The Latest in Diagnostic Imaging Modalities for Horses

An accurate diagnosis is imperative to pinpointing and treating lameness in horses. Fortunately, veterinarians today have more sound and accurate options for identifying specific issues than they did just 10 years ago, said Kent Allen, DVM, owner of Virginia Equine Imaging, a specialty sports imaging practice in The Plains, who presented April 26 at a University of Kentucky Department of Veterinary Science Equine Diagnostic and Research Seminar. That, he shared in his presentation, is good for the horse, the veterinarian, and the owner. 

"Absent a diagnosis, medicine is poison, surgery is trauma, and regenerative medicine is witchcraft," he said.

Allen touched on several imaging modalities during his presentation but stressed that, in most cases, the sole use of any one technique is not enough to reach a proper diagnosis. "Wait until all the information is in before you try to interpret results," he cautioned. "None of these (diagnostic imaging modalities) without a detailed clinical evaluation are a magic answer."

Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging is a technique that uses magnetic fields to create cross-sectional and three-dimensional images. While commonly used by physicians, MRI has only been used in equine clinical cases for the past decade and has come into widespread use just within the past five years.

"I've been waiting for MRI to catch up with the equine profession for about 20 years," Allen said.

This modality provides superior soft tissue and bone detail and often allows for detection of abnormalities earlier than some other modalities.

Allen said there are two types of MRIs used on horses: a standing low-field system and a recumbent high-field system. The low-field magnet allows veterinarians to image a horse using only standing sedation, but Allen relayed that it's difficult to get a good image of areas located above the foot using a low-field system. While the high-field magnet produces a clearer image and allows for imaging of additional body parts (including the pastern, fetlock, knee, hock, and skull), it requires the horse to be placed under general anesthesia.

"The high-field magnet has higher resolution, but the low resolution gives quite a lot of information without having to put the horse through general anesthesia," Allen said. "Typically it's a mix of the two we use in the end."

Allen relayed MRI (either high- or low-field) is indicated in several cases including:

  • When pain has been localized but a diagnosis hasn't yet been reached using other imaging modalities;
  • When a more detailed evaluation is needed to confirm diagnosis so treatment and prognosis decisions can be made more accurately; and
  • When very detailed images of both soft tissue and bone are needed.

He relayed that MRI is the only modality that can diagnose a few conditions, such as swelling in the navicular bone (navicular edema), so it should be considered during diagnostic lameness examinations.

It's not yet possible to obtain an MRI of a horse's entire body, so the problem must be localized to a specific area prior to MRI examination, Allen noted.

Nuclear Scintigraphy (Bone Scans)

Next Allen reviewed the technology behind nuclear scintigraphy, more commonly known as a bone scan. To run a bone scan, veterinarians inject horses with a radioisotope that is absorbed in excess by bony areas undergoing remodeling from injury. Then a gamma camera is used to scan the body, which can identify areas of increased radioisotope uptake.

Unlike some imaging modalities, veterinarians can take multiple bone scan views and combine them to get a picture of the horse's entire skeleton.

"The entire process takes two or three hours to put (a whole-body) picture together," Allen said, which is helpful as it allows comparisons of uptake in different body parts.

Bone scans are indicated in several cases, he relayed, including:

  • Difficult to diagnose and multiple limb lameness;
  • Axial skeleton (skull, vertebral column, ribs, and sternum) or pelvic injuries;
  • Severe lameness of unknown origin;
  • Poor performance of unknown origin;
  • Suspected fractures not visible on radiographs; and
  • Localized lameness with no radiographic or ultrasonographic evidence.

Allen recommended combining bone scans with nerve blocks, clinical evidence, and other imaging modalities to reach a diagnosis and treatment plan.

Digital Ultrasound

Upon its inception into the veterinary world, ultrasound was primarily used for reproductive purposes, but Allen discussed how the imaging modality is now being used in lameness diagnostics.

Sound waves are emitted by a handheld probe placed on the horse's body. Structures inside the body with different densities (e.g., ligaments, tendons, bone, and fluid) reflect some of the sound waves back to the probe. These reflections are used to create a digital image of the structures being scanned. Bone appears white, fluid appears black, and all other structures lie somewhere along a gray scale between these two limits.

Allen relayed that today's digital ultrasound machines and high-resolution probes provide veterinarians with detailed, high-quality images that can identify even the smallest problems that could contribute to lameness. Compared to older analog ultrasound machines, "the digital images are phenomenally better," Allen said. Additionally, he noted that ultrasound can detect defects up to two weeks sooner than can radiographs.

Allen said ultrasound is indicated in many cases but is particularly useful when evaluating soft tissue injuries and some joint issues. He also said ultrasound is very useful for monitoring healing progress and for guiding injections and other techniques.

Digital Radiography (X rays)

Veterinarians have used radiographs for years to visualize bony structures inside the body, Allen said, but today's radiograph machines are much more sophisticated than their predecessors. Portable, digital radiograph machines have taken the place of many old-fashioned machines.

Many veterinarians now employ digital radiography, which presents a radiographic image on a computer screen in about four seconds, he said. This improvement alone can rule out or confirm a catastrophic injury in seconds. Additionally, digital images are clearer and often more detailed, he said.

Allen said radiographs are most commonly used to visualize and bony tissues in a horse's body, and are now able to assist in radiograph-guided injections or procedures.

Lameness Locator

Allen touched briefly on a relatively new diagnostic tool: the lameness locator. Using three sensors placed on the horse's body (one on the poll, one on the pelvis, and one on the right front pastern), the machine yields a semi-quantitative measure of a horse's lameness, Allen said.

Allen said he's been using the commercially available lameness locator (manufactured by Equinosis) as an additional tool when conducting lameness examinations and has been pleased with the results it yields.

"This has turned into a very useful lameness tool," Allen said.

The lameness locator should be used in conjunction with other diagnostic imaging modalities, which can pinpoint and diagnose a problem identified by the new technology.

Thermography

Finally, Allen discussed the use of thermography as a diagnostic imaging tool. While it's not widely used for that purpose, Allen relayed that "it's useful as long as you're careful with the questions you ask of it. If you ask questions it can't answer, it's completely useless."

Allen uses thermography--which is 10 times more sensitive than the human hand in determining temperature differences--to identify inflammatory changes in hooves, joints, bones, tendons, ligaments, muscles, and vertebral columns. He noted that veterinarians should use thermography in conjunction with a physical examination and other imaging techniques but stressed that thermographic changes can be seen one to two weeks prior to the onset of clinical signs.

Take-Home Message

"It's fascinating how (diagnostic imaging modalities) have changed and developed," Allen said. "We've got a ton of tools. But it's also added to the confusion--which tool do I use?"

Although imaging modalities will likely continue evolving, there's no shortage of options for diagnosing lameness in horses. Understanding what the different techniques can accomplish can help an owner understand why a veterinarian recommends the one he does for each individual. 

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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