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Radiology and Scintigraphy
BACK PROBLEMS
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RADIOLOGY AND SCINTIGRAPHY Martin P. Weaver, BVMS, DVM, DVR, MRCVS, Leo B. Jeffcott, MA, BVetMed, PhD, FRCVS, DVSc, and Michael Nowak, DVM
Because of the inaccessibility of many of the structures in the spine that may be affected, ancillary diagnostic aids such as imaging are particularly important in the work-up of a horse with a suspected back problem. Radio graphic examination of the equine back has been used for several decades,6· 7 but the quality of the images that can be obtained has recently improved with the introduction of modern fast-screen film systems. Scintigraphic examination for assessing equine orthopedic conditions, including back problems, was first described 20 years ago. 19 These two imaging techniques complement each other. Although good quality radiographs give reasonable anatomical resolution of the thoracolumbar structures, some of the abnormalities that are detected may not reflect a current problem. The clinical signs associated with impinged spinous processes, for example, may resolve, but the radiographic changes usually remain. Scintigra phy portrays the metabolism of the different skeletal structures in the back. It is highly sensitive but has poor intrinsic anatomical resolution and is nonspecific. Radiographic examination of scintigraphically abnormal regions is usually re quired before a definitive diagnosis can be made. Scintigraphy is also useful as a screening tool. Not only can areas such as the caudal lumbar spine and sacroiliac regions be imaged in the standing horse; the bones of both hind limbs can also be imaged. This is important in many cases of suspected back pain which have a complex etiology, including low-grade hind limb lameness.
From Large Animal Surgery, Faculty of Veterinary Medicine, University College Dublin, Ballsbridge, Dublin, Ireland (MPW); Cambridge Veterinary School, University of Cambridge, Cambridge, England (LBJ); and Orthopaedic Department, Tierklinik Hochmoor, Aescher, Germany (MN)
V ETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 15 • NUMBER 1 • APRIL 1999
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RADIOLOGY Radiographic Techniques Standing Lateral thoracic and cranial lumbar radiographs of adequate quality can be obtained with the horse standing if care is taken with the technique. Low power portable machines can be used for radiographing the tips of the spinous processes, but if views of the bases of the processes, vertebral bodies, rib articulations, or articular facets are required, a machine capable of an output of up to 150 kV(p) and 500 mA is required. It is convenient to have the machine linked to a mobile stand with a cassette holder so that the beam is automatically aligned with the center of the cassette. Otherwise, the cassette should be mounted on a fixed stand or to a wall; it should never be handheld. The largest available cassette size (usually 35 X 43 cm or 14 X 17 in) reduces the number of views required for a full thoracolumbar examination, and it should be oriented perpendicular to the ground. At the high exposures used for the thoracolumbar spine, radiation scatter is an important factor which significantly reduces image quality. Several measures can help to reduce the effect of scatter. Rare-earth film/screen systems allow exposure factors to be reduced. A grid (crosshatched parallel grid with a ratio of 12:1) is necessary for all views, except for radiographic views of the spinous processes. Most radiographic film cassettes contain lead in the backing, but at exposures over 100 kV(p), it is advisable to place an additional lead sheet behind the cassette to absorb back scatter. As the cassette is oriented perpendicular to the ground with a film-focus distance of 150 cm, an air gap exists between the spine and the cassette, and this reduces the amount of scatter reaching the film. There is a considerable difference in tissue attenuation between the tips of the spinous processes and the vertebral bodies; thus, at least two radiographs of the same region at different exposures must be taken or some form of beam filtration is necessary. A "Dodger" beam filter using aluminum wedges has been de scribed,6 but this equipment is no longer commercially available. A similar appliance can be custom-made and attached to the front of the X-ray tube to allow a high exposure to be set for the deeper structures of the spine but filtered to the spinous processes. If no filtration is used, close collimation to the region of interest reduces scatter and enhances image quality. Exposure values vary depending on the system (Table 1). The horse should be sedated prior to radiography to reduce the degree of Table 1. TYPICAL RADIOGRAPHIC EXPOSURES TO DIFFERENT PARTS OF THE THORACOLUMBAR SPINE* View
Withers (T3-T8) spinous processes Midthoracic (T9-T15) spinous processes Thoracolumbar (Tl6, L3-L4) spinous processes Midthoracic articular processes and lumbar vertebral bodies
Kilovolt (peak)
Milliamperes
Grid
75 70 85
30 55 80
No Yes Yes
100 +
200-300
Yes
*Dupont Ultravision Rapid screens (Dupont UK, Stevenage, Herts, UK) were used in this study.
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movement and to allow the handler to stand well out of range of the beam. Stocks are also useful to prevent movement. Lead markers placed midline on the horse's back are useful for orientation on the radiographs; the marker sites can be lightly clipped to allow them to be found again later, for example, if local anesthetic is to be injected. As part of the clinical workup for suspected impinged spinous processes, the horse must be standing completely square, as the spine rotates if one leg is rested. For the apices/summit's spinous processes, the beam should be centered 5 cm ventral to the dorsal skin surface; for the region of the vertebral bodies, it should be centered 15 to 20 cm below the dorsum depending on the size of the horse. The beam should normally be horizontal, but for visualization of the dorsal articular facets in the cranial lumbar region, it is useful to angle the beam from a ventral to dorsal orientation at an angle of 20° to 30° to avoid superimposition by the transverse processes. As a grid is being used, the cassette must be tilted accurately at the same angle of 20° to 30°. Recumbent Radiography with the horse under general anesthesia is required in order to achieve optimum lateral views of the caudal lumbar spine (L3-L5), ventrodor sal views (apart from those in foals), and views of the sacroiliac joints. Prior scintigraphy is useful to localize areas to be radiographed and thus reduce the anesthesia time. In some large horses, the longer exposures required mean that recumbency is preferred for the caudal thoracic or cranial lumbar spine in order to obtain films of diagnostic quality. The horse is anesthetized and positioned in lateral recumbency. Ideally, a hoist should be available to rotate the animal into dorsal recumbency if ventrodorsal views are also required. Care must be taken in positioning to prevent axial rotation; for lateral views, this usually necessitates placing supports under the upper forelimbs and hind limbs to ensure that they are parallel with the trunk. With the horse recumbent and immobile, exposures of up to 125 kV(p) and 300 mA can be used for the lumbar spine. For ventrodorsal views, the horse should be placed in the "frog-legged" position, with the hind limbs flexed and abducted. Again, tilting should be prevented to avoid rotation of the spine. For views of the sacroiliac joint, a 35 X 43-cm cassette should be placed under the horse and centered just cranial to the tubera sacrale. Exposures of 130 kV(p) and 330 mA can be used for the sacroiliac joint and sacrum. Intestinal contents may be superimposed on these structures.
Normal Radiological Findings Foals have a slightly kyphotic midthoracic spine that becomes level by the time they reach adulthood. The thoracolumbar vertebral bodies have cranial and caudal physes that close radiographically by the time horses are 1 year and 4 years old, respectively. Separate centers of ossification begin to develop at the apices of the spinous processes in the withers from T2 to T8 when the horse is about 1 year of age; from then on, the tips retain a mottled irregular appearance. These features should not be mistaken for fractures or osteolysis. The spinous process of Tl has no separate center of ossification and that of T6 often has an irregular cranial surface.
