Magnetic resonance imaging (MRI) uses magnetism to evaluate tissue structures in detail. The patient is place in a magnetic field and a radio-frequency pulse is emitted from a coil. This pulse causes the patients tissues to return energy back to the radio frequency coli that is characteristic of a specific tissue type or disease process. These returned energy pulses are converted, by the computer, into a three dimensional image with detail and information that is superior to other imaging modalities.
These images allow soft tissue and bone structures to be evaluated in ways not previously possible. This method of imaging has quickly become the gold standard for evaluating many injuries in equine sports medicine. Unfortunately, due to the size of the machines available, the horses lameness must be localized to a region at or below the shoulder or stifle as higher locations will not fit into the opening of the machine.
Advantages of MRI include early detection of subtle changes in cartilage and bone before radiography. This method is also superior to ultrasound for imaging the soft tissue of joints. It is currently the only imaging modality that can evaluate soft tissue structures of the foot reliably and in detail. This method is critical for evaluation of navicular syndrome or palmar heal pain; in fact, many causes of foot lameness cannot be conclusively identified without MRI.
There are currently two different methods for obtaining MRI images; they can be performed with the patient lying down under general anesthesia or standing with the patient sedated. The MRI machine at the Equine Medical Center of Ocala is a standing MRI which removes the risk of general anesthesia and recovery from anesthesia.
The following are 2 examples of cases seen at our hospital during this past year.
A 4-yr- old Thoroughbred racehorse had won his first 2 career starts and was appearing to have a promising future. The colt was lame on his RF fetlock after his 3 career start where he finished 5th out of a field of 8 horses. The fetlock was radiographed but no significant abnormalities were identified. The horse was continued in light galloping exercise and the lameness resolved over a 3 week period.
Due to ongoing filling in his fetlock joint, a promising start to this colts career, and the trainer’s experience and knowledge on the merits of MRI, he elected to have the horse evaluated at our hospital. The results of the MRI revealed the start of a condylar fracture. It is possible that had he entered this horse in another race or done any speed work with the horse that he could have suffered a complete fracture of the cannon bone. The early identification of the lesion using the MRI allowed the placement of a single lag screw. After 5 months of stall rest and paddock exercise the colt has returned to a successful racing career.
Figure 1: The transverse slice to the left (cross - sectional) and the frontal slice (back to front) to the right are typical of the 5-mm slices that are generated by the machine. The red arrow identifies the fracture that is located at the back of the cannon bone towards the sesamoid bones. The white line demonstrates the location of the frontal image slice to the right. It is possible that if this horse continued in fast galloping exercise that the fracture at the back of the cannon bone would have propagated out the front of the cannon bone.
A 2 yr old Thoroughbred filly came up lame after her 1st career breeze, radiographs were taken the same day and no abnormalities were found so an MRI was recommended. The MRI revealed the start of a condylar fracture. It is possible that had she been put back to work based on the radiographic findings alone she could sustained a catastrophic fracture of her cannon bone.
The MRI revealed the lesion early enough that surgery was not needed and the mare was treated medically with stall rest. After her period of stall rest she returned to racing soundness.
As you can see from the images below, there is no evidence of a fracture on the radiographs and a clear fracture line can be seen on the MRI image.