Thermography

Recent advances in infrared imaging allow us to use this technology to assist in lameness cases.  The equipment measures minute changes in temperature at the skin surface and reports this information as a color-encoded photograph.  Underlying soft tissue and bony injuries raise the temperature of the surrounding tissues.  These increases in temperature are often readily detectable by direct palpation, however changes in temperature of less than 2 degrees are not detectable to human touch.  Thermography can detect temperature differences of less than one-half of a degree.

Scenario: Your event horse has just not been right. He seems to be getting a bit stiff to the right, and he's been grouchy when you are grooming him and getting him tacked up. No fever. No definite lameness. But something isn't right, you tell your veterinarian.

You haven't given your practitioner much to go on, and he isn't seeing much when you walk and jog the horse. Forelimbs? Back? Hind limbs? Feet? Everything is a question. He gets you to saddle the horse and walk him for a few minutes while he gets what looks like a video camera with a space-age lens out of his vehicle.

He instructs you to stand the horse facing him in the closed-off barn aisle way, asking you if you've had wraps on the horse's legs or used any kind of liniments on the animal in the past 24 hours. No. Remove the saddle, please.

He flips out a small viewing screen and proceeds to "shoot" your horse, from stem to stern, top to bottom. Then he stands up on a straw bale placed behind the horse and says, "Ah, ha!"

He returns to his truck and comes back with a TV/VCR combo that he sets up. He turns on the monitor and puts in a video tape to record. He wires up his camera to the VCR, all the while explaining his unusual behavior.

The camera is, in fact, an infrared detector. All living creatures give off infrared heat, he explains, and inflammation means greater heat than normal over injured areas. Decreased heat can mean injury, too, he adds.

Your horse on the television screen looks something like what a modern artist would paint--various shades of black, blue, yellow, green, and red, with some white. The white represents the hottest temperature, your veterinarian explains, and the blue the coolest. As it goes to shades of green to yellow to red to white, heat increases. (More than one color palate is available for the veterinarian to select the one he most prefers.)

Then, back on the straw bale, he shows you why your horse is "off." The whole left side of his back along the spine glows yellow--not matching the greenish pattern of the right side. Heat patterns that don't match translate into a problem.

A change in saddles and some therapy will have your horse back to his old self in short order.

Scenario: An obscure lameness is going on in a Warmblood stallion, somewhere in his hind end. This is not a stallion who is pleasant to work around. Blocking is low on the option list for fear of having someone injured.

Out comes the infrared camera, which picks up a problem in a tendon that didn't show anything on palpation. Ultrasound is brought into use and shows (on the now-sedated horse) a small disruption in the fiber patterns.

The two scenarios described above are actual cases related by an equine practitioner who has used infrared imaging on world-class equine athletes for several years. Veterinarians excited about bringing infrared imaging into common use in the barns and on the backstretches of our equine competitions have been working with a company to develop a system specifically for use on horses. 

Infrared Imaging

An animal's body creates heat so it can survive. That heat fluctuates throughout the body depending on blood flow. Blood flow, to some degree, is regulated by need; for example, injured tissues need more blood to bring in more helpful cells and take away the debris of repair. The body's recognition of injury and a subsequent increase in blood flow can happen even before the animal shows signs of pain, such as lameness. Enter thermography.

The infrared heat that a horse emits from its body can be "viewed" via a specialized camera and monitor. The heat patterns that can be seen show a trained practitioner how the blood flow is normal, or abnormal, in a particular horse. Blood flow can be either increased or decreased, both indications of health problems.

When horse owners hear the word imaging, most of the time they think of X rays, ultrasound, or perhaps scintigraphy. In fact, there are two divisions of imaging. Kent Allen, DVM, a member of the veterinary panel for the United States three-day event team, said that imaging should be thought of in terms of physiologic imaging and anatomic imaging. Anatomic imaging is what you can show--a broken bone or tear in a structure such as a tendon (X rays and ultrasound). This structural anomaly indicates a problem, but the image is static. Anatomic imaging only shows what has occurred. Anatomic images can be taken in a series over time and compared to help determine healing or lack thereof, and are needed for diagnosis after thermal (physiologic) imaging has pinpointed a problem area.

Physiologic imaging is a function of metabolic action. Physiologic images can change and might appear prior to anatomic disruption. Thermography (or thermal imaging) is considered physiologic imaging because as the horse's metabolism changes--a sore tendon heats up--that fact can be discerned. As one veterinarian said, "Thermography is a sifting device that you then go in and document the physiologic change (what you saw with thermography) with the anatomic change. It's not a stand-alone technique."

Thermography, according to Dorland's Medical Dictionary for humans, comes from the Latin words for "heat" and "to write." It is defined as "a technique wherein an infrared camera is used to photographically portray the surface temperatures of the body, based on the self-emanating infrared radiation; sometimes employed as a means of diagnosing underlying pathologic processes, such as breast tumors."

