MAGNETIC RESONANCE IMAGING (MRI)
In recent years, MRI use has become more widespread, especially with the availability of standing MRI, and is used to assess soft tissue (tendons, ligaments), bone (fractures), and cartilage. It is limited by its field strength (standing units have a small magnetic field leading to less resolution), patient motion, and the over-interpretation of the image results. On a positive note, MRI gives a tremendous amount of detailed information about both the anatomy and physiology of the area of interest. It is used when the area of pain is known, but the cause has not been well established using other techniques.
Nuclear Scintigraphy, which uses a radioactive isotope injected into the horse, can identify physiologic changes in bone and soft tissue metabolism and has been used for decades with equine lameness diagnosis. It is a simple and relatively noninvasive procedure that can screen the entire horse. Scintigraphy is often used when the region of lameness is unknown or indeterminate, using routine techniques such as nerve blocks to localize the area of pain.
Radiographs are probably the most commonly used diagnostic technique of these three and are essential to equine practice. With the use of digital radiography, the results can be viewed immediately. Radiography is somewhat limited to boney issues as it produces a two-dimensional image of a three-dimensional object (both a pro and a con, depending on what one is looking at). Considerable change in bone density has to occur before it can be picked up by a radiograph. Radiographs remain a very important diagnostic tool, especially when identifying boney issues such as osteophytes (“bone spurs”).
MRI is more sensitive than radiographs for identifying the pathology in the area of concern, and nuclear scintigraphy can help to localize difficult lameness, especially when it involves a bone-related issue. In conclusion, imaging the equine patient can be a complicated process. Negative results on one modality do not preclude the absence of a problem in the patient, and an alternative imaging technique may provide more insight into the pathology of a particular patient.
Equine lameness can be straightforward or complicated, can involve bone, soft tissue or cartilage, or any combination of these three, and can involve one joint/limb or multiple joints/limbs. Often more than one diagnostic tool will be needed by a veterinarian to diagnose the source of the problem such that a treatment plan can be put into place. Understanding the limitations of MRI, nuclear scintigraphy, and radiography can help both owner and veterinarian make informed decisions with respect to diagnostic choices as they move through a lameness examination together.
Scientific Article: Judy, CE. Comparison of Radiography, Scintigraphy, and Magnetic Resonance Imaging Techniques: How Does One Image Compare to Another? in Proceedings, Am Assoc Equine Pract 2014; 60:135-140.