Radiation Dose Reduction for Coronary Artery Calcium Scoring Using a Virtual Noniodine Algorithm on Photon-Counting Detector Computed-Tomography Phantom Data

Abstract

Background: On the basis of the hypothesis that virtual noniodine (VNI)-based coronary artery calcium scoring (CACS) is feasible at reduced radiation doses, this study assesses the impact of radiation dose reduction on the accuracy of this VNI algorithm on a photon-counting detector (PCD)-CT. Methods: In a systematic in vitro setting, a phantom for CACS simulating three chest sizes was scanned on a clinical PCD-CT. The standard radiation dose was chosen at volumetric CT dose indices (CTDI_Vol) of 1.5, 3.3, 7.0 mGy for small, medium-sized, and large phantoms, and was gradually reduced by adjusting the tube current resulting in 100, 75, 50, and 25%, respectively. VNI images were reconstructed at 55 keV, quantum iterative reconstruction (QIR)1, and at 60 keV/QIR4, and evaluated regarding image quality (image noise (IN), contrast-to-noise ratio (CNR)), and CACS. All VNI results were compared to true noncontrast (TNC)-based CACS at 70 keV and standard radiation dose (reference). Results: IN_TNC was significantly higher than IN_VNI, and IN_VNI at 55 keV/QIR1 higher than at 60 keV/QIR4 (100% dose: 16.7 ± 1.9 vs. 12.8 ± 1.7 vs. 7.7 ± 0.9; p < 0.001 for every radiation dose). CNR_TNC was higher than CNR_VNI, but it was better to use 60 keV/QIR4 (p < 0.001). CACS_VNI showed strong correlation and agreement at every radiation dose (p < 0.001, r > 0.9, intraclass correlation coefficient > 0.9). The coefficients of the variation in root-mean squared error were less than 10% and thus clinically nonrelevant for the CACS_VNI of every radiation dose. Conclusion: This phantom study suggests that CACS_VNI is feasible on PCD-CT, even at reduced radiation dose while maintaining image quality and CACS accuracy.

Keywords: calcium scoring; computed tomography; coronary artery disease; photon-counting detector; radiation dose; virtual noniodine.

Figure 1

Comparison of (A) image noise levels (SD background) and (B) contrast-to-noise ratio between VNI (55 keV, QIR 1 and 60 keV, QIR 4, respectively) and TNC (70 keV, QIR off) reconstructions. Range of IN_TNC and CNR_TNC at dose level 100 to 25% highlighted in green. Recommended Image Noise Target: 20 HU in small/medium phantoms (T1) and 23 HU in large phantoms (T2), respectively [33]. **** p < 0.0001. CNR = contrast-to-noise ratio; HU = Hounsfield units; IN = image noise; QIR = quantum iterative reconstruction; SD = standard deviation; VNI = virtual noniodine.

Figure 2

Comparison of Agatston scores between CACS_VNI (55 keV, QIR 1 and 60 keV, QIR 4, respectively) at different radiation dose levels and CACS_TNC (70 keV, QIR off) at standard radiation dose. ns = not significant; QIR = quantum iterative reconstruction; TNC = true noncontrast; VNI = virtual noniodine.