Image Characteristics of Virtual Non-Contrast Series Derived from Photon-Counting Detector Coronary CT Angiography-Prerequisites for and Feasibility of Calcium Quantification

Abstract

In photon-counting detector CT (PCD-CT), coronary artery calcium scoring (CACS) can be performed using virtual non-contrast (VNC) series derived from coronary CT angiography (CCTA) datasets. Our study analyzed image characteristics of VNC series in terms of the efficacy of virtual iodine "removal" and image noise to determine whether the prerequisites for calcium quantification were satisfied. We analyzed 38 patients who had undergone non-enhanced CT followed by CCTA on a PCD-CT. VNC reconstructions were performed at different settings and algorithms (conventional VNC_Conv; PureCalcium VNC_PC). Virtual iodine "removal" was investigated by comparing histograms of heart volumes. Noise was assessed within the left ventricular cavity. Calcium was quantified on the true non-contrast (TNC) and all VNC series. The histograms were comparable for TNC and all VNC. Image noise between TNC and all VNC differed slightly but significantly. VNC_Conv CACS showed a significant underestimation regardless of the reconstruction setting, while VNC_PC CACS were comparable to TNC. Correlations between TNC and VNC were excellent, with a higher predictive accuracy for VNC_PC. In conclusion, the iodine contrast can be effectively subtracted from CCTA datasets. The remaining VNC series satisfy the requirements for CACS, yielding results with excellent correlation compared to TNC-based CACS and high predicting accuracy.

Keywords: cardiac imaging; coronary artery calcium quantification; photon-counting detector CT; virtual non-contrast imaging.

Figure 1

Corresponding image examples for TNC and all reconstructed VNC series, respectively. VNC_Conv = conventional virtual non-contrast; VNC_PC = pure calcium.

Figure 2

Effectiveness of iodine removal. (A) demonstrates the segmentation of the left ventricle, right ventricle, and myocardium. (B) shows the segmentation of the whole heart including the atria. (C) exhibits the histograms based on the heart segments for CTA, TNC, VNC_Conv¹, and VNC_PC¹. For the whole heart, the histogram proportion exceeding 130 HU (marked by the dotted line) is given.

Figure 3

Boxplots of the noise levels measured as standard deviation of CT values in three circular regions of interest within the left ventricle comparing true non-contrast and virtual non-contrast (conventional and pure calcium) series, and differentiating between the different reconstruction settings of VNC^x (x = 1–4). Stars mark significant differences, as *** = p < 0.001, and n.s. marks no significant difference.

Figure 4

Boxplot of measured calcium quantities (score and volume) comparing TNC and VNC (conventional and pure calcium) series. (A) differentiates between the different reconstruction settings of VNC^x (x = 1–4) and (B) differentiates between the different coronary arteries considering only VNC² series. Stars mark significant differences, as * = p < 0.05 and *** = p < 0.001, and n.s. marks no significant difference.

Figure 5

Linear regression analyses of TNC and VNC (conventional and pure calcium) series shown for the Agatston score on a per-patient level for the different reconstruction settings of VNC^x (x = 1–4). r² = coefficient of determination.

Figure 6

Boxplots of the mean absolute error between the predicted Agatston scores based on VNC (conventional and pure calcium) and Agatston scores derived from TNC series on a per-patient level resulting from the 10,000-fold bootstrapping analysis for the different reconstruction settings of VNC^x (x = 1–4). Stars mark significant differences with *** = p < 0.001.