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Certain anatomical landmarks help in orientation when evaluating radio graphs of the thoracolumbar spine: • The tallest dorsal spinous process is usually TS or T6. • The cranial thoracic spinous processes are angled slightly caudad and the caudal thoracic spinous processes are angled slightly craniad. The anticli nal spinous process in most horses is TIS or Tl6. • On lateral radiographs, the caudad sloping diaphragm crosses the verte bral body of TIS or T16 during expiration or midinspiration. • The interspinous spaces are usually wider when the horse is radiographed under general anesthesia. In mature horses, the intervertebral disc space is narrow, and the caudal extremities of the vertebral body are relatively sclerotic. The ribs articulate dorsal to the vertebral body articulations. The orientation of the articular facet surfaces changes from almost horizontal in the cranial thoracic spine to near vertical in the caudal thoracic vertebrae. The tip of the third sacral spinous process (S3) may have a bifid appearance with an irregular outline. Many clinically normal horses have spinous processes whose apices are in close apposition (i.e., less than 2 mm apart), particularly in the vertebral region caudal to the anticlinal vertebra. This should not be misinterpreted as indicating significant pathology. In one survey of clinically normal horses, 34% showed some degree of spinous process impingement.7 Many clinically normal horses have a spur of new bone (i.e., enthesophyte) on the dorsal apex of T3 or T4 and also occasionally in the caudal thoracic spine. Radiological Findings in Horses with Back Problems
Congenital defects are occasionally seen in foals or as incidental findings in older horses. These include lordosis, kyphosis, scoliosis, hemivertebrae, and congenital fusion of the thoracolumbar vertebral bodies or processes. Impinged spinous processes ("kissing spines") range from spinous processes that are close at their summits to marked overlapping of the processes with advanced secondary bone changes. The vertebral sites most frequently affected are under the saddle region (Tl2-T17). In mildly affected cases, all that is seen is a narrow interspinous space, usually with sclerosis of the adjacent spinous process edges (Fig. 1). The nar rowing usually occurs at the summits but may rarely affect the lower portions of the spinous processes (Fig. 2). Radiographs of severe spinous process impinge ment show remodelling of the cranial and caudal edges, marked subcortical sclerosis, and cyst-like lucencies within the subcortical bone (Fig. 3). Spinous processes may also develop prominent rounded or pointed "beaks" of new bone. One or more interspinous spaces may be affected. Following surgery for impinged spinous processes, some horses may develop a fairly dramatic amount of irregular new bone formation locally, and this may cause a recurrence of clinical signs. Isolated periosteal reactions (i.e., enthesophytes) of bone fragments along the cranial or caudal edge of a spinous process are considered to be a consequence of interspinous ligament strain or injury (Fig. 4A and B). Osteoarthritis of the articular processes (i.e., zygapophyseal joints) is diffi cult to assess unless the radiographs are of excellent quality. The normal oblique radiolucent line of the articulation may become obscured by irregular new bone, which can become partly superimposed over the bases of the spinous processes (Fig. SA). Subchondral lucency may also occur giving the appearance of widened zygapophyseal joint spaces (Fig. SB). These changes are not commonly seen and
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Figure 1. Adult Warmblood. Impingement of midthoracic dorsal spinous processes (DSP). The cranial and caudal edges of the spinous processes are sclerosed dorsally (arrowheads) and have ill-defined zones of subcortical lucency ventrally (arrows).
- Figure 2. Adult Warmblood. Narrowing of second thoracic interspinous space, most pro nounced in the ventral third (small arrows). Sclerotic line superimposed over T2 (large arrow) is scapular spine. An aluminum wedge filter has been used over the apices of the spinous processes.
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Figure 3. Twelve-year-old Thoroughbred eventer with severe chronic back pain. Post mortem radiograph of DSPs T13-T17. Impingement with severe remodelling is present at T17-T18 (arrow). The other interspinous spaces are narrowed and the DSPs show subcorti cal sclerosis with lucencies in the impinging regions. See also Figure 12.
usually occur in the caudal thoracic and lumbar regions. They are usually clinically significant. The supraspinous ligament attaches to the apices of the spinous processes from the withers to the caudal lumbar spine. Changes are occasionally seen affecting only the apices of the spinous processes just caudal to the withers (T8-T12), and these are thought to indicate damage to the supraspinous ligament (Fig. 6). The changes may represent an enthesophyte or a flattened area of bone which appears to have been lifted from the apex of the spinous process (i.e., avulsion fracture).' A radiograph taken at lower exposure may show focal thickening of the ligament, but this is best evaluated with ultrasonography. Spondylosis (spondylosis deformans) in horses is uncommon, but it is regarded as a common aging process in dogs. The condition may occasionally be encountered as an incidental finding during radiographic examination of the equine thorax. In mildly affected horses, osteophytes develop on the ventral surface of two or more vertebral bodies close to the intervertebral disk. In some horses, the osteophytes fuse to form a complete osseous bridge on the ventrolateral vertebral bodies. Occasionally, an irregular vertical lucent line transverses the new bone formation adjacent to the intervertebral disk space (Fig. 7). These changes are only seen in the midthoracic to caudal thoracic spine and may affect up to five or six vertebral bodies. Vertebral body fractures are usually catastrophic, and the horse is often destroyed on humane grounds without radiography being attempted. Some fractures may result in only minimal fracture displacement, however, and the main clinical signs can be chronic back pain and hind limb ataxia. The most common sites for fractures are the first three thoracic vertebrae (Tl-T3), T9 to Tl6, and the lumbar vertebrae (Ll-L6). It may not be possible to assess the exact fragment alignment radiographically, but associated radiological signs include ventral displacement or apparent shortening of a spinous process, poor defini tion of the zygapophyseal joints due to callus formation (not to be confused with degeneration of these joints), and alteration in the normally evenly aligned vertebral bodies. The most common site for vertebral fractures is the spinous processes of the withers (Tl-T12), usually due to the horse rearing up and falling over backwards. Radiography is indicated to assess the number of spi-
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A
B Figure 4. A, Adult Warmblood. Radiograph of caudal thoracic DSPs. Flakes of bone (arrows) with associated irregularity in cranial borders of DSPs, indicating damage to the interspinous ligaments. An aluminum sheet has been used as a filter. B, Adult Warmblood dressage horse. Radiograph of caudal thoracic DSPs. New bone formation (arrows) on the caudal aspect of T17 secondary to interspinous ligament trauma.
nous processes affected and the extent of fragment displacement, as these may occasionally require surgical removal. Osteomyelitis of the vertebral bodies, although a rare condition, is seen most commonly in foals and young horses, and the spine is usually only one of several affected sites. Changes that have been described include proliferation of new bone around the vertebral body, soft tissue swelling ventral to the vertebral column, well-defined zones of radiolucency, and compressive pathological frac tures.11 Osteomyelitis of the spinous processes (fistulous withers) occurs most frequently at T4 to T7. Although the infection primarily involves the nuchal
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Figure 5. A, Nine-year-old Thoroughbred. Radiograph of caudal thoracic vertebral region. Degenerative joint disease of the zygapophyseal joints. There is subchondral sclerosis at two joints (arrows). B, Fourteen-year-old Warmblood. Radiograph of caudal thoracic verte bral region. Degenerative joint disease of the zygopophyseal joints. Subchondral lucency of the caudal articular surface results in a radiographically widened joint space (arrows).
ligament, supraspinous bursa, and other adjacent soft tissues, chronic cases have radiographic changes at the apices of the affected spinous processes. Irregular areas of radiolucency are present distal to the centers of ossification, and occa sionally, new bone formation is seen on the cranial or caudal surfaces. Radio graphs can be obtained with a metal probe in place to assess the depth and location of the fistulous tract(s). Damage to the sacroiliac joint is difficult to assess radiographically; even
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Figure 6. Fourteen-year-old Thoroughbred eventer with chronic back pain, radiograph of midthoracic vertebral region. Spondylosis. Smooth bone is present on the ventral surface of the vertebral bodies. There are two irregular lucent lines, one ventral to the intervertebral body articulation (T1 O-T11 ), the other ventral to T11 (arrows). See also Figure 11.
with the horse anesthetized and in dorsal recumbency, the quality of the resul tant radiographs is frequently poor. Additional factors reducing the radiographic resolution of the joints are that they are angled at about 30° to the horizontal and that ingesta may be superimposed over the area of interest. The exposure should be made during inspiration; this results in blurring of the overlying soft tissues. Radiographic features of chronic sacroiliac disease are smooth new bone
Figure 7. Eight-year-old Thoroughbred racehorse, low-exposure radiograph of midthoracic DSPs. Supraspinous ligament trauma. Apex of T1 O (arrow) shows periosteal irregularity suggestive of supraspinous ligament trauma. Ultrasound examination of the ligament con firmed the diagnosis.
