Computed Tomography and Magnetic Resonance Imaging in Equine Musculoskeletal Conditions

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COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING IN EQUINE MUSCULOSKELETAL CONDITIONS Russell L. Tucker, DVM, MS, and Ronald D. Sande, PhD, DVM

There is a growing interest in the use of computed tomography (CT) and magnetic resonance (MR) imaging in equine orthopedic patients. It is well established that CT and MR imaging offer superior diagnostic information in a wide variety of musculoskeletal injuries in humans and small animals. The highly detailed cross-sectional images obtained with these two modalities can often demonstrate pathologic changes undetected with other common imaging techniques. Based on their multiple applications in humans and small animals, CT or MR imaging may prove to be the optimal diagnostic imaging technique for several types of musculoskeletal disorders of horses. Several reports have advocated the use of CT and MR imaging for evaluation of equine orthOpedic conditions.2. •. 6. 7. 9. 11 -1 •• 16-20. 26, 31 Most reports have been from studies performed on isolated equine cadaver specimens, because the use of CT and MR imaging in live horses has been constrained by numerous practical and economic deterrents. The purchase price, installation costs, and service contracts for these systems are expensive. Furthermore, certain modifications are necessary to adapt CT or MR imaging systems for scarming live horses. 10• 19 Existing systems are designed solely for imaging human patients; thus, physical limita-

From the Department of Veterinary Clinical Scien~, College of Veterina ry Medicine, Washington State University, Pullman, Washington

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tions exist when scanning live horses. Typically, in adult horses, only the distal aspects of the legs fro m the carpus/tarsus to the tip of the foot can be placed within most units. The more proximal aspects of the upper limbs and thicker body parts cannot be positioned within current gantry designs. Another major drawback with cr and MR imaging in horses is the requirement for general anesthesia. Horses must lay motionless during the scanning procedure, which can take between 30 and 120 minutes depending on the type of examination. The risk of general anesthesia and the manipulation necessary to position the horse within the scanners must be considered worthwhile. Presently, only a few veterinary institutions and large referral centers worldwide offer cr or MR imaging of live equine patients. The physical principles and technical considerations for the use of cr and MR imaging in equine patients are described elsewhere in this issue (in the article by S. Kraft and P. Gavin). Recent publications have reviewed cr and MR imaging concepts and described their use in small animal patients. I. 27 This article outlines some of the potential applications and advantages of CT and MR imaging in horses with musculoskeletal injuries. COMPUTED TOMOGRAPHY

Equine orthopedic CT examinations usually are composed of a set of several thin (1- 10 mm) slices sequentially acquired through a specific region of interest. Similar to that obtained with conventional radiography, the cr information is based on the principle of differential X-ray absorption by each tissue. Whereas only fi ve fundamental opacities (air. fat, soft tissue, bone, and metal) make up the basis of the conventional gray-scale radiographic image, the cr system is capable of detecting thousands of slight differences in X-ray absorption within tissues. The cr gray-scale images can d isplay hundreds of different shades of gray between air and solid meta l. Viewing and integration of information from a series of individual CT slices lead to a three-dimensional perception of the structures (Fig. 1). Importantly, the tomographic slices eliminate the superimposition of soft tissues and bones, a problem inherent in conventional radiography. This is extremely helpfu l in examining complex regions and joints, where overlapping bones and articular surfaces are difficult to evaluate on standard radiographs (Fig. 2). Additionally. the image data are acquired and stored as digital information, and clinicians can review images in a variety of display formats to enhance visualization of specific structures. Orthopedic examinations are commonly reviewed in a "soft tissue window" display and in a "bone window" display to highlight the soft tissues and bones, respectively. Equine orthopedic CT examinations are made up of multiple parallel slices initially acquired in transverse orientation relative to the long axis of the leg. The original data set can be digitally manipulated (reconstructed) to create new tomographic images along any desired image plane (Fig. 3). The ability to view images in alternative image

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Figure 1. Transverse 2-mm CT section (Bone window: WW 2280, WL 470) at the level of the mid navicular bone (NB) and distal second phalanx (P2) in a normal adult horse. On this tomographic image the two bones are isolated and do not superimpose. Note the excellent visualization of the medullary and cortical bone distinction within the navicular bone. The articular surface (small arrowheads) and the flexor surface (large arrow) can be critically evaluated. The bone window display illustrates poor distinction between soft tissues contained in this section.

