DISEASES OF THE SPINE
0195-5616/92 $0.00 + .20
RADIOGRAPHY, MYELOGRAPHY, COMPUTED TOMOGRAPHY, AND MAGNETIC RESONANCE IMAGING OF THE SPINE Ronald D. Sande, DVM, MS, PhD
Study of the axial skeleton and spinal cord has traditionally been performed using radiography to record the images. Conventional radiology still remains the most convenient, cost-effective, and familiar modality available to practitioners. Regardless of its tenure as the preferred method of imaging, radiography remains a sophisticated procedure that requires exacting technique and application. Alternate methods of imaging are currently available to veterinary practitioners, the availability and cost-effectiveness of which are improving. Computed tomography (CT) is commonly used in veterinary diagnostics, and magnetic resonance imaging (MRI) is rapidly becoming a practical alternative to x-ray imaging.
RADIOGRAPHY
Spinal radiography may be a frustrating or a rewarding experience. Success requires that one observe fundamental rules of radiography." Survey radiographs of the spine must be performed with absolute perfect positioning and excellent diagnostic detail. The probability of From the Department of Clinical Medicine and Surgery, Washington State University College of Veterinary Medicine, Pullman, Washington
VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22· NUMBER 4· JULY 1992
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irreversible or fatal error is significant, and one must be certain of procedures, findings, diagnosis, and treatment.
Survey Radiography
Survey radiography should be performed after a thorough clinical examination. A complete neurologic examination should result in tentative location of the disease and perhaps even an expected anatomic description or etiology. Radiography may be performed according to a series of predetermined routine projections or a selective study based on the differential diagnoses derived during the neurologic and clinical examination. This radiologist prefers the latter because it encourages the diagnostic talents of clinical personnel, reduces unnecessary manipulation of the patient, and often avoids invasive procedures in a compromised patient. An added bonus is reduced cost to the client. It is preferable to have the patient heavily sedated or anesthetized to facilitate positioning. The radiographic projections should be made with the patient in perfect lateral and ventrodorsal position, respectively. The use of sandbags or other positioning aids will reduce exposure to attendants and result in more consistent positioning. Oblique projections are misleading and difficult to interpret owing to unfamiliar edge effects and other perceptual distortions. Films with suboptimal positioning should be retaken. Adequate positioning may be obtained in the lateral projection if the dorsal and ventral midlines of the patient are equidistant from the cassette. The axial skeleton should be extended and the central x-ray beam centered directly over the area of interest. The ventrodorsal projection should be made when the ventral midline and the dorsal midline are superimposed. In both projections, it is necessary to use foam pads and wedges to prevent "sagging" of the spine and insure that the spine is parallel to the cassette and film. Errors caused by parallax are the most frequent cause of misdiagnosis. Seven radiographic projections are necessary to complete a minimum examination of the cervical spine to the sacrum. The cervical spine, thoracic spine, thoracolumbar junction, and the lumbar segments should be visualized independently. Lateral projections centered on each area and ventrodorsal projections of the cervical, thoracolumbar, and lumbar spine are most informative. Ventrodorsal projection of the thoracic spine is confusing and usually not productive owing to superimposition of the sternum and the contribution of osseous detail provided by the dorsal spinous processes of the thoracic vertebrae. The central x-ray beam should be centered at each location and the beam limited to include only the axial skeleton; otherwise beam divergence and scattered radiation will result in loss of detail. Additional projections may be selected following evaluation of the survey films. Radiographic technique should be developed to maximize detail. High detail film and screen combinations using relatively higher mil-
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liamperage and moderate kilovolt peak will result in the best film detail. This type of technique will produce better film quality during contrast studies also.
Interpretation of Survey Radiographs
Interpretation of survey radiographs of the spine requires that the diagnostician understand the form and function of all components of the axial skeleton. Each vertebra or section of the spine performs a distinct function. Development (form) of a spinal segment has been derived to perform the required function. The spine has no paired anatomic structure such as the appendicular skeleton for comparison. Therefore the radiographer must compare radiographs of the spine to an expected mental standard. The atlanto-occipital junction provides flexion and extension of the head (the "yes" function). Fulcrum for this function is the joint, and the lever and moment arm of the function are the nuchal protuberance, the squamous part of the occipital, and basilar portion of the occipital bone. The atlantoaxial joint provides for rotation of the head about the axis of the "axial" skeleton (the "no" function). Leverage for performing this function is provided by the wings of the atlas. These vertebrae perform specific functions and are quite different from other vertebrae. The junction of C-3 with the axis is a transition of function, where lateral motion, elevation, and declination of the head and neck occur in the cervical spine. Cervical vertebrae exhibit gradual change in anatomic appearance as the transverse processes become progressively larger, being largest on C-6, and disappearing on C-7. C-7 to T-l is the next transitional space as the ribs replace transverse processes and the articular facets form sliding joints. The next transition occurs at T-IOT-Il, where the articular facets change from sliding joints to rotational joints. This site is usually narrower than the adjacent spaces and may be confused as having a protruded disc. The final transition of significance occurs at L-7-S-1. This segment often has congenital anomalies, usually of no clinical significance. Interpretation of radiographic signs must be made by recognition and recall when examining the atlas, axis, and each of the transitional spaces. The remaining vertebrae and their respective intervertebral spaces are best evaluated by comparing each one to the vertebral segment immediately cranial and immediately caudal. Therefore one evaluates the radiograph by studying groups of three vertebrae. Transition in anatomic features is gradual, and the vertebrae and their respective interspaces and articulations are sufficiently similar to use groups of three for controlled r.eference. The principles of interpretation can be used in any species. One needs only to recognize basic species differences. Figure 1 has examples of well-positioned projections of a canine spine demonstrating normal anatomic features and some degenerative disc disease.
