Speed, spatial resolution add clinical detail to CTA

Cardiovascular imaging has much to gain from developments in CT. The improved temporal and spatial resolution has already translated into superior image quality. Now the radiology community is waiting to see whether the promises made by proponents of dual energy-that it can remove calcified plaques from coronary CT angiography studies-will be kept.

Imaging with 64-plus-slice technology is making a real difference to the details that can be seen in diseased and damaged blood vessels. Radiologists can now get a much better picture of a patient's renal artery stenosis than would have been possible with 16-slice CT, for example. Small intracranial or hepatic artery aneurysms can also be visualized clearly.

Achieving good results is not a given, though. Earlier protocols used for contrast-enhanced studies may no longer be appropriate, said Dr. Elliot Fishman, director of diagnostic imaging and body CT at Johns Hopkins Hospital in Baltimore, Maryland.

"You really need to understand how quickly you scan," Fishman said. "Contrast delivery needs to be coupled with data acquisition for whichever phase you are interested in. If we don't do that, we aren't going to get the right study." Radiologists at the University Hospital Grosshadern in Munich, Germany, are taking advantage of the speed afforded by state-of-the-art scanners to perform dynamic CTA.

In a dynamic CTA acquisition, substantial anatomical volume is repeatedly covered by a periodic shuttle movement of the table. One potential application would be for patients who have had a stent fitted following discovery of an abdominal aortic aneurysm and may still have endoleaks.

"It is very hard to predict whether these endoleaks will fill early or late, and which vessel they will be filled from," said Dr. Christoph Becker, an associate professor of radiology at Grosshadern. "We want to see whether dynamic CT angiography could answer these questions."

Another application under investigation is peripheral artery imaging. A dynamic series is performed below the knee to check the time of contrast arrival on the left and/or right side. The optimum time to image blood vessels in the calf during a complete diaphragm-to-toe CTA study can then be determined.

Becker acknowledges that radiation is a concern when performing dynamic CTA. German regulatory authorities have set a 20-mSv limit.

"Below the knee, we are quite a distance from the main trunk, so I am less worried about the radiation," he said. "For patients with aortic aneurysms, we are trying to optimize our examination so that we don't waste too much radiation."

Dr. Geoffrey Rubin, chief of cardiovascular imaging at Stanford University in Palo Alto, California, would like to see the next generation of CT scanners make better use of contrast, too. He is hopeful that future systems will be able to acquire biometric data, allowing the contrast dose to be titrated automatically on an individual basis.

"It is a very simple concept that is not technically complex," he said.

But it is the potential of energy-dependent CT that is generating most debate within the cardiovascular imaging community. Some sites with access to dual-source systems are already in trials for clinical applications. Others are waiting to see whether the results will match up to the hype.

One problem in CTA is the presence of bones obscuring vessels of interest. This issue is common when imaging the skull base and can cause difficulties when viewing peripheral runoff CTA. When imaging is performed at two separate x-ray energy levels, the bone and contrast-filled vessels can be distinguished from each other. The bone can then be removed from the picture easily and automatically.

"People say: ‘I have been doing this for years with my 16-slice CT scanner. I have software that cuts away the bone.' This is true," said Dr. Konstantin Nikolaou, associate chair of radiology and CT section chief at Grosshadern. "But with dual energy it is easier and faster, in particular in complicated anatomical situations such as the base of the skull, and it may turn out to be more accurate."

The potential of dual-energy imaging to separate out plaque from the contrast-filled lumen in angiographic studies is also being promoted. This could be especially helpful when looking at the peripheral arteries or examining patients with severe coronary artery disease.

"Most patients undergoing coronary angiography who are over 70 years old will have coronary calcium. If we examine them with CTA, then we can't be as specific about their arterial disease in the presence of calcium as we might like," said Norbert Pelc, Sc.D., a professor of bioengineering and radiology at Stanford. "This is a potential limitation that affects all arterial regions."

Dr. Dennis Foley, a professor of radiology at the Medical College of Wisconsin in Milwaukee, is looking forward to testing this feature for himself. His department will shortly be receiving a single-source CT system from GE that will offer dual-energy imaging, thanks to the combination of new fast-response scintillator technology and rapid beam switching. This should make it possible for x-rays with different beam energies to acquire almost identical views of moving anatomy.

"It will be interesting to see if we can get an image that looks much more like a catheter coronary angiogram than what we can obtain currently," he said.