Production of the Radiographic Image and Radiation Protection

Recent advances in medical imaging have facilitated the use of radiography and imaging in the forensic sciences. Digital radiography has largely replaced analog imaging in diagnostic imaging and forensic radiology. X-rays used in both digital and analog systems are created in a sealed glass tube by directing high-speed electrons, released at the cathode, toward a tungsten target at the anode. The radiographic image is produced by differential absorption of the x-rays as they pass through the animal to a digital image detector (digital radiography [DR], direct digital radiography [DDR], computed radiography [CR]J or the analog film/screen system. The positive and negative features of the various digital imaging systems and the artifacts they produce are thoroughly described in many radiology references (Drost et al., 2008; Armbrust, 2009). The utilization of digital radiography in veterinary forensic radiology has decreased throughput time and greatly facilitated field radiography, archiving, image sharing, and chain of custody. The digital image is saved and shared between computers by a file exchange protocol called Digital Imaging and Communications in Medicine (DICOM). Patient information and equipment information are stored in a DICOM header which cannot be separated from the image. In the field of veterinary forensic radiology and imaging the ability to permanently store all modifications to the image within the DICOM protocol is a great advantage. The DICOM protocol is used in radiography, US, CT, and MR. The software and hardware, which control the retrieval, distribution, and display of images, the archiving of DICOM studies, and the integration of the DICOM heading with the report or medical record, are referred to as a picture archiving and communication system (PACS).

The exposure factors used in both digital and analog imaging systems are similar, and attention to radiation safety protocol is equally important in forensic radiography and clinical imaging. An effort to limit personnel exposure to as low as reasonably achievable (ALARA) during live animal radiography is a regulatory requirement and a basic tenet of radiation safety. Using the shortest exposure time possible, increasing personnel distance from the source of radiation (the tube and the animal) and wearing appropriate protective lead are the basic pillars of radiation protection known as time, distance, and shielding.

Interpretation of an imaging study requires attention to patient positioning, technical parameters, and labeling. Failure to obtain an adequate number of views, nonstandard positioning of the animal, absent or incorrect identifying data and markers of laterality, incorrect exposure factors, and technical artifacts degrade the quality of the image and diminish the amount of information available to the expert and the court. Any quality factor that limits the expert’s ability to interpret the images will be disclosed and can diminish the value placed on the imaging evidence. A minimum of two orthogonal views and additional tangential or oblique views for complex anatomy are standard procedure in diagnostic imaging and forensic radiology. To reduce scatter radiation, which is caused by the interaction of the x-ray beam with the patient, and to optimize image quality, images are centered on and collimated to the area of interest. Standard positioning and collimation performed in clinical diagnostic imaging is also utilized in forensic radiography. It should be recognized that improper or nonstandard centering of the x-ray beam or inadequate collimation may have a detrimental effect on the digital system’s production of the image.

Forensic radiography can differ from diagnostic imaging in the total number of images routinely obtained. Radiography of the entire body is indicated in cases of suspected nonaccidental injury (NAI). To reduce the number of required images needed in forensic evaluations, protocols for whole-body imaging are available (Table 18.1). If abnormalities are noted on standard views, additional detail images are performed. Although digital imaging has eliminated the need to apply an identifying or date label in the path of the x-ray beam, the identifying information must be electronically entered at the time of radiography. The identifying patient information and acquisition

Table 18.1 Recommended guidelines³ for small animal whole-body imaging in NAI

Number of Images

Anatomical Region

Views

19 Total

Abbreviations: NAI, nonaccidental injury; CrCd, craniocaudal; VD, ventrodorsal.

^a Guidelines are not intended as legal standards of care and may be modified in individual cases or based on available resources.

^b For larger animals, two lateral and two CrCd views are needed to image each limb.

date and time are available in the DICOM header, and this identifying information cannot be separated from the images. Application of right- and left-side markers are an essential part of the production of the x-ray image. To help reduce the potential human error of mislabeling and provide a secondary verification of laterality, right or left markers are placed within the path of the x-ray beam. Placing a left- or right-side marker in the primary x-ray beam at the time of radiography is preferred to computer-generated side labels. Single-marker techniques for labeling oblique views of the thorax and skull often produce confusion in interpretation. If the animal is in right lateral recumbency, the skull can be rotated ventrally or dorsally. Placing a single right marker indicating that the patient is in right lateral recumbency provides no benefit in differentiating the right and left skull. With the animal in right lateral recumbency and the dorsal skull rotated rightward, the right side of the patient is projected ventrally on the image and the left side of the patient is projected dorsally on the image. A right marker is placed on the ventral surface of the skull and a left marker is placed on the dorsal skull surface. Alternately, with the animal in right lateral recumbency and the dorsal skull rotated to the left, the image would be marked with a ventral "L” and a dorsal "R.”

Some variation in protocol is required for radiography of deceased animals. Decomposition and rigor mortis prevent standard positioning, increase the overall size of the animal body, and increase the distribution of gas and soft tissue in the body. An approximation of standard positioning may be needed, and collimation to larger areas of interest may be acceptable in postmortem radiography. Whole animal images, like whole baby images in the medical field, are not acceptable practice (Dwek, 2011), and the acquisition of orthogonal views is always required. Like all diagnostic images, forensic radiographic images will optimize conspicuity of the soft tissues and the osseous structures. Large volumes of intracavitary and subcutaneous gas associated with decomposition will adversely affect image quality and may require modification in the typical imaging algorithm or radiographic technique. Overexposure and underexposure are technical errors rarely acknowledged in digital imaging due to the digital post-processing capabilities and the broad exposure latitude of the digital system; however, image degradation in the digital system, more commonly recognized as diminished image contrast and increased quantum noise or mottle, can be caused by inappropriate exposure settings or post-processing settings. Postmortem increases in intracavitary and subcutaneous gas enlarge the body, which can result in an overestimation of body size, overexposure, and loss of image dynamic range or latitude, which refers to the digital imaging system’s ability to record a wide range of densities. Diminished soft tissues and increased gas in a body will decrease attenuation of the x-ray beam. Failure to recognize the increased gas to soft tissue ratio in body composition can result in an overestimation of the exposure factors and inappropriate post processing. Saturation

Forensic Applications of Computed Tomography, Magnetic Resonance, and Ultrasound of the detector in a digital system, resulting in loss of visualization of anatomical structure, is the most severe consequence of overexposure. Exposure factors in clinical imaging and live animal forensic radiography are optimized to produce a quality image, reduce exposure to personnel by scatter radiation, and minimize patient motion artifact. The digital detector may optimize image quality with increased radiation exposure. With no concern for motion and no need for personnel to be in the room during radiography of the deceased animal, postmortem images can be optimized for image quality.