Comparison of Image Quality between Virtual Monochromatic Images and Conventional CT Images

Objective: To compare the image quality between virtual monochromatic images (VMI) of dual-source dual-energy CT (DECT) and conventional CT images. Siemens SOMATOM Force dual source dual energy CT was adopted in the scanning of the high contrast resolution CTP 528 component and low contrast resolution CTP 515 component of image quality control phantom catphan 500. Dual-tube dual-energy scanning scheme was used to reconstruct low, medium and high-level VMI (group A: 40 keV, group B: 70 keV, group C: 100 keV) and a single tube scanning scheme was used to reconstruct conventional images (Group D: 120 kVp). Two radiologiss carried out the measurement of high resolution by the smallest line pair one could distinguish and low-contrast sensitivity was measured by the smallest diameter under 1% contrast. Mean noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were also evaluated for each group. Results: The image consistency of the two radiologists turned out well (high reolution and low-contrast sensitivity: Kappa coefficient=0.667; noise parameter: Spearman r = 0.920). Among all images, D group performed the best in high resolution (6 lp/cm) and A and B groups showed the best low-contrast sensitivity (diameter of 4 mm). C group had the lowest noise (2.333±0.289) and highest SNR (37.944±3.949). Mean noise and SNR were found significantly difference among the four groups. B group showed the highest CNR (3.361±0.875), but no statistical significance was found among the groups. Conclusion: Compared with conventional CT, VMI images from DECT could improve image quality while retain low-contrast sensitivity; image noise decreases while SNR increases as VMI elevates.