No animals were killed for the scientific purposes of this study. The animal models used in this study were obtained from an institute of veterinary pathology. Fresh cadavers were used as an addition to another study with ethical approval and are in accordance with the 3Rs (replacement, reduction, and refinement)—the guiding principles for the ethical use of animals in science. Additional ethical approval for using these animal cadavers was not required. Parts of this study were performed with human cadavers. Ethical approval was waived by the responsible ethics committee of the Canton of Zurich (waiver number: 2015-0686). This article does not contain any studies with (living) human participants.
Animal cadaver study and real forensic cases
Bullets (n = 12) from four different types of ammunition were selected for this study (Action 4, n = 3; QD-PEP, n = 3; Hydra-Shok, n = 3; 7.65 Browning, n = 3) (Fig. 1). The bullets were divided into two groups according to their core materials. One group (copper group, n = 6) included the unjacketed Action 4 and QD-PEP bullets, which are composed of copper. These solid copper bullets are deformation bullets that were developed for law enforcement units. The other group (lead group, n = 6) included the Hydra-Shok and 7.65 Browning bullets, which are frequently encountered lead bullets with jackets composed of copper-zinc alloys (copper/zinc). The Hydra-Shok bullet is a semi-jacketed hollow-point (deformation) bullet, while 7.65 Browning bullets (which are also referred to as .32 ACP bullets) are full metal-jacketed bullets. From each type of ammunition, three bullets were fired into animal cadaver models at a dedicated shooting range. Sheep legs were used as a substitute for human tissue. The shootings were performed by a ballistics expert from a forensics institute. After shooting, each sheep leg was scanned by CT.
Additionally, the distinction between copper and lead bullets was assessed in real forensic cases with fatal gunshot wounds and lodged bullets. The decedents (n = 15) underwent postmortem imaging as part of forensic judicial investigations. Ethical approval was waived by the responsible ethics committee. The bullets were removed during autopsy and identified by the forensics institute. Before the bullets were removed, the decedents underwent a CT examination using the same scanner used for the animal cadaver study. The CT scan protocol from the animal cadaver study was used. Decedents with a lodged Action 4 copper bullet (n = 3) and decedents with a lodged .22 LR lead bullet (n = 3) were selected for this study.
Scan protocol
Repeated CT scans using energy levels of 80, 100, 120, and 140 kVp were performed using a standard medical 128-slice CT scanner (SOMATOM Definition Flash, Siemens Healthcare GmbH, Forchheim, Germany). The tube current was adjusted to gain an almost equal volume CT dose index of 9 mGy at each energy level, which provides equivalent image noise. A standard pitch of 0.6 was used. The raw data were reconstructed using standard filtered back projection with a hard kernel (B70), a slice thickness of 1.5 mm, and a field of view of 140 × 140 mm (reconstruction matrix, 512 × 512; in-plane voxel size, 0.27 × 0.27 mm). Reconstructions were calculated in an extended CT scale (ECTS) to allow measurements beyond the standard range of HU values [22].
ROI measurements, CT numbers, and the dual-energy index
CT numbers were measured in a defined ROI at 80, 100, 120, and 140 kVp (Fig. 2). To assure identical ROI placement, the datasets were displayed side by side in a multiplanar reconstruction view using dedicated software (MM Reading, syngo.via, Version VB10B HF03, Siemens Healthcare GmbH, Forchheim, Germany) [23]. The software enables the mean and maximum CT numbers to be measured within an ROI at the exact same position on all four datasets with different energy levels. ROI circles were drawn at the centre (ROI: 1.6 mm2) and edge (ROI: 0.5 mm2) of the lodged bullet. Measurements were taken separately at these two positions to demonstrate the influence of the bullet’s caliber. For each bullet, six ROIs were positioned at different slices, i.e., different levels within the bullet or its fragments for the core and edge measurements (ROIs per bullet: core, n = 6; edge, n = 6). An ROI was repositioned on a new slice if the upper limit of 30,710 HU was displayed as the maximum CT number. The DEI was calculated for dual-energy pairs of 80/100 kVp, 80/120 kVp, 80/140 kVp, 100/120 kVp, 100/140 kVp, and 120/140 kVp using the mean CT numbers (DEImean) and the maximum CT numbers (DEImax) from the ROI measurements at the centre and edge of the lodged bullet. The following formula [24] was used to calculate the DEI:
$$ DEI=\frac{x_{\mathrm{low}}-{x}_{\mathrm{high}}}{x_{\mathrm{low}}+{x}_{\mathrm{high}}+2000} $$
The variable xlow represents the CT number measured at the lower energy level of the individual dual-energy pair, while xhigh is the CT number measured at the higher energy level.
Distinction between copper and lead bullets
The difference between using CT numbers from a single energy and the DEI for the distinction between copper and lead bullets within the animal cadaver models was assessed by considering the energy level, the use of the mean and maximum CT numbers, and the ROI position (core or edge). The two groups of bullets were compared using statistical analysis, standard deviations, and data overlap. Finally, the most suitable method with the lowest standard deviations and the smallest data overlap was applied and assessed in real forensic cases.
Statistical analysis and data overlap calculations
The overall mean values of the mean CT numbers, of the maximum CT numbers, and of the DEIs of each bullet were used for statistical analysis. The Shapiro-Wilk test was used to determine whether the data were normally distributed. The t test was used for normally distributed data, and the Mann-Whitney U test was used for non-normally distributed data to reveal statistically significant differences between the two groups of bullets (significance level, p < 0.05). The statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS, International Business Machines Corporation, IBM, Armonk, NY, USA).