For the first time in Oregon, horse owners have the option of new MRI diagnostic technology that does not require horses to be completely anesthetized and put on a surgery table. The Standing MRI can provide more valuable information about a horse’s leg than any other imaging technique, while reducing the risk of complications associated with anesthesia. As of this month, this advanced technology will be available through Drs. Revenaugh and Emerson at Northwest Equine Performance in Mulino, Oregon.

Most show horse owners in the Northwest are familiar with the use of MRI in horses. This is because Washington State University was among the first to use MRI in horses. More recently, Pilchuck Veterinary Hospital installed an MRI unit in the Seattle area. Why all the excitement? Because MRI uniquely provides information that can be extremely helpful in diagnosing and maintaining a working performance horse.

How Does It Work?
During an MRI, the horse is exposed to a very strong magnetic field. The MRI machine also uses pulses of radio waves. The MRI machine creates an incredibly detailed image, based on the way the horse’s hydrogen atoms respond to the magnetic field and radio waves. The MRI produces image “slices” of the leg, similar to slices of bread in a loaf. The slices are made in different orientations or ‘imaging planes,’ which allow for a three-dimensional appreciation of the area of interest.

How Long Does It Take?
An MRI study generally takes from one to three hours to perform. Each MRI study produces hundreds of images to be interpreted after the scan. With a standing MRI, the study should be able to be accomplished on an ‘out-patient’ basis (in and out on the same day).

What are the Risks?
There are no known risks associated with MRI studies. The risks involved are related to the anesthesia.

When Does My Horse Need an MRI?
MRI studies provide unparalleled diagnostic information when the exact area of concern has been determined. This means that the veterinarian has successfully located the problem area, typically with nerve blocks. MRI does not replace X-ray or ultrasounds; it is used when x-ray and ultrasound have not yielded a definitive diagnosis. This scenario is surprisingly common with horses. One of the most common problem areas is in the horse’s foot region.

The foot of the horse is an amazingly complex structure all contained within a hard shell, the hoof wall. The hoof wall interferes with imaging of many of the important structures located all or partially within it. However, MRI is able to image all of these deeper, and sometimes physically small, structures with a high level of detail.

Most clients are apprehensive about putting their horse under general anesthesia. You must weigh the benefits versus the risks, as well as the costs involved, before deciding to put a horse on a surgery table for one to two hours to obtain MRI images. Some horses are not good candidates for anesthesia at all, including many older horses, horses with Metabolic Disease or muscle conditions, horses with certain heart problems, or horses with COPD or other lung problems.

With the Standing (“Open”) MRI, the decision to do an MRI becomes not so much about ‘risk’ versus benefit as it is about “cost” versus benefit. We recently examined an upper level Dressage horse with a lameness that localized to the foot. Standard imaging, including radiographs and ultrasound, did not identify a cause. Because the lameness was interfering with the horse’s performance and it was unclear whether there was a significant underlying injury, an MRI under general anesthesia was performed. The findings were a simple ‘strain’ of the coffin joint, which would normally require only a couple of weeks rest to resolve. Although recovery from anesthesia did not result in any injury or other issues such as pneumonia, the effects of the general anesthesia on the horse’s performance may last significantly longer than the rest period required for the joint strain.

What are the differences between MRI machines?
MRI machines are either “high-field” or “low-field”, indicating the strength of the magnetic field created. High field units create thinner cross-sectioned images (i.e., thinner bread slices). All high-field units require general anesthesia at this time. ‘Low-field’ units may require general anesthesia (as at Pilchuck Veterinary Hospital), or can be performed in a standing horse, as at Dr. Revenaugh’s clinic at Northwest Equine Performance.

The main ‘downside’ to doing a Standing MRI is that standing sedated horses sometimes will move during the acquisition of the MRI images. Generally speaking, subtle motion can be accommodated for by the computer software involved. Major ‘motion artifact’ cannot be accommodated for. As you could imagine, the farther up the leg you image, the more problem the ‘motion’ or ‘sway’ during the study creates. This means that successful acquisition of a high quality study of the upper cannon area is not possible unless the horse is very still. Despite recent advances in technology of the standing MRI, acquiring diagnostic images of the upper cannon region is not always possible in all horses.

Currently, there is a debate among veterinarians about the necessity for general anesthesia when doing an MRI study on a horse. Specifically, is the risk of anesthesia worth the potential additional information gained with a ‘high-field’ unit? At this time, most (not all) veterinarians with ‘high field’ units believe that the risk is warranted. Conversely, most (not all) veterinarians with standing MRI are convinced that the general anesthesia is not necessary to obtain images at and below the fetlock. It should be noted that many of the top-level sport horse veterinarians internationally use a standing MRI in order to reduce risk to their valuable patients.The cost compares favorably for Standing MRI as well, with a total cost of under $2,000.

