Imaging
Imaging
High-tech diagnostic imaging techniques that have allowed physicians to explore bodily structures and functions with a minimum of invasion to the patient have been exploited for other uses. Forensic investigations have been one of the beneficiaries.
A forensic investigator may be faced with a body that displays no outward signs of trauma. Learning the cause of death may involve delving inside the body. Imaging techniques allow a detailed examination without the immediate need of a destructive autopsy .
The use of imaging techniques in forensics has followed the development of the techniques for other purposes. During the 1970s, advances in computer technologies, in particular the development of algorithms powerful enough to allow difficult equations to be solved quickly enough to be of real-time use in the clinical diagnostic setting and to eliminate "noise" from sensitive measurements, allowed the development of accurate, accessible, and relatively inexpensive (when compared to surgical explorations) non-invasive technologies. Although relying on different physical principles (i.e., electromagnetism vs. sound waves), all of the high-tech methods relied on computers to construct visual images from a set of indirect measurements. The development of high-tech diagnostic tools was the direct result of the clinical application of developments in physics and mathematics. These technological advances allowed the creation of a number of tools that made diagnosis more accurate, less invasive, and more economical.
The use of non-invasive imaging traces it roots to the tremendous advances in the understanding of electromagnetism during the nineteenth century. By 1900, physicist Wilhelm Konrad Roentgen's (1845–1923) discovery of high-energy electromagnetic radiation in the form of x rays were used in medical diagnosis.
The development of powerful high-tech diagnostic tools in the later half of the 20th century was initially the result of fundamental advances in the study of the reactions that take place in excited atomic nuclei. Applications of what were termed nuclear spectroscopic principles became directly linked to the development of non-invasive diagnostic tools used by physicians.
In particular, Nuclear Magnetic Resonance (NMR) was one such form of nuclear spectroscopy that eventually found widespread use in the clinical laboratory and medical imaging. NMR is based on the observation that a proton in a magnetic field has two quantized spin states. Accordingly, NMR allowed the determination of the structure of organic molecules and, although there are complications due to interactions between atoms, in simple terms NMR allowed physicians to see pictures representing the larger structures of molecules and compounds (i.e., bones, tissues, and organs) obtained as a result of measuring differences between the expected and actual numbers of photons absorbed by a target tissue.
Groups of nuclei brought into resonance, that is, nuclei absorbing and emitting photons of similar electromagnetic radiation (e.g., radio waves) make subtle yet distinguishable changes when the resonance is forced to change by altering the energy of impacting photons. The speed and extent of the resonance changes permits a non-destructive (because of the use of low energy photons) determination of anatomical structures. This form of NMR is used by physicians as the physical and chemical basis of a powerful diagnostic technique termed Magnetic Resonance Imaging (MRI).
MRI scanners rely on the principles of atomic nuclear-spin resonance. Using strong magnetic fields and radio waves, MRI collects and correlates deflections caused by atoms into images of amazing detail. The resolution of the MRI scanner is so high that they can be used to observe the individual plaques in multiple sclerosis.
Principles of SONAR technology (originally developed for military use) found clinical diagnostic application with the 1960s development of ultrasound. A sonic production device termed a transducer was placed against the skin of a patient to produce high frequency sound waves that were able to penetrate the skin and reflect off internal target structures. Modern ultrasound techniques using monitors allow physicians real-time diagnostic capabilities. By the 1980s, ultrasound examinations became commonplace in the examination of fetal development.
The advent of other imaging to supplant x rays provided for less potentially damaging forms of diagnosis. High photon energies found in x rays are ionizing and are thus capable of destroying chemical and molecular bonds in cells. In contrast, ultrasound relies not on electromagnetic radiation but rather on pressure waves that are non-ionizing.
Microscopes using ultrasound can be utilized to study cell structures without subjecting them to lethal staining procedures that can also impede diagnosis through the production of artifacts (extraneous bits of highlighted material). Ultrasonic microscopes differentiate structures based on underlying differences in pathology . Ultrasonic imaging devices are also among the least expensive of the latest high-tech innovations in diagnostic imaging.
During the early 1970s, enhanced digital capabilities spurred the development of Computed Tomography (derived from the Greek Tomos, meaning slice) imaging, also called CT, Computed Axial Tomography or CAT scans, invented by English physician Godfrey Hounsfield. CT scans use advanced computer-based mathematical algorithms to combine different readings or views of a patient into a coherent picture usable for diagnosis. Hounsfield's innovative use of high energy electromagnetic beams, a sensitive detector mounted on a rotating frame, and digital computing to create detailed images earned him the Nobel Prize. As with x rays, CT scan technology progressed to allow the use of less energetic beams and vastly decreased exposure times. CT scans increased the scope and safety of imaging procedures that allowed physicians to view the arrangement and functioning of the body's internal structures on a small scale.
