Incorporating Radiologic Imaging in the Study of Wound Ballistics

Abstract

Advances in radiologic imaging are being applied to the study of wound ballistics. Each of the modalities used (fluoroscopy, radiography, computed tomography, and magnetic resonance imaging) has strengths and limitations. Cross sectional digital based modalities have advantages over traditional radiography and fluoroscopy for localization and recovery of projectiles and documentation of wound tracks. Data from multiplanar two-dimensional and three-dimensional imaging can best be used by the forensic pathologist when combined with pathologic findings. With this information it may be possible to enhance the current understanding of wound ballistics.

Keywords

Forensic pathology Wound ballistics Forensic radiology Computed tomography Postmortem imaging

Introduction

Ballistics is the study of the dynamics of projectiles. When applied to firearms, it is further categorized into interior ballistics, exterior ballistics, and terminal ballistics. Interior ballistics is the study of the bullet within the firearm. For firearms that use a cartridge, this covers the time period from when the firing mechanism strikes the primer to when the bullet exits the barrel. Exterior ballistics is the study of the bullet in flight and covers the time period from when the bullet exits the barrel to when the bullet impacts a target. The target may be paper, wood, animal, human or the ground. Terminal ballistics is the study of the interaction between the bullet and the target.

Although the three ballistic categories are separated from one another based on the location of the bullet, they are interrelated by the laws of physics. However, this interrelation can be complex. In order to understand terminal ballistics, it is necessary to have at least a cursory understanding of interior ballistics, exterior ballistics, and bullet types. In-depth discussion of these topics can be found in Di Maio's book entitled “Gunshot Wounds: Practical Aspects of Firearms, Ballistics, and Forensic Techniques“ (1) and Rinker's book entitled “Understanding Firearm Ballistics: Basic to Advanced Ballistics, Simplified, Illustrated and Explained“ (2). Varying only a single factor such as bullet type (e.g., full metal jacket versus hollow point) may have a significant effect on how the bullet interacts with the target.

Wound ballistics is a subset of terminal ballistics that refers to the study of the dynamics of a bullet within biological tissue and the injuries caused. It is generally accepted that the first scientific investigations of wound ballistics were performed by the Swiss surgeon Emile Theodor Kocher in the 1870s. At the time of these studies, the “explosive” injuries made by conoidal bullets were thought to be due to the bullet's rotation, imparted by the barrel's rifling, which caused centrifugal forces that tore tissue apart. Kocher was able to show that there was no difference between wounds caused by conoidal bullets fired from a barrel with rifling and those from a barrel without rifling. He proposed that these “explosive” injuries were caused by hydrodynamic factors that came to be known as cavitation. In addition, he noted the importance of both bullet deformation and bullet velocity on determining the degree of wounding (3, 4). In England in 1898, Charles Woodruff proposed that it is the transfer of the kinetic energy from the bullet to the tissue and not just the bullet's amount of energy at entry that determines the degree of wounding (5).

Since that time, there have been numerous wound ballistics studies. These include, but are not limited to, studies by LaGarde and Thompson (6), Hatcher (7), Harvey (8), Fackler (9, 10), MacPherson (11), and Sellier (12) and the writings of Di Maio (1) and Bellamy and Zajtchuk (13). These studies have shown that wound ballistics depends on many factors, some controllable and some uncontrollable. Some of these factors include: type of firearm, construction of the cartridge, type of gunpowder, construction of the bullet, flight path of the bullet, intermediary target, angle of impact, location of impact on the body, deformation and fragmentation of the bullet after impact, “tumbling” of the bullet within the body, viscoelastic properties of the tissues, and nature of the permanent and temporary cavities. The permanent cavity is the actual track of tissue crushed by the bullet, including fragments. The temporary cavity is the displacement of tissue around a bullet's track that expands and collapses in milliseconds.

Although there is little controversy that the permanent cavity causes wounding, there is some controversy to the degree of wounding that the temporary cavity causes. For example, fracture of long bones in the extremities not hit by the bullet are mentioned in the literature but are rarely seen clinically (14, 15). Another area of controversy is the possible wounding caused by the sonic shock wave that precedes the bullet as it passes through tissue (11, 16). Information gathered from the traditional forensic autopsy has not been able to either support or reject most of the hypotheses. However, incorporating recent advances in radiologic imaging may lead to relevant information.

Radiologic Imaging

Since the development of the radiograph by Wilhelm Roentgen in 1895, radiologic imaging has been used in medicolegal death investigations (17). In terms of wound ballistics, these images have been used to document the presence and location of projectiles and the injuries caused by them. The National Association of Medical Examiners (NAME) has established minimum standards for the use of radiologic imaging in forensic autopsy performance. The second of the three standards states “Radiographs detect and locate foreign bodies and projectiles“ (18). However, this guideline does not delineate either the extent of imaging required or the imaging modality.