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— Case Studies —

Case 1: A 9 year-old barrel racing gelding had been fighting low-grade front limb lameness, which was localized to the foot using nerve blocks. When the problem first started, the gelding was initially only subtly off on a hard surface without a visible head nod. Radiographs of both front feet did not identify any significant abnormalities. Various corrective shoeing changes seemed to help only temporarily and, in fact, some changes appeared to worsen the discomfort. Therapeutic coffin joint injections, which are routinely performed in equine athletes experiencing low level generic foot pain without a significant injury, did help initially. However, these injections only lasted a few months. As the discomfort was continuing to progress to a lameness with a visible head nod even in softer footing, further diagnostics were performed (repeat radiographs, ultrasound examination, and even a bonescan of the front feet). The bonescan identified some inflammation of the navicular bone, although no radiographic changes were present. An MRI was recommended, which identified degenerative change of the navicular bone, as well as a tear in the deep digital flexor tendon near the level of the navicular bone. This combination of problems within the foot is not uncommon but can be very difficult to identify. Once identified an appropriate program of rest, therapeutic intervention and shoeing can be tailored to the individual case.

Case 2: MRI imaging is able to identify a condition commonly called ‘bone edema’ or ‘bone bruising,’ which is quite painful, but not associated with radiographic change. A successful middle-aged jumper mare began to have performance issues following a particularly grueling competition. A subtle lameness was present as she began regular exercise again, after a well-deserved short rest period. The lameness did not improve over a day or so of light riding and in fact appeared to be getting worse. The clinical examination noted some extra fluid associated with the fetlock joint and pain in response to flexion of the lower limb. Nerve blocks were performed, which confirmed the source of the discomfort as the fetlock joint. Radiographs and ultrasound did not identify any abnormalities other than some inflammation of the joint lining. At this juncture, it is not uncommon for owners to elect to perform intra-articular steroid therapy and return the horse to work after a short rest. The owners desired the most complete information they could obtain. A subsequent MRI examination revealed a significant ‘bone bruise’ to the cannon bone, where it articulates in the fetlock joint on the inside portion of the leg. The complete diagnosis allowed the appropriate response in this case, as the injury requires a significant rest before returning to work.

Case 3: Another example of the value of MRI is the case of the nine year-old Western Performance gelding that has been in full work and competition for a number of years. Recently he has been struggling with intermittent moderate lameness of either front limb, which seems to resolve with short rest periods of only a day or two. This has greatly frustrated the owner and trainer in that each time he is lame (often when away at a show); by the time they can arrange their schedule to bring him in for a thorough lameness exam he appears to be fine again. Front fetlock degenerative changes had been diagnosed and successfully managed for the past three years using IRAP (Interleukin-1 Receptor Antagonist Protein) therapy. The concern of another cause for this recurrent variable lameness led to an aggressive diagnostic approach. Although the horse appeared sound, when his shoes were removed for imaging he became clearly significantly lame in one forelimb. Diagnostic nerve blocks localized the pain to the foot, which allowed lameness in the opposite forelimb to be visualized and also localized to the foot region. Imaging (radiographs, ultrasound, nuclear scintigraphy) was able to identify a clear tear in the upper portion of a coffin collateral ligament in one forelimb. However, the cause of the lameness in the other forelimb was uncertain, as the only finding was mild inflammation in the navicular bone shown on the bonescan. Standard diagnostic techniques were able to reach a diagnosis and therapy plan for the injury in one forelimb. However, without a clear diagnosis in the other forelimb, prognosis and therapy cannot be determined. The owners do not wish to place the gelding under general anesthesia. Therefore this case will be one of the first to have an MRI at NWEP.

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Summary
Standing (“open”) MRIs have been used successfully in horses for almost 10 years, with over 20,000 studies successfully performed to date worldwide (http://hallmarq.net). The debate over high field versus low field MRI probably won’t go away soon. The results of an MRI study are often spectacular and extremely satisfying to the owner and veterinarian. When a complete diagnosis is made, it usually enables the owner to tailor a treatment plan or, in some cases, to not have to rest the horse at all!

About the Authors:
Since 1997, Dr. Mark Revenaugh has worked as a veterinarian for the United States Equestrian Team and has been Official Team Veterinarian at numerous competitions. He has earned accreditation in the FEI, and has been an official for multiple committees, including the Drugs and Medications committee for the United States Equestrian Federation.

Dr. Sue Emerson arrived in the Northwest in 2006 to join Northwest Equine Performance as an intern and subsequently stayed on as an associated. She has a special interest in lameness diagnosis and the associated diagnostic technologies, including radiography, ultrasound, nuclear scintigraphy, and MRI.