Photo: Stephanie L. Church, Editor-in-Chief

Magnetic resonance imaging (MRI) has become one of the most popular go-to tools for diagnosing the causes of lameness in horses, in large part because it allows us to see all the components of a horse's living limb for the first time. Now we can see pictures of a horse's limb that look eerily like the anatomy diagrams or dissections in a textbook, and diagnose problems in all the various structures (bone, tendon, ligament) with one imaging modality--MRI.

But as always, things aren't as easy as they seem. Rich Redding, DVM, MS, Dipl. ACVS, associate professor of equine surgery at North Carolina State University, feels that there are several concerns with the use of MRI in equine veterinary medicine today along with the great benefits it can offer.

On the Plus Side

First of all, Redding is a great proponent of MRI in veterinary medicine. "[MRI in horses today] is a lot like when I graduated veterinary school in the 80s and ultrasound was just starting to be introduced into veterinary medicine," he opines. "We used to treat all bowed tendons the same until we started ultrasounding them and found that they weren't all the same injury. That's where we're going with MRI, is finding they're all different injuries.

"For example, the hind end has really been a problem for diagnosing lameness," he explains. "When you block them out to an area (pinpoint the area of pain with nerve blocks) and take an MRI, you find that half of the cases have a bony lesion and the other half have a suspensory ligament lesion, which is important when you start talking about how to treat them. It's giving us a much more guided approach on how to manage injuries, and it's particularly helpful in the hind limbs."

Standing or Down Magnet?

NC State uses a down MRI, which offers a stronger magnet and a higher-resolution image.

Photo: TheHorse.com Staff

A significant advantage of MRI as it's done at NC State is the use of a high-powered 1.5-tesla unit (tesla is the unit of magnetic field strength), compared to the 0.3-tesla units found in many practices worldwide. NC State's unit was installed as a joint venture with IAMS, a company that is probably best known for its high-end dog and cat food. "They (IAMS) lease the space, remodeled it all and made it theirs, and we contract them to do our MRIs," explains Redding. The system is used for small animals as well as horses; imaging of horses began in late 2004.

There are more differences than just field strength between the two MRI systems, however--the 1.5-tesla unit is a "down magnet," or one for which the horse must be anesthetized. The 0.3-tesla units are "standing magnets," where the horse is imaged while standing and sedated. Both have a place, says Redding.

"The smaller units can't really go up as high on the leg, and then when we start looking at the images they're certainly suboptimal," he comments. "The resolution on this 1.5-tesla unit is so much better. (The 0.3-tesla units) are standing units, and that's one of the problems--the horses rotate (sway a bit) with sedation and motion restriction is extremely critical to acquiring a good image."

In comparison, "The 1.5-tesla unit is just like a large radiograph machine--you can take more power and generate a faster shot to virtually eliminate motion," he goes on. "This unit has the advantage of a very high resolution and the horses are under anesthesia, which is not necessarily an advantage to some people's minds. And it is an ordeal to get a horse in here. But it's an advantage in terms of eliminating motion. You do still get some motion artifacts just from the ventilator, particularly in the upper leg (the limb on the top side when the horse is lying down).

"You're only going to be able to get so much with the smaller system," he states. "I'm not trying to talk them down because I'm used to the big system; there's a place for them. For one, you can put a lot more horses into them (because they don't have to be anesthetized). They're best for the foot, because the foot's planted on the ground (compared to the leg, which can sway with changes in the horse's weight). The only thing that would really move in the foot with that sway might be the deep digital flexor tendon. Motion correction software is available, but I think motion correction software will never be as good as a good still image."

The latest down MRI units offer even more benefit to horses, as they have a slightly larger opening for the patient (4 cm wider) and a shorter bore. This means the body part being imaged doesn't have to go so far into the magnet to get a good image. "With that magnet, there is no restriction on the horse getting in there until it hits his chest," says Redding. "Alamo Pintado (in California) has one, and it can image stifles. Right now that's as good as it gets."

MRI Concerns

"The biggest challenge with using MRI in horses is just learning what's normal and abnormal and how does it apply?" says Redding. "That's the major source of discussion at veterinary meetings right now--what is a significant lesion and how does it relate to other imaging modalities? The learning curve is unbelievable."

