Usefulness Assessment of Virtual Monochromatic Image using Dual Layer Detector CT in Head & Neck Computed Tomography

Among the many functions of dual-energy computed tomography (CT), virtual monochromatic images provide various diagnostic values compared to conventional iterative reconstruction images, such as increasing visualization of intravascular iodine or reducing beam hardening artifact according to monochromatic image level. The optimal monochromatic image level for diagnosis differs for each applied anatomical site, In the neck CT protocol, we intend to find the optimal level of virtual monochromatic images for diagnosis through quantitative methods. We retrospectively studied 33 patients performed on dual layer detector type IQon Spectral CT (Philips, Amsterdam, Netherlands). The evaluation site was designated as the common carotid artery, internal jugular vein, sternocleidomastoid muscle, submandibular glands, and thyroid glands. As an indicator of image evaluation, the contrast-to-noise ratio (CNR) formula set against the background of sternocleido-mastoid muscle and signal-to-noise ratio (SNR) was used. The IMR and iDose iterative reconstruction images and the monochromatic Images 40 keV to 100 keV with the images at intervals of 10 keV were compared. After the normality test of the data was performed to check whether there was a significant difference in the mean of the comparative images, the Friedman test, a nonparametric response sample test, was performed, and all data were significantly different from each other. As a result of the experiment, SNR and CNR were measured the highest in the monochromatic 40 keV image of all evaluation sites except the sternocleido-mastoid muscle. Monochromatic 40 keV image is expected to help diagnose by providing better image quality and higher contrast in quantitative terms than the iterative reconstruction images used in existing clinical trials, and provides the possibility of reducing the amount of contrast medium used in the study.