Figure 2. Transverse 2-mm CT section (Bone window: WW 4275, WL 659) at the level of the proximal canon bones in both forelimbs of a 10-year-old Quarter Horse. The CT examination revealed a sagittal fracture (large arrow) of the medial aspect of the right fourth metacarpal bone. This fracture was not evident on standard rad iographs of this regioo. There is also active periosteal proliferation noted aloog the dorsal-lateral aspect of the third and fourth metacarpal bones (small arrows). The lett forelimb was imaged at the same time as the right forelimb and serves as a normal comparison. MC3 = third metacarpal bone; MC2 ., second metacarpal bone; MC4 = fourth metacarpal bone.

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Figure 3. Transverse 2-mm CT section (Bone window: WW 2164, WL 465) at the level of the proximal third phalanx (P3) in a 2-year-old horse with lameness localized to the coffin joint. A large subchondral lesion (arrows) was detected adjacent to the coffin joint (A). 2-D CT reconstNctions in the dorsal plane (8) and sagittal plane (C) help to demonstrate the communication of the subchondral lesion with the coffin joint (arrows). P2 _ second phalanx; P3 = third phalanx; N8 _ navicular bone.

planes may better delineate fracture orientation and assist in surgical planning. The surface contours of bones and joints can also be illustrated with three-dimensional software programs. The reconstruction images impart a perception of depth and volume to structures, which is useful for student and client education (Fig. 4). On the downside, reconstruction images always suffer from some loss of detail, and the reconstruction process can be time consuming. CT is exceptionally useful for evaluation of bone and, to a lesser degree, for evaluation of soft tissues. The tomographic information enables critical evaluation of bones and joints.s, 16. 26 CT imaging can reveal important characteristics about osseous lesions. For example, the proximity of a fracture to an articular surface can be precisely defined with CT (Fig. 5). Osteolysis and osteogenesis can be detected before any changes are perceived on conventional radiographs. CT has been helpful in identifying radiographically occult fractures of the coffin bone16 and in detecting subchondral erosions of the metacarpal and metatarsal bones. s CT imaging can also illustrate subtle internal and external osse-

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Figure 4. Three-dimensional surface reconst ruction image of the proximal articular surface of the third phalanx from the same horse in Figure 3A-C. The large subchondral lesion is clearfy delineated invading the articular surface (arrow). Note how the recoostruction image imparts the perception of surface topography to the proximal articular surface and palmar processes of the third phalanx.

Figure 5. Transverse 2-mm CT section (Bone window: WW 2045, Wl392) 01 both forelimbs at the level of the distal rad ius and proximal accessory carpal bones 01 a 4-year-old Thoroughbred racehorse that was reported to be lame 10f the previous 24 hours. There is an articular fracture of the right accessory carpal bone (black arrow) surrounded by multiple small osseous fragments. The fracture edges are fuzzy and indistinct, signifying that the accessory fracture is not an acute injury. There is also a proliferative periosteal fesponse along the dorsal-lateral cortex of the radius (small arrowheads) supporting a prolonged nature to the injury. Soft tissue swelling is present within the palmar aspect of the carpus (large arrowheads). The left carpus was imaged at the same time as the right carpus and serves as a normal comparison. R = radius ; AC - accessory carpal bone.

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remodeling and reactions. In specific sites, remodeling changes noted on CT are indicative of pathologic changes and impending fractures. n. 23, 2.8. JI Significant changes in bone density can be documented on CT systems using computer analysis software. Studies of equine bone response to implants, stresses, and conditioning regimens have used quantitative CT to measure osseous remodeling.5. 15. 22. 24 Soft tissues and fluids can be distinguished with CT better than with radiography. The tomographic slices eliminate superimposition inherent to standard radiographs. Individual muscles, tendons, and ligaments can be identified. Abnormal fluid accumulations and dystrophic mineralization can be recognized within most soft tissues; however, precise definition of soft tissues having similar density remains difficult with CT. OUS

