Equine Diagnostic Imaging 101

Horse owners faced with pinpointing lamenesses and other problems now have an arsenal of modern technologies at the ready. Think CT, ultrasound, MRI, and more.

Photo: Kevin Thompson/The Horse

Are CT, MRI, and X ray clear as mud? Learn about the appropriate uses for these imaging modalities and more.

Advances in medical technology aren’t just for people—our equine partners also benefit from an ever-increasing range of sophisticated diagnostic options. Case in point: Horse owners faced with pinpointing lamenesses and other problems now have an arsenal of modern technologies at the ready. Think CT, ultrasound, MRI, and more. So when is a simple radiograph sufficient, and when should you consider bringing in the big guns? 

The first thing to keep in mind is that each imaging modality doesn’t exist in a vacuum. Starting with a clinical examination, your veterinary team uses one or more diagnostic techniques to get a better idea of what’s going on inside your horse. Combining close observation and various examination techniques to reach a diagnosis means he or she can pursue a more precise and appropriate treatment plan.

Digital Radiography (X Rays)

Radiographs are the bread-and-butter of diagnostic imaging. Today’s portable machines are easy to use and produce high-resolution images that can be reviewed instantly on a laptop. Because radiographs are reasonably priced and it’s simple to send images electronically for evaluation, veterinarians rely on them for prepurchase and lameness exams. 

“With digital radiography, it’s very easy to share images to get other people’s opinions,” says Myra Barrett, MS, DVM, Dipl. ACVR, clinical instructor in Colorado State University’s radiology department and partner at Inside Information Radiology, in Fort Collins. “It’s a good first-line tool for a lot of conditions, whether you’re worried about a joint, a fracture, or even a soft-tissue injury. Sometimes using radiographs first and not jumping straight to ultrasound is good because we want to assess the bone associated with the soft tissue.” 

Veterinarians rely on radiographs extensively for prepurchase and lameness exams.

Photo: Kevin Thompson/The Horse

On the other hand, says Kent Allen, DVM, of Virginia Equine Imaging, in The Plains, while radiographs are useful for detecting bony conditions, they are less sensitive than some other modalities. “You can have tremendous inflammatory change going on within the bone, and the radiographs can still look completely normal,” he notes. “You can have a problem, and yet the radiographs are not going to reflect that problem for 14 to 21 days.”

Barrett echoes the sentiment that while radiography has its place, it’s not the most sensitive tool out there. “The downside to radiographs is that by the time we see secondary osseous -abnormalities—bone changes associated with osteoarthritis—generally there is already much more disease in the joint than we have any idea of because we can’t see cartilage, joint capsule, and synovium (joint lining).”

In addition, Barrett notes that not all digital radiography is up to snuff: “If you don’t have a good system, there will be limitations in interpretation.”

Computed Tomography (CT)

Often used to detect dental, sinus, and spinal issues, a standing CT scan produces 1-3 mm cross-sectional images of an area in a sedated horse. ”We get much more information from a skull CT than we do from radiographs because there are many structures (in the horse’s head that are difficult to distinguish on X rays),” Barrett says. “Also, being able to now image the standing cervical spine (the neck) is an exciting frontier.” 

Veterinarians can use computed tomography (CT) scans to detect dental, sinus, and spinal issues in horses.

Photo: Courtesy Washington State University

Practitioners can also use CT to image limbs. “In terms of the foot and joints, there’s a lot that has been published about looking at the deep flexor tendon injuries with CT using radiocontrast agents; the contrast tells us about the vascularity (blood supply) of lesions,” says Barrett. “CT is excellent for bone—it’s the best way to evaluate fractures.” 

Further CT options are on the horizon, notes Allen. “There is new technology being developed for standing CT of distal (lower) limbs, which may change how we view imaging the limbs,” he says. 

Magnetic Resonance Imaging (MRI)

With MRI, magnetic fields produce 3-D cross-sectional images, providing high-quality detail of both bone and soft tissue. Standing MRI, performed under sedation, captures images of the lower limbs. Recumbent (taken while lying down) MRI, which requires general anesthesia, offers a look at a greater anatomical range. 

“What we need to think about when we do an MRI is what structures are we are most worried about, as we have the option of (from most to least detailed) high-field anesthetized, low-field anesthetized, and low-field standing MRI,” Barrett says. “If I am worried about a joint in particular, I think it may be worth it to go straight to high-field MRI—you get a lot more bone and soft-tissue information. It allows us to see structures, especially in the hoof, that are otherwise difficult to see; it’s the only reasonable way to look for bone edema (fluid swelling) and contusions. Low-field MRI has its limitations; to scan for subtleties, smaller lesions, or any type of articular cartilage damage, you need high-field MRI.”

Nevertheless, low-field MRI is often a reliable, real-world choice. “The majority of MRIs done are low-field standing,” Allen says. “These do a good job of imaging the foot in particular and to a lesser extent structures above the foot and subtleties.” They also don’t come with the same anesthesia risks—such as stress to the body, injury during recovery, or other complications—as the other options.

“MRI is unusual amongst imaging modalities because it is both sensitive and specific,” Allen adds, meaning it offers few false negatives and positives. “It’s also unique in that you can see both soft tissue and hard tissue in the same image. It is not bothered by the hoof wall, so it turns out to be probably the ideal modality for looking at the foot, particularly if radiographs and/or ultrasound are negative.”

