Thermography: Use in Equine Lameness

Chapter 25 Thermography

Use in Equine Lameness

Infrared thermography has been used in equine orthopedics for a number of years.1 Improvements in imaging quality and technology now yield images that are easier to interpret. The systems are also more cost effective. Thermography pictorially represents the surface temperature of an object and is a noninvasive method of detecting superficial inflammation and thus can have a role in lameness diagnosis. It is a physiological imaging modality, as is gamma scintigraphy, and thus has a lower reproducibility than anatomical imaging modalities such as radiography and ultrasonography. Superficial blood flow is a dynamic system and is likely to be variable and is also prone to artifacts, which has led some people to doubt its clinical applicability. Others consider it useful in the diagnosis of a large number of diverse conditions.2 With experience and care in interpretation, thermography can be a useful adjunct to lameness evaluation, as part of an integrated clinical and imaging approach. Eddy and co-workers reported a 63% correlation among thermographic findings in 64 horses and ultrasonography, nuclear scintigraphy, and radiography.3

Heat is lost through the skin by radiation, convection, conduction, and evaporation.4 Thermal cameras assess infrared radiation from an object; this is optically focused, collected, and transformed by detector arrays into an electronic signal that in modern systems then generates a real-time video image. The skin and hoof derive their heat from tissue metabolism and local circulation, and because the former is generally constant, variation in superficial temperature normally relates to changes in local tissue perfusion. Radiation from the equine foot and limb is more important than reflection, and lighting levels are not a major concern. Thermographic evaluation of the foot and distal aspect of the limb is complicated by the thermoregulatory role of the distal aspect of the limb, whereby the blood supply can be dramatically reduced to conserve heat in cold conditions.5 Thus an understanding of the physiology of the distal aspect of the limb is essential for optimal image interpretation.

Several different thermal imaging systems are available. Older systems were primarily developed for military or industrial applications and are relatively cumbersome and have poor image quality. The top-end systems have a cooling system to ensure temperature stability of the detector, and this increases the fragility of the system and the maintenance costs. Uncooled camera technology is preferable in a veterinary environment because it is cheaper, lighter, and more robust. These systems have become more available recently, and the system cost for diagnostic-quality thermographic imaging is no longer prohibitive. There is still wide variation in the image quality among different systems, and it can be difficult to compare different systems, but the user becomes used to the particular thermal patterns produced by each machine. Some systems are also radiometers and allow an accurate temperature measurement to be obtained, whereas other systems do not measure the absolute temperature but are able to determine the difference between two areas within the same image. Although a radiometer is not essential, it does assist in comparisons among different examinations. Higher image quality leads to a greater ease of interpretation of the image. The quality of image processing software is also improving, but the majority of interpretation is carried out in real time. Images can be stored in a variety of digital formats for archiving and later comparison. Infrared thermographic instrumentation is far more sensitive than human hands in detecting temperature changes in an object. There may be variability of up to 1° C attributed to the camera in clinical imaging,6 and differences of over 1° C are normally deemed as being potentially clinically significant in image interpretation.

Image Acquisition

Imaging should be performed in a relatively bare room without radiant heat sources, drafts, or sunlight. Some clinicians recommend allowing the horse to thermally equilibrate in the environment in which it is to be imaged. Equilibration can take up to 1 hour, but although the absolute temperature changes, there is little change in the relative thermal pattern.6 Thus equilibration is not necessary for clinical imaging. The optimal ambient temperature for imaging is 20° to 25° C. Below this temperature the distal aspect of the limbs are more prone to thermal cutoff,5,7 and above this range the contrast between the horse and the background is lost. Thus in colder temperate climates it can be advantageous to have a “hot box” to raise the ambient temperature if thermal cutoff prevents diagnostic imaging. It can also be advantageous to image the feet after the horse has been trotted and lunged, which increases the blood supply to the limbs and hence increases the inherent contrast of the digit relative to the background.

A long hair coat acts as an insulator and reduces the contrast within the image. Irregular patterns of clipping, topical applications, and dirt can complicate interpretation of images. Bandages and rugs should be removed at least 20 minutes before imaging. The feet should be clean for thermographic imaging of the digit and should be picked out and brushed to remove external contamination.

The distal aspect of the limb should be imaged from a dorsal position, from a palmar or plantar direction, and from the left and right sides. The limbs are then lifted, and a solar view is obtained of the feet (Figure 25-1; Color Plate 1). The joints of the proximal aspect of the limbs should be imaged cranially and laterally. The neck is imaged from the side. The back and hindquarters should be imaged as dorsally as is possible. Close-up images of regions of interest can then be performed if necessary. Comparisons should be made between sides; it can be helpful to repeat the imaging on another occasion if the results are uncertain.

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Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Thermography: Use in Equine Lameness

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