The ability of virtual noncontrast images to remove intrinsic and external iodine from the thyroid via dual-layer spectral detector computed tomography

Background: Virtual noncontrast (VNC) images generated by dual-layer spectral computed tomography (DLCT) remove iodine influence from enhanced images to simulate true noncontrast (TNC) images. Previous research has demonstrated the high comparability of abdominal VNC images with TNC images, suggesting their potential as substitutes. Given the thyroid's significant iodine content, this study evaluated the efficacy of VNC images for removing both intrinsic and extrinsic iodine through an analysis of computed tomography (CT) attenuation and iodine density in TNC and enhanced VNC thyroid images.

Methods: A total of 115 thyroid nodules were classified into papillary thyroid carcinoma (PTC) and nodular goiter (NG) groups based on pathology. Patients underwent both noncontrast and enhanced neck scans, with attenuation values of CT and iodine density measured for nodules, normal thyroid, and the carotid artery on conventional and VNC images. The study calculated CT attenuation and iodine density for both intrinsic and external iodine content. The Wilcoxon rank-sum test was used to compare intrinsic and external iodine differences between the PTC and NG groups. The Spearman test was used for correlation assessment within each group, and Bland-Altman analysis was conducted to evaluate CT attenuation consistency between VNC images from nonenhanced scans (VNCn) and VNC images from enhanced scans (VNCe) within the groups.

Results: The CT attenuation values of intrinsic iodine showed no differences between nodules, normal thyroid, and carotid artery in the PTC and NG groups (P>0.05). However, there was a difference in the CT attenuation values of external iodine for nodules between the two groups (P<0.05), while no difference was found for normal thyroid or the carotid artery between the groups (P>0.05). The correlation between CT attenuation values of the nodules, normal thyroid, and carotid artery for intrinsic iodine and iodine density was high in both the PTC group (r=0.919, r=0.951, r=0.592, respectively; P<0.001) and the NG group (r=0.925, r=0.973, r=0.721, respectively; P<0.001). Similarly, there was a strong correlation between the CT attenuation values of the nodules, normal thyroid, and carotid artery for extrinsic iodine and iodine density in the PTC group (r=0.960, r=0.965, r=0.904, respectively; P<0.001) and the NG group (r=0.979, r=0.967, r=0.963, respectively; P<0.001). There was no statistically significant difference in the CT attenuation values of the normal thyroid in VNCn and VNCe images (P>0.05), while the CT attenuation values of nodules and the carotid artery in VNCn and VNCe images in the two groups were significantly different (P<0.05). Bland-Altman scatter plots showed that, apart from the normal thyroid in the NG group, the consistency of CT attenuation values on VNCn and VNCe images was generally poor (more than 5% of data points outside the consistency limit).

Conclusions: VNC images show certain ability to remove intrinsic and external iodine from the thyroid. In the diagnosis of thyroid nodules, enhanced VNC images of the thyroid cannot replace conventional nonenhanced images.