THERMOGRAPHY OF THE EQUINE BACK
Tracy A. Turner, DVM, MS, Dipl. ACVS, ACVSMR
Back problems are considered a major cause of altered gait or performance in horses. Unfortunately the characterization, localization, and identification of the painful area can be problematic. The incidence of back problems in general practice has been reported as 0.9%  and has been estimated as 2.2% of lameness cases . The equine back covers a large area, it includes the axial skeleton from the withers to the sacroiliac joint and sacrum; it is covered with thick muscles and fascial tissues and the numerous ligaments. It is the complex nature of this large structure that makes diagnosis so difficult. Clinical signs are highly variable and imaging, using radiography, requires special equipment. Specific treatment for back pain can only be performed after identification of the site of pain and nature of the injury. It is these problems that make thermography a valuable tool in the identification of back problems. The objectives of clinical examination of the horse’s back are to determine if back pain is present, the site or sites of pain, and the potential lesions for the pain. A number of confusing factors can make the diagnosis more difficult, specifically the effect of tack (saddles) and rider on the horse. Thermography aids each of these areas . However, before any clinical discussion can be made an understanding of the instrumentation and how it works and the principles of use and the physiology studied must be reviewed.
Thermographic instrumentation, in the past, has been divided into contacting and non-contacting.
- Contacting thermography
- Contacting thermography utilizes liquid crystals in a deformable base[4, 5].The crystals change shape according to the temperature that contacts them, and as they do they reflect a different colour of light. Therefore, the colour of a crystal represents a specific temperature. In order to use this technology for medical purposes, the liquid crystals are embedded into a flexible and durable latex sheet. This method has fallen out of favour because of numerous inherent problems in applying the technology. Because of this, non-contacting thermography is the method of choice.
- Non-contacting thermography.
- Two different technologies exist, cooled and uncooled. Cooled technology utilizes a detector of infrared radiation to measure temperature [6-8]. In addition, a series of focusing and scanning mirrors are used to systematically measure an entire field of view. The camera/detector is usually coupled to a cathode-ray tube and the intensity of the detected radiation is converted to an electrical signal. This signal is displayed on the cathode-ray tube as a black and white (gray scale) image of the object. The radiation intensity is directly proportional to the gray scale. Through the use of microchips, the black and white image can be made into a coloured image of the thermal picture (thermogram), hence a classic thermogram. Because of the heat generated by the camera the detector must be cooled to prevent interference from the machine heat. The complexity of the camera and the need to connect the camera to a computer makes this technology inconveniently portable.
The other technology available is uncooled technology. This technology employs a type of focal plane array that simply put means that the infrared radiation is focused and measured on a series of detectors. These cameras have no moving parts and are contained in instruments the size of a handheld flashlight.
There are several factors to consider before purchasing a thermographic camera. Among the most important factors is the spectral range. For medical use, the range of 8 to 14 microns is ideal because this is the peak emissivity of skin. From a practical standpoint, there is also less environmental artefact at this range. The author prefers real-time thermography versus still thermography because real-time eliminates any problems with motion, real-time also makes thermographic assessment more dynamic in that the operator can immediately observe change, and real-time allows for faster imaging. Sensitivity refers to the amount of temperature difference can be detected. Cooled units can differentiate within 0.01°C whereas most uncooled units can differentiate 0.08°C, although cooled is more sensitive, it is not applicable to veterinary thermography where 0.3°C is as sensitive as we need.
The final factor is portability and durability. In equine medicine, an instrument needs to withstand the rigors of daily use and it must be easy to carry. Relative to cooled units portability usually means increased cost. Also because of the detectors these units are usually more fragile. Uncooled cameras, utilizing the focal array technology, are very portable and durable because of no moving parts.
PRINCIPLES OF USE
Heat is perpetually generated by the body, and it is dissipated through the skin by radiation, convection, conduction, or evaporation . Because of this, skin temperature is generally 5oC (9oF) cooler than body core temperature (37oC). Skin derives its heat from the local circulation and tissue metabolism . Tissue metabolism is generally constant, therefore variation in skin temperature is usually due to changes in local tissue perfusion. Normally, veins are warmer than arteries because they are draining metabolically active areas. Superficial veins will heat the skin more than superficial arteries, and venous drainage from tissues or organs with a high metabolic rate will be warmer than venous drainage from normal tissues.
