Advances in capabilities
"MRI has evolved, image quality has improved and scanning times have been reduced," states Judy. This is due to the rapid
advance in computing power and gradient strength. The gradients are the portion of the MRI that cause the change in the magnetic
field temporarily and are responsible for beginning the whole imaging process. The stronger the gradients, the faster the
magnet can perform. "Combine this with advances in computer hardware and software technology, and many systems can gain a
significant image quality and speed advantage," Judy explains.
Photo 5: A transverse fast low angle shot (FLASH) image with fat saturation of a foot at the level of the insertion of the
deep digital flexor tendon to the distal phalanx of a horse with foot lameness. A hyperintense core lesion is present in the
central part of the tendon (arrow) and extends distally to the bone-tendon interface.
The protocols that are used with MRI can be tailored to target certain tissues. "We're seeing structures that we thought we
might not be able to see and with a lot more clarity," states Judy.
Photo 6: A transverse proton density image of the proximal metatarsal region of the right hindlimb of a horse with severe
proximal suspensory desmitis. There is a large abnormal area of diffuse signal hyperintensity in the dorsal and central parts
of the suspensory ligament (arrow).
The anatomical regions that equine practitioners are able to image with MRI have also increased. For equine limbs, that includes
higher regions such as stifles, knees and hocks in some horses. "It's not just a foot machine any more," Judy notes. "That's
still the bread and butter, but advances have opened up areas like fetlocks, where we see a lot of problems."
Photo 7: A sagittal proton density image of the tarsus of a horse with talocalcaneal joint osteoarthritis. There is localized
loss of joint space and subchondral bone margins in the center of the joint. This lesion is surrounded by a wide irregular
area of signal loss reflecting reactive osteosclerosis in both the talus and the sustentaculum tali of the calcaneus (arrows).
"I doubt that imaging the tendons and ligaments can get much better," says Chad Zubrod, DVM, MS, Dipl. ACVS, Oakridge Equine
Hospital, Edmond, Okla. "I think that we're probably every bit as good at looking at major tendons and ligaments as they are
Other applications of MRI are better than they use to be. Its use is no longer limited to the distal extremities. Although
it had been previously used for imaging the head on some horses, there is further potential for diagnosing neurologic conditions.
MRI has superior tissue contrast over CT, offering more exquisite contrast, for example, between the white and gray matter
of the brain. "Just as it has in small animals and people, MRI has really taken over neurology," says Tucker.
Equine practitioners are now using the high-field strength magnet and also getting some studies out on the necks of horses
— an interesting horizon for MRI. "It's got some inherent limitations with our current equipment, but it will be a revolutionary
application of MRI, as it has been in humans and in small animals." Using MRI to image cervical lesions such as those in Wobbler's
syndrome will revolutionize how these cases are diagnosed and treated.
Contrast MRI, similar to contrast CT is another expanding area. "We've done a lot of contrast MRI, which is increasing our
ability to detect subtle lesions that we couldn't see before," Judy states. A dye is injected into the jugular vein, and it
circulates throughout the body. The dye concentrates in areas of inflamed tissue or change. It can not only detect the subtle
lesions, but when you go back and recheck at a later time, you can see whether the dye accumulates in the same locations,
to give you an idea about prognosis and about how much the tissue has healed.
One of the current interests in human MRI has been weight-bearing imaging, in which a person is put through positional posturing,
for example, while looking at cartilage compression in a knee or shoulder. As the patient goes through a normal range of motion,
clinicians can develop some diagnostic criteria. "Some day, especially with all of the biomechanical pressure that horses
put on their lower legs and joints, this is going to be an important thing to do," Tucker suggests.