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Figure 1. See legend on opposite page
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MYELOGRAPHY Myelography should be considered only after thorough neurologic examination has indicated a profound or progressive disease of the central nervous system and survey radiographs have failed to provide substantive findings. Myelography should be considered if the neurologic signs and the survey radiographs appear to be in conflict; also myelography might be useful in providing an accurate indication of cord compression. Myelography is an invasive technique with a significant morbidity, although low mortality, and it is expensive for the client. The patient must be anesthetized for the procedure, and preanesthesia should include diazepam. Promazine derivatives are contraindicated because they decrease the convulsive threshold and contribute to unfavorable recovery and increased seizure activity. One should select the site for puncture according to the neurologic signs and the level at which the disease is expected. The cisternal or atlanto-occipital puncture is the simplest to perform, and the location is one from which cerebrospinal fluid (CSF) can be easily removed. Myelography using the cisternal tap is selected most often. It is not unusual, however, to select either the cisternal or lumbar site and fail to produce adequate opacification of the entire subarachnoid space. Failure to define a lesion may result owing to obstruction of the flow of contrast material to or around the lesion. When this occurs, one should not hesitate to perform myelography from the alternate location and introduce the contrast agent from the opposite direction. Using this method, one should be able to define the area or section of the cord with pathologic change. Myelography using the cisternal site requires surgical preparation of an area from the nuchal crest to the second cervical vertebrae and
Figure 1. A lateral survey study of the spine of a 5-year-old Dachshund with undefined back pain. A, This projection was centered at the mid-cervical spine. The atlanto-occipital and atlanto-axial junctions were difficult to interpret, and the shape of each vertebrae is unique to their respective function. The intervertebral spaces at C2-3 and at C7-T1 appeared narrow. Note the similarity and gradual change of anatomy of the articular facets, dorsal spinous processes, and transverse processes. B, Lateral projection centered on the thoracic spine. Note the gradual transition of the shape and direction of the dorsal spinous processes. Sliding articulations of T1-1 0 undergo transition at T10-11 to a rotational joint, and the intervertebral space was narrow as expected. There was degenerative mineralization of the intervertebral discs from T2-1 O. C, Lateral projection centered at the thoracolumbar junction. T10-11 appeared narrow, consistent with the transition of articular function at that site. T11-12 was narrow, and there was degenerative mineralization of the intervertebral disc at T12-13, either of which may have pathologic significance. D, Lateral projection centered on the lumbar spine. Note- the similarity but gradual change in the architecture of dorsal spinous processes and shape and size of the intervertebral foramina and vertebral bodies. There was degenerative mineralization of intervertebral discs at L3-6. Compare intervertebral foramina at L2-4 and note the subtle opacification at L3-4, where an intervertebral disc had herniated.