MAGNETIC RESONANCE IMAGING

MR imaging offers an exciting and novel imaging modality for equine orthopedic patients. MR imaging is accomplished using the magnetic properties of tissues and does not rely on the X-ray attenuation used in radiographic studies. In MR imaging, the limbs of horses are positioned within a strong magnetic field generated by the imaging gantry and are subjected to perturbing radiofrequency pulses. A unique radiofrequency signal, based on each tissue's magnetic characteristics, is emitted in response to the perturbing pulses, and these radiofrequency signals are collected to form the image. As a diagnostic imaging modality, MR imaging yields unparalleled tissue contrast and anatomic definition. MR imaging has several proposed advantages for imaging equine orthopedic diseases. 2. ~ · 7.9--1 4. 17_19. 28-31 MR imaging demonstrates excellent ana tomic and physiologic detail in osseous and soft tissue structures. Similar to CT, MR imaging yields sets of tomographic slices (usually 1-10 mm in thickness); however, a distinct MR imaging feature is the ability to acquire images in any anatomic plane. MR imaging orthopedic examinations are commonly acquired in at least three perpendicular imaging p lanes. Additional image planes may also be selected to better evaluate specific structures or areas of concern. As is the case with CT, the original MR imaging data can be reconstructed to create alternative two- or three-dimensional images. MR imaging of orthopedic d iseases is routinely performed in several different acquisition sequences. Each sequence displays slightly different anatomic and physiologic information. A conventional MR imaging protocol called spin-echo imaging usually includes these sequences: Tl-weighted, proton density, and TI-weighted images. Simplified, Tl weighted images are based on the magnetic properties of spinning protons responding to the influence of the external magnetic field. The Tl-weighted images highlight the structural characteristics of tissue and are most useful for the evaluation of bone and some soft tissues. Tl-

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weighted imaging is also used. for visualization of the MR imaging paramagnetic contrast agents. Proton density images are based. on the relative free proton concentrations of each tissue and offer excellent tissue contrast. Proton density images are useful for the evaluation of bones, ligaments, tendons, and articular cartilage. TI-weighted images are based. on the interactions of spinning protons and emphasize the fluid characteristics of tissue. T2-weighted images are quite sensitive to fluid and are thus helpful in detecting certain pathologic changes. T2weighted images are useful for displaying areas of inflammation or edema within injured tissues. Several additional imaging sequences are useful in orthopedic imaging. These include sequences such as fat suppression, gradient echo, or magnetic transfer contrast techniques. Additional sequences are selected to enhance or eliminate visualization of specific tissues. For instance, fat suppression sequences are frequently used. to eliminate the high fat signal within normal bone marrow so as to better visualize the presence of bone edema . Subchondral bone edema is an important diagnostic feature of several articular injuries that may go undetected if the fat suppression sequences are not obtained .II Gradient echo imaging allows for extremely thin tissue slices and volume acquisitions. Volume acquisitions are helpful when complex and multiplanar reconstructions a re necessary.) Magnetic transfer contrast imaging may enhance contrast between adjacent tissues such as the articular surface and synovia l fluid within joints. Abnormal cartilage may be more clearly visualized in the presence of joint effusion, allowing the detection of chondral fractures and fissures as well as all types of osteochondral fractu res. 3 Magnetic field strength is measured in Tesla, and current MR imaging systems used in clinical veterinary medicine range from low-field strengths (~O .064 n to midfield strength (0.1-0.5 T) to high-field strengths (2::1.5 n. The higher field strength magnets are capable of faste r scanning times and have better signal-ta-noise image quality. Unfortunately, high-field strength systems equipped with superconducting magnets are expensive to purchase and maintain . Low- and midfield strength systems use permanent magnets, which are less expensive to purchase and maintain. Newer MR imaging configurations are more open and allow easier positioning of equine patients. Future MR imaging designs targeted specifically at equine orthopedic applications may allow imaging of standing horses. Such technology is not yet available, however, and may have several practical limitations. Ferromagnetic metals are a serious hazard around MR imaging systems. Most metal horseshoes and nails need to be completely removed before imaging. Even small nail fragments remaining in the hoof wall cause magnetic distortions and degrade the imaging of the surrounding tissues. Nonferrous surgical implants (e.g., screws, plates, pins) can be safely introduced into the magnetic fie ld but still create imaging artifacts. Equine orthopedic MR imaging is valuable for evaluating osseous lesions. Cortical bone is normally hypointense (dark) on all imaging

