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.

MRI has traditionally been impossible or nearly impossible to use on the horse because of the small space for the subject in the scanner as well as the need for immobility. Horses need to be placed under general anaesthesia in order to be scanned in conventional human scanners. Most of these scanners are prohibitively expensive to purchase and to maintain, which limits their use to a small number of large institutions. However, in June 2002 Mair and colleagues at the Bell Equine Veterinary Clinic installed an open U-shaped scanner designed for imaging the standing horse's lower limb. It will image the limb up to the knee or hock. MRI works by exposing the area being scanned (which is placed inside a strong magnetic field) to pulses of radiowaves. The tissues emit a signal (also in the form of radiowaves), which is used to create the image. As a result, this and all other MRI units require radiofrequency shielding of the room to prevent outside interference from radiowavesand temperature control.

"Generally, the entire scan protocol (including pilot sequences to check positioning and setup for subsequent scans) takes between 60 and 120 minutes to scan both front feet or both hind feet," he said. "Each image sequence takes between 2.5 and 4.5 minutes. Images are viewed during the procedure, and repeat scans are taken to confirm the presence of any suspected lesion."

Motion Correction
Most of the time, Mair said, a horse's feet can be imaged without motion correction software as long as sedation is used. However, he said that scanning of areas above the foot generally requires motion correction.

He explained that a free-standing, heavily sedated horse will often sway up to 20 mm, pivoting about the distal joints. Also, sedation can cause transient trembling, and quick movements when the horse readjusts his balance also occur. However, supporting a leg in the scanner and employing careful head holding can help minimize movement in the leg being scanned.

"It has proved feasible to keep the fetlock, carpus (knee), and hock still for a sufficient number of the necessary 20- to 30-second periods within a four- to five-minute imaging time to produce high-quality motion-corrected images."

Lameness Observations
Mair and colleagues evaluated horses of various ages, breeds, and uses with lameness in one or both front feet. In most horses, pain had been localized to the foot, but no diagnosis had been made. A few horses had penetrating foot wounds to the frog or frog sulcus.

Additional imaging modalities for all cases included routine clinical observation with a 10-point lameness scale, nerve blocks, and radiographic examination. Some horses also were examined using intra-articular (within the joint) analgesia, analgesia of the navicular bursa, ultrasonography, and nuclear scintigraphy. Horses with foot wounds were also examined using synoviocentesis (sampling the synovial fluid in the coffin joint, navicular bursa and digital tendon sheath) and contrast radiography studies to determine any synovial cavity involvement in the wound.

The MRI observations of horses without foot wounds follow:

• Deep digital flexor tendon (DDFT) damage: Insertional damage, 13 horses; dorsal border damage: 15 horses; core lesion, three horses; multifocal tendonitis, five horses; sagittal tear, three horses; and thinning of tendon, four horses.
• Navicular bone changes, 31 horses.
• Navicular bursa changes, 13 horses.
• Navicular collateral ligament damage, five horses.
• Distal sesamoid impar ligament changes, six horses.
• Distal interphalangeal joint collateral ligament enlargement, seven horses.
• Distal interphalangeal joint distension, 19 horses.
• Sesamoidean ligament damage, three horses.
• No abnormalities detected, 13 horses.

In most wounded horses, the tract of the penetrating wound was identifiable in the MRI images.

Conclusions
"We believe that this technique (MRI) could revolutionize the assessment of certain musculoskeletal lesions of the distal limb," Mair said. "MRI has many advantages over other conventional imaging techniques. MRI does not use ionizing radiation and provides multiplanar, 3-D imaging capabilities. Both bone and soft tissues can be imaged simultaneously. In addition, MRI allows assessment of physiological differences between normal and abnormal tissues by use of various imaging sequences. DDFT lesions within the foot have not been reliably diagnosed in the live horse before the advent of MRI scanning.

"MRI is not considered to be a screening tool," he added. "For optimal results, the imaging should be focused on specific structures or regions as determined by previous clinical examinations, in cases of foot pain with no or equivocal radiograph and ultrasound findings."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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