AAEP Convention 2005: Magnetic Resonance Imaging

"I am here to convince you that MRI (magnetic resonance imaging) is in all your futures, so you are going to have to develop some familiarity with this," began Robert Schneider, DVM, MS, equine orthopedic surgeon at Washington State University, during his presentation on this imaging modality at the 2005 American Association of Equine Practitioners Convention, held Dec. 3-7 in Seattle, Wash. "It's been a valuable diagnostic tool at Washington State since 1997. MRI is changing the treatment of horses with lameness problems in the distal limb."

He explained that MRI is a hydrogen proton-based technology that generates images based on the biochemistry of the limb, not architecture (as does ultrasound). It provides detailed images of bone and soft tissue structures, highlighting different structures depending on the type of MRI image used. (Some highlight fluid, while others are best for showing anatomical structures.) At Washington State, horses are put under general anesthesia for MRI imaging and usually spend less than 90 minutes in the machine.

"MRI is indicated when a lameness diagnosis can't be made from radiographs, ultrasound, or scintigraphy, and we know the location is in the distal limb," Schneider said. "You must have the lameness pretty well localized; this is critical." In other words, one doesn't screen the whole limb with MRI looking for a problem. Rather, you focus on a specific problem area.
"Slice planes can be oriented in any direction through the leg," he said. "Orienting slice planes through specific anatomic structures has increased our ability to see some pathologic abnormalities.

"Clients will soon be bringing you horses with MRI images that you'll have to interpret," Schneider said. "It's like a cool computer game to look at these images of horses with lameness problems. We try to send CDs to all the referring veterinarians so they can develop some familiarity with looking at these things. Sometimes it's like looking at the ocean with all the abnormalities you can find."

Learning More About Old Problems

Schneider reported that MRI has been very useful in diagnosing specific problems in horses suspected to have navicular disease, although these horses often have more than one type or location of pathology. He commented that MRI evaluation of these horses "is an important direction for future research in horses with navicular disease."

Collateral ligament desmitis and small bone lesions are also much more easily diagnosed with MRI, he said.
"MRI evaluation of horses with tendon and tendon sheath injuries may prove to be valuable in the future, but few horses with these problems have been evaluated with MRI," he said. "This is because these injuries can be diagnosed and evaluated by clinical examination and high-resolution ultrasound.

Schneider described several cases he had worked up at Washington State, pointing out their lesions on various MRI images and correlating them with the clinical signs he had seen. "Abnormalities can be subtle," he commented.

New Problems?

Schneider said MRI also reveals problems that were not recognized without this technology, or that appear differently than would be expected. For example, he said, "I think prior to MRI, none of us really realized how far up the limb deep digital flexor tendonitis could go." He added that this problem has also been increasingly recognized in horses with heel pain.

"MRI has proven particularly valuable for horses with lameness problems in the foot, because it has allowed us to see inside the hoof capsule for the first time," Schneider noted. "As a result, diagnoses are being made that were not recognized or accurately diagnosed before the use of MRI. With a few exceptions, most of these abnormalities cannot be observed on radiographs."
Some horses with splint exostosis (popped splints) or even relatively small splint bone reactions have adhesions between the reaction around the splint and the suspensory ligament, he reported. This is another problem that was very difficult to see before MRI.

The Right Treatment for the Case

"An accurate diagnosis of a horse’s lameness problem is important in selecting treatment that will optimize the horse’s chances of returning to performance," Schneider stated. "MRI does affect treatment--would you really like to inject the coffin joint if the horse has deep digital flexor tendonitis?

"Lameness diagnosis is an important area that is rapidly changing, and it will continue to evolve in the future with more experience using MRI," he concluded.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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