American chemist Peter Alfred Wolf's (1923–1998) work with positron emission tomography (PET) led to the clinical diagnostic use of the PET scan, allowing physicians to measure cell activity in organs. PET scans use rings of detectors that surround the patient to track the movements and concentrations of radioactive tracers. The detectors measure gamma radiation produced when positrons emitted by tracers are annihilated during collisions with electrons. PET scans have attracted the interest of psychiatrists for their potential to study the underlying metabolic changes associated with mental diseases such as schizophrenia and depression. During the 1990s, PET scans found clinical usage in the diagnosis and characterizations of certain cancers and heart disease, as well as clinical studies of the brain.
MRI and PET scans, both examples of functional imaging (in addition to detailing structures they provide a view of dynamic functions), are the subject of increased research and clinical application. MRI and PET scans are used to measure reactions of the brain when challenged with sensory input (e.g., hearing, sight, smell), activities associated with processing information (e.g., learning functions), physiological reactions to addiction, metabolic processes associated with osteoporosis and atherosclerosis, and to shed light on pathological conditions such as Parkinson's and Alzheimer's disease.
During the 1990s, the explosive development of information technologies and the Internet allowed imaging data to be shared globally, both in real-time and by mining databases.
see also Alternate light source analysis; Biometric eye scans; Confocal microscopy; Polarized light microscopy; Scanning electron microscopy; Ultraviolet light analysis; Visible microspectrophotometry.
Digital Imaging
Digital Imaging
Digital imaging is the electronic recording, processing, enhancement, and storage of visual information. Its applications in forensic science range from documenting crime scenes to enhancing faint or indistinct patterns such as partial fingerprints. Although digital imaging is often considered to be synonymous with digital photography , digital imagery can also be obtained by conventional x-ray radiography, computed tomography (CT or CAT scans), magnetic resonance imaging (MRI), laser scanning, and infrared photography.
Both digital and film photography employ lenses to focus light rays into a sharp image, with the size of the image controlled by the focal length of the lens. Lenses with long focal lengths produce larger images than those with short focal lengths, although the magnitudes of long and short are relative to the film or sensor size. Focal lengths are generally given in millimeters. A diaphragm within the lens (in combination with shutter speed) controls the amount of light entering the camera as well as the depth of focus in the image. In place of the film used in a digital camera, however, a digital camera uses a light-sensitive electronic sensor. Two sensor types are commonly used: CCD, or charged coupled device sensors, and CMOS, or complementary metal oxide semiconductor sensors. Both types of sensors are composed of rows and columns of photosites that convert light into an electronic signal. Each photosite is covered with a filter so that it is sensitive to only one of the three (red, blue, or green) components of visible light. Digital image processing techniques can also be applied to film negatives or positives if they are digitized using a high-resolution scanner that operates in much the same way as a digital camera.
Two primary measures are used to characterize digital images: resolution and size. Resolution refers to the ability of a sensor to represent details, and is generally specified in terms of pixels per inch (ppi). Image size refers to the total number of pixels comprising an image, and is typically given in terms of megapixels. A pixel is the smallest possible discrete component of an image, typically a small square or dot, and one megapixel consists of one million pixels. As of early 2005, the best commercially available digital cameras had resolutions of approximately 20 megapixels and many professional quality digital cameras had resolutions of 5 or 6 megapixels.
Regardless of its origin, once an image is available in digital form it can be modified or enhanced using digital image processing techniques. Common image processing techniques include contrast stretching to expand the tonal range of an image, edge detection to outline areas possessing similar textural or tonal properties, and unsharp masking to increase sharpness. Unsharp masking derives its unusual name from a film photography technique in which an original negative was combined with a deliberately blurred negative to produce a sharp print. Although these image processing techniques can do much to enhance subtle features of an image, they cannot create information that does not already exist.
One of the aspects that distinguishes forensic digital imaging from non-forensic digital imaging arises from legal considerations. Images that are destined for use in a court of law must be obtained and processed using carefully documented procedures if they are to be allowed as evidence . The documentation typically includes the name of the photographer, the date the image was obtained, the names of anyone who had access to the image before it was introduced in court, the names of anyone who enhanced or altered the image, and the details of any enhancement procedures. One issue that is a particular concern when an image is obtained with a digital camera is originality. Whereas traditional photography produces a film negative or positive that cannot be easily replaced without detection, digital cameras produce electronic files that can be modified and overwritten either accidentally or deliberately. It is possible to open a file, make modifications, and then save it with the same file name even though the image has been altered. Computer systems used to store forensic digital imagery must therefore be secure enough to prevent accidental modification of or deliberate tampering with original files.
The possibility of image tampering was raised during a 1995 murder trial in Seattle. The only evidence that linked the defendant to the crime scene consisted of a digitally enhanced image of a bloody palm print taken from a mattress pad. Prosecutors used a digital image that had been sharpened and filtered to remove the fabric texture, and the defense unsuccessfully claimed that the image could have been altered by the computer operator. The possibility of image manipulation was also raised in the O.J. Simpson murder trial, during which the Simpson defense suggested that photographs of him wearing a particular brand of shoes had been fabricated.
see also Automated Fingerprint Identification System (AFIS); Cameras; Crime scene investigation; Evidence; Forensic science; Geospatial imagery; Photo alteration; Photography; Remote sensing; Simpson (O. J.) murder trial.