Forensic Anthropology MODULE NO.24: Facial Superimposition-I
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SUBJECT FORENSIC SCIENCE Paper No. and Title PAPER No.11: Forensic Anthropology Module No. and Title MODULE No.24: Facial Superimposition-I Module Tag FSC_P11_M24 FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Two-Dimensional Images 4. Three-Dimensional Skull Models from Limited Two-Dimensional Data 5. Three-Dimensional Skull Models from Clinical Imaging 6. Craniofacial Superimposition 7. Video Superimposition 8. Summary FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I 1. Learning Outcomes After studying this module, you shall be able to- Know about the importance of facial superimposition Learn about the various techniques of facial superimposition 2. Introduction In some forensic investigations the usual methods utilized for human identification can be unsuccessful, and the police may have few clues as to the identity of an individual. The majority of identification techniques require a known individual with whom to compare data, such as DNA, fingerprints, or dental records, and where there are no suspects for identification, it is practically impossible to compare data with records from an entire population. In these circumstances the police may employ less definitive methods in an attempt to focus on a population from which the individual may be identified. Facial reconstruction is one of the methods that are frequently employed in such investigations. Facial reconstruction (otherwise known as facial approximation) is the process utilized to reproduce the facial appearance of an individual and includes a number of different procedures. Traditionally, facial reconstruction has involved the analysis of skeletal detail to determine facial morphology. However, facial soft tissues may be present as partially decomposed, damaged, distorted, or preserved remains. In these cases it may not be appropriate to present images of the remains to the public, and facial reconstruction is utilized to visualize facial appearance. In these circumstances the practitioner will analyse the soft tissues rather than skeletal detail, although it may be possible to analyse both soft and hard tissues where clinical imaging or dissection are employed. Different approaches in this field have created some confusion with regard to the reliability and presentation of a facial reconstruction within a forensic scenario. Some practitioners attempt to reproduce a facial “type,” an approximation of the facial proportions and morphology that relies on sets of average tissue data and facial templates relating to the sex, age, and ethnic group of the individual (Evison et al. 1999, Vanezis et al. 2000, Stephan 2004). Facial superimposition is one of the techniques that come under forensic facial reconstruction. It is the technique by which a particular skull recovered can be matched with the photograph. It is very helpful for identification. More recent photograph is better. Even photograph of lateral view can be used. Two negatives are prepared one of the person and the other of the skull. FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I Appropriate magnification is done to achieve maximum alignment and where inter-pupillary distance of both exactly matches. It is a known fact that the inter-pupillary distance of a person never changes regardless of his age. The two negatives are then superimposed by keeping over each other and various points are compared. The canthus, the nasion, the nasal spine and lower border of nose and upper jaw, the supraorbital ridges, angle of jaw, external auditory meatus and the teeth etc. are compared and a photograph is taken which is analysed for similarities and dissimilarity to arrive at a conclusion. This test has more of negative value as it can definitely rule out a combination of a photo and a skull though only a possibility of a match can be established. 3. Two-Dimensional Images In certain circumstances, photographs or x-rays of the skull may be the only available information, and access to the original specimen may be restricted due to legal matters, inaccessibility, destruction, or damage. Where images of the skull are considered adequate, two-dimensional facial reconstruction can be performed. Ideally, the skull should be in the Frankfurt Horizontal Plane (FHP), and for 35mm photography, a lens with a focal length of 100–200mm and a camera position of no less than 12 ft/3 m from the skull will avoid distortion. When there is access to the original specimen, the facial anthropologist should take scaling measurements or include a visual scale in the foreground of the images. If possible, tissue depth markers should be attached to the skull prior to production of the images to provide the maximum number of tissue indications. When this is not possible, but a scale has been included, some of the tissue depth markers can be indicated on overlays superimposed onto the images of the skull. 4. Three-Dimensional Skull Models from Limited Two-Dimensional Data When access to the original specimen is not possible, either directly or via three-dimensional clinical imaging, methods of three-dimensional model production from two-dimensional data (such as radiographs, photographs, and craniometrics) may be utilized. Radiographs and/or photographs are used as templates, and multiple views are aligned using cranial points as registration marks. Computer modeling software is then employed to create a three- dimensional model (Wilkinson 2005) or distort a template mesh (Davy et al. 2005) to reproduce the skull morphology. Extrapolation of surface morphology between the views is inevitable, and the more views that are available, the more accurate the resulting three- dimensional model of the skull will be. FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I As these methods include a certain degree of estimation and loss of detail on the surface of the bone, any resulting facial reconstruction or approximation would require photographic records of the skull and an appreciation of the decrease in accuracy of any resulting face. Care must be taken when establishing anatomical landmarks. 5. Three-Dimensional Skull Models from Clinical Imaging Where soft tissues are present, access to the original specimen is restricted or computer-based facial reconstruction or approximation is planned. Cross-sectional data created by x-ray computed tomography (CT) or surface scans can be employed to produce a three dimensional digital model of the skull (Spoor et al. 2000). Sometimes replica skulls may be produced from digital data using stereo-lithography (Hjalgrim et al. 1995) or another form of three- dimensional model manufacture (Seitz et al. 2005). Where digital data from surface scans and CT scans are employed the limitations of the data must be taken into account. With CT data, the slice thickness, scan plane, spatial resolution, filters, and angle of rotation will all affect the resulting three-dimensional model. In addition, dental filling and appliances will cause artefacts and may require some manual intervention. A variety of software packages render the surface cross-sectional CT data by extracting selected tissues and visualizing the tissues as three-dimensional models for use in computer- based facial reconstruction or approximation systems, or for production of a physical replica. Surface rendering involves segmentation (isolation of the tissue by thresholding), interpolation between the slices to create a smooth surface, and illumination of the surface. Spoor and his colleagues (2000) state that “However, improved visual representation [is, this] does not imply that the image is more accurate as well. The extent to which the reconstruction reflects reality primarily depends upon limitations inherent to CT or MRI.” FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I Fig.1: Common facial anthropometry points With surface scan data morphology, details may also be lost due to discrepancies at apertures, fossae, and holes where the scanning mechanism cannot visualize the surface. In addition, hair on the face and the head may cause surface artefacts. As surface scanners produce a series of profiles from a number of viewpoints, which then require computational reconstruction of the object and subsequent manual editing to ascertain clean, raw data sets, there may be inconsistencies to the original specimen. Where possible, attention should be given to the original specimen, so that errors are not perpetuated to the facial reconstruction. There are many surface scanners currently available, including laser (Kau et al. 2005, Moss et al. 1989, Bush and Antonyshyn 1996), photographic projection (Yamada et al. 1999, Motoyoshi et al. 1992, Techalertpaisarn and Kuroda 1998, Curry et al. 2001, Siebert and Marshall 2000), or holographic (Bongartz et al. 2000, Giel et al. 2004), and three-dimensional models will be created that can be imported into computer-based facial reconstruction or approximation systems or employed to produce a three-dimensional replica. FORENSIC SCIENCE PAPER NO.11 : Forensic Anthropology MODULE NO.24: Facial Superimposition-I Where the skull is fragmented and each piece has been scanned, reassembly can take place using some computer-based systems. There are computer-based systems (Wilkinson 2003, Sanghera et al. 2002, Bibb et al. 2000) that allow manual reassembly