There are four radiologic imaging modalities currently being used in medicolegal death investigations involving ballistic wounds: fluoroscopy, radiography, computed tomography (CT), and magnetic resonance imaging (MRI). All modalities are now available in digital format allowing for electronic recording, viewing, manipulation, and transmission of images. Each modality has its own unique advantages and disadvantages for investigating wound ballistics postmortem. A fifth modality, sonography, can be considered. There are limitations to this modality, mainly soft tissue air that degrades the images, and consequently experience has been limited.

Fluoroscopy displays a continuous X-ray image on a monitor in real-time. The commonly used configuration by medicolegal systems is the “C-arm” unit where X-ray source and receptor are linked and the object studied is placed in between. The main use of the fluoroscope is to facilitate the localization and recovery of projectiles. When viewing the images real-time, as the body or fluoroscope is moved, the field of view is limited and motion obscures detail. The scanning technique may allow one to “miss” a bullet or bullet fragments. The fluoroscope only displays where the projectile is in two dimensions ( Image 1 ). In order to determine the exact location of the projectile it is necessary to either rotate the fluoroscope to obtain an orthogonal view or probe the surrounding tissue with a radio-opaque instrument to determine the plane of the projectile. Still, the “C-arm” is very helpful in recovering minute fragments difficult to view at autopsy. Although fluoroscopy can detect skeletal injuries, the limited field of view makes documentation of the entire skeleton extremely difficult. Another limitation is the concern of radiation exposure to the individuals performing the scan and recovery.

Image 1:

Fluoroscopic (“C-arm”) image of a round projectile showing excellent edge detail.

Radiography uses electromagnetic radiation, “X-rays”, to generate and record a static image after termination of exposure. The medium for viewing these images has changed from film, to receptor plates in computed radiography, to computer monitors in direct digital radiography (DDR) where the X-ray tube and receptor are linked. In order to produce a “whole body” image and not overlook any projectiles or injuries, individual radiographs of the head, neck, torso, pelvis and extremities must be taken separately and then viewed in succession, ensuring all areas of the body are covered. A recent development in the field of radiography has been the introduction of planar whole body digital scanning systems (Lodox Systems Ltd, Saudton, Johannesburg, South Africa). This type of system uses a unique geometry detector configuration and linear slot scanning radiography technology to produce an entire full-body image in seconds. High quality images can be obtained from any angle between the sagittal and coronal planes.

For decades, radiography has been used to document the presence or absence of projectiles, location of projectiles, skeletal injuries, and additional findings such as air in the right atrium and pulmonary outflow tract in an individual with a gunshot wound to the head (19). As long as the projectile is radio-opaque, there is excellent edge detail of the projectile's borders ( Image 2 ). It must be noted that precise measurement of the dimensions of the projectile is limited by geometric and physical factors during the capture of the image. Although tempting, the caliber of the bullet cannot be determined on radiography. It can only be determined when it is recovered (1, 19). The edge detail allows for the visualization of deformation and/or fragmentation of the projectile prior to recovery ( Image 3 ). Similar to fluoroscopy, the image displayed is two dimensional. Orthogonal views are necessary to determine the location of the projectile in all three planes of the body. The location and extent of long bone skeletal injuries caused by projectiles is usually readily apparent ( Image 3 ). However, without orthogonal views, it is often difficult to determine the exact location and extent of axial skeleton injuries. The main limitations of radiography are the inability to determine the location of wound tracks and to detect soft tissue injuries. Radiographs may not detect non-metal fragments such as plastic or paper wadding which can be projectile components.

Image 2:

Penetrating gunshot wound of the chest from a rifle. Radiographs show a bullet projecting on the scapula. Note scapular fracture (arrow) and right lung opacity.

Image 3:

Radiograph of the forearm with a single penetrating gunshot wound. Note bullet fragmentation with comminuted fractures of the radius and ulna. The penetrator tip (arrow) is recognized by its conical configuration.

In computed tomography (CT), the X-ray source and detector rotate around the individual as they are moved through a circular opening on a motorized table. As the detector records the X-rays passing through the individual, this data is sent to a computer that maps the attenuation measurements to picture elements using grayscale. The reconstruction of these data sets produces a series of “slices” or cross-sectional images. Post processing workstations can be used to create multiplanar two-dimensional and three-dimensional images that can be manipulated and reformatted. With multi-detector, helical CT scanners, whole body imaging can be completed in minutes.

Similar to radiography, CT can be used to document the presence or absence of projectiles. In contrast to radiography, the location of projectiles relative to adjacent anatomical structures can be determined in any plane ( Image 4 ). A disadvantage of CT is that metallic objects create a streak artifact and the edge detail of the projectile's borders is blurred ( Image 4 ). When there is significant fragmentation of the projectile, it may be difficult to individually identify the location of each fragment. CT is effective in documenting skeletal injuries including the axial skeleton ( Image 5 ). Three-dimensional images can be reformatted to demonstrate the extent of skeletal injury. Caution must be taken when creating these images as the “smoothing” software can mask injury. Additional findings, such as a hemothorax due to injury of a major vascular structure, are readily apparent on CT ( Image 5 ). The ability of CT to detect soft tissue injury depends on the organ and the ballistic characteristics of the projectile.

