Veterinary applications of thermography on cats and dogs
How thermography works
Thermal, or infrared, energy is a part of the electromagnetic spectrum that is invisible because its wavelength is too long to be detected by the human eye (Figure 1); instead, we perceive it as heat. Unlike visible light, everything with a temperature above absolute zero emits heat. Even very cold objects, such as ice cubes, emit infrared radiation.
Thermography: a method for noninvasive examination
Thermal cameras are small and easy to use.
An important concept is the "color palette." A color palette is the set of colors that is used in a thermal image, with the specific colors varying with temperature. For example, a palette may display the coldest areas in blue and the hottest areas in white, with red and yellow in between.
Thermal cameras allow a wide choice of color palettes. It is important to select a palette that is easy to interpret when examining animals: for example, 'high rainbow' has easily distinguishable colors (Figure 2).
Lastly, it is important to remember that clinical examination is also required with any diagnostic method.
Thermography can detect many things that change the normal thermal pattern, as it can show differences in thermal symmetry and abnormally warm or cold areas in patients. If the thermal pattern is not symmetrical or there are unexpected thermal anomalies, this may indicate, for example, infection, a soft tissue (tendon, muscle, etc.) problem, a joint problem, nervous dysfunction, or even a viper bite. Looking at this list of examples, it can be seen that thermography is particularly useful in pain localization, pain management, and follow-up for patients with these types of problems.
Patients that are presented for thermography usually have some kind of musculoskeletal problem – either the specific localization cannot be found or the patient is in follow-up for rehabilitation. Patients that have clear pain with an unknown localization are typical.
The following examples illustrate what can be accomplished with thermography.
The definition of infection is an inflammation process with, for example, a bacterium or virus causing the inflammation. Inflammation and infection cause an exothermic reaction: in other words, the infected part of the body is warmer than the surrounding tissues. Thermography can detect the inflammation process, whereas the naked eye cannot.
Inflammation can be determined by the response of the body to possible harm. The signs of inflammation include warmth, redness, swelling, pain, and loss of function. Inflammation does not involve bacteria or viruses. As the inflamed region is hotter than the surrounding area, thermography can detect this indication of inflammation.
A German shepherd was presented for thermography after two inconclusive orthopedic examinations. The lameness was fairly mild, but the owner of the dog was concerned, particularly because the orthopedic surgeon did not find anything specific, and the animal was still not functioning normally. An abnormally warm left right paw sole was seen on thermography (Figure 12). The owner later found and removed a small splinter from the paw, after which the symptoms disappeared. This diagnosis of localized inflammation would not have been possible without thermography.
Tendons usually appear colder than other structures in thermal images. Small injuries can be seen with thermography before any anatomical changes have occurred – after anatomical changes, injuries can be observed with ultrasonography or radiography.
A 4-year-old castrated male Staffordshire bull terrier was presented after ongoing slight right hind limb lameness. The symptoms included occasional unevenness of walk with the patient not putting any weight on the leg. The dog was examined by neurologists and an orthopedic surgeon, but nothing specific was found on clinical examination, radiography, or magnetic resonance imaging (MRI). A lumbosacral problem was suspected but never confirmed.
After taking thermograms, abnormalities were seen in the plantar area of the right hind leg (Figures 14–15).
Different joint problems cause distinct types of functional problems for the animal and different types of pain.
Osteoarthritis is painful when active. After the acute phase, the animal might not feel much pain, but the disease may affect the use of muscles. Osteoarthritis can also be the consequence of another disease/process in the joint.
A young adult female working (herding) border collie presented with undulating pain and lameness in the right front leg. Previously, she had been diagnosed with osteochondrosis dissecans in the right elbow, and the joint was operated on. The operation is often curable, or at least allows the dog to function normally.
No specific cause of the lameness could be found through the usual examinations, and the owner wanted a second opinion and a thermographic examination. An obvious thermal difference was found in the right leg elbow area (Figure 16). Radiographs taken and evaluated by an orthopedic surgeon showed osteoarthritic changes. Herding with this dog must be limited.
Nonacute osteoarthritic processes can also be seen with thermography. A rottweiler had osteoarthritis in the left shoulder and was being treated with NSAIDs and acupuncture. Thermography was used to visualize the problems (Figures 17 and 18).
Nervous system problems
Nervous system problems are seen as cold areas where the nerve in question innervates the tissues. Nerve dysfunction can be due to trauma, compression, or a degenerative process. When the nerve does not work properly, the innervated areas (muscles, etc.) are not as active as usual, so the area is seen as colder than surrounding tissue in thermograms.
A male greyhound was presented because of continuous problems and pain. The owner did not want large-scale examinations but felt comfortable with thermography. Previously, calcification of the cervical vertebrae had been diagnosed, and the dog had had a rock lodged in one foot pad. The dog was limping on both front legs, and the left hind leg was shaking.
Thermography indicated that the previous nerve problem is still ongoing (Figure 19). Nerve problems in the neck area could also explain the lameness in the front legs. A cold spot in the pelvic area could explain the shaky left hind leg.
A pain clinic veterinarian found the cause of the shaking left hind leg to be sciatic nerve entrapment. Thermograms showed a colder area that correlated anatomically to the sciatic nerve area (Figure 20).
Pain management with acupuncture and pain medication is ongoing in this dog since the problem can only be alleviated, not cured.
Thermography can be very useful in detecting pain. Cats are a good example since they do not show pain easily. Pain can be seen using thermography as warmer areas or areas of uneven temperature in the animal. This warmth could indicate inflammation or infection, or be caused by excess use of the less painful limb.
This cat will be monitored frequently by thermography so that any pain that develops can be treated as early as possible.
As with any method of examination, there are aspects to the use of thermography that it is necessary to be aware of in order to avoid misdiagnosis. If the patient is wet (e.g., from rain), the moisture in the fur will affect the appearance of thermal images. Windy locations are not suitable for thermography because heat escapes via evaporation, radiation, and conduction. Places where there is direct sunlight or other heat sources should also be avoided.
One of the many positive aspects of thermography is that it does not radiate and does not cause any harm to the animal (or its owner). Also, the animal can be fully awake during the procedure, and very sick and scared animals can have a stress-free experience since they are not handled during thermography.
Courses on small animal thermography are being developed by Mari Vainionpää at the University of Helsinki to meet the needs of clinicians interested in using this unique tool for diagnosis and follow-up.
Those using thermography to examine animals need to familiarize themselves with the techniques of thermography, especially the interpretation of thermal images, before making decisions about the treatment of patients or further procedures. First, knowledge of the physiology and anatomy of the animal in question is needed – the person doing the examination should be a veterinarian, or the findings discussed with one.
Department of Equine and Small Animal Medicine, Pharmacology and Toxicology Faculty of Veterinary Medicine, PO Box 57, Koetilantie 7, FI-00014, University of Helsinki, Finland
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