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Figure 6. MR transverse proton density section at the level of the mid navicular bone in a normal adult horse. The medullary bone is hyperintense (white area) and the cortical bone is hypointense (dark area) within the navicular bone and the second phalanx. Note the distinctive contour of the flexor cortex of the normal navicular bone. The synovial fluid (black arrow) and the navicular bursa (white arrow) is hyperintense on this sequence. The hypointense deep digital flexor tendon is bi-lobed as it passes over the flexor surface of the navicular bone. P2 "" second phalanx; NB "" navicular bone; DDF = deep digital flexor tendon.

sequences. Medullary bone has a hyperintense (bright) signal on most sequences because of the high fat content of bone marrow (Fig. 6). Osseous lesions, which cause subchondral sclerosis and trabecular remodeling, create hypointense zones within the normally hyperintense cancellous bone (Fig. 7). MR imaging is also quite sensitive for the

Figure 7. MR transverse proton density section at the level of the mid navicular bone of an 8-year-old Quarter Horse. This horse had lameness with pain localized to the heel bulb area that greatly diminished with a regional palmar digital nerve block. There is advanced navicular bone degeneration within the medullary cavity and along the flexor cortex (arrow). There is loss of visualization of th e central navicular bursa. and the deep digital flexor tendon is in direct contact with the flexor cortex. Contrast this image to the appearance of this region in the normal horse in Figure 6.

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Figure 8. MR sagittal proton density section at the level of the pastern and coNin joints in a 3·year·old horse with lameness associated with the pastern. Regional anesthesia 01 the pastern joint completely abolished the lameness. Conventional radiographs were negative for any abnormalities of the distal limb. A large circular subchondral lesion is demonstrated with the MR image (large arrow). The hyperintense cystic lesion contains smaller hypointense Circular and linear signals. There is a zone of sclerosis surrounding Ihe hyperinlense defect (small arrows). The articular cartilage overlying the subchondral lesion also has a local defect (arrowheads). Osteochondrosis was confirmed on arthroscopy. P1 ,. first phalanx; P2 = second phalanx; P3 = third phalanx.

detection of osteochondrosis, subchondral bone necrosis, and osteomyelitis (Fig. 8). Most inflammatory lesions of bone have increased fluid signal on TI-weighted sequences. NondispJaced or stress fractures can be detected on MR imaging by the disruption of the normal bone signal. Fat-suppressed sequences can be used to demonstrate abnormal inflammatory reactions and marrow edema associated with such fractures. The osseous remodeling associated with navicular bone degeneration has been clearly demonstrated with MR imaging. 28 • 31 The abnormal signal from navicular lesions contrasts remarkably with the normal bone pattern. The increased number and abnormal shapes of the synovial invaginations along the distal navicular margin are evident on MR imaging sequences even before abnormalities can be recognized on radiographic studies (Fig. 9). Early detection of such lesions may favorably influence treatment decisions. One of the most useful applications of MR imaging is the ability to evaluate soft tissues of the equine limb. Ligaments and tendons are well defined and are normally hypointense on all imaging sequences (Fig. 10). With MR imaging, it is possible to evaluate the size and contour of tendons and ligaments as well as their origin and insertion sites. Evaluation of difficult structures such as the distal sesamoid and impar liga-

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Figure 9. MR (magnetic transfer contrast) images of sagittal (A) and dorsal (8) sections of the foot of a 6-year-old horse with navicular degeneration. The horse had symptoms of navicular disease but radiographs were inconclusive for the detection of navicular bone changes. On the MR image there is definitive enlargement and remodeling of the synovial invaginations along the distal navicular margin (arrows). The navicular bursa is incompletely visualized, and the deep digital flexor tendon appears adhered to the flexor surlace of the navicular bone (arrowhead). MR image findings were confirmed on necropsy. PI = firsl phalanx; P2 = second phalanx; P3 = third phalanx; NB _ navicular bone.

