Equine Thermography - Hot or not?
Updated: Jun 16, 2020
The potential uses for thermography in equestrian care have been outlined historically, as far back as Von Schweinitz (1999) its sensitivity in equine pathophysiological detection was highlighted, it stated thermography as the most sensitive screening modality for back pathology in the horse (Fig.1), but why has it not become part of mainstream diagnostics?
Fig.1 Thermogram of a horses spine.
Thermography is the least invasive of any equine diagnostic modality (Readelli et al 2014), this is an obvious advantage, however this benefit creates certain downfalls of the modality: It is not regulated, anyone who decides to go out and purchase a thermographic tool can point it at an animal and provide a “Thermography service”, without any training or understanding of how the images need to be acquired in order to make them reliable; There is a plethora of different grades of equipment available, some designed for industrial use, some low spec some high spec, again, this lack of industry standardisation creates an unreliability; Vets receive very limited training in thermography, so when these unqualified people use non-standardised equipment to acquire unreliably taken images, they then give those images to a vet who has no training in reading them, over time this has given thermography a reputation for being a non-tool amongst the veterinary community, however we can see that in the medical world the modality is widely used (Lahiri et al 2012), so is thermography a useful tool in equestrian care or not?
Temperature has been used as an indication of pathology for thousands of years, since 400BC it has been used as a diagnostic tool, Galileo invented the thermoscope in the 17th century which was later adapted to become the thermometer and today radiometers are widely used in the measurement of inner ear temperatures. In 1868 Carl Wunderlich studied the temperatures of fever sufferers and found increased temperatures to correlate with illness, it was with the discovery of infrared radiation in 1800 that the path to thermography as a diagnostic tool began (Lahiri et al 2012), Hardy (1934) described the emissivity of the human body and confirmed the efficacy of reading surface body temperatures by way of a radiometer. Lahiri et al (2012) outlined the science behind creating a thermal image, using Boltzmann’s constant the emissivity of the skin can be measured and turned into a colourful image representing the differences in surface temperature, it was in 1963 that these differences in colour were identified as potentials for recognising physical illness (Barnes 1963), since then its uses have become more numerous as human medicinal research studied its potentials and applications, these are extensively outlined in Lahiri et al (2012), which concluded that the use of thermography is likely to see a surge in the medical field as the equipment and interpretation becomes more precise. The unique applications of thermography have also been studied in human medicine, its use in diagnosing neurological pathologies is outlined in Neves et al (2015) and many other studies have proven its applications in pathologies that are often difficult to detect with other screening modalities (Fig.2).
Fig.2 Unique applications of Thermography in human medicine include vascular issues such as thrombosis, hormonal issues such as thyroid dysfunction, neurological issues such as Reflex Sympathetic Dystrophy, complex regional pain syndrome and paralysis (Shaydakov and Diaz (2017),Cojocarul et al (2015),Hooshmand (2018),Moran et al (2018).
These pathologies often do not show structural changes therefore can be difficult to diagnose through other screening modalities, however as thermography is a test of physiological dysfunction it can see the bodies response. Thermography is often thought of as showing “hot spots” however as you can see from this Fig.2 often abnormal thermal patterns and in fact “cold spots” can present as indications of pathology.
So again we ask, with this wide use of the technology in the medical field, why are we not more widely utilising the technology to care for our horses? Lets address the questions most frequently asked of the modality, the reliability and the science, furthering on from what was outlined earlier in this article.
Dyson (2013) expressed the complexity of equine diagnostics, emphasising the systematic, correct selection of diagnostic tools, the precise and reliable acquisition of diagnostic imagery and the specialist, experienced interpretation of those images, this set of necessary protocols in achieving valuable diagnostics also applies to thermography, it is a veterinary diagnostic tool and should be treated as such. To assess thermography’s place in equine care and veterinary medicine we must understand diagnostics in general (see my article on common diagnostics), no diagnostic modality is perfect, artefacts are common place and are a result of equipment and human error, even in the most established of diagnostic modalities. Jimenez et al (2008) produced 5 tables of potential artefacts in the process of producing and reading radiographs, Thrall et al (2013) expressed the importance of correct image acquisition and how radiographs can become unreliable in the event of inexperienced acquisition and interpretation (Fig. 3).
Fig.3 Poor image acquisition can mean the images are sub-clinical even in established modalities such as radiographs, this is the same for every modality including thermography.
All veterinary diagnostic processes have a careful set of protocols to minimise error, however artefacts and generally poor images are sometimes inevitable, it is only an experienced eye that can recognise these and not make the mistake of them becoming part of their interpretation. Thermography is a highly sensitive modality, again this is an obvious benefit of the technology, however as well as internal influences on temperature readings, environmental factors can hugely effect reliability of results (Soroko and Howell 2018) from mud, sweat and water, to scars, recent veterinary care and even tack and boots, if these environmental factors are not addressed by strict protocols artefacts are created. Fernandez-cuevas et al (2015) outlined the potential influencing factors, expressing the importance of a controlled environment and an equilibration period to ensure stabilised measurements, 8-10 min was recommended for human equilibration. Studies found that humidity levels didn’t have significant effect on skin temperature as long as the ambient temperature was within a comfortable range (Atmaca and Yigit 2006), however it is clear that outside of this range, if sweating or shivering occur the reliability of results can become questioned.
Thermography will essentially and potentially detect any and all physiological dysfunctions occurring within the patient, once again a huge benefit, however when addressing a particular lameness for example, which of these findings is clinically relevant? Let’s take an MRI, MRI’s produce highly detailed images and will inevitably show evidence of multiple pathological changes (Fig 4-5)
Fig4. MRI enables a high tissue contrast image and provides both structural and physiological data, it can be presented as a 2D “slice” of the subject enabling the viewer to see intended structures without the possible superimposition of superficial structures (Murray 2011).
Fig.5 MRI findings will often show pathological changes that are not clinically significant.
Experienced specialised veterinarians can distinguish between clinically relevant findings and non-relevant findings by understanding the pathogenesis of the clinical history presented, this is also true of thermography, without the clinical history and veterinary knowledge the relevance of thermographic findings can not be established and misdiagnosis is probable.
The importance of veterinary knowledge in interpretation goes beyond clinical relevance, thermography is often thought of as “looking for hotspots” this is inaccurate as thermography is a test of physiological dysfunction by assessing the autonomic, sympathetic and parasympathetic nervous systems effect on dermal tone and the study of the disruption in homeostasis. Von Schweinitz (1999) highlighted the importance of understanding neuroanatomy and neurophysiology, outlined in detail the interpretational considerations in particularly spinal thermograms and stated thermography as portraying the physiological status of neural outflow and reading the bodies control of vasomotor tone.
Fig.6 Diagram expressing the sympathetic nervous systems influence on the peripheral vascular system. Interactive-biology.com
The mammalian inflammatory response to injury and disease has been widely studied and documented and is similar in systemic and musculoskeletal systems, this inflammatory response creates heat (Fig. 7) and this heat can be detected by thermography, either by direct heat transference in superficial musculoskeletal injury or by the sympathetic nervous system in deeper pathologies as described previously.