Heavy metal in radiology: how to reliably differentiate between lodged copper and lead bullets using CT numbers

Abstract

Background The in situ classification of bullets is of interest in forensic investigations when the bullet cannot be removed. Although computed tomography (CT) is usually performed on shooting victims, visual assessment, or caliber measurements using CT can be challenging or infeasible if the bullets are deformed or fragmented. Independent from the bullet’s intactness, x-ray attenuation values (CT numbers) may provide information regarding the material of the bullet. Methods Ethical approval was not required (animal cadavers) or waived by the ethics committee (decedents). Copper and lead bullets were fired into animal cadavers, which then underwent CT scanning at four energy levels (80, 100, 120, and 140 kVp). CT numbers were measured within regions of interest (ROIs). In addition to comparing CT numbers, the dual-energy index (DEI), representing the ratio between the CT numbers of two energy levels, was calculated. The most appropriate method was applied for decedents with fatal gunshot wounds. Results CT numbers demonstrated no significant difference between copper and lead bullets, and false classifications can easily occur. DEI calculations revealed significant differences between the two groups of bullets. The 120/140 DEIs calculated from the maximum CT numbers obtained from ROIs at the edge of copper versus lead bullets presented a significant difference (p = 0.002) and a gap between the CT numbers of copper and lead bullets and was successfully applied for the decedents. Conclusions This study presents a viable method for distinguishing copper and lead bullets in situ via CT and highlights the potential pitfalls of incorrect classifications.

Lodged bullets on computed tomography: Three classification procedures for the virtual investigation of bullets or their fragments that cannot be recovered from the living patient

“…On CT, a fragment may turn out to be a separated jacket, in which case the lodged bullet is certainly not an unjacketed solid bullet. 14 The shape of individual fragments may indicate special types of bullets that are designed to fragment in a predictable manner, such as the Radical Invasive Projectile (RIP), which has been frequently encountered in recent years. [15][16][17][18] This frangible bullet consists of a solid base ('base slug') and six or eight trocar-shaped petals ('trocars'), depending on its calibre.…”

“…However, it is not recommended to draw conclusions on the metallic components of a bullet based on CT numbers from a single energy, since single-energy CT numbers from different metals demonstrate large variations and can be strongly affected by beam hardening from bones in close proximity. 14 An additional CT scan with a different energy is required for the reliable differentiation of lodged bullets. 14,22 The ratio of CT numbers from two different energies is related to the k-edge energy of the material, which allows better differentiation between materials than single-energy radiodensity, and it provides internal referencing, which bypasses the large variations in CT numbers and the influence of beam hardening from bones.…”

“…14 An additional CT scan with a different energy is required for the reliable differentiation of lodged bullets. 14,22 The ratio of CT numbers from two different energies is related to the k-edge energy of the material, which allows better differentiation between materials than single-energy radiodensity, and it provides internal referencing, which bypasses the large variations in CT numbers and the influence of beam hardening from bones. The dual-energy index describes the ratio of CT numbers from two energy levels and was successfully utilised for the differentiation of lodged bullets according to their metallic components.…”

“…The dual-energy index describes the ratio of CT numbers from two energy levels and was successfully utilised for the differentiation of lodged bullets according to their metallic components. 7,14,22 So far, this recently developed method allows only a rough classification into bullets composed of metals with 'low' atomic numbers and bullets composed of metals with 'high' atomic numbers, for instance to distinguish between copper and lead, the most common core metals of bullets. 14,22 Furthermore, a separated copper jacket could be distinguished from the lead core according to its dualenergy index.…”

“…7,14,22 So far, this recently developed method allows only a rough classification into bullets composed of metals with 'low' atomic numbers and bullets composed of metals with 'high' atomic numbers, for instance to distinguish between copper and lead, the most common core metals of bullets. 14,22 Furthermore, a separated copper jacket could be distinguished from the lead core according to its dualenergy index. 14 However, it is not yet feasible to distinguish ferromagnetic (steel-jacketed) from nonferromagnetic (copper-jacketed) lead bullets if the jacket still encases the bullet, as was originally assumed for magnetic resonance imaging safety purposes.…”