This sophistication, however, comes at a price: MRI leads the way as the most expensive imaging modality. Regardless, Barrett feels its benefits are cost-effective: “When we first started doing MRI, we would do it way down the line—we would do radiographs and ultrasound first, and we would wait to do MRI,” she explains. “I think we’re learning that by not waiting so long, we can actually end up saving the client money; we can have a much more directed treatment and rehabilitation protocol. I think there is a paradigm shift that needs to continue to shift toward performing MRI early in the lameness work-up and not waiting.”

Our sources do agree that MRI has changed the way veterinarians look at all imaging modalities. “The more we do MRI, the more we learn about what the tip of the iceberg (looks like) on our other imaging modalities and what we can and cannot get out of them,” says Barrett. “I pick up on subtleties on radiographs that I didn’t pick up on before.”

Digital Ultrasound 

No longer relegated solely to prenatal care, ultrasound has enjoyed a steady progression in both equipment quality and application since the mid-1980s. A hand-held probe pressed against the horse’s body emits sound waves, which reflect back to the probe as they bounce off body structures. These reflections create a dynamic digital image of the scanned structure. 

“In the hands of a trained ultrasonographer, ultrasound is a very useful tool for evaluating soft tissue injuries and bone margin,” Barrett says. “While we can’t see deep into the bone, it’s excellent for looking at the attachment of soft tissues to bone or changes on the surface of bone. In fact, I prefer ultrasound for looking at the insertion of soft tissue onto bone (e.g., the suspensory ligament branch onto the sesamoid bone) over MRI.” 

Like radiography, ultrasound is accessible and affordable. However, it’s only as good as the technician performing the scan. “Angling the tail of the probe slightly can cause shadows and artifacts in the picture,” Allen says. “An experienced ultrasonographer constantly moves the probe, changing from on-angle to off-angle, looking for more than one plane.”

As Barrett puts it, you can send radiographs and MRIs to other experts to help interpret the results, and they can adjust contrast and other levels to better view the structures, but an ultrasound’s usefulness in a consult depends on the technician’s skill at the time of the capture. 

Nuclear Scintigraphy (Bone Scan)

This imaging approach is as high-tech as it sounds. It involves injecting a radioactive substance into the vein before using a gamma camera to scan the body and reveal bony changes. 

“Nuclear scintigraphy is a very good screening tool that gives us a physiological rather than anatomic form of imaging,” notes Barrett, meaning it reveals bone activity rather than detailed anatomic information. “If we see an increased radiopharmaceutical uptake in the hock, for example, we can get a general but not precise sense of where to look; it helps to localize areas. After we see these areas of increased uptake, we have to explore them further with radiographs, nerve blocks, and so on. It’s part of the whole diagnostic toolkit.” 

Allen says this option is one of the most sensitive modalities for reflecting changes in the bone and where ligaments attach to bone. “It’s also one of the few modalities which can produce a view within an hour or so of the inflammatory issues taking place,” he adds. “You can also see areas of bony inflammation in which the horse is not clinically unsound, but could become so later.” 

This technology’s downside is it lights up all locations with bony changes. “Experts must determine which areas are actually clinically significant,” Barrett says. “The information you get depends a lot on breed, age, and activity of the horse.” 

She says nuclear scintigraphy is a particularly important screening tool in young racehorses predisposed to stress fractures. It can be much trickier with middle-aged sport horses that have wear and tear from years of competition. Allen also notes that young horses have areas of natural bony change—a caveat that must be considered as routine when interpreting their images.

Lameness Locator

With lameness locator technology, sensors on the horse’s poll, pelvis, and pastern allow the practitioner to note gait irregularities. “A lameness locator uses a combination of accelerometers and inertial gyroscopes to determine head bob, pelvic tilt, and lame limb in the stance-phase of the stride,” explains Allen, noting that while this technology might not yet be in widespread use, it is a useful tool.

Barrett finds lameness locators to be valuable for multilimb lameness evaluations and teaching situations. 

Learning Curve

Becoming a diagnostic imaging expert doesn’t happen overnight. Knowing which modality to choose and being skilled in that modality comes with extensive experience. “It’s a complex question of what to do when,” Allen says. “It takes a lifetime to figure out.” 

One thing Barrett would like horse owners to keep in mind is that equine practitioners can’t be experts in everything. “There are people who are good at a lot of things and people who are experts in certain things,” she says. To that end, general equine practitioners consult with diagnostic imaging experts, just as primary care physicians refer patients to a variety of specialists. 

Whether for a backyard horse in for a lameness work-up or a high-end jumper undergoing a prepurchase exam, Barrett says specialists with intrinsic knowledge of equids and their craft can take imaging to the next level.

About the Author

Natalie DeFee Mendik, MA

Freelance journalist Natalie DeFee Mendik is a multiple American Horse Publications editorial and graphics awards winner specializing in equestrian media. She holds an MA in English from Colorado State University and an International Federation of Journalists' International press card, and is a member of the International Alliance of Equestrian Journalists. With over three decades of horse experience, Natalie’s main equine interests are dressage and vaulting. Having lived and ridden in England, Switzerland, and various parts of the United States, Natalie currently resides in Colorado with her husband and two girls.

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