Bullet characterization using Photon-Counting detector CT: A phantom study with intact bullets

Photon-counting detector CT (PCD-CT) is expected to substantially improve and expand CT-imaging applicability due to its intrinsic spectral capabilities, increased spatial resolution, reduced electronic noise, and improved image contrast. The current study aim is to evaluate PCD-CT efficacy in characterizing bullets based on their dimensions, shape, and material composition.

This is an observational phantom study examining 11 unfired, intact bullets of various common calibers, placed in ballistic gelatin. Ultra-high resolution and spectral images were acquired on a clinical PCD-CT-scanner. CT-measurements of the dimensions (length and diameter) were independently determined by 2 readers and compared to previously obtained manual caliper measurements. Bullet shape (round nose, flat nose, or spitzer bullet) was assessed using 3D-volume rendered (3D-VR) CT-image reconstruction. Bullet composition was examined based on dual-energy ratios (DER) which were color-coded accordingly and overlaid on high resolution CT-images.

Compared to caliper measurements, mean differences (SD) in bullet diameter were + 0.40 mm (0.16) and + 0.38 mm (0.13) for readers 1 and 2; mean differences in length were + 0.49 mm (0.19) and + 0.45 mm (0.20). The observed proportion of accurately categorized 3D-VR reconstructions was 90.9 % (10/11) for both readers with moderate interrater agreement (Cohens κ: 0.59, P = 0.004). DER color-coded images showed clear distinction between jacket and core and enabled correct identification of bullets with and without jackets.

These results indicate that PCD-CT is a promising tool for reliable radiological characterization of bullets.