray angiogram, also poses a slight risk of cancer due to radiation exposure.

Current MR angiography is noninvasive and effective for producing detailed images of blood vessels, but MR angiography is not able to see and capture images of calcium deposits. In addition, the clarity of MR angiography images of some arteries does not match those obtained with conventional catheter-based angiography.

While Botnar’s approach is at least two years away from human trials, the ESMA contrast MRI overcomes the limitations of current MRI techniques and has the added advantage of plaque characterization by quantifying intraplaque elastin content using signal intensity measurements.

“The doctor could image his or her (patient’s) coronary arteries, (measuring) both the lumen and plaque burden as well as adjacent large vessels such as the aorta and potentially the carotid arteries,” Botnar says. “Due to the properties of the contrast agent, it would also allow simultaneous visualization of myocardial infarction and thus not interfere with the conventional cardiac MRI examination of a patient.”

By: Michael O'Leary, contributing editor Health Imaging Hub

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