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on the caudal aspect of the sacroiliac joint or joint space widening. 8• 9 Neverthe less, many cases with clinical signs of sacroiliac disease do not show detectable radiographic changes. SCINTIGRAPHY Technique for Nuclear Medicine Scanning The radiopharmaceutical most commonly used for equine scintigraphy is 99m technetium-labelled methylene diphosphonate(99mTc-MDP). The procedures required for bone scanning in horses have been extensively described. 2• 10• 17• 19 A weight-dependent dose between 3 and 6 GBq (100-150 mCi) of 99mTc-MDP is injected intravenously, and images of the bone phase are obtained 2 to 3 hours later. Due to the initial high renal and bladder radioactivity, vascular phase and soft tissue phase images of the thoracolumbar spine cannot be obtained. Dam aged skeletal muscle can be detected by abnormal soft tissue uptake of 99mTc MDP during bone phase imaging but usually only if the horse has been strenu ously exercised before the scintigraphic examination. 12 Overlapping views of the entire thoracolumbar spine are obtained from the caudal cervical spine caudad; this requires a total of three to five views de pending on the gamma camera field of view. A high-resolution, low-energy, parallel-hole collimator should be used; this gives a geometrical resolution of 4.4 mm at a distance of 5 cm from the collimator face. 15 For a complete examination, the horse should be imaged from both the left and the right; otherwise, some subtle lesions, for example, those located at the zygapophyseal joints, can easily be missed. For the caudal thoracic and lumbar regions, the camera is positioned obliquely at an angle of 50° to 65° to the horizontal. This allows the distance between the camera and spine to be mini mized. Positioning the camera horizontally above the spine results in superimpo sition of all the vertebral structures. The camera can be oriented perpendicular to the floor for views of the cranial thoracic spine. When the camera is positioned over the lumbar spine, the image quality may be improved by shielding the kidney using a lead sheet. The images are acquired on the basis of either counts or time. One million counts or more are optimal for maximal resolution of the caudal regions of the equine back and the pelvis. 20 This large number of counts requires several minutes of immobility on the part of the horse, however, and thus generally cannot be achieved without general anesthesia. The clinician must weigh the advantages of scintigraphic examination under general anesthesia (improved image resolution and better lesion detection) against the cost, time, effort, risk of complications, and increased personnel exposure to radiation that is involved with general anesthesia. Movement of the standing horse can be minimized by appropriate sedation and quiet handling of both the horse and camera. Blinkers and cotton ear plugs for the horse may help. Recently, nuclear medicine software that is able to correct for motion has become available, and this compensates for the slight swaying of the horse and respiratory movements. The authors always scan both hind limbs first prior to the back and pelvis. If the images are acquired on the basis of time (e.g., 2 minutes per image in the standing animal), the images or regions of interest can be directly compared. If counts are used, image manipulation using appropriate software is required for direct comparison between images. With high-quality images, it is possible to create regions of
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interest and to quantitate differences in uptake by comparison to a reference site such as a rib (Gottlieb Ueltschi, personal communication 1992). Scintigraphic Findings In normal horses, regions of bone close to the skin surface such as the tuber coxae and tuber sacrale normally appear "hotter" (i.e., appearance of increased radiopharmaceutical uptake) than adjacent bone structures, as there is less soft tissue attenuation. Other superficial sites that normally appear slightly "hot" are the apices of the spinous processes, particularly at the withers (Fig. 8). In good quality images, the individual vertebra can be clearly distinguished. The spinous processes and articulations have a slightly higher uptake, but this should be equal for all joints in the region. On lateral oblique views, each rib is located cranial to its respective vertebra. As an aid to orientation, on left lateral views, the 16th rib usually overlies the cranial pole of the right kidney (Gottlieb Ueltschi, personal communication 1992). Any scintigraphic abnormality appears as increased radiopharmaceutical uptake (a "hot spot"), whereas areas of diminished uptake are difficult to detect due to poor image resolution. The most common sites for increased uptake are at the apices of the spinous processes (Fig. 9). Similar sites to those seen on radiography are affected, although the incidence of abnormal findings is less with scintigraphy than with radiography (M.P. Weaver and L.B. Jeffcott, unpub-
Figure 8. Seven-year-old Thoroughbred. Scintigraphic view (motion-corrected) of normal caudal thoracic vertebral region. Cranial is to the right. Cranial pole of the right kidney is seen caudoventrally (K). The DSPs (D), vertebral bodies (V), and ribs can be identified. Cranially, the spinous process apicies appear brighter as there is less soft tissue attenua tion.
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Figure 9. Nine-year-old showjumper with chronic, low-grade back pain. Scintigraph (motion corrected} of midthoracic vertebral region. An intense uptake is present at the tip of DSP T1O. There was pain on palpation of this site. Radiographic and ultrasonographic examinations were normal.
lished data 1994). In a review of horses with a history of back pain scanned by the authors (M.P.W. and L.B.J.) about 65% had increased uptake of varying intensity in the back or the pelvis. This included 35% with changes at the spinous processes, 15% at the articular processes, and 20% with changes at the level of the vertebral bodies. Other workers have described a much higher incidence of increased activity at the spinous processes: 18 48 of 50 horses scanned had spinous process uptake and only two of 50 showed vertebral body in creased uptake. Focal or diffuse increases in uptake have been described abaxial to the tubera sacrale on dorsal views and attributed to sacroiliac joint disease.17 Al though the sacroiliac joint is ventral to the ilium at this site, the ilial wing between the sacroiliac joint and the tuber sacrale is a common site for stress remodelling or fracture. 14 These lesions can be confirmed by ultrasonography (Fig. lOA and B) or dorsal oblique scintigraphy views, where the camera is placed midway between the tuber sacrale and tuber coxae of the ilium being imaged. 4 It would seem unlikely that low-grade sacroiliac joint pathology in the adult horse would result in detectable increased bone metabolism. New bone on the ventral surface of the vertebral bodies (spondylosis) usually has the same level of uptake as the adjacent vertebra, but can occasion ally be considerably more active (Fig. 11). Lesions such as vertebral fractures or osteomyelitis result in marked in creases in uptake, but the final diagnosis requires radiographic examination to distinguish between several possible causes. Although the bone scan is effective
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Figure 1 0. A, Ten-year-old polo pony with acute left hind lameness. Dorsal view of lumbar vertebral region and cranial pelvis. Cranial is to the left. There is moderate bladder activity (small arrow), normal lumbar spine (LS) uptake, and an area of intense uptake at the left tuber sacrale (large arrow) compared to the right. A fractured ilial wing was confirmed on ultrasonography. B, Three-year-old normal Thoroughbred. Dorsal view of pelvis (motion corrected), similar orientation to A. Moderate bladder activity (arrow). Note symmetrical appearance of both iliae.