Image 4:

Axial CT of the bullet in Image 2 shows precise location of the bullet posterior to the scapular fracture. Note loss of edge detail by streak artifact. There is a right hemothorax and a left pneumothorax.

Image 5:

Gunshot wound of the thorax from a rifle shatters the vertebral body with disruption of the posterior elements and spinal canal (arrow A). Bilateral pneumothorax and hemothorax (arrow B) are present.

In many cases, the path of the bullet through the body can be determined using CT. For soft tissue, gas and hemorrhage are the principal markers of the wound track. For bone, fractures and bone fragments in adjacent soft tissue are the principal markers of the wound track (20). In some cases, the direction of the wound track can be determined based on the pattern of fractured bone. The beveling of the skull that has been described in the literature (1, 21, 22) can be demonstrated on CT images. Since the images can be viewed in any plane, it is possible, in select cases, to view the track of the bullet on a single image (Images 6A, 6B, and 6C). However, it must be understood that many factors affect the trajectory of a bullet within the body and this may not always be a linear path. In addition, the body position at the time of injury may be completely different from position at the time of imaging. As a consequence, individual injuries such as fractures of different bones may not form a straight line even though the trajectory of the bullet through the body was linear. One limitation of CT is the difficulty in determining the location of the entrance and exit wounds on the surface of the skin. Optical surface scanning (23) and the placement of radio-opaque markers on the skin surface at the location of the wounds (24) have been used to overcome this limitation (Images 7A and 7B). Another significant limitation of CT is its inability to discriminate specific vascular injury. A technique to overcome this limitation is postmortem angiography, in which radio-opaque contrast material is injected into a vessel and images obtained (20, 25, 26).

Image 6A:

Sagittal reconstruction in the plane of the track shows the submandibular entry wound (arrow A), defect in the clivus (arrow B), and exit at the skull vertex (arrow C).

Image 6B:

Three-dimensional reconstruction with vertex digitally removed shows the bullet hole in the clivus (arrow).

Image 6C:

Sagittal reconstruction at the exit wound. Margins of defect demonstrate external beveling.

Image 7A:

Axial CT performed without marker does not show the surface wound on the left side of the back.

Image 7B:

Repeat CT after metallic marker defines the precise location (arrow). Note that a wound in the right chest (not visible on this image) has produced diffuse soft tissue air.

Computed tomography has the potential to help resolve some of the controversies of ballistic wounds such as the degree of wounding by the temporary cavity. For example, Image 8 depicts a perforating gunshot wound of the lower extremity caused by a full metal jacket bullet fired from a rifle. The wound track is immediately adjacent to the bone and there is no evidence of fracture. In cases where the bullet has injured the lung, CT imaging depicts a zone of hemorrhage around the wound track ( Image 9 ).

Image 8:

Axial CT of a perforating gunshot wound of the left thigh from a rifle. The soft tissue track (arrows) passes adjacent to the femur without fracture. Note a tiny bullet fragment in the soft tissue (arrowhead).

Image 9:

Perforating gunshot wound of the thorax from a rifle. Coronal CT reconstruction demonstrates parenchymal hemorrhage in both lungs (outlined arrows) attributed to the temporary cavity. Note soft tissue air in the left chest wall and axilla.

Magnetic resonance imaging (MRI) uses a magnet to align protons in the body. A radio frequency current is briefly turned on that flips the spin of the protons in the field. After this is turned off, the spin of the protons return and receiver coils capture the radio frequency signal generated during this relaxation phase. Different biological tissues have different relaxation times. The distribution of protons in the body can be mathematically recovered from the signal to create an image. The brightness of the tissue depends on the type of relaxation that is measured. For example, in T₁–weighted scans the images depict the differences in spin-lattice relaxation time and water appears darker and fat appears brighter. In T₂–weighted scans the images depict the differences in spin-spin relaxation time and water appears brighter and fat appears darker. As with CT, this image can be displayed in any plane. MRI has superior soft tissue contrast resolution as compared to CT. This resolution may allow for further delineation of soft tissue injury created by the temporary cavity (27). In the literature there has been sparse documentation of the use of postmortem MRI in ballistic injury because of the possible presence of ferromagnetic fragments in the body. A main concern is the movement of the ferromagnetic projectiles during the scan as this can distort the forensic findings as well as damage the equipment.

Conclusion

The use of radiologic imaging to study wound ballistics requires integration of imaging data with all information available to the forensic pathologist, in particular the scene investigation and the autopsy itself. Experience to date has demonstrated imaging alone does not allow definitive conclusions about ballistic wounds. Imaging does, however, provide the forensic pathologist with a tool to improve understanding of ballistic wounds both on a case specific basis and as a basic principle. Advancements in imaging have yielded data that can best be understood when it is combined with pathologic findings through careful scientific study.

Footnotes

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the view of the Departments of the Army,Air Force or Defense.

The authors,reviewers,editors,and publication staff do not report any relevant conflicts of interest.

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