Implementing a coronary CT angiography protocol based on the body mass index: Radiation dose reduction, image quality, and diagnostic performance

Objectives

To evaluate the relation between the coronary calcium score and the posterior choice of kilovoltage according to radiologists’ criteria in a standard coronary CT angiography protocol to rule out coronary disease.

To quantify the reduction in ionizing radiation after linking kilovoltage to patients’ body mass index in a low-dose protocol with iterative model reconstruction.

To evaluate the image quality and diagnostic performance of the low-dose protocol.

Material and methods

We compared anthropometric characteristics, calcium score, kilovoltage levels, size-specific dose estimates (SSDE), and the dose-length product (DLP) between a group of 50 patients who were prospectively recruited to undergo coronary CT angiography with a low-dose protocol and a historical group of 50 patients who underwent coronary CT angiography with the standard protocol. We correlated these parameters, the number of coronary segments that could not be evaluated with and without temporal padding, the attenuation, and the signal-to-noise ratio in the ascending aorta in the low-dose protocol with excellent imaging quality according to a semiquantitative scale. To calculate the diagnostic performance per patient, we used 24-month clinical follow-up including all tests as the gold standard.

Results

In the standard protocol, the presence of coronary calcium correlated with the selection of high kilovoltage (p = 0.02); this correlation was not found in the low-dose protocol (p = 0.47). Median values of SSDE and DLP were significantly (p < 0.001) lower and less dispersed in the low-dose protocol [9.22 mGy (IQR 7.84–12.1 mGy) vs. 26.5 mGy (IQR 21.3–36.3 mGy) in the standard protocol] and [97 mGy cm (IQR 78–134 mGy cm) vs. 253 mGy cm (IQR 216–404 mGy cm) in the standard protocol], respectively.

The overall quality of the images obtained with the low-dose protocol was considered good or excellent in 96% of the studies. The parameters associated with image quality in a multivariable model (C statistic = 0.792) were heart rate (estimated coefficient, −0,12 [95% confidence interval: −0.2, −0.04]; p < 0.01) and the SSDE (estimated coefficient, −0,26 [95% confidence interval: −0.51, −0.01]; p < 0.05).

The CAD-RADS modifier for a not fully evaluable or diagnostic study was used on two occasions (4%); the final measures for the diagnosis of coronary disease were sensitivity 100%, specificity 94%, and efficacy 94%.

Conclusions

In the standard protocol, the radiologist selects higher kilovoltage for CT angiography studies for patients whose previous calcium score indicates the presence of coronary calcium. In the low-dose protocol, linking kilovoltage with body mass index enables the dose of radiation to be reduced by 65% while obtaining excellent or good image quality in 96% of studies and excellent diagnostic performance.