Rapid scanning shows benefits in cardiac CT

The clinical impact of new technology is often difficult to quantify. In cardiac CT, however, the connection between diagnostic performance and latest generation scanning technology is crystal clear.

The most obvious benefit from CT's technological evolution is the overall reliability of cardiac studies.

Radiologists agree that their diagnostic confidence has been boosted by the clarity of images generated by 64-slice and greater CT. Put simply, the more anatomy that can be seen, the less chance that pathological findings will be missed.

"Today, up to 95% of our coronary CT studies are typically of excellent quality. When we were using 16-slice CT, that figure was more like 50% to 60%," said Dr. Elliot Fishman, director of diagnostic imaging and body CT at Johns Hopkins Hospital in Baltimore, Maryland.

Overall image quality improves further when you move from single-source scanning to dual-source CT, according to Dr. Ioannis Vlahos, an assistant professor of radiology at New York University Hospital. The number of coronary segments that cannot be visualized because of artifacts or cardiac or respiratory motion is considerably less, giving coronary CT an impressively high negative predictive value.

"If there are certain segments that you can't evaluate, then you can't real-ly say that the examination is normal," Vlahos said. "With dual-source CT imaging, fewer segments are indeterminate. Not only will you be able to make more positive diagnoses, but when you say the examination is normal, you can do so with much greater confidence."

The main advantage of dual-source CT is the exquisitely fast-83 msec-temporal resolution, made possible by running the system's two x-ray tubes at the same energy. The temporal resolution of single-source 64-slice CT is still impressive at 164 msec. But when it comes to freezing the small, rapidly moving structures in the heart, dual-source scanners appear to have the edge.

This step up in speed is having an impact on the way cardiac CT is performed. For the majority of examinations performed on 64-slice CT systems, patients whose pulse is faster than 65 to 70 beats per minute will be given beta blockers to slow the heart's motion. Dual-source CT obviates the use of these drugs in many patients (Figure 1). The examination is simpler and, ultimately, safer.

"Previously, we had to lower the heart rate of most patients undergoing cardiac CT," said Dr. Christian Fink, head of CT at the University Medical Center Mannheim, part of the University of Heidelberg in Germany. "Now we are able to perform most examinations without beta blockers. This is better for us and more comfortable for the patient."

Another upside is that patients who are contraindicated from taking beta blockers-perhaps because of asthma, known left ventricular dysfunction, or recreational drug use-can now undergo a cardiac CT study. The dual-source system also adjusts to cope with fluctuating heart rates, adding patients with arrhythmias to the "can scan" list.

"The thing about dual-source is that it is much more robust. Basically, you can scan somebody and do a great job up to a heart rate in the low 90s," Fishman said. "I'm always looking for things that make examinations easier and less prone to error. Dual-source technology appears to be doing that for cardiac CT."

Patients with a very high resting heart rate will sometimes still be given beta blockers. Previously, there would have been no point even trying to perform cardiac CT on this group, Vlahos said. Dual-source CT offers a much greater chance of achieving good-quality diagnostic images free from motion artifacts.

LOWERING DOSE

Speed is not the only important factor in cardiac CT, though. Dose-reduction strategies are important as well. This is particularly true if the noninvasive scans are to be used for screening purposes.

One method of minimizing the dose is to use tube current modulation algorithms. These work by altering the dose depending on the phase of the cardiac cycle during a standard breath-hold spiral acquisition. The dose is lowered when the heart is moving rapidly and data are unlikely to be used. It is then raised again to boost image quality when the organ is momentarily still. This technique has most dose-lowering impact when used with a dual-source system, according to Fink.

"With the dual-source scanner, you have high temporal resolution so you can use tube current modulation more effectively," he said. "You can't stop the dose altogether, but you can lower it at certain points in the ECG cycle."

An alternative radiation-reduction strategy is to use prospectively triggered ECG gating (Figure 2). With this approach, termed "snapshot pulse" by developer GE, x-ray radiation is administered in predefined bursts during the cardiac cycle. Unlike retrospectively gated coronary CT studies, in which patients are irradiated continuously, the x-ray beam may be on for only 25% of the scan.

The results are impressive, according to Dr. Geoffrey Rubin, chief of cardiovascular imaging at Stanford University in Palo Alto, California. He has seen substantial reductions in radiation exposure when performing cardiac CT on a 64-slice system equipped with prospective triggering software.

"We have achieved a 10- to 20-fold reduction in dose," he said. "In our institution, patients' exposure has dropped from 20 to 50 mSv with retrospective gating down to around 2 mSv when we use prospective gating."

Prospectively triggered ECG gating is also available with Siemens' dual-source CT and with Siemens' work-in-progress 128-slice adaptive imaging CT scanner, said Dr. Christoph Becker, an associate professor of radiology at the University Hospital Grosshadern in Munich, Germany.

"The temporal resolution with the adaptive 128-slice system is not as high as we get with dual-source CT, so we need beta blockers to scan patients with high heart rates or arrhythmias, as before" he said. "But the really exciting thing in my eyes is that cardiac CT scans can now be performed using very little dose, but still achieve very high image quality."