The development of an anatomically accurate human head model for the analysis of cranial backspatter

Abstract

Firearm fatalities feature prominently in crime statistics worldwide. An understanding of the elements related to a gunshot event, especially bloodstain evidence, is crucial. Blood spatter generated from a gunshot wound is subdivided into forward spatter and backspatter. Forward spatter travels in the same direction of the bullet and is ejected from the exit wound. Backspatter travels opposite to the direction of the fired shot, ejected from the entrance wound. Backspatter generated following a gunshot event has high evidential value for crime scene investigators. Despite this fact, no existing studies have utilised an anatomically accurate human head model to study backspatter. This project developed the first anatomically accurate human head model to be used for quantitative and qualitative bloodstain pattern analysis resulting from a gunshot event. The construction of a full cranial human model with internal cavities was a complex process. Models went through a series of developmental stages prior to achieving a final reliable and valid model. It consisted of the current best simulant materials which emulated real human tissue. The models incorporated five layers: brain, bone, blood, tissue and skin. Full cranial models were tested at a firing range. The gunshot event was filmed with high-speed video cameras and blood spatter stains were collected. High-speed photography captured key backspatter events on film. These videos were quantitatively assessed to measure backspattered droplet diameters and velocities. Deposited backspatter stains were also quantitatively assessed with various software programs. This enabled the identification of several backspatter pattern characteristics which included: distribution, maximum travel distance, stain number and stain size. A maximum travel distance range was identified as 121 mm to 1692 mm, with a possible maximum of 1971 mm from blood spattered off skin. Over 90 % of all deposited stains were less than 2.2 mm in diameter. Characteristics of backspatter droplet diameters and velocities lacking from the literature were identified. Measured droplet diameters ranged from 0.2 mm to 3 mm and velocities ranged from 1 m/s to 27 m/s. Multiple ejections of backspatter were observed with three of the final models, including four separate ejections observed with one.