AJR: Rad dose may be underestimated in CCTA studies

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 Source: U. Joseph Schoepf, MD

Published effective radiation dose values may be underestimated and current methods of dose assessment may not be suitable for cardiac CT angiography (CCTA) acquisitions, according to a multicenter trial published in the May issue of the American Journal of Roentgenology. The researchers also found that the use of protocols and newer technologies can help reduce dose.

“Dose assessment is particularly relevant for cardiac CT because it is increasingly used in patient groups with various risk profiles,” wrote Jacob Geleijns, MD, of the Leiden University Medical Center in Leiden, Netherlands, and colleagues.

During the CORE 64 study (Coronary Artery Evaluation Using 64-Row Multidetector Computed Tomography Angiography Study), researchers assessed the diagnostic accuracy of a 64-slice CT to identify coronary artery stenosis (CAS). Different acquisition protocols were used to take into account CT scanner (Aquilion 64, Toshiba Medical Systems) characteristics and patient size and sex. The researchers also reported patient dose exposure.

The cardiac CT protocol and organ and effective radiation doses were reported for six patient models. The average dose for CCTA was calculated by Report 103 of the International Commission on Radiological Protection (ICRP) and was 19 mSv with a range from 16-26 mSv, Geleijns and colleagues reported.

The average effective dose for whole cardiac CT protocol—CT scanograms, bolus tracking and calcium scoring—was reported to be 22 mSv with a range of 18 to 30 mSv. Sex-averaged effective dose was 16 mSv and size-averaged dose was 18 mSv using ICRP definitions.

Average normalized weighted CTDI 100 for CCTA was 0.086 mGy/mAs with a range of 0.080-0.094 mGy/mAs across the nine participating centers. “The good agreement between sites confirms that variation in radiation output was only minimal between the nine different CT scanners that were installed at the centers participating in the CORE 64 study.”

For women, the average volume CTDI 100 was reported to be 41 mGy and for small, normal-sized and obese men these numbers were 52, 59 and 62 mGy, respectively.

Geleijns recommended that an E/DLP dose conversion factor of 0.030 mSv/mGy cm be used for cardiac CT. The CORE 64 trial was based on ICRP 60 and the extensive literature published on effective dose in cardiac CT also uses ICRP 60 dose definitions. “Effective dose becomes much higher for cardiac CT when assessed according to the now preferred ICRP 103 publication because of the higher weighting factor for breast tissue,” the authors noted.

In fact, the researchers estimated that sex-averaged effective dose is estimated to be 25 percent higher when ICRP 103 was used compared with ICRP 60.

The researchers said that of the total 49 reported dose values only nine values were traceable to actual dose measurements. And for 10 of the values the researchers said that is unclear whether dose measurements were performed or if the technology was calibrated properly. Thirty of the 49 reported dose values were untraceable and the effective dose was derived from the CLP or CTDI indicated in the operator’s console.

The researchers also reported dose values for different protocols including helical acquisition and retrospective ECG-gating reconstruction, helical acquisition with ECG-triggered modulation of tube currents and ECG-gated reconstruction, a helical acquisition of a small phantom (55-kg adult) and retrospective ECG-gated reconstruction and the step-and-shoot method.

The average effective dose for the aforementioned protocols was 15 mSv, 12 mSv, 22 mSv and 4 mSv, respectively.

“The published values show a trend of substantial dose reduction when a helical acquisition with tube current modulation is used (12 mSv) and even more when prospectively triggered axial step-and-shoot acquisitions are used (4 mSv),” the authors wrote. “This observation suggests that with the wider implementation of these dose-saving acquisition techniques, patient dose in cardiac CT decreases substantially.”

Lastly, the researchers concluded that newer technologies that allow the entire heart to be scanned within one heartbeat, and high-pitch dual-source CT and volumetric CT, can help further minimize dose in CCTA exams.