Objective:To compare the difference of image quality between virtual non-contrast(VNC)with different iteration levels of spectral iteration algorithm and true non-contrast(TNC)with iDose4 algorithm,which to explore the application value of spectral iteration algorithm on optimizing the image quality of chest VNC.Methods:54 patients with chest plain scan combined with enhanced scan by dual-layer spectral detector CT were prospectively collected.TNC was reconstructed using iDose4 algorithm to iterate level 4.The spectral iteration algorithm was used to reconstruct 14 groups of VNC images with level 0 to level 6 from the arterial and venous enhanced scan data.The CT value and noise in the aorta and erector spinae of VNC and TNC images were measured and used to calculate SNR and CNR.VNC image quality was evaluated by two radiologists.One-way ANOVA analysis and Kruskal-Wallis test were used to analyze the objective evaluation indexes of TNC and VNC images.Bland-Altman plots were used to analyze the consistency of CT value in TNC and VNC images.Spearman correlation analysis was used to analyze the correlation between SNR,CNR and spectral iteration levels.Wilcoxon test was used to compare the subjective scores of VNC.Results:Compared with TNC,the CT value of the aorta in VNC-a had a statistical difference(P<0.05),while the CT value of the erector spinae in VNC-a and the CT values of the aorta and erector spinae in VNC-v had no statistical differences(all P>0.05).There were no statistical differences in the CT value of each tissue between the same phase VNC images of different iteration levels(all P>0.05),and no statistical differences of noise,SNR or CNR at spectral iteration level 3 in the arterial phase or venous phase between VNC and TNC(all P>0.05).Bland-Altman analysis showed that the proportion of out-of-boundary data points in the aortic CT value of VNC-a was 3.7%when the consistency limit was 15 HU,and the CT values of VNC and TNC in other tissues had a good consistency.The SNR and CNR of VNC-a and VNC-v images were positively correlated with the spectral iteration levels(r=0.792,0.263,0.761,0.255;all P<0.01).The subjective scores of VNC-v images of spectral iteration level 3 and level 4 were the highest,with no statistical difference(P>0.05).The consis-tencies of subjective scores of 14 groups of VNC images were high(all K≥0.898).Conclusions:In the chest scan of the dual-layer spectral detector CT,the objective index of the VNC-v image with the spectral algorithm iteration level 3 is the closest to the TNC image,and the image score is the highest,which can replace TNC for routine use.
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