Infrared thermography (IRT) represents a useful imaging modality when used in conjunction with traditional lameness and poor performance investigation. All diagnostic imaging techniques have their limitations and IRT is no exception, however if attention to detail is taken during the image collection phase in particular, inaccuracies can be minimised. Successful interpretation of physiological imaging is reliant on the ability of technicians to collect reliable data, solid clinical histories and of course veterinary surgeons that have experience in analysing this data.
Physiological imaging comes with its own particular set of challenges because (like scintigraphy) it looks at function rather than just structure. Images of the patient are analysed pre- and post-exercise in order to assess dynamic blood flow. Normal (asymptomatic) animals have fluctuating skin temperature depending on a long list of factors both environmental (wind, solar loading, ambient temperature, humidity etc) and host related (coat colour, hair coverage, circadian rhythm, plasma cortisol etc). The former can be limited if the patient is imaged in a controlled, stable environment, whereas the latter factors give further emphasis on the need to interpret patterns of symmetry (before and after exercise) rather than outright quantitative readings.
Thermography provides a valuable insight into the function of the autonomic nervous system (ANS), which controls blood flow to the skin. In normal subjects there is a symmetrical ‘dermal pattern’ which is consistent and reproducible for any individual providing environmental factors are controlled. One arm of the ANS is the sympathetic nervous system (SNS), which has nerve fibres running to and from the central nervous system (CNS). Inflammation at any tissue depth is detected by sensory fibres. This information is processed by the CNS and leads to a sympathetic response in the area of skin corresponding to the affected site (dermatome). This reflex arc causes a decrease in the sympathetic motor tone of capillary bed sphincters, leading to increased dermal blood-flow. Other than assessment of dermatomes that represent deep inflammation, superficial, localized inflammation can also be detected to a temperature sensitivity of 0.01°C by modern IRT cameras. Further, abnormally low surface temperatures can be indicative of poorly perfused or compromised tissues, or of nerve dysfunction.
Veterinary interpreters of IRT images are gradually getting to grips with the inevitable pitfalls of using a new technology as clinical follow-up allows confirmation of its strengths and limitations. The growing body of literature to support the use of IRT in veterinary medicine (see below), combined with its obvious practical advantages mean that this is an imaging modality of increasing popularity amongst clinicians and owners alike. It is important to remember (as with most imaging modalities) that diagnosis cannot be reached using IRT alone, rather made upon further clinical examination and in concurrence with other imaging techniques.
References:
Rekant S.I. et al (2015) Veterinary applications of infrared thermography. American Journal of Veterinary Research 77 98-107
Cetinkaya M.A. & Demirutku A. (2012) Thermography in the assessment of equine lameness. Turkish Journal of Veterinary Animal Science 36 43-48
Turner T.A. (1991) Thermography as an aid to the clinical lameness evaluation. Veterinary Clinics of North America Equine Practice 7 311-338
Stokes J.E. (2012) An investigation into the use of infrared thermography (IRT) as a rapid diagnostic tool for foot lesions in dairy cattle. Veterinary Journal 193 674-678
