ASNC: CTA may have a role for a-fib patients, but still tricky
DENVER—Coronary CT Angiography (CCTA) is not indicated for use in atrial fibrillation (AF) patents due to the problems it causes in terms of diagnostic accuracy, Thomas C. Gerber, MD, PhD, of the division of cardiovascular diseases at the Mayo Clinic, Jacksonville, Fla., said during a presentation Sept. 10 at the annual scientific sessions of the American Society of Nuclear Cardiology (ASNC). However, cardiac CT may be beneficial in some instances for this patient population when the right methods are used.

When CCTA is used in AF patients, coronary motion artifacts are present due to the temporal resolution of the scanners. “You can’t do much for this unfortunately,” Gerber said. “For most contemporary scanners, temporal resolution is half the gantry time.”

During cardiac scans, coronary motion velocity varies throughout the cardiac cycle with various peaks and troughs. Low temporal resolution does not get around these issues, however, scanning with a high temporal resolution can resolve these peaks and troughs, helping to avoid motion artifacts.

An additional problem is “misregistrations” during ECG triggering. “This is when the slices of the heart don't exactly line up,” Gerber said. “This creates stair-step artifacts of the heart.” Additionally, problems with ECG include the different sizes and shapes of the left ventricle (LV) at varying positions of the cardiac cycle. In fact, Gerber added that the "percentage of R-R intervals translates into difference portions of the cardiac cycle (between normal and premature beats).”

What are some solutions?
The fast and irregular heart beats of AF patients cause coronary artifacts during CCTA exams. Therefore, appropriate patient selection becomes important to prevent these problematic imaging artifacts. “For patients with fast, uncontrollable heart rates, CCTA is not the right test,” Gerber noted. However, oftentimes beta-blockers can be administered prior to exam to help reduce and regularize heart rate.

As far as imaging quality problems go, high temporal resolutions can decrease the risk of motion artifacts. Currently, contemporary scanners have a fast gantry rotation of 165 msec, while dual-source scanners can be as low as 83 msec. Gerber said that high z-axis coverage can also decrease misregistration. “Here the whole heart is imaged within one gantry rotation.”

Gerber added that while prospective triggering may be a great way to decrease radiation, there is no projection of data during certain parts of the cardiac cycle. On the other hand, during retrospective gating, projection data is acquired throughout the entire cardiac cycle, allowing for choice reconstruction intervals. Reconstructing the data set multiple times during a cardiac cycle can also decrease motion artifacts, Gerber said.

And while ECG editing may be cumbersome, it can help address the problems of misregistration. 

“Again, CCTA is not an ideal first line choice of diagnostic test for AF patients,” Gerber said. However, if no possible alternative is feasible, Gerber offered that a scanner with fast gantry rotation and/or wide z-axis coverage, along with meticulous patient care, will be necessary to image this patient population.

Gerber concluded that there may be some good uses for cardiac CT in the AF population, including: preprocedural pulmonary vein anatomy; post-procedural pulmonary vein stenosis; atrioesophageal fistula; and possibly left atrial thrombus.