Determining the Radiation Dose Reduction Potential for Coronary Calcium Scanning With Computed Tomography

Abstract

This study describes a method to determine the lowest possible thresholds for volume computed tomographic dose index (CTDI(min)) and maximum tolerable pixel noise (SD(max)) values for coronary calcium scanning while maintaining accurate Agatston score values. The method was applied to a comparison between the iterative reconstruction (IR) and filtered backprojection (FBP) image reconstruction algorithms in a phantom study. An anthropomorphic thoracic phantom with a calibration insert for the quantification of coronary calcium, containing 200, 400, and 800 mg HA/cm of calcium mass spheres of 1, 3, and 5 mm diameter (QRM GmbH, Moehrendorf, Germany), was scanned without (G1) and with (G2) an additional 2 cm-thick wrap of muscle-equivalent material. Electrocardiographically simulated volume scans were performed on a 320-row computed tomographic scanner (Aquilion ONE, Toshiba Medical Systems, Otawara, Japan) set to 120 kilovolt peak [kVp] and 10 to 580 mA variations in 21 steps. For the IR, the Adaptive Iterative Dose Reduction 3-dimensional algorithm (AIDR 3D) was used. Agatston scores were calculated semiautomatically on the computed tomographic console. Inclusion tests to assess the accuracy of the Agatston scores were performed to determine the CTDI(min) thresholds and the associated maximum pixel noise SD(max) for FBP and IR from identical raw data. The inclusion tests were as follows: (1) the semiautomatic identification of the 1 mm sphere with 800 mg HA/cm, (2) the exclusion of false-positive lesions, and (3) a statistical outlier test. Statistical differences between the Agatston score means from both image reconstruction algorithms were evaluated using the paired t test. All Agatston scores using both reconstruction methods were normally distributed (P > 0.49). For FBP and IR, the mean ± 1σ of Agatston score, CTDI(min), and SD(max), respectively, were determined as follows: 697.8 ± 7.7, 7.5 mGy, and 24.4 Hounsfield unit (HU) (G1-FBP); 678.8 ± 14.3, 1.5 mGy, and 20.1 HU (G1-IR); 677.0 ± 11.6, 14.5 mGy, and 27.3 HU (G2-FBP); and 643.9 ± 13.4, 2.6 mGy, and 20.0 HU (G2-IR). The mean Agatston scores obtained using IR (both with and without the additional 2 cm muscle shell) were slightly (approximately 5%) but significantly lower (P ≤ 0.001) than those obtained using FBP reconstruction. The Adaptive Iterative Dose Reduction algorithm AIDR 3D shows potential to reduce dose exposure by approximately 80% in comparison with the dose currently applied with FBP image processing. On the basis of phantom evaluation, a target noise of 20 HU for the application of this method in coronary calcium scanning is recommended to avoid loss in accuracy.