An example of an MRI image of the foot.
and fat molecules, are very prevalent in biological tissue which is why there is no need to add dye or any other substance in the bloodstream for the procedure to be effective. The interaction between the external magnet and the
biological magnet in the hydrogen nuclei helps to identify areas of concern. Signals from the hydrogen atoms are re- ceived, improved and analyzed by a computer to give a de- tailed image. Water, in areas where it’s not supposed to be, is an indica-
tor of disease. As the MR “slices” straight through tissue and bone, it collects a series of different types of images. (Learn how images are sliced on p. 78.) By comparing these images, the reader can determine where there is water and fat in bone and soft tissue where it’s not supposed to be. Therefore, an accurate diagnosis can be made.
High Field Versus Low Field Machines are rated based on magnetism strength measured
in units called Tesla. If a machine is greater than .6 Tesla, it is considered a high field magnet and any machine with less than .6 Tesla is considered a low field magnet. A supercon-
Different Diagnostic Procedures
Radiography: Primary method for imaging of bone. Ultrasound: Primary method for imaging of soft tis- sues, especially tendons and ligaments. With suitable probes it is possible to image some tendons of the foot.
Scintigraphy (bone scanning): Varying uses across different horse types, including detection of stress fractures in Thoroughbreds. Good for necks and backs where bony lesions are suspected. Good for corrobo- rating location of pathology in sport horses.
CT: Limited availability of equine CT but good for head and necks and also for analysis of complex fractures pre-surgery.
MRI: Good for feet where other imaging modalities can’t reach and for other parts of the limb where clini- cal exam, nerve blocks or scintigraphy indicate the site of pathology but where there are no visible abnormali- ties on radiography.
Foot Injury Study
The U.K.’s Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, world-renowned for her expertise in equine orthope- dics, has had specific interest in lameness diagnostics which led her to conduct a study from January 2001 to December 2003. She completed MRIs on 199 sport horses to document the variety of injuries in the foot and to establish treatment plans depending on her findings. According to her study, the results were: Deep digital flexor (DDF) tendonitis was the most common injury (59%) with primary injury in 65 horses (33%) and a further 27 horses (14%) having lesions of the DDF tendon and navicular bone. Desmitis of a collateral ligament (CL) of the distal interphalangeal (DIP) joint was the second most com- mon injury (62 horses, 31%). Seventeen percent of horses had injuries to multiple structures, including 24 with DDF tendonitis.
Warmbloods Today 77
ducting 1.5 Tesla whole body magnet (as you might find in a human hospital) costs approximately ten times that of a low field open magnet. Annual maintenance costs are also very high for the high field systems because of the need for liquid helium to cool the magnet. To compensate for higher costs of purchasing and maintaining a high field machine, as well as the resources needed to put a horse under gener- al anesthesia, the cost to the owner is greater than a stand- ing MRI (low field). There are currently no standing equine, high field MRI systems and all ‘high field horses’ have to be scanned under general anesthesia. Traditionally, of all the high field MRI examinations performed under anesthesia, 75 percent are of the foot. Your initial assumption may be that high field is better
than low field, but in situations where you need images of the fetlock, pastern or hoof, that is not necessarily the case. It is true that a high field magnet can produce a better qual- ity image; however, a low field magnet will produce a good image of high enough quality to make accurate diagnostic evaluations. The lower the problem is on the leg, the better the standing machine can do because there is less move- ment by the patient. In cases where the problem is in the hock or knee, a high field machine might be the better option to complete- ly diagnose the cause of lameness as patients are unable to move while under anesthesia and higher quality images can be gathered. Of course, with general anesthesia comes the possibility of complications as well as an increase in cost, however it is harder to gather diagnostic images by using a low field magnet higher up on the leg. When at all possible, the standing procedure is preferred by owners and trainers, especially when lameness originates from the lower limb. When there is no need for general anesthesia, mortality risk is eliminated and day patient scheduling is possible, making it easier for horses to undergo the process. Depending on
Equine Medical Center Ocala
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