Equine imaging update: Magnetic imaging resonance

Equine imaging update: Magnetic imaging resonance

Apr 01, 2010

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

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. Kane)
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 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 the magnet.
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 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.
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 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.
Obviously, the limitation of MRI is the bore of the MRI gantry, which, unfortunately, may never accommodate the entire equine anatomy.