Magnetic resonance imaging provides veterinarians with a new way to investigate injuries and diseases

by Carter E. Judy, D.V.M., and Douglas Herthel, D.V.M.

AFTER a workout, a three-year-old colt returns to the barn lame on the right front limb. The examining veterinarian evaluates the horse and isolates the source of pain to the fetlock region using nerve blocks. Radiographs, ultrasound, and nuclear scintigraphy (bone scan) are negative as to the cause of the lameness.

The horse is rested for ten days, but it remains lame. A magnetic resonance imaging (MRI) scan of the fetlock is performed, and the clinician identifies a contusion (bruising injury) of the cannon bone in the lateral condyle, the knuckle at the lower end of the bone. The horse is treated, a catastrophic injury is avoided, and the horse returns to the track to continue a successful career.

MRI is changing the way veterinarians evaluate, diagnose, and treat complex orthopedic cases in the horse, but MRI evaluation has been used in human medicine since 1973. The technology has evolved from grainy, incoherent images to detailed, precise models of the anatomical and functional components of the body. MRI has revolutionized imaging in human medicine and is revolutionizing veterinarians' understanding of complex injuries that just a few years ago could not be diagnosed accurately.

MRI technology

First applied to the horse in 1997 by the veterinarians at Washington State University, MRI utilizes a strong magnetic field to orient the atoms in the body. The modality employs radio-frequency waves, a powerful computer, and some tricky physics and mathematical calculations to produce detailed images.

MRI images are much different from those produced by other imaging techniques, such as radiographs, computerized axial tomography (CAT) scans, or nuclear scintigraphy. While MRI images often look similar to CAT scan images, they are a result of the body's different responses to magnetism.

Because a majority of the body is water, MRI is very sensitive to changes in the amount of water in tissues. This allows for a detailed evaluation of fluid accumulation, edema, swelling, and tearing of soft and hard tissues in injured limbs. It provides not only an anatomical depiction but also a physiological evaluation of changes in the composition of tissues.

MRI images can be acquired and oriented in many different planes, which allows for a complex evaluation of the area in question.

MRI is the imaging modality of choice when others have failed to identify a cause for the lameness after the area of interest has been identified. Most trainers, owners, and veterinarians are familiar with the commonly used diagnostic tests, including nerve blocks, radiographs, ultrasound, and nuclear scintigraphy. Each test provides information as to a potential cause for a lameness or problem.

Nerve blocks are useful in identifying a location for the lameness, but they fail to identify the underlying cause. Radiographs are best suited to evaluate the bony structures of the body, but they require a 10% difference in surrounding density for the area radiographed to be clinically visible. Using radiographs, low-grade problems often can require several weeks to months for a change to be identifiable. This makes radiographs relatively insensitive to subtle changes.

Ultrasound is useful to evaluate the soft tissues of the horse, but it only reveals anatomical changes, and ultrasound cannot penetrate the hoof capsule or bony tissue. This makes ultrasound ineffective at evaluating problems of the foot and in evaluating problems where a physiological abnormality exists and not an anatomical problem.

CAT scans are more sensitive in detecting subtle anatomical abnormalities, but they rely on radiographic principles and, therefore, are best suited for bony problems. A CAT scan is unable to distinguish subtle fluid shifts within the tissues and is usually only able to produce cross-sectional images.

The MRI procedure

The ability to perform a full-body MRI of the horse is the goal, but limitations of the equipment currently available restrict the evaluation to the limbs, head, and neck of the horse. In smaller horses (those less than 500 pounds), a full-body MRI is possible and has been successfully performed.

Currently, MRI is able to examine the following regions:

•Feet;

•Pastern;

•Fetlock (ankle);

•Cannon and suspensory region;

•Carpus (knee);

•Hock;

•Head; and

•Neck.

The process of performing a high-field MRI involves anesthetizing the horse and placing the area of interest within the magnet. For comparison purposes, the same area of interest is imaged in both limbs. Average time required for the scan is about one hour, during which about 700 images per limb are obtained.

When is an MRI indicated?

An MRI is suited for a patient when the area of pain is localized and traditional methods of diagnosis have failed. MRI also is indicated in cases where an alternative modality has obtained a diagnosis, but the problem is more severe, complicated, or surgical intervention may be required. A few case examples below help to illustrate MRI's application in the racehorse:

Case #1: A three-year-old Thoroughbred racehorse experiences an acute onset of lameness that is isolated to the fetlock region. The trainer and veterinarian have noticed significant swelling of the fetlock joint. Radiographs and ultrasound are negative for the cause of the lameness. A nuclear scintigraphy scan reveals marked uptake of the radioactive isotope in the fetlock region, which indicates a problem exists, but the activity is diffuse throughout the region and not definitive as to cause.

Clinicians perform a high-field MRI scan, and they are able to identify a short tear in the sesamoidean ligament with a small fracture of the base of the sesamoid bone where the ligament attaches and generalized distension of the fetlock joint.

Exploratory surgery is avoided. Instead, clinicians prescribe a conservative treatment of shock-wave therapy, intra-articular (joint) therapy, and a therapeutic exercise program.

Case #2: A three-year-old Thoroughbred racehorse exhibits a chronic, grade-2, front-limb lameness that has been isolated to the foot multiple times with diagnostic nerve blocks. Radiographs, ultrasound, and nuclear scintigraphy reveal no abnormalities. The horse is rested for 30 days, but when it is returned to light training, the lameness returns within three days. A high-field MRI reveals a tear of the deep digital flexor tendon. The horse is treated with autogenous (harvested from its own body) stem cell injection and given a prolonged period of rehabilitation.

Case #3: A two-year-old Thoroughbred racehorse with an acute, grade-4, front-limb lameness is diagnosed via radiographs with a fracture of the base of the sesamoid bone. Prior to surgery for possible repair, the surgeon employs an MRI to evaluate the injury fully. A cartilage defect with a severe bone contusion of the lateral condyle of the cannon bone is identified. Accordingly, the surgeon develops a different surgical plan, and the horse is retired as a broodmare after successful surgery.

As these cases reveal, MRI provides a detailed, noninvasive, sensitive technique to image the affected region that other techniques are not able to provide. In each case, traditional techniques reveal the presence of an abnormality, but all are unable to specify what that abnormality is. MRI provides that specificity, complements the imaging techniques performed, and directs the appropriate treatment.

The future of MRI

MRI is revolutionizing the diagnosis of orthopedic problems in the horse. It is already making possible the diagnosis of disease processes that were not obtainable just a few years ago. While standard MRI is providing a unique look into the horse, advanced MRI and the future of the modality are revealing what once was not considered possible, such as techniques including:

•MRI angiography, which utilizes the inherent properties of blood flow to detect abnormalities in blood vessels and blood flow;

•MRI contrast arthrography to detect subtle cartilage defects not presently detectable;

•MRI spectroscopy to detect chemical changes within the tissues of the limb, thereby increasing the sensitivity of the modality; and

•Preventive MRI to establish the presence of abnormalities before they become clinical problems.

These techniques currently are being performed in the early stages and are showing great promise in helping to treat difficult disease, such as laminitis, neuronal problems, cartilage damage, and bone abnormalities.

The future of MRI promises a more detailed, functional evaluation of the horse with a look into the anatomy that has never been seen and at a more detailed level than ever was possible.

Carter E. Judy, D.V.M., is a staff surgeon at Alamo Pintado Equine Medical Center, one of America's premier equine referral hospitals, located in Los Olivos, California. Douglas Herthel, D.V.M., is director and founder of Alamo Pintado.