Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to produce detailed, various plane, cross-sectional images. It allows a simultaneous examination of both bone and soft-tissue structures and can identify injuries to tendons and ligaments as well as bones and joints. Generally, MRI is limited to the lower part of the limbs, meaning up to and including the carpus, or knee, and hock.
MRI was first performed on live horses in 1997 at Washington State University. As its availability has become more commonplace, MRI has increasingly become the gold standard for the diagnosis of musculoskeletal injuries of the distal limb. It has become especially helpful in the equine foot, a particularly common site of lameness where other imaging techniques are sometimes limited.
MRI is indicated when a lameness problem has been localized to a specific anatomic area, generally through diagnostic nerve or joint blocks. Through limitations, other imaging modalities (X-rays, ultrasound, bone scan) may have failed to provide a specific diagnosis. MRI is also useful in interpreting the significance of findings that were previously identified, particularly those that have responded poorly to treatment. It is not a survey technique or a substitute for a thorough clinical investigation, including conventional imaging modalities. The interpretation of MRI findings is enhanced by the information obtained from these previous clinical examinations.
Front and/or rear shoes need to be removed prior to the exam depending on which limb/limbs are being imaged. Metal produces MRI image artifacts, significantly affecting image quality. If an examination of the feet is being performed, the feet are generally radiographed prior to the exam. This confirms that any metal from a nail has not been left behind in the hoof. If present, its removal is facilitated by this finding.
MRI examinations may be performed in a standing (low-field MRI) or recumbent (high-field MRI) position. Standard protocols result in the generation of hundreds of very detailed images that require time and specialized training for interpretation. Horses placed in lateral recumbency (down) require general anesthesia. Examinations take approximately an hour and a half. Those performed under general anesthesia generally image both the clinical and the opposite (“normal”) limb for comparison purposes. Time constraints with respect to sedation and keeping the horse still make this much more difficult in the standing individual. Younger or fractious individuals may not be able to be kept quiet enough to perform this procedure standing. Individuals undergoing general anesthesia are generally dropped off the day prior to the procedure for preanesthetic blood work and nail check radiographs, if required. Horses typically spend the night of the exam under observation before being discharged the following day.
The main disadvantages of MRI involve cost, its somewhat limited availability, the limited accessibility to areas above the distal limbs (head, neck, stifle), and the need for general anesthesia in high-field magnets. The accurate evaluation of cartilage lesions using MRI in the distal limbs of horses remains difficult.
MRI has greatly advanced our diagnostic capabilities and has enabled us to prescribe more focused treatments and more accurate prognosis. It is certainly not indicated in all situations, but may be considered in selected cases through consultation with your veterinarian.
Stay tuned: Dr. Haines is writing a follow-up post with MRI case study examples.