Areas for improvement
The sensitivity of MRI for cartilage lesions is high but certainly not perfect, so veterinary and human orthopedic clinicians
are interested in improved cartilage imaging. New MRI sequences are being explored all the time, specifically to try to improve
the contrast of cartilage compared with the subchondral bone and the joint fluid.
"The thickness of equine articular cartilage is a huge limiting factor compared with our counterparts in human medicine,"
says Zubrod. "We can get articular images in the horse today, but we have to be very critical about evaluating or interpreting
the images." Small irregularities in the cartilage seen on MRI may be a cartilage lesion, an erosion or lack of cartilage.
"Other difficulties in imaging equine cartilage are also related to how thin these structures are," says Zubrod. "It may also
be impossible to discern very small lesions because in the same voxel (volumetric pixel) you could have both normal tissue
and damaged tissue, and within that voxel, the signal intensity will be averaged out, making you lose the definition of that
lesion. The signal change in that small cartilage lesion will, therefore, be averaged out by the surrounding normal tissue.
So you're unable to see it."
In order to reduce voxel size enough to have a really sensitive MRI protocol for small cartilage lesions, you need to use
inordinately long imaging times. You can achieve higher resolutions by running longer sequences. The longer the sequence has
to run, the longer the horse has to be anesthetized. "So there comes a point where it is no longer practical to try and run
more detailed, higher-resolution sequences because you just can't keep the horse asleep for three hours or more without increasing
the risk to the horse exponentially," says Michael Schramme, DVM, PhD, Dipl. ECVS, North Carolina State University College
of Veterinary Medicine.
There are three general ways to try to improve cartilage imaging. The first is to try to find better sequences with better
cartilage contrast, such as the VIBE (volumetric interpolated breath-hold) sequence, the DESS (dual echo steady state) sequence
and others that are currently being evaluated in human medicine. The second is to use contrast media in the joint such as
saline or gadolinium. The third is to use higher-field strength magnets such as the 3-tesla magnets used in human hospitals
for cartilage imaging. But these high-field strength magnets are also inherently more susceptible to artifacts and side effects.
These artifacts include motion from the patient's breathing or signal voids from metal dust in the nail holes in the horse's
foot. In addition, side effects of 3-telsa MRI that have been reported include tissue heating and headaches in people.
Standing MRI, low-field vs. high-field strength magnets
There has been substantial improvement in the low-field strength magnet, with a couple of companies marketing to veterinarians.
Also, low-field strength magnet manufacturers are now making systems applicable to imaging the anesthetized horse since removing
the motion results in higher resolution and better diagnostic images. "Just a little bit of swaying makes a big difference
and was one of the major problems," says Tucker. Without anesthesia, motion is addressed by improving the acquisition parameters
and by being able to go back and recalculate the data, excluding those images in which motion added to the blurriness of the
image. Once corrected, the images are clearer.
Although MRI for standing horses serves a valuable role in many cases, especially on follow-up examinations, there are still
limitations for initial diagnosis that must be recognized and respected. Low-field strength does not, and will not ever, equal
high-field strength in diagnostic capacity. "Though a high-field strength magnet will most often be the best imaging method
and the most efficient way to get a diagnosis and the proper treatment, for those patients that require follow-up to monitor
the healing process, practitioners may be able to use low-field strength magnets and, therefore, not have to anesthetize the
horse," says Tucker. For certain limited applications, standing MRI techniques are useful because of the availability of smaller,
low-field strength magnets, along with improving technology.
However, equine veterinarians are not in complete agreement on the usefulness of low-field strength magnets. "If people want
follow-up information, if it's worth repeating the MRI, then it's worth repeating with a high-field strength magnet," says
Zubrod. "The minimal risk of anesthesia combined with the superior quality of the images makes high-field strength MRI the
better choice. I think that at this time, there is still enough difference in the images between a high- and low-field MRI
that when you perform a follow-up examination with a different machine, you add another variable and have to determine if
all of the change is due to the healing process or possibly due to the variability in equipment."
"Eventually, it will be possible to do high-quality MRI on standing horses, but the technology has yet to mature," Tucker
states. "The diagnostic quality of the systems currently is not as good as the higher-field strength systems."