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Figure 11. Same horse as Figure 6. Lateral oblique scintigraph of midthoracic vertebral region; cranial to left. There is a focal hot spot associated with the ventral aspect of the vertebral bodies at approximately T10 (arrow), corresponding to the site of spondylosis on the radiograph (see Fig. 6). As this smooth new bone formation does not normally result in hot spots, the more caudal lucent line on the radiograph in Figure 6 was thought to be a fracture.
in detecting fractures, it should be noted that acute lesions of the axial skeleton in humans may take up to 10 days to become scintigraphically apparent. 1 6 CORRELATION OF IMAGING TECHNIQU ES, CLINICAL FINDINGS, AND PROGNOSIS
In studies of human patients with back pain who have been imaged using both radiography and scintigraphy, a relatively low correlation between the two modalities has been reported/' 5 with only about half the findings of one tech nique corresponding with those of the other. This is not surprising, as metabolic bone activity (scintigraphy) often does not mirror anatomical changes (radiogra phy). Scintigraphy is used in human orthopedics to assess the significance of radiographic findings. 1 3 In a series of 65 horses imaged due to signs of back pain, only 19 had findings confirmed by both techniques. Sixteen showed only scintigraphic changes, although radiographic changes were not matched by scintigraphic abnor malities in 36 animals (M.P. Weaver and L.B. Jeffcott, unpublished data 1994). Horses with a history of back pain but no current problems showed a similar incidence of radiographic changes compared to those with current evidence of back pain. In contrast, the number of scintigraphic abnormalities was consider ably lower in "normal" horses compared to those with back pain. In other
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Figure 1 2. Same horse as in Figure 3. Lateral oblique view scintigraph of the thoracolumbar vertebral region, with horse anesthetized. Caudal (CD) is right. Diffuse activity from the left kidney is seen ventral to L 1, partly superimposed by the last rib. The site of DSP impinge ment seen on the radiograph corresponds to a pronounced increased uptake (white arrow). However, there are several additional hot spots, including one at the T18-L1 zygapophyseal joint (arrow). Severe osteoarthritis at this joint was confirmed post-mortem.
words, radiography detects a higher number of "false positives." In the authors' experience, radiographic changes that are scintigraphically active are likely to be clinically significant, while radiographic findings without increased bone metabolism represent a nonactive (chronic or healed) lesion. Horses with impinged spinous processes showing moderate or no scinti graphic activity have a good prognosis for eventual recovery. Many horses with clinical signs of back pain have more than one radiographic or scintigraphic finding, however (Fig. 12). For example, it is more common to find several vertebrae with impinged spinous processes than a single vertebral lesion. The long-term prognosis for animals with metabolically active (i.e., increased uptake) degenerative joint disease at the articular facets is poor. The outlook is also significantly worse for those animals with concurrent lameness whether or not this correlates with imaging findings. Scintigraphic and radiographic findings alone are poor indicators of whether the horse is likely to return to work or not. Of 27 horses followed for longer than a year after examination, which were still not being ridden, the same incidence (55%) of scintigraphic and radiographic abnormalities had been de tected. A further 16 animals that returned to full work had shown more radio graphic (75%) and scintigraphic (63%) changes than the nonrecovered group. Therefore, imaging findings should only be assessed in conjunction with careful clinical examination.
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CONCLUSIONS Radiography and, more recently, scintigraphy are important diagnostic aids in the diagnosis of a horse with back pain. Although the image quality is restricted by the size of the patient, these imaging techniques allow assessment f of the anatomy and physiology of structures that are dificult or impossible to palpate. In this article, the techniques and limitations of imaging the equine back have been described, and the correlations between the imaging results and clinical findings have been discussed. ACKNOWLEDGMENTS The authors would like to thank Hester McAllister, David Johns, and Christy King for their help with the illustrations.
References 1 . Butler J, Colles C, Dyson S, et al: Clinical Radiology of the Horse. Oxford, Blackwell Science, 1993 2. Devous M, Twardock A: Techniques and applications of nuclear medicine in the diagnosis of equine lameness. JAVMA 184:318-325, 1984 3. Gelfand M, Strife J, Kereiakes J: Radionuclide bone imaging in spondylosis of the lumbar spine in children. Radiology 140:191-195, 1981 4. Homof WJ, Stover SM, Koblik PD, et al: Oblique views of the ilium and the scinti graphic appearance of stress fractures of the ilium. Equine Vet J 28:355-358, 1996 5. Humphreys R, Gilday D, Ash J, et al: Radiopharmaceutical bone scanning in pediatric neurosurgery. Childs Brain 5:249-262, 1979 6. Jeffcott L: Radiographic examination of the equine vertebral column. Vet Radiol 20: 135139, 1979 7. Jeffcott L: Radiographic features of the normal equine thoracolumbar spine. Vet Radio! 20:140-147, 1979 8. Jeffcott L: Radiologic appearance of equine lumbosacral and pelvic abnormalities by linear tomography. Vet Radio! 24:201-213, 1983 9. Jeffcott L: Technique of linear tomography for the pelvic region of the horse. Vet Radio! 24: 194-200, 1983 10. Lamb C, Koblik R: Scintigraphic evaluation of skeletal disease and its application to the horse. Vet Radio! 29:16-27, 1988 11. Markel M, Madigan J, Lichtensteiger C, et al: Vertebral body osteomyelitis in the horse. JAVMA 188:632-634, 1986 12. Morris E, Seeherman HJ, O'Callaghan MW, et al: Scintigraphic identification of skeletal muscle damage in horses 24 hours after strenous exercise. Equine Vet J 23:347-352, 1991 13. Papanicolaou N, Wilkinson R, Emans J, et al: Bone scintigraphy and radiography in young athletes with low back pain. AJR Am J Roentgenol 145:1039-1044, 1985 14. Pilsworth R, Shepherd M, Herinckx B, et al: Fracture of the wing of the ilium, adjacent to the sacroiliac joint, in Thoroughbred racehorses. Equine Vet J 26:94-100, 1994 15. Sharp P, Gemmell H, Smith F: Practical Nuclear Medicine. Oxford, IRL Press, 1989 16. Spitz J, Lauer L, Tittel K, et al: Scintimetric evaluation of remodeling after bone fractures in man. J Nucl Med 34: 1403-1409, 1993 17. Steckel D: The role of scintigraphy in the lameness evaluation. Vet Clin North Am Equine Pract 7:207-239, 1991 18. Steckel R, Kraus-Hansen A, Fackelman G, et al: In Proceedings of the 37th Annual Convention of the American Association of Equine Practitioners, San Francisco, Cali fornia, 1991, pp 583-592
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19. Ueltschi G: Bone and joint imaging with 99mTc labelled phosphates as a new diagnostic aid in veterinary orthopedics. J Am Vet Radiol Assoc 18:80-84, 1977 20. Ueltschi G: Die Skelettszintigraphic beirn Pferd [PhD thesis] . Berne, Universitat Berne, 1980
Address reprint requests to Martin P. Weaver, BVMS, DVM, DVR, MRCVS Large Animal Surgery Faculty of Veterinary Medicine University College Dublin Ballsbridge, Dublin, Ireland
0749-0739/99 $8.00 + .00
RADIOLOGY AND SCINTIGRAPHY Martin P. Weaver, BVMS, DVM, DVR, MRCVS, Leo B. Jeffcott, MA, BVetMed, PhD, FRCVS, DVSc, and Michael Nowak, DVM
Because of the inaccessibility of many of the structures in the spine that may be affected, ancillary diagnostic aids such as imaging are particularly important in the work-up of a horse with a suspected back problem. Radio graphic examination of the equine back has been used for several decades,6· 7 but the quality of the images that can be obtained has recently improved with the introduction of modern fast-screen film systems. Scintigraphic examination for assessing equine orthopedic conditions, including back problems, was first described 20 years ago. 19 These two imaging techniques complement each other. Although good quality radiographs give reasonable anatomical resolution of the thoracolumbar structures, some of the abnormalities that are detected may not reflect a current problem. The clinical signs associated with impinged spinous processes, for example, may resolve, but the radiographic changes usually remain. Scintigra phy portrays the metabolism of the different skeletal structures in the back. It is highly sensitive but has poor intrinsic anatomical resolution and is nonspecific. Radiographic examination of scintigraphically abnormal regions is usually re quired before a definitive diagnosis can be made. Scintigraphy is also useful as a screening tool. Not only can areas such as the caudal lumbar spine and sacroiliac regions be imaged in the standing horse; the bones of both hind limbs can also be imaged. This is important in many cases of suspected back pain which have a complex etiology, including low-grade hind limb lameness.
