Fig. 2
Cross sections of a lodged Action 4 copper bullet at different energy levels indicated in kilo-voltage-peaks (kVp) (a, 80 kVp; b, 100 kVp; c, 120 kVp; d, 140 kVp). Region-of-interest-based measurements were carried out in the hyperdense ring at the edge (area 0.5 mm2, highlighted in blue) and in the centre of the bullet (area 1.6 mm2, highlighted in red). Each pixel (i.e., voxel) contains a single CT number. The measurements indicate the mean computed tomography (CT) number, the standard deviation (SD), the minimum CT number, and the maximum CT number of all pixels within the ROI. The CT numbers (i.e., the x-ray attenuation values) are influenced by the energy level. CT numbers obtained from two different energy levels can be used to calculate the dual-energy index (DEI), which represents the ratio of the CT numbers of the two energy levels