Taking a Peek Inside : Diagnostic Imaging Options for the Sport HorseJuly 17, 2020
By Christina Keim
When a horse goes lame, owners want to know what is wrong and how to fix it as quickly as possible. Veterinarians can employ several different forms of imaging to determine a diagnosis and make recommendations for treatment. The type of imaging they choose will depend on the part of the body affected, the specific tools at their disposal, and the owner’s budget.
Dr. Grant Myhre, owner, hospital director, and senior surgeon at Myhre Equine Clinic, a referral hospital in Rochester, N.H., says that fully evaluating lameness in the field can sometimes be difficult, especially if the horse is painful on more than one limb. These animals are often sent to a referral hospital for more advanced diagnostic imaging, which Myhre divides into two categories: physiologic and structural. Physiologic imaging reveals whether there is activity such as inflammation or remodeling going on within the limb. Structural imaging shows the condition of the “construction” of the limb itself.
“The two [categories] cross over sometimes, but it is important to differentiate between these because it is too easy to look at just one or the other,” says Myhre. “I think you have to look at both.”
A visual examination to evaluate obvious sources of physiological change (such as swelling or excess joint or tendon fluid), combined with a traditional lameness exam can help to further pinpoint the probable source of the lameness. The veterinarian will use all of this information when determining the best imaging option(s) to get a diagnosis.
Radiographs, better known as x-rays, are a structural imaging modality that is used to examine bones and evaluate joint health. In horses, they are most commonly used to image the distal limb and hooves. A radiograph provides a 2-D image of one angle of the anatomy; usually several radiographs are taken from different angles to give a more complete picture. Ever improving technology means that veterinarians can get excellent images in the field, but machines at referral hospitals are usually larger, more powerful, and can image harder to reach areas.
Dr. Kirstin Bubeck, Dr. Med. Vet., DACVS-LA, DACVS-MR, clinical assistant professor at Tufts University in Massachusetts, explains that radiography is most valuable when evaluating joint inflammation or when looking for evidence of fractures.
“If there is longer standing inflammation in the joint, you can see some of the changes in the bone, or where the joint capsule attaches,” says Bubeck. “You can also see subchondral bone cysts on radiographs.”
Bubeck notes that injuries such as fissure lines (an area of bone damage that can precede a fracture), avulsion fractures (when a torn ligament pulls away a small piece of bone), and more significant fractures can all usually be seen on a radiograph.
What radiographs cannot show definitively is whether an area of bony change is actively contributing to the horse’s lameness. This is where nerve blocks play an important role in the evaluative process.
“90% of front limb lameness is in the foot, and the majority block out with a palmar digital nerve block,” says Myhre. “Let’s say we block [the lameness] out there, then we know it is in the back third or bottom of the foot.”
Radiographs showing signs of bony changes in an area, combined with improvement after the administration of a nerve block, may be enough to confirm the source of a lameness. But sometimes, further diagnostics are indicated, particularly in cases where a block helps somewhat, but does not restore full soundness. Veterinarians will look for supporting evidence through evaluation of soft tissue structures. One of the most common tools for this is ultrasound.
In diagnostic ultrasonography, small pulses of high frequency sound waves are sent into the body and their echoing waves are measured. The resulting image reveals irregularities in soft tissue, including changes in density, areas of fluid accumulation, and degraded quality of attachments to muscles or bone.
“Ultrasound is very commonly used to diagnose superficial and deep digital flexor tendon tears and for evaluation of the suspensory ligament,” says Bubeck.
Ultrasound is frequently used in combination with radiography; for example, to further evaluate avulsion fractures, or to assess the health of larger joints such as the hip, shoulder, and elbow. In addition, ultrasound is an important tool for evaluating wounds near a joint, as it can reveal if the tract enters the joint capsule. Ultrasound is also useful when assessing hematomas, or evaluating a joint for signs of sepsis.
“Joint fluid should normally be really black on ultrasound,” says Bubeck. “When a joint goes septic, there are speckles from the gas.”
Ultrasound is commonly performed in the field, but assessing the images takes a bit of practice. For this reason alone, veterinarians may refer clients to a hospital like Tufts, where two doctors specialize in ultrasound diagnostics and evaluation. The expertise gained from seeing a variety of different cases gives greater confidence of an accurate assessment.
Nuclear scintigraphy, also called a bone scan, is an example of a physiological diagnostic tool available only at certain referral hospitals. It is used most often to better isolate the specific area of an injury, and to determine if the body is actively reacting to stress or trauma through inflammation or bone remodeling. The horse is injected with the radioisotope technetium-99, which accumulates in areas of the body with active inflammation or bony changes. The veterinarian uses a gamma camera to create a black and white image on which active areas “light up”.
Nuclear scintigraphy is an invaluable tool in situations where the veterinarian suspects an issue, but cannot see enough detail on radiograph or ultrasound.