From Large Animal Surgery, Faculty of Veterinary Medicine, University College Dublin, Ballsbridge, Dublin, Ireland (MPW); Cambridge Veterinary School, University of Cambridge, Cambridge, England (LBJ); and Orthopaedic Department, Tierklinik Hochmoor, Aescher, Germany (MN)
V ETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 15 • NUMBER 1 • APRIL 1999
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RADIOLOGY Radiographic Techniques Standing Lateral thoracic and cranial lumbar radiographs of adequate quality can be obtained with the horse standing if care is taken with the technique. Low power portable machines can be used for radiographing the tips of the spinous processes, but if views of the bases of the processes, vertebral bodies, rib articulations, or articular facets are required, a machine capable of an output of up to 150 kV(p) and 500 mA is required. It is convenient to have the machine linked to a mobile stand with a cassette holder so that the beam is automatically aligned with the center of the cassette. Otherwise, the cassette should be mounted on a fixed stand or to a wall; it should never be handheld. The largest available cassette size (usually 35 X 43 cm or 14 X 17 in) reduces the number of views required for a full thoracolumbar examination, and it should be oriented perpendicular to the ground. At the high exposures used for the thoracolumbar spine, radiation scatter is an important factor which significantly reduces image quality. Several measures can help to reduce the effect of scatter. Rare-earth film/screen systems allow exposure factors to be reduced. A grid (crosshatched parallel grid with a ratio of 12:1) is necessary for all views, except for radiographic views of the spinous processes. Most radiographic film cassettes contain lead in the backing, but at exposures over 100 kV(p), it is advisable to place an additional lead sheet behind the cassette to absorb back scatter. As the cassette is oriented perpendicular to the ground with a film-focus distance of 150 cm, an air gap exists between the spine and the cassette, and this reduces the amount of scatter reaching the film. There is a considerable difference in tissue attenuation between the tips of the spinous processes and the vertebral bodies; thus, at least two radiographs of the same region at different exposures must be taken or some form of beam filtration is necessary. A "Dodger" beam filter using aluminum wedges has been de scribed,6 but this equipment is no longer commercially available. A similar appliance can be custom-made and attached to the front of the X-ray tube to allow a high exposure to be set for the deeper structures of the spine but filtered to the spinous processes. If no filtration is used, close collimation to the region of interest reduces scatter and enhances image quality. Exposure values vary depending on the system (Table 1). The horse should be sedated prior to radiography to reduce the degree of Table 1. TYPICAL RADIOGRAPHIC EXPOSURES TO DIFFERENT PARTS OF THE THORACOLUMBAR SPINE* View
Withers (T3-T8) spinous processes Midthoracic (T9-T15) spinous processes Thoracolumbar (Tl6, L3-L4) spinous processes Midthoracic articular processes and lumbar vertebral bodies
Kilovolt (peak)
Milliamperes
Grid
75 70 85
30 55 80
No Yes Yes
100 +
200-300
Yes
*Dupont Ultravision Rapid screens (Dupont UK, Stevenage, Herts, UK) were used in this study.
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movement and to allow the handler to stand well out of range of the beam. Stocks are also useful to prevent movement. Lead markers placed midline on the horse's back are useful for orientation on the radiographs; the marker sites can be lightly clipped to allow them to be found again later, for example, if local anesthetic is to be injected. As part of the clinical workup for suspected impinged spinous processes, the horse must be standing completely square, as the spine rotates if one leg is rested. For the apices/summit's spinous processes, the beam should be centered 5 cm ventral to the dorsal skin surface; for the region of the vertebral bodies, it should be centered 15 to 20 cm below the dorsum depending on the size of the horse. The beam should normally be horizontal, but for visualization of the dorsal articular facets in the cranial lumbar region, it is useful to angle the beam from a ventral to dorsal orientation at an angle of 20° to 30° to avoid superimposition by the transverse processes. As a grid is being used, the cassette must be tilted accurately at the same angle of 20° to 30°. Recumbent Radiography with the horse under general anesthesia is required in order to achieve optimum lateral views of the caudal lumbar spine (L3-L5), ventrodor sal views (apart from those in foals), and views of the sacroiliac joints. Prior scintigraphy is useful to localize areas to be radiographed and thus reduce the anesthesia time. In some large horses, the longer exposures required mean that recumbency is preferred for the caudal thoracic or cranial lumbar spine in order to obtain films of diagnostic quality. The horse is anesthetized and positioned in lateral recumbency. Ideally, a hoist should be available to rotate the animal into dorsal recumbency if ventrodorsal views are also required. Care must be taken in positioning to prevent axial rotation; for lateral views, this usually necessitates placing supports under the upper forelimbs and hind limbs to ensure that they are parallel with the trunk. With the horse recumbent and immobile, exposures of up to 125 kV(p) and 300 mA can be used for the lumbar spine. For ventrodorsal views, the horse should be placed in the "frog-legged" position, with the hind limbs flexed and abducted. Again, tilting should be prevented to avoid rotation of the spine. For views of the sacroiliac joint, a 35 X 43-cm cassette should be placed under the horse and centered just cranial to the tubera sacrale. Exposures of 130 kV(p) and 330 mA can be used for the sacroiliac joint and sacrum. Intestinal contents may be superimposed on these structures.
Normal Radiological Findings Foals have a slightly kyphotic midthoracic spine that becomes level by the time they reach adulthood. The thoracolumbar vertebral bodies have cranial and caudal physes that close radiographically by the time horses are 1 year and 4 years old, respectively. Separate centers of ossification begin to develop at the apices of the spinous processes in the withers from T2 to T8 when the horse is about 1 year of age; from then on, the tips retain a mottled irregular appearance. These features should not be mistaken for fractures or osteolysis. The spinous process of Tl has no separate center of ossification and that of T6 often has an irregular cranial surface.
