February 3, 2009
Have you ever seen a horse get an MRI?
By The Journal’s Christine Hamilton, with contributions from Kristin Syverson
MRI technology expands veterinary understanding of many lower limb and hoof lamenesses by giving vets an in-depth view of structures that X-ray and ultrasound can’t show. The diagnoses that vets are now able to make using MRI can have a significant impact on the end result for many horses.
The key to using MRI is to know the lameness location: a standing MRI can only be used on the hoof and lower leg. A veterinarian must first use nerve blocks to pinpoint the lameness and then decide whether an MRI would help diagnose the problem.
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The MRI Procedure
1. Perform clinical exam. When a horse arrives for an MRI, the veterinarian first performs a clinical exam to evaluate the horse’s health for sedation and observe the lameness.
2. Begin sedation. A variety of sedation techniques are used, but premedication with acepromazine and maintenance of sedation with dormosedan or Sedivet (romifidine) is one example.
3. Pull shoes. The MRI’s magnet will attract horseshoes, and any metal in the hoof, such as a nail fragment, will distort the image.
4. Place IV catheter. An intravenous drip allows the veterinarian to administer a level amount of sedation at a constant rate and to be able to instantly adjust it.
5. Enter MRI. The horse is loaded into the stocks – they are wooden because of the magnetic field – and the veterinarian begins the sedation drip. A muck bucket is nearby in case the horse urinates. Ear balls and blinders may also be used to help the horse stay quiet.
6. Ready the magnet. The foot is placed inside the magnet. A wrap on the foot holds the signal receiver coil in place. The magnet creates images from radio frequency signals.
7. Run a “pilot.” Once everything is in place, an initial scan called the “pilot” is run to make sure the foot and coil are positioned properly. If adjustments are made, a second pilot is run.
8. Run MRI sequences. A sequence is a series of eight to 24 images only 5 millimeters apart, taken along four different planes of the limb or hoof: one up and down; one front to back; and two diagonally. In each plane, the veterinarian runs three types of sequences that slightly alter the signal and image the magnet produces. The veterinarian gets the same views of a structure with a slightly different emphasis: One signal might be better for imaging bone lesions, another for tendon lesions, etc.
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The procedure typically lasts a little more than an hour.
9. Recover horse. The catheter is pulled and the horse goes back to his stall. He will typically recover from sedation within an hour. Horses are usually hauled in and out for scans.
Unfortunately, the MRI cost is still prohibitive for many owners. Even so, the procedure can have big benefits. First, it enables a veterinarian to better target the treatment to the exact problem. Second, the MRI shows an exact picture of where the lameness is in its healing process, and a horse is less likely to be returned to work before healing is complete, risking further damage.