Forensic Investigations: Using Micro-CT to Examine Toolmarks in Bone

Micro-CT is proving valuable in the analysis of crimes and the presentation of evidence by helping establish the weapon class used for sharp force injuries and/or to link a specific object to a crime, making it an important emerging technology within forensic laboratories¹. In recently published work, researchers extracted detailed knife toolmark properties and statistically differentiated the marks left from straight and serrated blades². Using micro-CT we can non-destructively image the sample, preserving the sample for future testing. Special micro-CT software allows for detailed volumetric analysis of the data for comparison to other evidence. Micro-CT data is also readily converted to a file type suitable for use with 3D printers so that highly accurate physical representations of key pieces of evidence can be produced at any scale.

While clinical CT scanning has been used in forensic studies for some time, micro-CT has the capacity to image forensic specimens at a much higher resolution and can be used to identify and classify microscopic injuries and marks. When compared to clinical CT, micro-CT provides increased spatial resolution leading to higher resolution images, albeit on smaller sample sizes. Clinical CT represents an effective method to image large samples, such as a body undergoing autopsy, while micro-CT provides a means to examine a specific feature of interest in greater detail.

For our demonstration, we chose to utilize our SkyScan 1173 system due to its large sample capacity and fine spatial resolution.

Micro-CT Scan of Bone Samples

After reconstructing the X-ray attenuation data for both bones, we are able to isolate our view on specific regions of interest such as locations with possible toolmark impressions (Figure 1). In both of the views comprising Figure 1, the image from each coordinate plane focused at the point of impact between the tool and the bone is highlighted. The ability to view the mark from all three coordinate planes provides us the unique ability to visualize the cut from multiple views, which is impossible using traditional light microscopy.

If we more closely examine the marks left in the cortical bone by both the knife and the saw we can readily visualize notable differences in the marks left behind by each instrument (Figure 2). For the knife strike to the bone, a pronounced and sharp wedge shaped incision is present in the cortical bone. This matches well with expectations based on previous scientific studies of knife wounds to bone³.  Similarly, examination of the mark left by the saw shows a wide, flat impression in the cortical bone representing the kerf of the blade used to create this cut.

Likewise, if we examine the 3D renderings of the toolmarks in the bone samples we see a similar difference between the two samples as observed for the 2D view (Figure 3). The mark left by the knife in the bone is well defined and wedge-shaped while the marks from the saw blade are more irregularly defined and show the expected flat bottomed profile representing the width of the blade which passed through the bone.

Conclusion

Micro-CT excels as a tool to nondestructively examine toolmark impressions in bone and produces a highly detailed copy suitable for evidentiary comparison. We hope you found this image of the month interesting. If you have an image of the month sample that you would like us to scan, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com

Scan Specifications

All scans completed on our large capacity SkyScan 1173 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using DataViewer and CTVox.

Works Cited

1. http://dx.doi.org/10.1016/j.forsciint.2012.10.030
2. https://doi.org/10.1016/j.forsciint.2017.12.039
3. https://www.ncjrs.gov/pdffiles1/nij/grants/232227.pd