Journal Wrap | While computed tomography (CT) angiography is the accepted standard of diagnosis for coronary artery disease (CAD), pulmonary embolism (PE), and aortic disease (AD), the growing use of CT scans could place patients at a higher lifetime risk of cancer from diagnostic radiation exposure.

Recently, novel scanner technologies capable of acquiring low-dose, prospectively triggered scans in most patients—or high-pitch spiral acquisition protocols in selected patients—have been shown to reduce doses by more than 50% compared to previous-generation systems. These results, however, come from small-scale studies and there is a lack of "real-world" data surrounding these new scanners and protection against radiation.

In a study published recently in the Journal of Cardiovascular Computed Tomography, Kavitha M. Chinnaiyan, MD, and colleagues evaluated the effect of scanner technology on radiation dose in a large population of patients at multiple centers undergoing coronary, pulmonary, or aortic CT angiography. Investigators compared retrospective scan data (including dose and image quality parameters) from first-generation 64-slice scanners with data after next-generation 128-slice dual-source scanners with high-pitch capability were installed at the centers.

From April 2011 to March 2012, 2,085 patients at nine sites completed the study:

• 1,051 coronary artery disease (509 control [older-generation scanners], 542 study [newer-generation scanners])
• 528 pulmonary embolism (267 control, 261 study)
• 419 aortic disease (268 control, 151 study)
• 87 triple rule-out or TRO (53 control, 34 study)

In the control group, 79% of patients were scanned on a previous-generation dual-source system, and the remaining 21% on a 64-slice system. In the study group, the high-pitch scanner mode was used in 48.2% of patients; the high-pitch mode was more often used for evaluation of PE and less often for CAD and triple rule-out. Notably, there was a significant dose difference between high-pitch and non-high-pitch modes (median dose = 2.4 vs. 5.1 mSv; p < 0.0001).

There was a significant overall reduction in median volume CT dose index—from 24 (interquartile range [IQR]16-40 mGy) in the control group to 9.1 (IQR 5.8-21; p < 0.0001) in the study group. In addition, patient radiation exposure dropped 61% when the newer-generation scanners were used: median dose-length product (DLP) was 669 mGY-cm (IQR 419-1,026) versus 260 mGy-cm (IQR 159-441). Total DLP reduction within each subgroup also was significant.

In terms of factors that predicted meeting the target of total DLP <360 mGy-cm (and effective dose <5 mSv), the investigators found that use of a high-pitch capable scanner (OR = 16.40; 95% CI 1.64-23.11); a tube potential ≤100 kV (OR = 4.21; 95% CI 2.02-6.08); and a body mass index <30 kg/m2 (OR = 1.62; 95% CI 1.13-2.31) were associated with better radiation dose reduction.

A lower radiation dose is all well and good, but the scans also need to provide accurate and reliable data for a patient's diagnosis. When Dr. Chinnaiyan and investigators examined core laboratory image analysis data, they found that more patients in the study group had mild or no motion (p < 0.001), but there was slightly but significantly lower noise level in the control group in three of five anatomic regions. However, there was no difference noted in the signal-to-noise ratio, suggesting that the lowered tube voltage and resulting higher contrast attenuation may have compensated for the overall image quality.

Ultimately, there were no significant differences noted in image quality between the two modes). While this study demonstrates the magnitude of real-world major radiation dose made possible by new advanced scanner technology, the results also stress the need for standards for radiation safety quality control in cardiovascular imaging. "In the present study, no preset scan acquisition protocols were set, and all scans were performed according to institution-specific protocols," the authors noted.

Furthermore, "there were no recommended settings dictated by the study or the manufacturer to explain these findings."

Defined reference levels would help with ensuring radiation safety in the real-world clinical practice of cardiovascular CT, particularly given the rapid increases in multidetector row thoracic CT angiography. Using a conventional 75th percentile dose level, the investigators proposed the following reference levels:

• 453 mGy-cm, 6.3 mSv for CAD
• 326 mGy-cm, 4.6 mSv for PE
• 491 mGy-cm, 6.9 mSv for AD
• 898 mGy-cm, 12.6 mSv for TRO

These differ substantially from the current reference levels for effective dose, suggesting that quality targets for total doses from cardiovascular CT can be reset to lower reference standards for the advanced CT scan technology.

While this study only looked at the impact of adoption of a new scanner technology on radiation dose reduction, the authors noted the many other techniques (such as lowered tube voltage) that played a role in reducing radiation exposure: "This study provides testimony to the increased awareness among CT practitioners regarding dose reduction."

Chinnaiyan KM, Bilolikar AN, Walsh E, et al. J Cardiovasc Comput Tomogr. 2014;8:205-14.

Keywords: CardioSource WorldNews Interventions

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