“If you suspect a stress fracture but can’t see it on radiograph, you can do a bone scan,” says Bubeck. “If it is there, it would light up.”
Scans can be focused on a particular area (which costs less) or cover the entire body. Scintigraphy is sometimes the best tool for evaluating regions that are difficult to radiograph, such as the dorsal spinous processes, sacroiliac joint, or vertebral bodies. It is also able to detect subtle damage that is hard to see on ultrasound, such as small fiber tears in the attachments of the proximal suspensory ligament. These common types of injuries present a diagnostic challenge to veterinarians in the field.
“The horse blocks in that area, flexes positive, but nothing shows up on the ultrasound,” says Bubeck. “If the horse has all those signs, do a bone scan and see if there is inflammation.”
This diagnostic tool is not without some unique concerns. Depending on the state, horses that have been injected with technetium-99 are considered radioactive for a period of up to 24 hours. Handlers must wear protective gear and the horse cannot be released from the hospital until the pharmaceutical has been mostly excreted from the body. However, with proper precautions, nuclear imaging is quite safe and is used extensively in human and animal medicine.
“A nuclear scan is so exacting that in the human field they use it to see cardiac function in a non-invasive way,” says Myhre.
Veterinarians use the results of a nuclear scan in concert with other tools to make a diagnosis.
“The gold standard is if I see structural damage, and can block the lameness out in that area, then I can say that is probably the cause of the lameness,” says Myhre. “A nuclear scan with uptake in an area is confirmation.”
Computed Tomography (CT Scan)
Computed tomography uses a computer to process multiple x-ray images taken in slices. The result is a computerized 3-D image modeling the affected area, allowing for more clear analysis.
“There is no overlap of the structures, and you can see the anatomy or changes, and that gives you better insight,” says Bubeck.
CT is used mostly to evaluate bony changes, especially in the distal limb. It often aids equine surgeons in the specific placement of screws, or when making other corrections within the hoof capsule, such as with fractured coffin or navicular bones. Cysts and other growths suspected but not confirmed on radiograph may also be diagnosed and treatment supported with a CT scan.
“For example, take a horse with a keratoma, a benign growth within the hoof capsule,” says Bubeck. “To remove that you want to do an opening in the hoof capsule right over that keratoma. You see it on CT, drill the hole, and remove it.”
CT scans are also particularly useful for evaluating problems in the horse’s skull, including tooth abnormalities, tumor growths within the sinus cavities or temporomandibular joint issues.
Due to the specific body positioning required in a CT scan, horses are most often fully anesthetized and the exam is performed with the animal lying down. A CT scan is extremely efficient; most are completed within 15 to 20 minutes. A limited number of specially constructed facilities offer standing CT scans with just sedation; this perhaps will be the trend in the future.
Although CT scans can be used to evaluate soft tissue injury (sometimes via the injection of a contrast dye), they do not provide as fine or detailed of an image as an MRI. Therefore, its use in soft tissue diagnostics is generally reserved for those situations in which MRI is not available.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging is a diagnostic procedure that uses strong magnetic fields and radio waves to take extremely small slice images of both bones and soft tissue. A computer assembles the images to create an incredibly detailed and precise look at the physiology of internal structures in the targeted area. In general, MRIs take longer to complete than CT scans, during which time the animal must remain completely still. Therefore, the best images will be taken with the animal under general anesthesia.
“We are talking about a very small area,” says Myhre. “This [machine] will take 250 slices in one sequence, and we do about 12 to 15 sequences. We can build them into different views. We can angle it to whatever angle we want. It really helps to define where [the problem] is.”
MRI is used extensively to evaluate issues in the distal limb, especially the hoof and fetlock joints, and can help distinguish between acute and chronic bone inflammation. But it is also an excellent tool with which to evaluate soft tissue.
“An MRI can show acute or chronic inflammation in ligaments and tendons,” says Bubeck. “It can see through the hoof capsule, and allow us to evaluate the collateral ligaments of the coffin joint, deep digital flexor tendon tears that are really down there, and navicular bursa inflammation.”
MRI is available only at some referral and teaching hospitals, and styles of machine vary in quality and ability. While the best images are obtained when the horse is under general anesthesia, this also increases the cost of the procedure. The Myhre Equine Clinic has recently installed a high intensity MRI that allows for a bigger field of imaging (this machine can scan from the stifle down) as well as for standing images, with the horse under sedation. It is one of only three machines in the world that have the capacity to do both wide range and standing MRI procedures.
“It is cheaper and quicker to do a standing MRI,” says Myhre. “You get a lesser quality image, you can’t see detail as well, but you can still get diagnostic images from that.”
When it comes to taking a peek at your horse’s internal structures, veterinarians have a range of options from which to choose. Hard to solve lameness cases may benefit from referral to a specialty hospital, where more advanced imaging options exist.