The circulatory pattern and the relative blood flow dictate the thermal pattern, which is the basis for thermographic interpretation . The normal thermal pattern of any area can be predicted on the basis of its vascularity and surface contour. Skin overlying muscle is also subject to temperature increase during muscle activity. Based on these findings, some generalizations can be made regarding the thermal patterns of a horse: the midline will generally be warmer [6, 8], this includes the back, the chest, between the rear legs, and along the ventral midline. Heat over the legs tends to follow the routes of the major vessels, the cephalic vein in front and the saphenous vein in the rear.
The thermal pattern of the back is warmest down the midline from withers to tail. There is a corresponding warm area from tuber coxae to tuber coxae. There will be isothermic bands of decreasing temperature on either side of the midline. In the lumbar area the isothermic bands are colder than they are over the thoracic area. The croup has a “T” pattern with a warm isothermic band from across the tuber coxae and down the midline. The areas over the gluteal are isothermic cold zones.
Injured or diseased tissues will invariably have an altered circulation . One of the cardinal signs of inflammation is heat which is due to increased circulation. Thermographically, the “hot spot” associated with the localized inflammation will generally be seen in the skin directly overlying the injury [4, 10]. However, diseased tissues may in fact have a reduced blood supply either due to swelling, thrombosis of vessels, or infarction of tissues. [5, 11, 12]. With such lesions the area of decreased heat is usually surrounded by increased thermal emissions, probably due to shunting of blood.
USE IN VETERINARY MEDICINE
In order to produce reliable thermographic images the following factors need to be controlled: motion, extraneous radiant energy, ambient temperature, and artefacts.
- Motion can be controlled by immobilizing the horse in stocks or using a qualified handler. The use of real-time thermography decreases the need for complete immobilization. Chemical restraining agents to keep the horse from moving should be avoided because these drugs affect the peripheral circulation and cardiovascular systems which could cause false thermal patterns to be produced; however, the author has not encountered this.
- Extraneous radiant energy.
- To reduce the effects of extraneous radiant energy, thermography should be performed under cover shielded from the sun . Preferably, thermography should be done in darkness or low-level lighting. Ideally, ambient temperature should be in the range of 20oC (68oF) but any temperature as long as the horse is not sweating is acceptable. Heat loss from sweating does not occur below 30oC (86oF), as radiation and convection are responsible for heat loss below that temperature. Very cold environmental temperatures may cause vasoconstriction of the lower legs and interfere with imaging. In these cases, low level exercise to stimulate vasodilatation is necessary. The thermographic area ideally should have a steady, uniform airflow so that erroneous cooling does not occur. Practically, the horse should be kept from drafts. Likewise, the horse should be allowed time to acclimate to the environment or room where thermography is performed. Typically, 10-15 minutes is adequate when going from warm temperature to a cool one. However, up to 2 hours maybe necessary when going from a cool environment to a warm one. In these cases, the acclimation can be sped up with low level exercise.
- Artefacts are extraneous sources on the skin that can cause irregular images. Among these are debris, scar tissue, hair length, liniments, leg wraps, blankets, and other equipment . To avoid artefacts, make sure all subjects are groomed and free of leg wraps, blankets or sheets for two hours whenever possible. Hair insulates the leg and blocks the emission of infrared radiation. But, as long as the hair is short and of uniform length, the thermal image produced is accurate. The skin should always be evaluated for changes in hair length that may cause false “hot spots” in the thermogram.
Multiple thermographic images of a suspect area should be made [13, 14]. The area in question should be evaluated from at least two directions approximately 90o apart, to determine if a “hot spot” is consistently present. The horse’s extremities should be examined from 4 directions (circumferentially) . Significant areas of inflammation will appear over the same spot on each replicate thermogram.
There are at least three ways in which thermography can be utilized in equine veterinary practice :
- A diagnostic tool
- In these cases, thermography is a physiologic imaging method where a 1oC difference between two anatomically symmetrical regions indicates a region of inflammation and a decrease in temperature is as important as an increase in temperature. The image will identify an area of interest to pursue with an anatomic imaging method such as ultrasonography and /or radiography. The author finds that temperature differences as little as 0.3°C are significant.
- To enhance the physical examination.
- In this case, thermography is used to identify changes in heat and therefore locate “areas of suspicion”. Thermographic cameras are easily 10 times more sensitive that the hand in determining temperature differences. This method simply helps identify asymmetry and then the practitioner must utilize the information to determine the actual cause and significance of the temperature difference.
- In a wellness program.
- In this method, horses in training are followed on a routine basis, once weekly. Thermographic changes will occur 2 weeks prior to clinical changes. In these cases, thermography can be used to identify subclinical problems then training alterations can be made so that injury may be avoided altogether.
