The imaging technique called computed tomography (CT) found its way into human medical practice in the mid 1970s. The first reports of its use in equine patients were published a decade or so later. Since its inception in veterinary medicine the 1980s, the use of equine CT to diagnose injuries in horses has increased enormously alongside evolutions in technology that have improved our ability to use this modality safely in clinical patients.

What Is Computed Tomography?

Computed tomography is a diagnostic imaging modality that uses the same technology as radiographs but to produce three-dimensional information. As a result, CT scans give us a much more detailed look at the horse’s anatomy and any abnormalities present than X rays do. And unlike magnetic resonance imaging (MRI), CT allows veterinarians to take scans in a matter of minutes, making this modality very appealing for use in the standing patient. This rapidity and ease of acquisition also lends itself nicely to screening for low-grade pathology (disease or damage), monitoring injuries, and even assessing surgical results in the operating theater.

Type of CT Scanners for Horses

The two main types of CT scanner are the fan beam and the cone beam. As the name suggests, a fan-beam CT is one in which a thin, fan-shaped beam of radiation rotates several times around the patient to obtain “slices” of his or her anatomy, which it stitches together to create the image. With a cone-beam CT, the beam is wide and cone-shaped. It usually passes around the patient once, capturing a large volume of anatomy it then reconstructs to produce the final image.

A fan-beam CT produces images that are generally of higher diagnostic quality than cone-beam CT images, with particularly good soft tissue detail. However, fan-beam CT scanners require a closed system, resembling a doughnut shape, through which the horse’s anatomy must pass to obtain the image. Conversely, cone-beam CT scanners can be configured in a variety of ways. This means veterinarians have been limited by which regions of a horse’s anatomy we can safely capture in a fan-beam CT scanner without requiring general anesthesia. As a result, we have relied on cone-beam setups, which manufacturers have optimized to image the horse in a standing position, to perform scans of the horse’s limbs. More recently, the advent of new configurations for fan-beam CT scanners has facilitated their use for scanning equine patients without general anesthesia. These scanners are becoming more widely available at equine hospitals worldwide.

Why Would a Horse Need a CT Scan?

Your veterinarian might recommend performing a CT scan on your horse for several reasons, from scanning limbs and the neck for bone pathology to investigating causes of dental or sinus disease. CT scans enable us to rapidly capture a large volume of anatomy, such as the horse’s head, and provide excellent, detailed information about the horse’s teeth, bony structures of the head, nasal passages, and sinuses. With the advent of CT systems designed to image the limbs in the standing horse, more veterinarians are using this modality as part of a lameness work-up, particularly when they suspect bony abnormalities. Several systems can also image the horse’s neck, which is useful for investigating the spine for bone abnormalities that could be associated with neurologic disease. Surgeons might perform CT scans prior to, during, and/or following a complex surgical procedure, such as a fracture repair, to assist with the planning and successful execution of these intricate operations. It is also worth noting that owners and veterinarians sometimes base their choice to send a horse to CT in part upon which advanced imaging modalities are most locally accessible, since generally only larger equine hospitals offer these types of scans.

The Latest Research About CT in Horses

We’ve organized the following information regarding recent research according to the various uses for CT outlined above.

CT of the Neck

A few clinics have installed the equipment required to perform CT of the neck, either under general anesthesia or in the standing horse. Veterinarians might use it to diagnose and assess conditions such as neck arthritis. They might also use neck CT to assist with diagnosing neurologic diseases such as Wobbler syndrome (cervical vertebral stenotic myelopathy). This condition can be challenging to diagnose—historically, it has involved a combination of X rays and ruling out other causes of neurologic disease. With Wobbler syndrome, the spinal cord is compressed within the horse’s neck. To try to identify regions of compression, the veterinarian injects a contrast dyelike substance into the space surrounding the spinal cord and takes X rays—a procedure known as a myelogram—which can reveal focal narrowing of that space. With two-dimensional X rays, we can only assess the dye that sits above and below the spinal cord and, thus, view possible cord flattening in this plane. However, the cord can also be compressed from side to side; 3D imaging such as CT can detect this type of compression.

CT of the Skull

The skull is a particularly challenging area to image successfully due to the complex anatomy of the facial bones, sinus system, teeth, and soft tissues in this region. With X rays, which, again, generate 2D images, these skull components are projected onto a flat surface and appear superimposed in the resulting image, making it difficult to see exactly what is going on.

Several studies have demonstrated that CT is superior to X rays for detecting and characterizing disease within the horse’s skull, including dental and sinus disease as well as diseases of the sensory tissues such as the inner ear.

