Saturday, November 23, 2013

How accurate is thermography of horses’ legs?

Infrared thermography is increasingly being applied to investigate the cause of lameness in horses.  The equipment is easy to handle and the method is fast and safe, both for the animal and for the vet.  But is it accurate? 

Recent work by Simone Westermann at the University of Veterinary Medicine, Vienna shows that the technique is surprisingly tolerant of variation in the position of the equipment, i.e. how far from the horse and at what angle to the animal the infrared camera is held. 

In fact, the results were almost completely unaffected by 20° changes in camera angle and increases of up to 50 cm in the distance of the camera from the animal.  At a distance of 1m from the horse a 20° change in camera angle corresponds to about 35 cm.  This represents the effective horizontal tolerance in positioning of the camera.  As Westermann says, “vets should have little difficulty in remaining within this limit, so the method is applicable in practice”. 

Surprisingly, the results showed that horses’ left and right forelimbs show minor differences in temperature and Westermann cautions that “it might be important to take these into account before reaching a final diagnosis.”

The technique is thus reliable and robust, at least in terms of variation in where the camera is located. 

However, it turned out to be extremely sensitive to even very gentle drafts.  A wind speed of less than 1 m/s causes a drop in measured temperature of about 0.6°C, while winds of 1.3-2.6 m/s cause a drop of 1.5°C and winds of 3-4 m/s cause a drop of 2.1°C.  The discrepancies are more than sufficient to lead to a wrong diagnosis, although even the highest wind speed tested is hardly perceptible:  it would barely cause leaves on trees to move.

Westermann is keen to note the relevance of her work for vets who work on horses.  As she says, “It turns out that it is not too important to be sure that the camera is in exactly the correct position before taking measurements.  But it is essential to perform thermography on horses in a room that is completely free of draughts.  If you don’t, your diagnosis will be completely unreliable.”

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