Coronary MRA struggles against success of CTA

The failure of MRA to make major inroads into coronary angiography is not due to lack of trying. Many different techniques have been attempted, often with encouraging results. Additional clinical trials, involving both healthy volunteers and patients, are ongoing. But promising research results in a controlled environment are no guarantee of clinical viability.

"Quite frankly, MRA of the coronary arteries is not a valid imaging tool at the moment," said Dr. Mathias Goyen, a radiologist at the University Medical Center Hamburg-Eppendorf in Germany. "It is very hard to get good image quality in a routine patient."

Acquiring diagnostic-quality images of the coronary arteries is undeniably a technical challenge. Visualizing the small vessels and their pathology requires extremely high spatial resolution. Image quality may also be compromised by motion artifacts from the heart and chest. The latest generation of multislice CT scanners appears capable of overcoming these issues, prompting a renewed focus on coronary CT angiography. Even devotees of MRA admit that their preferred radiation-free technology cannot compete in this territory.

"We have active research interests in coronary MRA, but we don't do much clinical coronary MRA," said Dr. Paul Finn, a professor of radiology and medicine at the University of California, Los Angeles Cardiovascular Research Laboratory. "CTA is very powerful and, at the moment, better than MRA for coronary artery imaging. However, for just about everything other than the coronary arteries, I believe MRA is preferable to, and often better than, CTA."

As a result, coronary MRA is not part of clinical practice at many healthcare institutions. Others are choosing to use the technique for specific indications.

Dr. Martin Prince, chief of MRI at the Presbyterian Hospital and Weill Cornell Medical Center in New York City, chooses MRA when investigating coronary anomalies in pediatric patients. This means that younger patients are spared the high dose of ionizing radiation from CTA and the invasive nature of diagnostic coronary catheterization. Administration of MR contrast is also unnecessary, owing to fast flow in the blood vessels of pediatric patients, which ensures a strong MR signal with flow-based pulse sequences.

Radiologists at Maastricht University Hospital in the Netherlands also do without contrast when using MRA to study coronary anomalies. The problem is not lack of signal, but synchronizing acquisition with the diastolic rest period to minimize motion artifacts, said Dr. Tim Leiner, a radiologist at Maastricht. Getting this right for each and every case requires considerable patience. Although CTA protocols do not generally involve this time-consuming step, modern CT scanners are still not rapid enough to freeze motion completely.

"The treacherous thing about coronary CTA is that there are motion artifacts, but you don't recognize them as easily as with MR," he said.

The advent of blood pool contrast agents that stay in the circulation longer may remove some of the obstacles to contrast-enhanced MRA. The longevity of these agents relieves the need to catch a bolus in a single rapid pass (see "Expanding contrast options widen clinical utility of MRA," page 29.) Increasing imaging times in this way should lead to higher resolution pictures of narrow, difficult-to-image vessels.

Studies of the impact of blood pool agents on coronary arteriography are in progress. The pharmacological assistance offered by this class of contrast will be essential if MR is to make any serious inroads into the coronary angiography arena, Finn said.

"In five years, I hope that we will be doing coronary artery imaging routinely, and for that, I do think we need a blood pool agent," he said.