Optimal virtual monochromatic images for assessing metastatic lateral cervical lymph nodes in patients with papillary thyroid carcinoma using dual-layer spectral detector computed tomography

Purpose: To determine the optimal virtual monochromatic images (VMIs) from dual-layer spectral detector computed tomography for the visualization and diagnosis of metastatic lateral cervical lymph nodes (LNs) in patients with papillary thyroid carcinoma (PTC). Methods: Ninety-five lateral cervical LNs (49 metastatic and 46 non-metastatic) derived from 24 patients (16 females; mean age, 40.0 +/- 13.4 years) were included. 40-100 kiloelectron voltage (keV) VMIs, 120 keV VMI and conventional 120 kV peak (kVp) polyenergetic image (PI) were reconstructed. Five-point scale of subjective image quality, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of LNs were assessed and compared among each VMI and 120 kVp PI. Receiver operating characteristic (ROC) curves and Delong tests were used to assess and compare the diagnostic efficacy of arterial enhancement fraction (AEF) based on each VMI and 120 kVp PI. Results: 40 keV VMI showed significantly higher SNR and CNR in both arterial and venous phases, and better image quality in arterial phase than 70-100 keV VMIs, 120 keV VMI, and 120 kVp PI (all p < 0.05). In all sets of images, AEF values of metastatic LNs were significantly higher than those of non-metastatic LNs (all p < 0.05). When using AEF value of 40 keV VMI to diagnose metastatic lateral cervical LNs, an area under ROC curve (AUC) of 0.878, sensitivity of 87.8 % and specificity of 80.4 % could be obtained, while the AUC of AEF value of 120 kVp PI was 0.815 (p = 0.154). Conclusion: 40 keV VMI might be optimal for displaying and diagnosing the metastatic lateral cervical LNs in patients with PTC.