Magnetic resonance imaging (MRI) was first performed on live horses at Washington State University's College of Veterinary
Medicine in the late 1990s. Available for almost 15 years, this diagnostic modality has seen a steep increase in clinical
use during the past five years. MRI images have expanded our thinking on many types of equine lameness and greatly improved
veterinary diagnostic ability.
MRI equipment and processing technology have significantly improved during this time period as well. Clinicians now have a
number of choices to make when contemplating obtaining an MRI evaluation of a patient, including the type of machine, magnet
strength, image quality, anesthesia risk and a number of other factors.
A standing sedated horse getting an MRI scan of its right distal forelimb. Scans such as this can produce good images of the
navicular bone, coffin joint, pastern, fetlock and associated ligaments and related structures in the lower limb. (PHOTO COURTESY
OF ROSSDALE & PARTNERS, NEWMARKET, U.K.)
A look at your options
Bigger is generally better, and in the case of MRI, bigger is measured in teslas. MRI machines come in three different sizes
based on the magnet's strength, which is measured in tesla units or Ts; 1 T is nearly 30,000 times stronger than the earth's
gravitational pull. High tesla values can be obtained by placing a magnetic field in a confined or enclosed space.
Equine MRI evaluations are generally done with one of three types of systems. There are two high-field systems, which have
3-T and 1.5-T magnets, and a low-field system with a 0.27-T magnet. To get stronger magnetic fields and higher teslas, MRI
units have been designed in a tubular shape with a core space that decreases as magnetic field increases. The 3-T and 1.5-T
units are generally known as "down" magnet systems because the horse must be anesthetized to have its distal limbs or head
and neck safely placed as deeply as possible into the machine's tubular core. Horses do not like the somewhat claustrophobic
aspect of the magnetic coil or the recumbent positioning, so anesthesia is required.
Additionally, in order to get an optimal scan with high-field systems, there must be minimal to no motion. Even a fully anesthetized
horse will still have some limb movement because of its heartbeat and respiratory efforts, but this is controlled in down
MRI systems by padding and careful positioning.
An MRI view of a hoof taken with a standing MRI showing a problem with the deep digital flexor tendon within the hoof. Newer
motion-reducing software has greatly advanced the quality of scans taken in the standing horse to the point where diagnostic
rates for lesions in the distal limb are very comparable between "up" and "down" MRI systems. (PHOTO COURTESY OF HALLMARQ
The 0.27-T MRI is referred to as an "up" or "standing" MRI. This low-field system features a more open C- or U-shaped clam
shell design, and a sedated horse is kept standing as the magnetic coil is placed around its leg, foot or other affected area.
Each system has distinct strengths and weaknesses, and these MRI units have been in use long enough now to make an interesting
and informative comparison.