The images were analyzed using a proprietary image workstation (IntelliSpace Portal 6.5, Philips Healthcare). Virtual monochromatic images (VMIs) at 40 keV and 80 keV and iodine density maps were used. Three regions of interest (ROIs) were manually outlined at the center of nodules in the middle of the homogeneous tissue. The copy-and-paste function was used to ensure that the size and position of the ROIs were the same between reconstructions obtained from the AP and VP scans. To verify the reliability and consistency of the measurement, 70 cases were randomly selected in a blinded manner and measured independently by two radiologists with 13 years and 10 years of experience in thoracic imaging, respectively. The Intraclass correlation coefficient (ICC) between the two radiologists was calculated. The final results of all cases were measured by the radiologist with 13 years’ experience.

The following parameters were used in our study: (I) CT value (HU) from 40 keV reconstructions (CT40keV); (II) the slope of spectral HU curve (λHU): CT values of VMI at 40 keV (CT40keV) and 80 keV (CT80keV) were selected and λHU = |CT40keV − CT80keV|/(80-40) (the changes in CT attenuation in this range were prominent in contrast to the higher energy levels, where the spectral curve exhibited smaller changes); (III) normalized iodine concentration (NIC): the iodine concentration (IC) values of the SPN (ICSPN) and of the thoracic aorta (ICaorta) were measured from the same iodine-based slice and NIC= ICSPN/ICaorta; (IV) normalized arterial enhancement fraction (NAEF) was defined as the NIC in the AP divided by the NIC in the VP. The values of CT40keV, λHU, and NIC were measured from both the AP and VP scans.

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