Department of Wildlife Clinical Sciences, Wildlife Conservation Society, Southern Boulevard, Bronx, NY, USA
Skin serves as a thermoregulatory interface between the metabolic processes of the body and the state of the environment. Skin temperature depends on many internal and external factors, including: body metabolic state, thermal conduction from internal heat sources, vascular activity adjacent to skin surface, evaporative heat losses, air current convection, and reflection from environmental energy sources. Heat radiation is distinctive at anatomic sites and a high degree of symmetry is present between right and left sides of the normal whole body. Inflammatory processes (e.g., trauma or infection) and neurologic processes (e.g., nerve damage or irritation) may alter circulation and metabolism, and thus, the surface temperature of the skin.
Thermal imaging identifies and quantifies comparative skin surface temperature. Invisible infrared energy emitted from the skin surface is captured by an infrared scanning device and converted into electric impulses that are displayed in color on a monitor as a map of body temperature, or thermograph. Increased or decreased emissions of heat radiation are indicated by a spectrum of colors. Thermography is a real-time, non-invasive diagnostic aid to identify abnormalities in comparative body temperature. Early stages of injury may be detected as a “hot spot” since heat is one of the cardinal signs of inflammation.
The Diagnostic Thermal Imaging System (DTIS)-500 (eMERGE Vision Systems [eVS], XL Vision and Safeguard Scientifics, Inc, Sebastian, FL, 32958) features an uncooled, handheld infrared camera, proprietary image processing and database software, and access to a webbed clinical database to provide state of the art technology in thermography for practicing veterinarians. The camera produces a color video image, which is recorded as a digital image that may be stored on a FLASH Memory card. This card may be used to download images into the eVS image processing software, where images may be viewed, catalogued, annotated, enhanced, and archived. The software allows the generation of written reports and image prints. In addition, images that are produced in any video or digital format can be archived in the software system. The webbed clinical database is designed to be a reference tool for the customer base which will obtain and manage information, share knowledge, and document images and the imaging environment.
The DTIS-500 detects and maps temperature differences as small as 0.2°C as it measures the surface temperature of an animal or object from −15°C to +100°C. Control buttons on the camera allow specification of settings for optimal data collection related to patient and environment. Several color palettes with multiple discriminatory shades in each are available for user selection: black/white, rainbow, blue, red-hot black/white, and lava. For example, in the commonly used rainbow color scheme, areas of increased heat (i.e., injury and inflammation) appear as bright reds, oranges, and yellows; areas of decreased heat (i.e., nerve damage and scar tissue) appear as greens, blues, and purples. The red-hot black/white palette facilitates the rapid location of “hot spots” during a quick scanning procedure. The distance range of the DTIS-500 falls between 1–20 m, thus allowing remote examination of unrestrained animals.
For collection of optimal thermograms, several factors must be addressed (some of which are difficult to control in the zoological setting). Animal motion should be controlled where possible or images should be collected when the animal is relatively still. Low-level lighting with elimination of extraneous radiant energy sources (e.g., the sun) should be sought. An ambient temperature of (68°F) is ideal, while temperatures below (86°F) are recommended. Debris on the haircoat, scar tissue, irregular hair lengths, ointments, and wraps may produce artifacts. Plexiglass and glass interfere insurmountably with image reception, while many screens and cage bars may be focused away for minimal interference. Elimination of artifacts and optimization of the viewing situation is important as asymmetry of even 0.5°C merits consideration for possible pathologic change.
In general symmetry is essential to image interpretation. Identical anatomic images should be compared so that thermal changes can be accurately evaluated. Normal thermal patterns can often be predicted by vascular patterns and surface contour. For example, the “normal” warmer areas of an ungulate include the dorsal midline, ventral midline, between rear legs, and the routes of major vessels through the appendicular musculature (i.e., forelimb cephalic vessel, and rear-limb saphenous vessel). The operator must strive for objectivity in symmetry evaluation of animals, in particular for the great diversity of zoological specimens.
Thermography is limited to the measurement of skin surface temperature as it reflects local conditions. Deeper structures may have increased radiant heat but may be shielded by massive musculature. Coat color pattern and hair coat length cause differential external energy absorbency and reflectivity. Motion and position are difficult to control in zoological patients and so the optimal views to evaluate symmetry (left to right, side to side, front to back) may be very challenging to capture. In contrast to radiography, but similar to ultrasonography, thermograms are not collected at distinct camera settings. The subjective adjustment of camera settings may cloud the comparison of structures or abnormalities over time and dependent upon operator.
Diagnostic thermal imaging serves as a complementary tool for evaluation of soft tissue structures to be used in conjunction with visual examination and other imaging modalities. Critical evaluation of areas of asymmetry in the thermogram may allow zoo veterinarians to more accurately direct attention towards a traumatized or diseased region and highlight regions worthy of further diagnostic study. Thermography has applications in zoo animal medicine in the areas of general health assessment, lameness evaluation, wound management, dental evaluation,1 localized inflamation,1 and reproduction.2
1. Walsh, M., T. Turner, S. Dover, C. Wood, and L. Wood. 1993. Proc Intl Assoc Aquat Anim Med 24: 96.
2. Hilsberg, S. and K. Eulenberger. 1997. Infrared-thermography in zoo animals: preliminary experiences with its use in mammalian pregnancy diagnosis and avian and reptilian egg control. Proc Amer Assoc Zoo Vet. 343–345.