In a study published in 2017, Liuti, Smith, and Dixon found that X rays permitted them to identify only 53% of infected teeth in equine patients, while with CT they identified 100%. Sinusitis, which often accompanies tooth infections, can be difficult to localize on X rays. One sinus compartment in particular is challenging to view on X rays: In one study, veterinarians were able to identify just 17% of conditions involving this sinus on X rays versus 100% on CT (Manso-Diaz et al., 2015). Knowing which sinuses contain infected material is important for targeting treatment and, where necessary, planning for surgical procedures to treat sinusitis.

While horses are much less likely to suffer from cancerous growths than our other pet species, the skull is an area in which veterinarians frequently find tumors. The CT features of the types of growths commonly seen in the equine skull (including more benign masses such as cysts) have been well-characterized within the literature. This means that while X rays might reveal the presence of a mass, CT can often provide more evidence about the likely type of mass and how it’s acting in the body, informing possible treatment options and potential outcomes.

Finally, in a 2019 study Crijns et al. found that while X rays can usually detect skull fractures, CT more accurately demonstrates the fracture’s size and complexity, the number of fracture fragments, and the degree of associated soft tissue injury. These features might change how veterinarians manage the injury.

Orthopedic CT

As with skull fractures, sometimes it takes CT to fully understand the nature of limb fractures and any associated soft tissue injury. This is because radiographs generally underestimate the degree of fracture. CT allows veterinarians to diagnose more subtle bone injuries such as bone bruising and incomplete or hairline fractures that are nearly impossible to detect using X rays alone. For example, in a study published this year Steel et al. demonstrated that CT was superior for assessing some key features of fractures of one of the horse’s carpal (knee) bones, such as the number of fragments and involvement of the joint surface.

In addition to fracture identification and characterization, veterinarians can use CT before and during surgical fixation of these fractures to plan and guide the placement of orthopedic implants designed to stabilize the fracture while it heals. Using CT, surgeons can map out the precise trajectory of, for example, an orthopedic screw, to provide the most robust fixation. In a recent study Taylor et al. (2022) reviewed the use of CT for surgical planning in a large equine hospital and concluded that “CT has a significant role in surgical planning and in the majority (62%) of cases added additional information or significantly changed the surgical plan.”

Advancements in Standing CT

The development of novel CT machine configurations has enabled veterinarians to start imaging previously inaccessible body parts using this modality. These include the horse’s elbow, back, and pelvis. Recently, a team of vets at the Université de Lyon, in France, and an equine hospital in Belgium successfully positioned horses within a fan-beam CT scanner to obtain images of the elbow in 99 equine patients. From these images, the team diagnosed conditions such as arthritis, bone cysts, and fractures.

Another enormous benefit of our increasing ability to perform CT scans in standing patients is the option to screen high-risk horses for early signs of injury before they result in severe lameness. This is of particular interest in racehorses, whose training regimens lead to specific bone remodeling patterns. Veterinarians can use CT to visualize these patterns and identify horses at risk of injury so their workload can be decreased to allow bones to recover and prevent catastrophic injury. Some clinics are offering these types of screening CTs in conjunction with another newly emerging technology within the equine sphere: positron emission tomography (PET). This imaging modality uses radioactive tracers that localize to regions of bone remodeling to provide information about the location of “active” injuries. However, PET imaging does not give us good anatomic detail, which is why veterinarians must combine it with another modality, most commonly CT.

The use of PET/CT in combination offers great potential for identifying pathology early and reducing horses’ injury rates (Spriet, 2022). Veterinarians can also use this combination of imaging modalities not only as a screening tool but also to diagnose a variety of orthopedic diseases in equine athletes. The rapid growth and development of CT technology in the equine sphere opens exciting new doors for imaging horses quickly and safely and to generate high-resolution, highly diagnostic images.

References

Crijns et al. (2019) Comparison between radiography and computed tomography for diagnosis of equine skull fractures. Equine vet. Educ. 31(10) 543-550.

Klopfenstein Bregger et al. (2019) Cone-beam computed tomography of the head in standing equids. BMC Vet. Res. 2019 Aug 13;15(1):289.

Liuti, Smith & Dixon (2018) Radiographic, computed tomographic, gross pathological and histological findings with suspected apical infection in 32 equine maxillary cheek teeth (2012–2015) Equine Veterinary Journal 50, 41–47.

Manso-Diaz et al. (2015) The role of head computed tomography in equine practice. Equine vet. Educ. 27(3) 136-145.

Riggs (2019) Clinical Commentary: Computed tomography in equine orthopaedics – the next great leap? Equine vet. Educ. 31(3) 151-153.

Steel et al. (2023) Comparison of Radiography and Computed Tomography for Evaluation of Third Carpal Bone Fractures in Horses. Animals, 13, 1459.

Stewart (2021) Use of cone-beam computed tomography for advanced imaging of the equine patient. Equine Vet J. Sep;53(5):872-885.