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Substantial reductions in radiation dose with coronary computed tomographic angiography (CTA) can be achieved using a novel algorithm that does not compromise diagnostic image quality, according to a small study published online February 19, 2014, ahead of print in the European Heart Journal. The improved technology results in patient radiation exposure in the range of that delivered by standard chest X-ray, the authors say.   

Investigators led by Philipp A. Kaufmann, MD, of University Hospital Zurich (Zurich, Switzerland), assessed 42 consecutive patients with known or suspected CAD using both standard low-dose CTA and ultra-low-dose coronary CTA.    

Standard low-dose CTA was reconstructed using a form of adaptive statistical iterative reconstruction (ASIR), while ultra-low-dose CTA was reconstructed using a new processing algorithm called model-based iterative reconstruction (MBIR; both GE Healthcare, Waukesha, WI).  

Radiation Dose Cut by Four-Fifths   

The median dose-length product from ultra-low-dose CTA was 14.9 mGy-cm compared with 84.7 mGy-cm from standard low-dose CTA, resulting in an estimated median effective radiation dose of 0.21 mSv (IQR 0.18-0.23 mSv) vs 1.19 mSv (IQR 1.07-1.30 mSv; P < 0.0001), for an 82% dose reduction.   

The calcium score in the 20 patients with coronary calcifications (calcium score > 0) averaged 335 ± 499.   

A total of 168 vessels and 551 coronary artery segments with a diameter of a least 1.5 mm were evaluated. Both CTA protocols yielded good to excellent image quality per segment, coronary, and patient. On a 4-point Likert scale, the median per-segment image quality score was identical (3.5) for the 2 methods.  Moreover, interobserver agreement regarding image quality was good. Overall, 97.8% of ultra-low-dose CTA and 98.7% of low-dose CTA coronary segments were of diagnostic image quality.  

Mean image noise decreased and mean attenuation increased with ultra-low-dose CTA compared with standard low-dose CTA (table 1). Thus, ultra-low-dose CTA with MBIR resulted in an increase in signal-to-noise ratio despite a substantial dose reduction.

 Table 1. Comparison of Image Quality Parameters^a

HU = Hounsfield units.

^a P < 0.001 for difference between ultra-low-dose and low-dose CTA for all metrics.

In a telephone interview with TCTMD, James K. Min, MD, of Weill Cornell Medical College (New York, NY), called the dose reductions achieved with the new technology “pretty impressive.” They are “lower than [those of] any other noninvasive cardiac imaging test that uses ionizing radiation,” he said, noting that the average dose from a nuclear stress test is 12 to 15 mSv. “With model-based iterative reconstruction, the doses are just going to continue to go down,” he added.  

This first study of the novel algorithm in a fairly broad range of patients “certainly shows promise,” Andrew J. Einstein, MD, PhD, of Columbia University Medical Center (New York, NY), told TCTMD in a telephone interview.  

He cautioned, however, that the study’s claim regarding image quality is not definitive. Readers deeming an image ‘good’ or ‘excellent’ does not guarantee that it has all the necessary diagnostic information, Dr. Einstein said, adding that the new technology still has to be compared against the gold standard of invasive angiography. Dr. Min agreed that such validation will be important.   

Opening New Doors for Coronary CTA  

Dr. Einstein echoed the authors in asserting that achievement of a very low radiation dose is likely to broaden the horizons for CTA use, even in lower-risk patients. “We have been hesitant to expose otherwise healthy people to radiation if there is no good reason to do a heart scan,” he observed, but ultra-low-dose CTA appears to shift the harm-to-benefit balance in favor of benefit in these patients.   

“…The radiation exposure [with this technique] is lower than for calcium scoring,” he continued, “yet CTA has the potential to provide more [data] than does calcium scoring. It shows not just calcified plaque but soft plaque,mixed plaque, and coronary stenosis. And now there is even FFR determined by CT. So if we can provide all that information at very low cost in terms of radiation, then it opens the door for CTA as a potential screening test. Of course, that would require further demonstration.”  

Dr. Min was more cautious. “I think we’re a ways from having the data to support screening with coronary CTA,” he said. “But you could consider tacking on CT perfusion so that you can couple [evaluation of] angiographic stenosis severity with physiological assessment of myocardial perfusion. If you could do both at about 0.2 mSv [per procedure], you’re talking about 0.4 mSv, which is 20 times lower than today’s stress test.”  

One unknown for the new technology is the effect of higher BMI, given the low tube voltage employed, Dr. Einstein said, and that is an important question since obesity is prevalent among US patients.     

As to the feasibility of incorporating the novel algorithm into existing CT scanners, Dr. Einstein observed that it is “computationally demanding.” That could limit implementation because many institutions will have neither the space nor the economic resources to bring it onboard, he said.    

While agreeing that the MBIR algorithm is “computationally intense,” Dr. Min said that the extra time required to reconstruct data will no doubt be reduced in the future. For now, the main issue is the software is not yet available, he said, adding that “invariably it will be over time.”  

Other CT vendors are already pursuing similar software, Dr. Einstein noted. 

Study Details

All examinations were performed with a 64-slice CT scanner (Discovery HD 750, GE Healthcare) using prospective ECG triggering during inspiration breath hold.  

The mean age of the study cohort was 55 years and the mean BMI was 25.2 kg/m². 

Source:

Fuchs TA, Stehli J, Bull S, et al. Coronary computed tomography angiography with model-based iterative reconstruction using a radiation exposure similar to chest X-ray examination. Eur Heart J. 2014;Epub ahead of print. 

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