Abstract
This chapter provides an overview of the Virtopsy procedure, a computerised approach to autopsy, lessening the need for invasive examination. Invasiveness results in the loss of evidence, and of the structural integrity of organs; it is also offensive to some worldviews. At the Institute of Forensic Medicine of the University of Bern, the Virtopsy project has unfolded during the 2000s, its aim being the application of high tech methods from the fields of measurement engineering, automation and medical imaging to create a complete, minimally invasive, reproducible and objective forensic assessment method. The data generated can be digitally stored or quickly sent to experts without a loss of quality. If new questions arise, the data can be revised even decades after the incident. This chapter describes technical aspects of the Virtopsy procedure, including imaging modalities and techniques (the Virtobot system, photogrammetry and surface scanning, post-mortem computer tomography, magnetic resonance imaging, post-mortem CT angiography, tissue/liquid sampling), then turning to the workflow of Virtopsy, and to a technical discussion of visualisation. Medical image data are for either radiologists and pathologists, or medical laypersons (such as in a courtroom situation). The final part of this chapter discusses Virtopsy in relation to the Swiss justice system.
*This chapter is contributed by Lars C. Ebert, Thomas Ruder, David Zimmermann, Stefan Zuber, Ursula Buck, Antoine Roggo, Michael Thali, and Gary Hatch.
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Notes
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Luhmann et al. (2006) is a standard reference on close range photogrammetry, “which uses accurate imaging techniques to analyse the three-dimensional shape of a wide range of manufactured and natural objects. Close range photogrammetry, for the most part entirely digital, has become an accepted, powerful and readily available technique for engineers and scientists who wish to utilise images to make accurate 3-D measurements of complex objects” (ibid., from the blurb). The mathematics of close range photogrammetry handles orientation, digital image processing, and the reconstruction of a model in three dimensions. Imaging technology includes both hardware and software. Important topics include targeting and illumination.
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Schafer and Keppens (2007), who were discussing computer animation in the context of computer-assisted teaching in evidence courses, have remarked: “In this animation, two competing theories that both claim to account for the evidence are modelled side by side. According to the prosecution, the evidence found was produced by a cold-blooded killing, according to the defence, it was caused by events more consistent with the assumption of self defence. As we can see from the models, only the defence hypothesis produces the type of evidence that was found, in particular it accounts for the bullet trajectory found in the victim. The user can directly change the position of the people involved, the computer calculates how this would have affected the evidence that was created. The scientific knowledge that underlies these models is complex. To calculate the relevant trajectories requires knowledge of geometry and kinetics, to reason about the ability of the accused to shoot from a specific position requires biological, biomechanical and medical knowledge. How much can a hand holding a heavy gun rotate? What would the recoil do to the ligaments? Moreover, it is not contested knowledge, and hence of little interest to the lawyer pleading the case. Nonetheless, the manipulation of the relevant parametric and geometric equations is taken care of by the computer. The user only needs to manipulate the physical objects (victim, gun, accused) to test different theories and explanations. To hide expert knowledge in this way does create problems if these models are used as evidence, in particular if they are used in an adversarial, partisan context. There is also the danger that computing constraints add facts that are either not established, or not established in legally permissible ways (Selbak, 1994; Kassin & Dunn, 1997; Menard, 1993). In our example for instance, the jury may be subconsciously swayed by the facial expressions of the animates, even though they have not been introduced through a witness into the court proceedings. These problems in using computer models in courts are however an advantage when using them for teaching. Without the need for time consuming mathematical preparation, students can be directly exposed to critical scientific thinking and substantive forensic subjects.”
- 5.
Hounsfield units are a measure of density. For example Aamodt et al. stated (1999, p. 143): “Our aim was to assess in Hounsfield units (HU) the CT density of the inner cortical surface of the proximal femur after this bone had been removed. One HU is defined as a number on a density scale in which the X-ray absorption of water has been assigned the value of zero and the air the value of –1000.”
http://en.wikipedia.org/wiki/Hounsfield_scale provides this definition: “The Hounsfield scale, named after Sir Godfrey Newbold Hounsfield, is a quantitative scale for describing radiodensity. […] The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as –1000 HU. For a material X with linear attenuation coefficient μX, the corresponding HU value is therefore given by
$$\displaystyle HU=\dfrac{\mu_X-\mu_{\textrm{water}}}{\mu_{\textrm{water}}-\mu_{\textrm{air}}}\times 1000$$where μwater and μair are the linear attenuation coefficients of water and air, respectively. Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero. It is the definition for CT scanners that are calibrated with reference to water. Rationale[:] The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. humans. […] The Hounsfield scale applies to medical grade CT scans but not to cone beam computed tomography (CBCT) scans.”
