Abstract
PurposeForensic investigations could benefit from non-invasive in situ characterization of bullets. Therefore, the use of CT imaging was explored for the analysis of bullet geometry and composition. Bullet visualization with CT is challenging as the metal constituents suffer from excessive X-ray attenuation due to their high atomic number, density, and geometry.MethodsA metal reference phantom was developed containing small discs of various common metals (aluminum, iron, stainless steel, copper, brass, tungsten, and lead). CT images were acquired with 70–150 kVp and 200–400 mAs and were reconstructed using an extended Hounsfield unit (HU) scale (− 10,240 to + 30,710). For each material, the mean CT number (HU) was measured to construct a metal database. Different bullets (n = 11) were scanned in a soft tissue-mimicking phantom. Bullet size and shape were measured, and composition was evaluated by comparison with the metal database. Also, the effect of bullet orientation within the CT scanner was evaluated.ResultsIn the reference phantom, metals were classified into three groups according to their atomic number (Z): low (Z ≤ 13; HU < 3000), medium (Z = 25–30; HU = 13,000–20,000), and high (Z ≥ 74; HU > 30,000). External bullet contours could be accurately delineated. Internal interfaces between jacket and core could not be identified. Cross-sectional spatial profile plots of the CT number along bullets’ short axis revealed beam hardening and photon starvation effects that depended on bullet size, shape, and orientation within the CT scanner. Therefore, the CT numbers of bullets were unreliable and could not be used for material characterization by comparison with the reference phantom.ConclusionCT-based characterization of bullets was feasible in terms of size and shape but not composition.
Highlights
In the last decade, medical imaging modalities have emerged as a valuable tool for postmortem examination of victims in shooting incidents by providing non-invasive information prior to and complementary to conventional forensic autopsy [1,2,3]
Bullet characterization using computed tomography (CT) is not straightforward, as bullets are predominantly composed of metals, with high Xray attenuation compared with organs and bones [5]
At 70 kVp (Fig. 1a), image artifacts were observed for both medium Z-materials and high Z-materials, whereas at 150 kVp (Fig. 1b) artifacts only were apparent for high Z-materials
Summary
Medical imaging modalities have emerged as a valuable tool for postmortem examination of victims in shooting incidents by providing non-invasive information prior to and complementary to conventional forensic autopsy [1,2,3]. With standard reconstruction techniques, the characterization of metals is hampered by the limited range of CT numbers that can be distinguished (− 1000 to + 3095 HU [6]) This can be overcome to some extent by reconstructing images using an extended HU scale (− 10,240 to + 30,710 [6]), which allows quantification of X-ray attenuation values over a much larger range. This has been useful for the identification of heavy and dense materials [7, 8]. The strong Xray attenuation of the bullets’ materials may still result in artifacts that could hamper accurate measurement of bullet geometry and X-ray attenuation [6, 9]
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