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Certain anatomical landmarks help in orientation when evaluating radio graphs of the thoracolumbar spine: • The tallest dorsal spinous process is usually TS or T6. • The cranial thoracic spinous processes are angled slightly caudad and the caudal thoracic spinous processes are angled slightly craniad. The anticli nal spinous process in most horses is TIS or Tl6. • On lateral radiographs, the caudad sloping diaphragm crosses the verte bral body of TIS or T16 during expiration or midinspiration. • The interspinous spaces are usually wider when the horse is radiographed under general anesthesia. In mature horses, the intervertebral disc space is narrow, and the caudal extremities of the vertebral body are relatively sclerotic. The ribs articulate dorsal to the vertebral body articulations. The orientation of the articular facet surfaces changes from almost horizontal in the cranial thoracic spine to near vertical in the caudal thoracic vertebrae. The tip of the third sacral spinous process (S3) may have a bifid appearance with an irregular outline. Many clinically normal horses have spinous processes whose apices are in close apposition (i.e., less than 2 mm apart), particularly in the vertebral region caudal to the anticlinal vertebra. This should not be misinterpreted as indicating significant pathology. In one survey of clinically normal horses, 34% showed some degree of spinous process impingement.7 Many clinically normal horses have a spur of new bone (i.e., enthesophyte) on the dorsal apex of T3 or T4 and also occasionally in the caudal thoracic spine. Radiological Findings in Horses with Back Problems
Congenital defects are occasionally seen in foals or as incidental findings in older horses. These include lordosis, kyphosis, scoliosis, hemivertebrae, and congenital fusion of the thoracolumbar vertebral bodies or processes. Impinged spinous processes ("kissing spines") range from spinous processes that are close at their summits to marked overlapping of the processes with advanced secondary bone changes. The vertebral sites most frequently affected are under the saddle region (Tl2-T17). In mildly affected cases, all that is seen is a narrow interspinous space, usually with sclerosis of the adjacent spinous process edges (Fig. 1). The nar rowing usually occurs at the summits but may rarely affect the lower portions of the spinous processes (Fig. 2). Radiographs of severe spinous process impinge ment show remodelling of the cranial and caudal edges, marked subcortical sclerosis, and cyst-like lucencies within the subcortical bone (Fig. 3). Spinous processes may also develop prominent rounded or pointed "beaks" of new bone. One or more interspinous spaces may be affected. Following surgery for impinged spinous processes, some horses may develop a fairly dramatic amount of irregular new bone formation locally, and this may cause a recurrence of clinical signs. Isolated periosteal reactions (i.e., enthesophytes) of bone fragments along the cranial or caudal edge of a spinous process are considered to be a consequence of interspinous ligament strain or injury (Fig. 4A and B). Osteoarthritis of the articular processes (i.e., zygapophyseal joints) is diffi cult to assess unless the radiographs are of excellent quality. The normal oblique radiolucent line of the articulation may become obscured by irregular new bone, which can become partly superimposed over the bases of the spinous processes (Fig. SA). Subchondral lucency may also occur giving the appearance of widened zygapophyseal joint spaces (Fig. SB). These changes are not commonly seen and
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Figure 1. Adult Warmblood. Impingement of midthoracic dorsal spinous processes (DSP). The cranial and caudal edges of the spinous processes are sclerosed dorsally (arrowheads) and have ill-defined zones of subcortical lucency ventrally (arrows).
- Figure 2. Adult Warmblood. Narrowing of second thoracic interspinous space, most pro nounced in the ventral third (small arrows). Sclerotic line superimposed over T2 (large arrow) is scapular spine. An aluminum wedge filter has been used over the apices of the spinous processes.
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Figure 3. Twelve-year-old Thoroughbred eventer with severe chronic back pain. Post mortem radiograph of DSPs T13-T17. Impingement with severe remodelling is present at T17-T18 (arrow). The other interspinous spaces are narrowed and the DSPs show subcorti cal sclerosis with lucencies in the impinging regions. See also Figure 12.
usually occur in the caudal thoracic and lumbar regions. They are usually clinically significant. The supraspinous ligament attaches to the apices of the spinous processes from the withers to the caudal lumbar spine. Changes are occasionally seen affecting only the apices of the spinous processes just caudal to the withers (T8-T12), and these are thought to indicate damage to the supraspinous ligament (Fig. 6). The changes may represent an enthesophyte or a flattened area of bone which appears to have been lifted from the apex of the spinous process (i.e., avulsion fracture).' A radiograph taken at lower exposure may show focal thickening of the ligament, but this is best evaluated with ultrasonography. Spondylosis (spondylosis deformans) in horses is uncommon, but it is regarded as a common aging process in dogs. The condition may occasionally be encountered as an incidental finding during radiographic examination of the equine thorax. In mildly affected horses, osteophytes develop on the ventral surface of two or more vertebral bodies close to the intervertebral disk. In some horses, the osteophytes fuse to form a complete osseous bridge on the ventrolateral vertebral bodies. Occasionally, an irregular vertical lucent line transverses the new bone formation adjacent to the intervertebral disk space (Fig. 7). These changes are only seen in the midthoracic to caudal thoracic spine and may affect up to five or six vertebral bodies. Vertebral body fractures are usually catastrophic, and the horse is often destroyed on humane grounds without radiography being attempted. Some fractures may result in only minimal fracture displacement, however, and the main clinical signs can be chronic back pain and hind limb ataxia. The most common sites for fractures are the first three thoracic vertebrae (Tl-T3), T9 to Tl6, and the lumbar vertebrae (Ll-L6). It may not be possible to assess the exact fragment alignment radiographically, but associated radiological signs include ventral displacement or apparent shortening of a spinous process, poor defini tion of the zygapophyseal joints due to callus formation (not to be confused with degeneration of these joints), and alteration in the normally evenly aligned vertebral bodies. The most common site for vertebral fractures is the spinous processes of the withers (Tl-T12), usually due to the horse rearing up and falling over backwards. Radiography is indicated to assess the number of spi-
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A
B Figure 4. A, Adult Warmblood. Radiograph of caudal thoracic DSPs. Flakes of bone (arrows) with associated irregularity in cranial borders of DSPs, indicating damage to the interspinous ligaments. An aluminum sheet has been used as a filter. B, Adult Warmblood dressage horse. Radiograph of caudal thoracic DSPs. New bone formation (arrows) on the caudal aspect of T17 secondary to interspinous ligament trauma.
nous processes affected and the extent of fragment displacement, as these may occasionally require surgical removal. Osteomyelitis of the vertebral bodies, although a rare condition, is seen most commonly in foals and young horses, and the spine is usually only one of several affected sites. Changes that have been described include proliferation of new bone around the vertebral body, soft tissue swelling ventral to the vertebral column, well-defined zones of radiolucency, and compressive pathological frac tures.11 Osteomyelitis of the spinous processes (fistulous withers) occurs most frequently at T4 to T7. Although the infection primarily involves the nuchal
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Figure 5. A, Nine-year-old Thoroughbred. Radiograph of caudal thoracic vertebral region. Degenerative joint disease of the zygapophyseal joints. There is subchondral sclerosis at two joints (arrows). B, Fourteen-year-old Warmblood. Radiograph of caudal thoracic verte bral region. Degenerative joint disease of the zygopophyseal joints. Subchondral lucency of the caudal articular surface results in a radiographically widened joint space (arrows).
ligament, supraspinous bursa, and other adjacent soft tissues, chronic cases have radiographic changes at the apices of the affected spinous processes. Irregular areas of radiolucency are present distal to the centers of ossification, and occa sionally, new bone formation is seen on the cranial or caudal surfaces. Radio graphs can be obtained with a metal probe in place to assess the depth and location of the fistulous tract(s). Damage to the sacroiliac joint is difficult to assess radiographically; even
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Figure 6. Fourteen-year-old Thoroughbred eventer with chronic back pain, radiograph of midthoracic vertebral region. Spondylosis. Smooth bone is present on the ventral surface of the vertebral bodies. There are two irregular lucent lines, one ventral to the intervertebral body articulation (T1 O-T11 ), the other ventral to T11 (arrows). See also Figure 11.
with the horse anesthetized and in dorsal recumbency, the quality of the resul tant radiographs is frequently poor. Additional factors reducing the radiographic resolution of the joints are that they are angled at about 30° to the horizontal and that ingesta may be superimposed over the area of interest. The exposure should be made during inspiration; this results in blurring of the overlying soft tissues. Radiographic features of chronic sacroiliac disease are smooth new bone
Figure 7. Eight-year-old Thoroughbred racehorse, low-exposure radiograph of midthoracic DSPs. Supraspinous ligament trauma. Apex of T1 O (arrow) shows periosteal irregularity suggestive of supraspinous ligament trauma. Ultrasound examination of the ligament con firmed the diagnosis.