THERMOGRAPHY of the BACK
Thermography has been used to identify 6 different back injuries, over-riding dorsal spinous processes (ORDSPs, kissing spines), dorsal spinous ligament injuries, muscle pain, withers injuries, sacroiliac problems, and saddle fit problems . In order to obtain a readable thermogram of the back, one must get high enough behind the horse to get a full view of the back. To be 90° to the back is ideal but not always positive, but less than 60° is not high enough to produce a good thermogram of the back. An alternative is to stand the horse under a reflective mirror and capture the back image through the horse’s reflection.
The author uses 2 different thermal images to assess the back, one a thoracolumbar view and the second a croup view (Fig. 12.1). The thoraco-lumbar view shows the withers and the sacrum and
is especially good for looking at the mid back. The croup view is best for evaluating the sacroiliac region. The thermal pattern is the most important aspect of assessing the thermogram of the back. It must be remembered that thermography establishes the location of a possible problem it does not characterize the lesion. However, we will show that there are certain patterns that have been seen consistently with particular back problems.
The normal back pattern is: the warmest areas are down the midline, cooling slightly in the lumbar region and then a warm cross from tuber coxae to tuber coxae and over the tuber sacrale, then same warmth down the middle of the croup (Fig. 12.1). The warm area has symmetric isothermic bands on either side. Any variation of this pattern is either due to an artefact (thin hair, rubbed area) or pathologic process. If the lesion occurs along the midline of the thoraco-lumbar area, radiographs of the thoraco-lumbar spine is indicated. If the lesion occurs off the midline or in the croup region, we usually image the area with ultrasonography.
ORDSPs has been associated with any one of three different thermal patterns .
- Pattern 1: horses showed a “hot streak” perpendicular to the thoracic spine (Fig. 12.2A)
- Pattern 2: horses showed a “cold streak” perpendicular to the thoracic spine (Fig. 12.2B)
- Pattern 3: (the most common): horses showed a combination “hot spot”-“cold streak” pattern over the back (Fig. 12.2C).
Our practice has evaluated 95 cases of ORDSPs over 3 years. In these cases the sensitivity of thermography for overriding spinous process was 99%; however, the specificity was only 75%. This results in a positive predictive value of 94%. Compared to the positive predictive value of palpable pain in the thoracolumbar area for kissing spines which is 74%, thermography performs well.
In contrast injuries to the supraspinous ligament or dorsal sacro-iliac ligament injuries have not presented with this type of identifiable pattern . Thermography changes associated with these injuries have been abnormal thoraco lumbar thermal images. The specific changes may be increased heat, decreased heat, or simply an abnormal back thermogram. Following the identification of an abnormal thermogram, ultrasonography is used to identify the lesion (Chapter 17).
Muscle injuries of the thoracolumbar region; likewise do not have characteristic thermograms . Typically, however, the thermal patterns will show either “hot spots” or “cold spots” off the midline (Fig.12.3). It is in these regions we have concentrated ultrasonography to determine if muscle lesions can be seen. Withers injuries have all shown “hot spots” in the area of the withers but nothing more characteristic than that. The croup muscles will show some specific changes .
Sacroiliac region thermography has shown several different patterns . The most common pattern is a cold area centred over the region of the tuber sacrale (Fig. 10.4). This finding of cold is hypothesized to be due to lack of normal movement in the sacroiliac region. This lack of movement can be either due to primary pathology or secondary to other causes of the horse not moving normally through the pelvic region. This has been corroborated by our clinical findings as well. Specifically, we find only about half the horses exhibiting this thermal pattern actually show pain in the sacroiliac area. In addition, sonographic evaluation reveals pathology in only about half the cases and this pathology usually looks chronic in nature.
We have seen either thinning of the cross sectional diameter of the dorsal sacroiliac ligaments or in a few cases thickening of the ligaments. In these cases with the cold area, generally, if there is pain in the area we will ultrasonographically examine the sacroiliac region. On the other hand, if there is no pain or the horse moves normally through the pelvis we look for other causes of loss of back mobility. Pathology is seen much more commonly if the area over the tuber sacrale is hot or there is a hot spot centred over one tuber sacrale or the other (Fig. 12.4). The pathology varies from hypoechoic areas within the dorsal sacral ligaments to generalized oedema in the region. This thermographic pattern is almost always associated with either pain or marked stiffness in the sacroiliac region.
