Equine imaging update: Magnetic imaging resonance

During the last few years, magnetic resonance imaging (MRI) and computed tomography (CT) have made significant improvements in assessing equine tissue damage and diagnosing disease. "Both modalities have made substantial advances, and there is a role for both in modern equine imaging decisions. They are complementary, not competitive," says Russell Tucker, DVM, DACVR, associate professor and chief of radiology at Washington State University's College of Veterinary Medicine. This article discusses MRI. The next issue will discuss CT and computed tomography angiography (CTA).

The use of MRI is increasing in equine practice. "Quite a few more magnets are available for horses, and there are more MRI systems throughout the country than there were just a few years ago," says Carter Judy, DVM, Dipl. ACVS, Alamo Pintado Equine Medical Center, Los Olivos, Calif.

Advantages and disadvantages

In the past, equine imaging was limited to ultrasonographic and radiographic techniques. Ultrasonography works well for some problems but has limitations, for example, in imaging all the structures of the foot. Radiographs can detect major bone injuries clearly, but MRI is more sensitive, provides an even clearer picture and can identify accompanying soft-tissue injury.

Photo 1: This equine MRI unit from Hallmarq Veterinary Imaging, Guilford, United Kingdom, is an open, low-field magnet unit mounted vertically at floor level and allows MRI to be performed on standing, sedated horses. (Photos: Courtesy of Dr. Ed. Kane)

MRI has exceptional sensitivity and specificity, providing physiologic and anatomical information about bone and soft tissue (Photos 1-7). It is useful to assess lameness and lower limb damage and to identify injury to the bone, tendons and ligaments that could not be identified in horses previously. It produces a clear display of anatomy on any plane and allows for visualization of different tissue structures. In addition to its use for detecting injuries of the equine limbs, it is being used to assess head injury and disease, and recent advances have enabled its use for neck problems as well.

Photo 2: The 1.5-tesla Siemens Symphony high-field magnet (Siemens, Malvern, Pa.) is used for MRI of horses under general anesthesia at the Veterinary Teaching Hospital of North Carolina State University. The horse is positioned in lateral recumbency with the lame limb lowermost. The region of interest in the limb, in this case the foot, is positioned in the isocenter of the magnet.

One of the biggest advantages of MRI is the ability to image cartilage. CT and radiography can image cartilage indirectly if you use contrast agents. MRI is the only imaging modality that can directly image cartilage.

Photo 3: A sagittal short-tau inversion recovery (STIR) image of the foot of a horse with lameness that is abolished by anesthesia of the palmar digital nerves. There is marked STIR hyperintensity of cancellous bone in the medullary cavity of the navicular bone (arrow), indicating the presence of abnormal medullary fluid probably due to a bone bruise.

Obviously, the limitation of MRI is the bore of the MRI gantry, which, unfortunately, may never accommodate the entire equine anatomy.

Photo 4: A sagittal STIR image of a 6-year-old Thoroughbred with a focal cartilage injury of the proximomedial aspect of the proximal phalanx in the left forelimb. There is a focal hyperintensity in the articular cartilage layer near the dorsal margin of the joint caused by pooling of synovial fluid in the cartilage defect.

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 in people."

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.

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."

Bigger magnets

The low-field strength units run at 0.2 to 0.3 tesla, and the high-field strength units at 1.5 tesla. Newer units now have field strengths that run at 3 tesla or higher. The advantage of a higher magnetic field is that you get more signal, so you can get the same detail and resolution with a much shorter imaging time. The potential capability of those newer, higher-field, 3+ tesla systems is that it may be possible to get information with a much shorter scanning time for an anesthetized patient. "It really hasn't worked out that way," Schramme says. "Because of the risk of tissue overheating, they need to lengthen the sequences to let the tissue cool down."

The University of Minnesota College of Veterinary Medicine Leatherdale Equine Center has a 3-tesla magnet, the only such unit for horses in the country. The unit's disadvantage is that you lose the ability to do some portions of the equine limb because the bore gets smaller and smaller, and although you can still look at feet, and possibly ankles, it's hard to get up any higher than that. "I haven't looked at 3-tesla MRIs of horses, but if you ask our counterparts on the human side if they can tell the difference in their diagnostic abilities between a 1.5- and a 3-tesla unit, most of them will admit that they don't know if they can see a difference."

Conclusion

MRI is proving to be a vital tool in veterinary diagnostic testing and is giving new hope to many equine athletes. Recently, MRI was able to diagnose a problem in the famous racehorse Lava Man. Doug Herthel, DVM, Alamo Pintado Equine Medical Center, treated the horse aggressively, which enabled it to get back to racing after previously being retired. "The MRI gave us an idea of what we could do," Judy remarked. "We can be a lot more specific, more focused and a lot more directed with our therapies."

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.