Figure 10. MR transverse proton density section at the level of the proximal row of carpal bones in a normal adult horse. The superficial digital flexor (SOF) and deep digilal flexor (OOF) are clearly visualized within the carpal canal in this section. It is also possible to identify the extensor carpi radialis (ECR) and long digital extensor (lDE) tendons, and the medial (MCL) and lateral (LCL) collateral ligaments. Note the fine linear Interosseous ligaments between the radial , intermediate, and ulnar carpal bones (arrows). RC = radial carpal bone; IC = intermediate carpal bone; UC '" ulnar carpal bone; AC .. accessory carpal bone.

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ments is possible with MR imaging. Injuries of tendons and ligaments are typically characterized by increased signal within and around the h ypointense structures. Adhesions and focal necrosis of the deep digital flexor tendon adjacent to the navicular bone have been identified using MR imaging. 28• 31 The muscles and fa scia of the distal limb can also be evaluated; however, potential applications have yet to be explored in horses. Muscle strains and tears are commonly detected in human patients, and similar findings are anticipated in equine patients. One promising application of MR imaging is the early detection of articular cartilage destruction. The recognition of initial cartilage pathologic changes would be a tremendous diagnostic advantage of MR imaging over other more invasive imaging modalities. The optimal imaging sequences for articular cartilage evaluation are still under investigation.3. 21, 30 Development of quantitative MR imaging assessment techniques, including measurement of magnetic transfer, changes in signal intensity, and physical diffusion parameters, may offer methods to detect early stages of cartilage abnormalities in the future. 21 It is possible to improve visualization of the articular cartilage and synovial structures using MR imaging contrast arthrography. In one method, sterile saline is infused into joints. The expanded synovial fluid volume may better delineate synovial proliferation and intra-articular filling defects. Unfortunately, the articular cartilage remains difficult to differentiate from the synovial fluid . Alternatively, MR paramagnetic intravenous contrast agents can be used for arthrography. The contrast mixes with the synovial fluid and is well visualized using Tl-weighted sequences (Fig. 11). Gadolinium, a common MR imaging contrast agent, can be diluted with sterile saline (1:25{}-1:500) and injected into joints. Contrast arthrography with diluted gadolinium yields better d efinition of the articular surfaces and may reveal cartilage fissures or defec ts. If desired, it is also possible to combine intravenous iodinated radiographic contrast agents with the diluted gadolinium solution to allow radiographic arthrography and MR imaging arthrography to be performed simultaneously.25 There are several important limitations to the use of MR imaging in equine patients. The requirement of general anesthesia and the time required for scanning must be considered an acceptable risk. The cost of the purchase and maintenance of the systems currently limits availability of live equine MR imaging to only a few referral centers and teaching institutions. MR imaging examinations result in large data sets of serial images from multiple sequences. Each image set must be carefully reviewed and compared. Interpretation of MR imaging examinations requires detailed knowledge of the regional anatomy and an understanding of pathologic mechanisms. Optimal imaging sequences and parameters still need to be determined for equine orthopedic applications. With an increase in the accessibility to MR imaging systems, however, the technology is likely to become an integral part of d iagnostiC imaging in equine orthopedic patients.

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Figure 11. MA transverse fat-suppressed n-weighted image at the level of the mid navicular bone following arthrography of the coHin joint. The medullary bone is intermediate In signal (gray area) and the cortical bone remains hypointense (dark area) because of Ihe fat-suppression sequence. Note the single enlarged synovial fossa within the central navicular bone (while arrow). The contrast-enhanced synovial fluid within the dorsal and palmar recess of the coHln }oInt Is hyperintense on this sequence (black arrows). The hypointense deep digital flexor tendon is normal as it passes over the flell.OI" surface of the navicular bone. An enemal contrast-filled marker has been placed along the dorsal hoof wall (arrowhead). P2 - second phalanx; NB - navicular bone; OOF - deep digital nexor len-

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Address "print requests to Russell L. Tucker, DVM, MS Veterinary Clinical Sciences College of Veterinary Medicine Washington State University Pullman, WA 99164-6610