The Science Behind The Image

Thermography has been used in human and veterinary medicine (as well as in imaging for other purposes, mostly military reconnaissance) for many decades. All living creatures give off self-created infrared heat. (Machinery also gives off "heat signatures," which are used by military analysts to determine whether equipment or ships are being readied for travel or have been moved recently, and to detect numbers.)

There are different types of cameras or detectors that read infrared heat images. Some are quite bulky and require a cooling element to keep the camera at a set temperature. Cooled cameras require a "cool-down" period of 10 minutes or more prior to use. Others cameras are more modern, hand-held devices specifically designed for use around horses. Cost also is a consideration; cold systems could cost $75,000 or more. However, the newer infrared imaging systems can be purchased for about what a high-end ultrasound machine would cost ($10,000).

Thermography has been around and studied since the 1960s. Early machines were costly, and cumbersome, and the maintenance was delicate.

Other uses in the livestock industry have been tried, especially inside buildings where the rising body temperature of an individual animal in confinement could be an indicator of illness.

Thermography is a qualitative assessment of temperatures. In other words, the camera can be set to detect differences in temperatures and show those differences as colors. Usually, veterinarians like to set the temperature/color changes for every 0.5-1° Celsius. It should be remembered that skin temperature in the normal horse is about five degrees cooler than core body temperature. The skin derives its heat from local circulation and tissue metabolism. Also, a horse is warmer down the inside of its body than the outside.

A "hot spot" indicates inflammation or increased circulation. Hot spots generally are seen in the skin directly overlying injury. A cold spot is a reduction in blood supply usually due to swelling, thrombosis, or scar tissue.

Uses Of Thermography

At the 1996 Olympic Games in Atlanta, where there was millions of dollars worth of equipment available to the equestrian teams, the most-requested diagnostic tool was thermography. It was fast. It was portable. It was non-invasive. It could detect injury sites before they became lameness problems, and could guide practitioners to specific anatomic areas for study using other diagnostic techniques. And it was extremely accurate when used by an experienced practitioner.

There can be "artifacts" found when using a thermography camera, so experience is a key to diagnosis. If legs have been wrapped, or blisters or liniments have been used, they will show up as areas with increased heat. If there is a strong breeze blowing through the barn, it also can affect imaging. Long hair coats are not as good as short hair coats in allowing infrared heat to escape (hair is an insulator). Horses which are clipped for winter with patterns of long and short hair also can present problems with imaging, as can horses with mud caked over parts of their bodies.

The motion of the horse must be controlled, as must be extraneous radiant energy (don't stand the horse with one side next to the open door of a warm tack room or in direct sunlight). The horse should be given 10 minutes or more to acclimatize to the area where the thermography imaging will be done.

Thermal symmetry is the rule--you compare one anatomic area with the same area on the other side (i.e., outside foreleg to outside foreleg). Changes of one degree Celsius over 25% of the comparable anatomic structure usually are clinically significant.

Thermography can be used to determine if there is inflammation in an area that was sore on palpation, or to detect an area of increased blood flow when there is no specific pain or signs (subclinical inflammation). Most horses don't have just one problem associated with a lameness. Thermography also helps in detecting the secondary areas with problems.

Tracy Turner, DVM, MS, Diplomate ACVS, noted that tendons and joints will show inflammatory changes as much as two weeks before clinical lameness is apparent.

Thermography also can be used to assess the vasculature and blood flow to tissues before and after exercise.

Other uses include prepurchase examinations, saddle fit, a training aid to avoid injury (i.e., detecting hot shins before they buck), pre-race examinations, hoof balance, track design or footing (based on hoof heat of horses performing over the surface), detecting early laminitis, palmar heel pain, subsolar/submural abscesses, diagnosing capsulitis/synovitis in joints, tendinitis, following tendon healing after injury, viewing muscle injury, detecting muscle atrophy, muscle strain, and nerve injury.

Turner noted that with capsulitis/synovitis, as the joint becomes inflammed, the thermal pattern changes to an oval area of inflammation just over the joint. This pattern might become evident two weeks prior to the onset of clinical signs. He said that the pattern is similar for tendonitis. A hot spot occurs over the injury site that can be detected about two weeks before there is evidence of swelling and pain over the tendon. He also noted that as tendons heal, the temperature becomes more uniform, but remains elevated.

"Thermal changes correlate well to structural reorganization," he noted.

The most valuable use of thermography, according to Turner, is in detecting muscle injury.

"It locates the area of inflammation associated with a muscle or muscle group," he said. "It shows atrophy before it becomes apparent clinically. Atrophy is seen as an area of consistent decrease in circulation when compared to the opposite side."

Turner said that while muscle strain in the forelimbs is rarely seen, he does find strains in the pectoralis muscle and shoulder extensors. The most common muscle strain in the hind limb is best described as in the croup region, and the caudal and cranial thigh region.

Nerve injury due to direct trauma or secondary to another injury or disease can affect blood flow and can be visualized with thermography, said Turner.

As can be seen, there are many different uses for this non-invasive, but reliable diagnostic tool. It is thought that many practitioners will become well-versed in the subtleties of using thermography, and their patients will benefit from this advance in equine veterinary medicine.