Magnetic resonance imaging (MRI) is becoming more available and helpful as researchers learn more about MRI in the horse. Tim Mair, BVSc, DEIM, DESTS, Dipl. ECEIM, MRCVS, of the Bell Equine Veterinary Clinic in the United Kingdom, presented the basics of MRI and his experience imaging 96 horses (83 of which had pathology) at the 2003 American Association of Equine Practitioners convention.

"Magnetic resonance imaging uses the body's natural magnetic properties to produce detailed images of the tissues," he began. "Magnetic resonance signal intensity varies widely in different musculoskeletal tissues because of differences in the proton density and the status of the chemically free versus chemically bound molecular water.

"Most diseases manifest themselves by an increase in water content, and therefore MRI is a sensitive test for the detection of disease," he continued. "The high soft-tissue contrast afforded by MRI makes it ideal for assessment of articular cartilage, ligaments, tendons, joint capsules, synovium (the membranes that line joint capsules and produce synovial fluid), and bone marrow."

There are different types of MR imaging (different sequences and image weighting) that provide different types of information. By combining and comparing these different sequences, the clinician can build up a picture of the tissues and the disease processes occurring within them; it's a bit like putting together various pieces of a jigsaw puzzle until the whole picture becomes clear."The T1-weighted imaging provides the best anatomical detail and is considered to be the standard accepted sequence for baseline information of the musculoskeletal system," Mair said. "T2-weighted images are particularly useful for evaluating synovial structures and areas of fluid accumulation." The STIR sequence is used to detect early damage and fluid fluid accumulation in bone