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on the caudal aspect of the sacroiliac joint or joint space widening. 8• 9 Neverthe less, many cases with clinical signs of sacroiliac disease do not show detectable radiographic changes. SCINTIGRAPHY Technique for Nuclear Medicine Scanning The radiopharmaceutical most commonly used for equine scintigraphy is 99m technetium-labelled methylene diphosphonate(99mTc-MDP). The procedures required for bone scanning in horses have been extensively described. 2• 10• 17• 19 A weight-dependent dose between 3 and 6 GBq (100-150 mCi) of 99mTc-MDP is injected intravenously, and images of the bone phase are obtained 2 to 3 hours later. Due to the initial high renal and bladder radioactivity, vascular phase and soft tissue phase images of the thoracolumbar spine cannot be obtained. Dam aged skeletal muscle can be detected by abnormal soft tissue uptake of 99mTc MDP during bone phase imaging but usually only if the horse has been strenu ously exercised before the scintigraphic examination. 12 Overlapping views of the entire thoracolumbar spine are obtained from the caudal cervical spine caudad; this requires a total of three to five views de pending on the gamma camera field of view. A high-resolution, low-energy, parallel-hole collimator should be used; this gives a geometrical resolution of 4.4 mm at a distance of 5 cm from the collimator face. 15 For a complete examination, the horse should be imaged from both the left and the right; otherwise, some subtle lesions, for example, those located at the zygapophyseal joints, can easily be missed. For the caudal thoracic and lumbar regions, the camera is positioned obliquely at an angle of 50° to 65° to the horizontal. This allows the distance between the camera and spine to be mini mized. Positioning the camera horizontally above the spine results in superimpo sition of all the vertebral structures. The camera can be oriented perpendicular to the floor for views of the cranial thoracic spine. When the camera is positioned over the lumbar spine, the image quality may be improved by shielding the kidney using a lead sheet. The images are acquired on the basis of either counts or time. One million counts or more are optimal for maximal resolution of the caudal regions of the equine back and the pelvis. 20 This large number of counts requires several minutes of immobility on the part of the horse, however, and thus generally cannot be achieved without general anesthesia. The clinician must weigh the advantages of scintigraphic examination under general anesthesia (improved image resolution and better lesion detection) against the cost, time, effort, risk of complications, and increased personnel exposure to radiation that is involved with general anesthesia. Movement of the standing horse can be minimized by appropriate sedation and quiet handling of both the horse and camera. Blinkers and cotton ear plugs for the horse may help. Recently, nuclear medicine software that is able to correct for motion has become available, and this compensates for the slight swaying of the horse and respiratory movements. The authors always scan both hind limbs first prior to the back and pelvis. If the images are acquired on the basis of time (e.g., 2 minutes per image in the standing animal), the images or regions of interest can be directly compared. If counts are used, image manipulation using appropriate software is required for direct comparison between images. With high-quality images, it is possible to create regions of
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interest and to quantitate differences in uptake by comparison to a reference site such as a rib (Gottlieb Ueltschi, personal communication 1992). Scintigraphic Findings In normal horses, regions of bone close to the skin surface such as the tuber coxae and tuber sacrale normally appear "hotter" (i.e., appearance of increased radiopharmaceutical uptake) than adjacent bone structures, as there is less soft tissue attenuation. Other superficial sites that normally appear slightly "hot" are the apices of the spinous processes, particularly at the withers (Fig. 8). In good quality images, the individual vertebra can be clearly distinguished. The spinous processes and articulations have a slightly higher uptake, but this should be equal for all joints in the region. On lateral oblique views, each rib is located cranial to its respective vertebra. As an aid to orientation, on left lateral views, the 16th rib usually overlies the cranial pole of the right kidney (Gottlieb Ueltschi, personal communication 1992). Any scintigraphic abnormality appears as increased radiopharmaceutical uptake (a "hot spot"), whereas areas of diminished uptake are difficult to detect due to poor image resolution. The most common sites for increased uptake are at the apices of the spinous processes (Fig. 9). Similar sites to those seen on radiography are affected, although the incidence of abnormal findings is less with scintigraphy than with radiography (M.P. Weaver and L.B. Jeffcott, unpub-
Figure 8. Seven-year-old Thoroughbred. Scintigraphic view (motion-corrected) of normal caudal thoracic vertebral region. Cranial is to the right. Cranial pole of the right kidney is seen caudoventrally (K). The DSPs (D), vertebral bodies (V), and ribs can be identified. Cranially, the spinous process apicies appear brighter as there is less soft tissue attenua tion.
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Figure 9. Nine-year-old showjumper with chronic, low-grade back pain. Scintigraph (motion corrected} of midthoracic vertebral region. An intense uptake is present at the tip of DSP T1O. There was pain on palpation of this site. Radiographic and ultrasonographic examinations were normal.
lished data 1994). In a review of horses with a history of back pain scanned by the authors (M.P.W. and L.B.J.) about 65% had increased uptake of varying intensity in the back or the pelvis. This included 35% with changes at the spinous processes, 15% at the articular processes, and 20% with changes at the level of the vertebral bodies. Other workers have described a much higher incidence of increased activity at the spinous processes: 18 48 of 50 horses scanned had spinous process uptake and only two of 50 showed vertebral body in creased uptake. Focal or diffuse increases in uptake have been described abaxial to the tubera sacrale on dorsal views and attributed to sacroiliac joint disease.17 Al though the sacroiliac joint is ventral to the ilium at this site, the ilial wing between the sacroiliac joint and the tuber sacrale is a common site for stress remodelling or fracture. 14 These lesions can be confirmed by ultrasonography (Fig. lOA and B) or dorsal oblique scintigraphy views, where the camera is placed midway between the tuber sacrale and tuber coxae of the ilium being imaged. 4 It would seem unlikely that low-grade sacroiliac joint pathology in the adult horse would result in detectable increased bone metabolism. New bone on the ventral surface of the vertebral bodies (spondylosis) usually has the same level of uptake as the adjacent vertebra, but can occasion ally be considerably more active (Fig. 11). Lesions such as vertebral fractures or osteomyelitis result in marked in creases in uptake, but the final diagnosis requires radiographic examination to distinguish between several possible causes. Although the bone scan is effective
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Figure 1 0. A, Ten-year-old polo pony with acute left hind lameness. Dorsal view of lumbar vertebral region and cranial pelvis. Cranial is to the left. There is moderate bladder activity (small arrow), normal lumbar spine (LS) uptake, and an area of intense uptake at the left tuber sacrale (large arrow) compared to the right. A fractured ilial wing was confirmed on ultrasonography. B, Three-year-old normal Thoroughbred. Dorsal view of pelvis (motion corrected), similar orientation to A. Moderate bladder activity (arrow). Note symmetrical appearance of both iliae.
