During the last few years, magnetic resonance imaging (MRI) and computed tomography (CT) have made significant improvements
in assessing equine tissue damage and diagnosing disease. "Both modalities have made substantial advances, and there is a
role for both in modern equine imaging decisions. They are complementary, not competitive," says Russell Tucker, DVM, DACVR,
associate professor and chief of radiology at Washington State University's College of Veterinary Medicine. This article discusses
MRI. The next issue will discuss CT and computed tomography angiography (CTA).
The use of MRI is increasing in equine practice. "Quite a few more magnets are available for horses, and there are more MRI
systems throughout the country than there were just a few years ago," says Carter Judy, DVM, Dipl. ACVS, Alamo Pintado Equine
Medical Center, Los Olivos, Calif.
Advantages and disadvantages
In the past, equine imaging was limited to ultrasonographic and radiographic techniques. Ultrasonography works well for some
problems but has limitations, for example, in imaging all the structures of the foot. Radiographs can detect major bone injuries
clearly, but MRI is more sensitive, provides an even clearer picture and can identify accompanying soft-tissue injury.
Photo 1: This equine MRI unit from Hallmarq Veterinary Imaging, Guilford, United Kingdom, is an open, low-field magnet unit
mounted vertically at floor level and allows MRI to be performed on standing, sedated horses. (Photos: Courtesy of Dr. Ed.
MRI has exceptional sensitivity and specificity, providing physiologic and anatomical information about bone and soft tissue
(Photos 1-7). It is useful to assess lameness and lower limb damage and to identify injury to the bone, tendons and ligaments
that could not be identified in horses previously. It produces a clear display of anatomy on any plane and allows for visualization
of different tissue structures. In addition to its use for detecting injuries of the equine limbs, it is being used to assess
head injury and disease, and recent advances have enabled its use for neck problems as well.
Photo 2: The 1.5-tesla Siemens Symphony high-field magnet (Siemens, Malvern, Pa.) is used for MRI of horses under general
anesthesia at the Veterinary Teaching Hospital of North Carolina State University. The horse is positioned in lateral recumbency
with the lame limb lowermost. The region of interest in the limb, in this case the foot, is positioned in the isocenter of
One of the biggest advantages of MRI is the ability to image cartilage. CT and radiography can image cartilage indirectly
if you use contrast agents. MRI is the only imaging modality that can directly image cartilage.
Photo 3: A sagittal short-tau inversion recovery (STIR) image of the foot of a horse with lameness that is abolished by anesthesia
of the palmar digital nerves. There is marked STIR hyperintensity of cancellous bone in the medullary cavity of the navicular
bone (arrow), indicating the presence of abnormal medullary fluid probably due to a bone bruise.
Obviously, the limitation of MRI is the bore of the MRI gantry, which, unfortunately, may never accommodate the entire equine
Photo 4: A sagittal STIR image of a 6-year-old Thoroughbred with a focal cartilage injury of the proximomedial aspect of the
proximal phalanx in the left forelimb. There is a focal hyperintensity in the articular cartilage layer near the dorsal margin
of the joint caused by pooling of synovial fluid in the cartilage defect.