Saddle fit thermography is very interesting (Chapter 23) and requires multiple examinations . Thermography is an imaging modality that offers objective insight into saddle fit but has multiple other veterinary applications. In evaluating the dynamic interaction between the saddle and the horse’s back, thermography will show not only the heat generated in contact areas on the saddle (Fig. 12.5) but also the physiologic effects of the saddle on the horse’s back (Fig. 12.5). A consistent technique provides the most useful information. Our protocol is to perform a baseline thermographic examination of the horse’s back. Then the horse is saddled over a simple cotton pad with the girth as tight as it would be for riding. Then the horse is lunged for at least 20 minutes. The horse should be exercised at its normal gaits (walk, trot, canter) paying attention to divide the lunging time equally in both directions. The saddle is then removed and the panels of the saddle are assessed thermographically. The most important criterion is thermal symmetry. After the saddle, the back is reassessed. Again thermal symmetry is important. Most commonly because of the heat generated by the saddle, the horse’s midline is now colder than the other structures under the saddle. In addition, the examiner is looking for focal hot spots, particularly along the spine, or hot or cold spots over the musculature. These abnormalities indicate problems caused by the saddle. The assessment is then repeated after a similar exercise session with the rider mounted. This evaluation allows consideration of the effect of the rider on the horse’s back.
As our technological capabilities for equine practice increase and improvements are made in imaging the biologic organism, our ability to make accurate diagnoses continues to improve. It is important to understand that no imaging techniques can replace or be used in lieu of the physical examination. Rather, all imaging techniques only enhance the database established by the physical examination, and each imaging modality offers unique specific information. Similarly, each imaging modality correspondingly has its own limitations.
Thermography is a physiologic imaging modality . Thermography provides information about tissue physiology: specifically it gives insight into the circulation. Thermography provides information as to the location of injury or disease as well as viability of the tissue. But it does not provide information as to the specific nature of the problem. This must be reserved for an anatomic imaging modality that identifies the structure of the tissues in question.
Radiography generally evaluates changes in bone. Identifications of these changes are used to determine injuries to bone. Unfortunately, excepting fractures, most radiographic changes in bone often take 10-14 days to become evident [11, 16].Further, many of the bone changes are often permanent, so it can be difficult to determine if a change, especially a chronic change, is the cause of pain and lameness.
Thermography essentially images inflammation which usually implies pain [6, 8]. In this respect, thermography can directly help determine if a radiographic change is associated with inflammation, and therefore the possible cause of lameness.
Thermography and ultrasonography are complementary. Whereas, thermography may be used to locate an injury, ultrasonography evaluates the injured structure’s morphology and the size and shape of the injury. Ultrasonography can be used to follow the healing, but thermography evaluates “when” the inflammatory process is resolved [5, 11].
Thermography is a practical aid in the clinical evaluation of the equine patient. It is of particular help in the assessment of the horse’s back. This modality specifically increases the accuracy of diagnosis. Thermography is an excellent adjunct to clinical examination, as well as being complementary to other imaging techniques such as radiology, ultrasonography and scintigraphy.
Figure 12.1 Two thermograms to show the normal pattern of distribution of body surface temperature (A) Thermogram of the thorac-lumbar region of the back of a normal horse, the midline is the warmest (yellow stripe) (black arrows). (B) Thermogram of the lumbo-sacral and gluteal region of the back of a normal horse, the warmest area (yellow) is along the midline and from tuber coxae to tuber coxae (white arrows).
Figure 12.2 Three thermograms to show the pattern of distribution of body surface temperature in horses with over-riding dorsal spinous processes (A) A horse with over-riding dorsal spinous processes showing “hot spot” (circle). (B) A horse with over-riding dorsal spinous processes showing the “cold streak” (white arrow). (C) A horse with over-riding dorsal spinous processes showing the “hot” (black arrow) “cold” (white arrow) pattern.
Figure 12.3 Two thermograms to show the pattern of distribution of body surface temperature in horses with suspected muscle pathology (A) A horse with increased heat over muscle. The black line signifies midline, black box is the region of “heat” on the left side and the white box is the corresponding “cooler” area on the right. (B) A horse with a “cold area” over the epaxial muscle just left of the midline (white arrow).
Figure 12.4 Two thermograms to show the pattern of distribution of body surface temperature in horses with suspected sacro-iliac pathology (A) A horse showing the “cold spot” (white circle) over the tuber sacrale indicating possible sacroiliac disease. (B) A horse showing “hot spot” (black circle) over the left tuber sacrale indicating possible sacroiliac disease.
Figure 12.5 Two thermograms to show the pattern of distribution of body surface temperature in horses associated with the saddle (A) A thermogram of the panels of a saddle, the right side of the saddle shows more heat (white arrow). (B) A thermogram of the horse where the saddle had been. The black arrow shows increased heat, correlating with the same region as the saddle in (A)