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Figure 11. Same horse as Figure 6. Lateral oblique scintigraph of midthoracic vertebral region; cranial to left. There is a focal hot spot associated with the ventral aspect of the vertebral bodies at approximately T10 (arrow), corresponding to the site of spondylosis on the radiograph (see Fig. 6). As this smooth new bone formation does not normally result in hot spots, the more caudal lucent line on the radiograph in Figure 6 was thought to be a fracture.
in detecting fractures, it should be noted that acute lesions of the axial skeleton in humans may take up to 10 days to become scintigraphically apparent. 1 6 CORRELATION OF IMAGING TECHNIQU ES, CLINICAL FINDINGS, AND PROGNOSIS
In studies of human patients with back pain who have been imaged using both radiography and scintigraphy, a relatively low correlation between the two modalities has been reported/' 5 with only about half the findings of one tech nique corresponding with those of the other. This is not surprising, as metabolic bone activity (scintigraphy) often does not mirror anatomical changes (radiogra phy). Scintigraphy is used in human orthopedics to assess the significance of radiographic findings. 1 3 In a series of 65 horses imaged due to signs of back pain, only 19 had findings confirmed by both techniques. Sixteen showed only scintigraphic changes, although radiographic changes were not matched by scintigraphic abnor malities in 36 animals (M.P. Weaver and L.B. Jeffcott, unpublished data 1994). Horses with a history of back pain but no current problems showed a similar incidence of radiographic changes compared to those with current evidence of back pain. In contrast, the number of scintigraphic abnormalities was consider ably lower in "normal" horses compared to those with back pain. In other
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Figure 1 2. Same horse as in Figure 3. Lateral oblique view scintigraph of the thoracolumbar vertebral region, with horse anesthetized. Caudal (CD) is right. Diffuse activity from the left kidney is seen ventral to L 1, partly superimposed by the last rib. The site of DSP impinge ment seen on the radiograph corresponds to a pronounced increased uptake (white arrow). However, there are several additional hot spots, including one at the T18-L1 zygapophyseal joint (arrow). Severe osteoarthritis at this joint was confirmed post-mortem.
words, radiography detects a higher number of "false positives." In the authors' experience, radiographic changes that are scintigraphically active are likely to be clinically significant, while radiographic findings without increased bone metabolism represent a nonactive (chronic or healed) lesion. Horses with impinged spinous processes showing moderate or no scinti graphic activity have a good prognosis for eventual recovery. Many horses with clinical signs of back pain have more than one radiographic or scintigraphic finding, however (Fig. 12). For example, it is more common to find several vertebrae with impinged spinous processes than a single vertebral lesion. The long-term prognosis for animals with metabolically active (i.e., increased uptake) degenerative joint disease at the articular facets is poor. The outlook is also significantly worse for those animals with concurrent lameness whether or not this correlates with imaging findings. Scintigraphic and radiographic findings alone are poor indicators of whether the horse is likely to return to work or not. Of 27 horses followed for longer than a year after examination, which were still not being ridden, the same incidence (55%) of scintigraphic and radiographic abnormalities had been de tected. A further 16 animals that returned to full work had shown more radio graphic (75%) and scintigraphic (63%) changes than the nonrecovered group. Therefore, imaging findings should only be assessed in conjunction with careful clinical examination.
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CONCLUSIONS Radiography and, more recently, scintigraphy are important diagnostic aids in the diagnosis of a horse with back pain. Although the image quality is restricted by the size of the patient, these imaging techniques allow assessment f of the anatomy and physiology of structures that are dificult or impossible to palpate. In this article, the techniques and limitations of imaging the equine back have been described, and the correlations between the imaging results and clinical findings have been discussed. ACKNOWLEDGMENTS The authors would like to thank Hester McAllister, David Johns, and Christy King for their help with the illustrations.
References 1 . Butler J, Colles C, Dyson S, et al: Clinical Radiology of the Horse. Oxford, Blackwell Science, 1993 2. Devous M, Twardock A: Techniques and applications of nuclear medicine in the diagnosis of equine lameness. JAVMA 184:318-325, 1984 3. Gelfand M, Strife J, Kereiakes J: Radionuclide bone imaging in spondylosis of the lumbar spine in children. Radiology 140:191-195, 1981 4. Homof WJ, Stover SM, Koblik PD, et al: Oblique views of the ilium and the scinti graphic appearance of stress fractures of the ilium. Equine Vet J 28:355-358, 1996 5. Humphreys R, Gilday D, Ash J, et al: Radiopharmaceutical bone scanning in pediatric neurosurgery. Childs Brain 5:249-262, 1979 6. Jeffcott L: Radiographic examination of the equine vertebral column. Vet Radiol 20: 135139, 1979 7. Jeffcott L: Radiographic features of the normal equine thoracolumbar spine. Vet Radio! 20:140-147, 1979 8. Jeffcott L: Radiologic appearance of equine lumbosacral and pelvic abnormalities by linear tomography. Vet Radio! 24:201-213, 1983 9. Jeffcott L: Technique of linear tomography for the pelvic region of the horse. Vet Radio! 24: 194-200, 1983 10. Lamb C, Koblik R: Scintigraphic evaluation of skeletal disease and its application to the horse. Vet Radio! 29:16-27, 1988 11. Markel M, Madigan J, Lichtensteiger C, et al: Vertebral body osteomyelitis in the horse. JAVMA 188:632-634, 1986 12. Morris E, Seeherman HJ, O'Callaghan MW, et al: Scintigraphic identification of skeletal muscle damage in horses 24 hours after strenous exercise. Equine Vet J 23:347-352, 1991 13. Papanicolaou N, Wilkinson R, Emans J, et al: Bone scintigraphy and radiography in young athletes with low back pain. AJR Am J Roentgenol 145:1039-1044, 1985 14. Pilsworth R, Shepherd M, Herinckx B, et al: Fracture of the wing of the ilium, adjacent to the sacroiliac joint, in Thoroughbred racehorses. Equine Vet J 26:94-100, 1994 15. Sharp P, Gemmell H, Smith F: Practical Nuclear Medicine. Oxford, IRL Press, 1989 16. Spitz J, Lauer L, Tittel K, et al: Scintimetric evaluation of remodeling after bone fractures in man. J Nucl Med 34: 1403-1409, 1993 17. Steckel D: The role of scintigraphy in the lameness evaluation. Vet Clin North Am Equine Pract 7:207-239, 1991 18. Steckel R, Kraus-Hansen A, Fackelman G, et al: In Proceedings of the 37th Annual Convention of the American Association of Equine Practitioners, San Francisco, Cali fornia, 1991, pp 583-592
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19. Ueltschi G: Bone and joint imaging with 99mTc labelled phosphates as a new diagnostic aid in veterinary orthopedics. J Am Vet Radiol Assoc 18:80-84, 1977 20. Ueltschi G: Die Skelettszintigraphic beirn Pferd [PhD thesis] . Berne, Universitat Berne, 1980
Address reprint requests to Martin P. Weaver, BVMS, DVM, DVR, MRCVS Large Animal Surgery Faculty of Veterinary Medicine University College Dublin Ballsbridge, Dublin, Ireland