With a new modality comes a lot of learning, but also a great need for education about its capabilities and training in its use, says Redding. "One of the things that really concerns me is that everyone wants to embrace this technology and wants to get it (in their practices)," he comments. "But they don't realize when we're showing graphic representations of these lesions at meetings that we had to go through 600 images (all the images from a typical hour-long MRI study) to get that. It's not that easy. It takes a lot of time both to take these and to read them. I'm fortunate that I can sit in here while [the images] are coming in, but then I'll go back later and read them again to make sure I haven't misinterpreted something. I might have an hour and a half, two hours in reading these things by the time it's done."

Dr. Redding says that a challenge presented by MRI is learning what is normal and what abnormalities are clinically relevant.

Photo: TheHorse.com Staff

Along with the challenge of taking and interpreting MRI images is the current discussion of just who really should be interpreting the images--boarded radiologists or equine veterinarians. "Most people want a boarded radiologist to look at these images, well guess what? A boarded radiologist has no training in lameness," says Redding. "So you really need someone who knows the lameness side as well to put the whole picture together. Both sides are important. There is a lot of talk right now about how to put together a training program to teach people how to image and certify them, so they have the training in lameness, they have the training in imaging, and they are much more attuned to what needs to be realized in practice.

"We have an advantage here in that we have two boarded radiologists who are very interested in MRI, and Dr. Michael Schramme and myself who are very interested in MRI," he goes on. "It's always amazing to me to look at everyone's interpretations; you have to get together and look at the clinical picture. You'll find three, four, five things wrong, and now you have to figure out what is the clinically relevant problem. Human orthopedic surgeons are constantly telling me they always have false positives--there's always a lesion there, but we're not sure if those are significant. So that's what we're trying to figure out in the horse right now, is what's real and what is clinically relevant.

"My fear is that a lot of people are getting these standing magnets who have no MRI training whatsoever and are trying to interpret all these different things," Redding comments. "Add that to the fact that they have a lease they have to pay, so they're throwing high numbers of horses in these things. And I think MRI is going to get a bad name, because it takes a trained eye to see some of these things, and they're not having a boarded radiologist looking at them. [Interpreting MRI images] is not always as easy as you want it to be."

Targeted Tool

One thing Redding makes very clear is that MRI is not a tool you'll use to scan the whole horse and see what you can find. Instead, he localizes a horse's pain to a particular area with nerve blocks or other imaging methods, then investigates further with MRI.

"You have to know the reason why you're going to MRI," he says flatly. "We have people call up and say they want their horse MRIed, and they want everything MRIed, but I won't allow a horse be anesthetized for more than an hour for this. And they're paying by the hour (which costs about $2,500-$2,600 and will cover both lower front or hind limbs, not including the feet), so we have yet to have someone want to go for the two hours. We work them up before they come in, basically just double checking the blocks done by the referring veterinarian. We don't do a lot of outpatient CT or MRI."

Continuing to Investigate MRI

MRI is far from figured out--Redding and many other researchers continue to investigate its worth in equine veterinary medicine. Topics include comparing data gained from standing magnets to information from down magnets, comparing MRI images of various lesions to those gained with other imaging modalities, and continuing to correlate clinical evaluation with MRI imaging of all types of lesions.

Redding notes that the suspensory apparatus has been an area of particular interest to the clinicians/researchers at NC State. "For example, with hind limb suspensory injuries, if you do nothing about 20% will come back sound," he says. "If you do shock wave therapy on horses that block out to the deep plantar nerve (narrowing down the injury to the suspensory region, although not necessarily the suspensory apparatus), 40% will go sound. If you do a deep plantar neurectomy and retinacular release, you'll get 70-80% sound. That's why we feel it's really critical to do MRI to see just what is damaged--those aren't minor procedures, and I certainly don't think neurectomy is indicated for a bony lesion."

One of the problems facing MRI research is that the technology is quite expensive, both in terms of equipment and trained personnel to run it. NC State and many other veterinary hospitals/clinics are always grateful for donations to help them investigate better ways to treat our ailing equine friends. 

The Bottom Line

Whenever a new type of imaging, treatment, or product becomes available to equine veterinary medicine, there is an initial period when the learning curve is still quite steep, and practitioners must figure out how it can be most effectively applied. This is where MRI is right now for horses--we've learned a lot, and the technology offers great promise. However, we are still going through growing pains as we learn how to use it to the greatest benefit.

As with many things in horses, not enough studies have yet been done to hammer out all the details. But work is ongoing at many clinics and institutions like NC State, and the value of MRI to lameness and other diagnostics for horses will continue to grow exponentially.