High-field pros and cons
The primary advantage of the high-field system is image quality. Large tesla machines will produce a more detailed image of
a larger area in a single scan, giving the clinician a clearer look at a horse's anatomy, generally leading to optimum diagnosis.
Numerous research projects have evaluated equine distal limb images produced from cadaver forelimbs using 3-T, 1.5-T and 0.27-T
systems. There is no disagreement that larger units produce more detailed images. A study comparing the three different types
of equine MRI units concluded that "anatomical structure scoring values of images obtained with 3 T and 1.5 T were significantly
greater than scores of images obtained at 0.27 T."1 Not surprisingly, this research also indicated that 3-T images scored significantly higher than 1.5-T images.
The main disadvantage of high-field systems is that patients must be anesthetized. Many horses that are being considered for
MRI evaluation have an injury or some degree of lameness that has led to the need for diagnosis. Clinicians often have concerns
about the induction and recovery aspects of general anesthesia and the potential risk to an already compromised horse. Many
owners share this concern, and horses with noncrucial problems or non-life- or career-threatening problems may not be evaluated
by MRI because of anesthesia concerns. In these cases the information potentially gained is not judged to be worth the risk.
MRI may also be excluded in sports medicine evaluation of elite performance horses. Subtle issues could be identified and
treated to yield even better performance, but owners and trainers are reluctant to risk anesthesia on a still highly functional
equine athlete. A recent report on equine perioperative fatality rates concluded that there is a 1 percent risk of death from
anesthesia in healthy horses.2 Older horses and horses with pulmonary, cardiovascular, metabolic or musculoskeletal issues had a higher degree of risk.
Those percentages are significant in the eyes of many owners, and high-field systems may consequently not be fully utilized.
Low-field pros and cons
Low-field MRI systems do not carry an anesthesia risk, but traditionally these units have not been seriously considered as
diagnostic options because of their lower decreased image quality. Scanning for longer time periods allowed these lower-T
systems to improve image quality, but longer scans in standing tranquilized horses meant more potential swaying motion, which
ultimately negated any image improvement.
However, significant advances have been made to low-field MRI systems in the past few years. Hallmarq Veterinary Imaging,
the only manufacturer of MRI systems for the standing horse, has introduced 0.27-T scanners internationally that use improved
software features to help control motion and provide other improvements that now may make standing MRI a viable choice.
An investigation of these newer motion-correction techniques for standing equine MRI outlined the promise of standing MRI
saying, "MRI of the distal extremities of the standing, sedated horse would be desirable if diagnostic quality images could
be obtained. With the availability of extremity and special purpose magnet designs on the market, a system to safely accommodate
the standing horse may gain increasing popularity."3
The study tested a number of motion correction techniques on scans of the carpus and tarsus in five standing sedated horses.
It is significant that this research was done on carpal and tarsal scans since these relatively more proximal joints are usually
associated with more "wobble motion" than distal limb structures, which are far more commonly imaged with MRI. Standing MRI
scans of the equine foot, pastern and fetlock already have generally good diagnostic correlation to high-field scans because
of the decreased motion seen in the distal limb. This study, however, concluded that in standing MRI scans of the equine carpus
and tarsus, "the motion artifacts were nearly eliminated. Although significant hurdles remain, these results suggest promise
for allowing diagnostic quality MRI of the carpus and tarsus in the standing horse."3