Figure 1: An axial CT image of the low-contrast module. It contains three groups of cylinders with various diameters from 2.0 to 15.0 mm. The nominal contrast levels of these groups are 0.3%, 0.5%, and 1.0%. In our study, only the 15.0 mm diameter object with a contrast difference of 1.0% (white arrow) was chosen to be analyzed.