Printing: the STL files were then l

Printing: the STL files were then loaded to the 3-D printer to print the phantom. Each segmented object was assigned a specific printing material, which was a mixture of two base printing materials (two cartridges). Note that to construct phantoms that mimic patient anatomy in CT images, materials with appropriate attenuation properties are needed. Therefore, cylindrical rods made from various printing materials were scanned on a CT scanner (Definition Force, Siemens Healthcare) at nine different tube potentials (the most available on a CT scanner), from 70 to 150 kV at 10 kV steps. Based on the measured CT numbers of the materials, appropriate printing materials were assigned to each object in the phantoms. The final selection of printing material for each object is reported in the Results section.