The HU of air is –1000; the HU of fat is –120; the HU of water is 0; the HU of blood is +30 to +45; the HU of muscle is +40; the HU for contrast is +130; the Hu of bone is +400 or more (ibid.). “A practical application of this is in evaluation of tumors, where, for example, an adrenal tumor with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign adrenal adenoma” (ibid.). Something about the history of the technology: “CT machines were the first imaging devices for detailed visualization of the internal three-dimensional anatomy of living creatures, initially only as tomographic reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-ray images are two-dimensional projections of the true three-dimensional anatomy, i.e. radiodensity shadows. It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineers and developers of computed axial tomography (CAT, or CT scans).” (ibid.).
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http://en.wikipedia.org/wiki/Orthopantomogram states the following concerning the equipment: “Dental panoramic radiography equipment consists of a horizontal rotating arm which holds an X-ray source and a moving film mechanism (carrying a film) arranged at opposed extremities. The patient’s skull sits between the X-ray generator and the film. The X-ray source is collimated toward the film, to give a beam shaped as a vertical blade having a width of 4–7 mm when arriving on the film, after crossing the patient’s skull. Also the height of that beam covers the mandibles and the maxilla regions. The arm moves and its movement may be described as a rotation around an instant center which shifts on a dedicated trajectory. The manufacturers propose different solutions for moving the arm, trying to maintain constant distance between the teeth to the film and generator. Also those moving solutions try to project the teeth arch as orthogonally as possible. It is impossible to select an ideal movement as the anatomy varies very much from person to person. Finally a compromise is selected by each manufacturer and results in magnification factors which vary strongly along the film (15–30%). The patient positioning is very critical in regard to both sharpness and distortions.”
The image is formed as follows (ibid.): “Normally, the person bites on a plastic spatula so that all the teeth, especially the crowns, can be viewed individually. The whole orthopantomogram process takes about one minute. The patient’s actual radiation exposure time varies between 8 and 22 seconds for the machine’s excursion around the skull. The collimation of the machine means that, while rotating, the X-rays project only a limited portion of the anatomy onto the film at any given instant but, as the rotation progresses around the skull, a composite picture of the maxillo-facial block is created. While the arm rotates, the film moves in a such way that the projected partial skull image (limited by the beam section) scrolls over it and exposes it entirely. Not all of the overlapping individual images projected on the film have the same magnification because the beam is divergent and the images have differing focus points. Also not all the element images move with the same velocity on the target film as some of them are more distant from and others closer to the instant rotation center. The velocity of the film is controlled in such fashion to fit exactly the velocity of projection of the anatomical elements of the dental arch side which is closest to the film. Therefore they are recorded sharply while the elements in different places are recorded blurred as they scroll at different velocity.”
There is image distortion (ibid.): “The dental panoramic image suffers from important distortions because a vertical zoom and a horizontal zoom both vary differently along the image. The vertical and horizontal zooms are determined by the relative position of the recorded element versus film and generator. Features closer to the generator receive more vertical zoom. The horizontal zoom is also dependent on the relative position of the element to the focal path. Features inside the focal path arch receive more horizontal zoom and are blurred; features outside receive less horizontal zoom and are blurred. The result is an image showing sharply the section along the mandible arch, and blurred elsewhere. For example, the more radio-opaque anatomical region, the cervical vertebrae (neck), shows as a wide and blurred vertical pillar overlapping the front teeth. The path where the anatomical elements are recorded sharply is called ‘focal path’.”
Digital dental radiology, using electronic sensors and computers, offers advantages (ibid.): “One of the principal advantages compared to film based systems is the much greater exposure latitude. This means many fewer repeated scans, which reduces costs and also reduces patient exposure to radiation. Lost X-rays can also be reprinted if the digital file is saved. Other significant advantages include instantly viewable images, the ability to enhance images, the ability to email images to practitioners and clients (without needing to digitize them first), easy and reliable document handling, reduced X-ray exposure, that no darkroom is required, and that no chemicals are used.”
- 7.
See Niklaus Schmid’s (2009) Handbuch des Schweizerischen Strafprozessrechts, N 380.
- 8.
Article 253 Section 3 Swiss Code of Criminal Procedure (SCCP).
- 9.
Articles 249–252 Swiss Code of Criminal Procedure (SCCP).
- 10.
Schmid Niklaus, N 944 f.
- 11.
Article 140 Swiss Code of Criminal Procedure.
- 12.
Article 76 Section 4 Swiss Code of Criminal Procedure (SCCP).
- 13.
Schmid Niklaus, N 929 f.
- 14.
Article 253 Section 3 Swiss Code of Criminal Procedure (SCCP).
- 15.
Schmid Niklaus, N 936.
- 16.
Schmid Niklaus, N 937 f.
- 17.
Schmid Niklaus, N 940 f.
- 18.
SCHMID, N 951 f.
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Nissan, E. (2012). Virtopsy: The Virtual Autopsy*. In: Computer Applications for Handling Legal Evidence, Police Investigation and Case Argumentation. Law, Governance and Technology Series, vol 5. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-8990-8_9
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