The utility of a particular medium in the identification of individual skeletal remains is directly related to the availability of that medium, in other words the likelihood that antemortem data will be available for comparison to the same type of evidence collected from the deceased. In its various forms, radiography is frequently used in the diagnosis of a myriad of diseases, and for this reason, has become increasingly common in the antemortem record. In recent years, the invention of new radiographic modalities has increased the ease and accuracy with which various clinical diagnoses can be made, and there has been a concomitant increase in the employment of these radiographic techniques as means to make forensic identifications.
However, the petrous part of the temporal bone has been the subject of surprisingly little research as far as development of techniques for individual identification using radiographic imagery. At least in part, this is due to its relatively poor representation on most types of radiographic imagery. In recent decades, however, a variety of imaging modalities have become available for use in clinical evaluation of the petrous part of the temporal bone, including polytomography, angiography, magnetic resonance imaging (MR), and
are also seen incidentally on radiographs intended to portray other portions of the skull or cervical spine.
The most significant problem associated with the use of plain
radiographic imaging of the petrous portion of the temporal bone is related to the properties of the x-ray beam that passes through the skull, and the composition of the image that results. Unlike, CT and MR imagery, the images resulting from plain radiography are not cross-sectional, and as a result, the features through which the x-ray beam passes appear superimposed. This complicates the visualization of small individual features. It has also been demonstrated that abnormalities seen on plain radiographs often correlate poorly with the actual dimensions of intracranial abnormalities (Schaffer et al., 1980; Taylor, 1982; Virapongse et al., 1982). For these reasons, conventional radiography has largely been replaced by MRI and CT as the most frequently used techniques in cranial imaging, and is limited presently, to use in diagnosing acute sinusitis and cranio-facial trauma, both of which are better demonstrated with CT when
available.
Computed tomography scanning provides the most accurate graphic representation of bony and air space anatomy and has also replaced
polytomography in imaging of soft tissue abnormalities because of its lesser susceptibility to artifact and greater image accuracy. Another advantage of CT imaging is the decreased level of radiation to which it exposes patients,
under a variety of circumstances that has resulted in an exponential increase in its usage relative to other radiographic modalities, and a consequent increase in its pervasiveness in the antemortem record. The result is a considerable and ever increasing number of antemortem CT images that a forensic scientist can expect to be available for comparison. The typical hospital in a moderate sized city performs between 30 and 50 routine head CT scans per week (Dr. Gill Naul, personal communication). This chapter will: (1) define CT technology, both mechanically and historically, (2) discuss the advantages of CT relative to other modalities, both in the clinical and forensic settings, (3) discuss its limitations, (4) outline the solutions to those limitations, and finally, (5) relate the advantages and limitations of its use in the forensic sciences generally, and the present research specifically.
Computed tomography (CT) was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories, England, and independently by Allan Cormack of Tufts University during the same year (Haaga et al., 1994). The earliest CT systems were dedicated to head imaging only and "whole body" systems with larger patient openings became available in 1976. CT was widely available by about 1980. As in traditional radiography, CT employs ionizing radiation, projected through the body onto a photographic plate. The CT apparatus, however, allows for the acquisition of images from many different angles. CT scans result in digital data, which are then processed by a computer and used to create cross-sectional images of selected body tissues, organs, or
bones. Early CT systems often required several hours to assemble a single image, and these images were restricted to the axial (horizontal) plane of the body. This is the derivation of the original term Computed Axial Tomography or "CAT scan”. Modern CT scanners are much faster, and provide images of far greater detail. The first CT scanner developed by Hounsfield in his lab at EMI took several hours to acquire the raw data for a single scan or "slice" and took days to reconstruct a single image from this raw data. The latest multi-slice CT systems can collect up to 4 slices of data in about 350 ms and reconstruct a 512 x 512-matrix image from millions of data points in less than a second (Kalendar, 2006). An entire chest (40 8 mm slices) can be scanned in five to ten seconds using the most advanced multi-slice CT system (Kalendar, 2006).
The CT scanning process consists of moving the CT apparatus around a stationary patient by means of a mobile table, which moves the patient through a circular shaped scanner (Kalendar, 2006). The scanning portion of the machine is then rotated around the selected portion of the patient’s body. The CT
apparatus rotates a narrow x-ray beam around a stationary patient, and the portion of the x-ray energy transmitted through the body is quantified on the opposite side of the body. Tissues of the body absorb variable amounts of energy based on their physical properties, particularly density. The apparatus uses a series of simultaneous mathematic equations to quantify the amount of x- ray energy absorbed by each of the tissue areas. A numerical density value is attributed to each volume unit. These densities are then translated by digital-
analog converter from numerical units into shades along a gray scale. CT slices through the body consist of collections of large numbers of these units according to the pattern represented by the variation in tissue densities within the area exposed to radiation within that slice. Slice thickness can typically vary from 1.5 to 10mm based on the clinical goal of the scan (Hsieh, 2003). Conventional CT technology typically requires several seconds for the acquisition of the slice and for the subsequent computer interpretation. Newer technologies are making the process considerably shorter, which by minimizing artifacts related to patient movement, enhances both the comfort of the patient and the quality of the images (Kalendar, 2006).
The resultant data are adjusted according to specific parameters called "windows." The term window refers to the range of grey shades incorporated into an image. Adjustment of the window settings allows for the better
visualization of variable tissue types. This process is discussed in detail in later paragraphs. The cross sectional images that result are then interpreted by a radiologist, either on printed films or on digital computer images. The higher resolution and enhanced diagnostic utility that result constitute the main
advantage of computed tomography over traditional x-ray technology. The use of a cross sectional perspective allows the doctor to more accurately locate an injury or tumor in relation to the surrounding anatomy, and to more accurately estimate its size and severity. Another advantage of CT technology is the ability to visualize a variety of tissue types on a single image, including bones, soft
tissues and blood vessels. This enhanced detail often eliminates the need for invasive exploratory surgery and surgical biopsy.
Computed tomography usage is generally divided into two categories, CT of the head and CT of the body. The uses of body CT vary widely and include cancer diagnosis, diagnosis and treatment of spinal problems, evaluation of skeletal injuries, measurement of bone mineral density for the detection of osteoporotic bone loss, identification of traumatic injuries to internal organs, and the diagnosis and treatment of vascular diseases. Head CT is used in a variety of diagnoses, including recognizing blood clots, detecting some varieties of brain tumors, diagnosis of problems associated with the eyes and the optic nerves, detecting fractures of the bones around the eyes or foreign objects in the eye, evaluating cranial sinuses for inflammation or other changes, investigating problems associated with the bone and joints of the temporomandibular joints, detection of enlarged brain cavities, locating potential skull fractures, identifying brain damage resulting from traumatic injury, and finally, in investigating the petrous part of the temporal bone as related to middle ear and auditory nerve problems. CT provides information that aids in the evaluation of symptoms such as confusion, paralysis, numbness, vision problems, vertigo or headaches that might indicate a brain injury, brain tumor, aneurysm, or bleeding within the skull. If bleeding is known to have begun, CT is also useful in assessing its severity, and the prognosis of potential surgical intervention. CT is also routinely used to
evaluate the extent of damage caused by a stroke and to determine the effectiveness of stroke treatment.