Cardiac specialists herald advances in multislice CT and MRI

The future of cardiac imaging lies with noninvasive cross-sectional imaging techniques, according to a speaker at this year's U.K. Radiological Congress. Advances in CT and MRI technology will ensure that both modalities take an expanding role in imaging the heart and great vessels, said Dr. Andrew Taylor, a radiologist at the Institute of Child Health and Great Ormond Street Hospital for Children in London.

Requests for cardiovascular MRI in his hospital's cardiac department doubled over the past year, he said, while CT studies increased eightfold. The benefits of both modalities will be best realized through a team approach.

"There is going to be integration across imaging modalities, from the catheter lab to CT and MRI. And, I hope, there is going to be integration between the specialties," Taylor said. "This should be a joint venture, with cardiologists, physicists, radiologists, cardiothoracic surgeons, and radiographers all contributing to driving cardiac imaging forward."

Continued evolution of cross-sectional imaging techniques will likely require increased use of 3D data sets. Radiologists at Great Ormond Street, for example, use MRI to develop 3D anatomic models prior to pulmonary valve stent insertions.

This percutaneous procedure can save patients with congenital heart disease from invasive bypass surgery. But not all patients can be considered as candidates, due to current restrictions on the stent's diameter.

"When we plan each of these studies, we are really looking for the 3D anatomy of the outflow tract. We are trying to custom-design the treatment to the individual patient," Taylor said.

If the virtual view is ambiguous, his team uses the 3D data to create physical models. This enables cardiologists to rehearse complicated interventions and provides working models for the development of new medical devices.

Three-D data could be similarly employed to improve the success rate of aortic valve stent placement, he said. Morbidity rates for aortic valve stent placement are relatively high, at 50% to 75%. Better understanding of the complex relationship between the coronary arteries and mitral valve anatomy could help cardiologists insert the valve stent safely.

Work is under way to develop this technique, using CT, for the predominantly elderly patient population. Although many studies have demonstrated the potential of MR angiography for assessing coronary artery disease, CT is more likely to become the radiologists' and clinicians' choice as a replacement for diagnostic cardiac catheterization, Taylor said.

"The problem with the coronary arteries is that they are small, they are widespread, and they are also affected by cardiac motion and respiratory motion. The MR sequences that have been designed to deal with this are relatively complicated and quite time-consuming," he said.

MRI produces high-quality images of coronary anatomy, but its impressive results are not guaranteed, especially for patients unable to lie still in the scanner.

Coronary MRA in one patient, who had undergone surgery for aortic dissection, took about 40 minutes, Taylor said, while coronary 16-slice CTA produced images of comparable quality in just 20 seconds.

"It will be images from 64-slice scanners that really convince us," he said. "Not only can we see calcification in these images, we can see plaque. That will give us not just lumenography but, hopefully, some information about the vessel wall in patients who may have more acute coronary syndromes."

The use of cross-sectional modalities for interventional imaging will probably increase as well, albeit at a far slower pace. MRI is preferred over CT because of its lack of ionizing radiation, but fully interventional cardiovascular MRI involves other practical hazards. Wires used to place angioplasty balloons and catheter tips are generally unsuitable for use in an MR scanner. Interventionalists must use passive tracking techniques with MRI, switching to x-ray guidance when greater accuracy is essential.

Superimposing 3D MRI data onto the catheter data set also improves the accuracy of functional measurements. Electrophysiological measurements can be located more precisely, and MR flow data can be combined with catheter pressure readings to determine pulmonary vascular resistance.

"Although the drive to do interventional MRI is to reduce dose, there are additional pieces of information we can acquire in these patients," Taylor said.