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between the ears. A 20- to 22-gauge, 1- to 1V2-inch short bevel spinal needle is adequate even for large canine patients. The patient is placed in lateral recumbency, the head should be elevated, and the head and neck should be placed in flexion. The needle is inserted percutaneously on the dorsal midline of the neck at its intersection with a line between the rostral limits of the wings of the atlas. The needle should be directed slightly rostral. Entry into the subarachnoid space is recognized by a change in the "drag" or friction as the needle penetrates the ligamentum flavum. The bevel of the needle should be inserted into the dorsal subarachnoid space and placed such that the bevel is toward the desired direction of flow of the contrast agent. Removal of the stylet is usually followed by drip or steady flow of CSF unless low pressure is encountered, in which case gentle aspiration may be required. Passing the stylet across the back of the rubber glove should leave a faint wet streak if the needle has penetrated the subarachnoid space (CSF will ascend the needle by capillary action, and the stylet will leave a wet line on the glove). Hemorrhagic CSF or frank blood indicates the needle has deviated lateral to the midline and entered a vertebral sinus. The needle should be withdrawn and discarded and the procedure begun anew. A volume of CSF equal to the intended dose of contrast agent may be removed if excessive intracranial pressure is not a problem. Injection of the contrast agent should be made slowly, and one should expect little back pressure. Volume for injection is variable depending on the area of the spinal cord to be visualized and is based on experience with normal patients. A contrast dose of 0.3 mL/kg was found to be adequate for visualization of the caudal extent of the dural sac when administered by cisternal tap.17 Personal experience supports a dosage rate of 0.25 mL/kg, not to exceed a total dose of 10 mL in larger patients. Distribution of the contrast agent requires elevation of the patient's head, neck, and thorax approximately 20 degrees, using gravity to disburse the media to the caudal end-sac. This procedure should result in successful opacification of the caudal limit of the subarachnoid space within 10 minutes. 2 Even using larger volumes, the contrast agent may displace cranial and not flow caudal without the use of gravity distribution. Unobstructed, the contrast should flow to the caudal end-sac of the subarachnoid space in the sacrum of dogs and perhaps even into the coccygeal space in cats. 31 , 32 Myelography in the cat, using the cisternal tap, is performed using the same landmarks and procedures as described for the dog. Injection volume for high-quality opacification in an adult cat requires 1.5 to 2 mL of contrast material. 31 Volume of contrast may also be based on patient weight, using 0.2, 0.45, and 0.5 mL/kg for cervical, thoracolumbar, and complete spinal studies, respectively.23 Lumbar myelography in the dog is usually performed at the intervertebral space between L-4-L-5-L-6. These spaces were selected because L-4-L-5 has been shown to be the caudal limit of the spinal cord in nonchondrodystrophic dogs and L-5-L-6 in the chondrodystrophied breeds. 21 It may be difficult to obtain CSF at this cord level. Therefore one may wish to select L-4-L-5 routinely. Surgical preparation
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of the caudal lumbar region should be made. A 20-gauge, 2- or 21/2inch needle is usually adequate depending on the size and obesity of the patient. The patient should be in lateral recumbency, and the back may be slightly flexed. The needle is placed through the skin at the intervertebral space caudal to the intended space selected for puncture (Le., at L-5-L-6 when the intended space is L-4-L-5). The needle should be advanced until it contacts the dorsal spine of the more cranial vertebrae (L-5) and then directed along the lateral surface of the spinous process at approximately 45 degrees to the topline of the patient. This procedure should direct the needle toward the articular fossa and through the dorsal intervertebral foramen. The hindlimbs and tail will "jump" or "twitch" as the needle passes through the filum terminale of the cord to the floor of the spinal canal. The bevel of the needle should be placed facing the desired direction. The stylet is removed and passed across the surgical glove to see if the needle has entered the subarachnoid space. CSF may be withdrawn for analysis. The quantity of CSF will be less, and it will be more difficult to withdraw than at the atlanto-occipital junction. The contrast material should be injected slowly with no back pressure. Volume of contrast agent required to reach the thoracolumbar junction and the rostral cervical spine is 0.3 mL/kg and 0.5 mL/kg, respectively. Even the largest dog should require less than 10 mL total volume. Lumbar myelography in the cat is performed using the same landmarks and procedures as described in the dog. Percutaneous puncture of the subarachnoid space is advised at L-5-L-6 interspace. Volume of contrast material may be based on patient weight, using 0.2, 0.35, and 0.4 mL/kg for lumbar, thoracolumbar, and complete spinal studies, respectively. 23
Contrast Agents Suitable contrast agents for use as a myelographic drug should be nontoxic to the central nervous system; water-soluble and therefore miscible with the CSF; radiopaque while remaining iso-osmolal; easily and rapidly eliminated from the subarachnoid space; and in veterinary medicine, reasonably affordable. 34 Iohexol, iopamidol, and metrizamide are nonionic, water-soluble, triiodinated benzoic acid-based contrast agents that approach the requirements outlined. Metrizamide has been reported to have adverse postmyelographic effects in humans and other animals. 1, 9,34 The concentration of iodine in the contrast agent accounts for the relative opacification of the subarachnoid space following intrathecal injection of the contrast agent. Concentration of iodine in the range of 180 mg/mL is iso-osmolal, although 240 mg/mL will provide better opacification and does not seem to increase problems following myelography.34 Personal experience favors the use of the lower concentration of iodine augmented by radiographic techniques using lower kVp, higher mAs, and high detail screen-film combination.
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Contrast agents that are used for veterinary myelography are designated for human use. Extra-label use of contrast agents is not an unusual practice in veterinary medicine. Medicolegal aspects may be of little significance since there have been several studies documenting the use of these products in animals and describing the presence or absence of side effects and documenting their safety.2, 6,10,11,24,26,28,31,32,34 The development of iso-osmotic, nonionic contrast agents has revolutionized the practice of myelography in humans and animals. Despite the success rate using current agents, there will soon be a new generation of hypo-osmolar compounds that may be even less reactive. 7 Following is a list of contrast agents currently used in veterinary myelography. 1. Metrizamide (Amipaque, Winthrop-Breon, Laboratories, New York, NY, and Nycomed Oslo, Norway) was the first nonionic, water-soluble contrast agent used for human and veterinary myelography. This agent is no longer listed as advised for myelography and is used for urography and angiography. Because the use of metrizamide is associated with adverse postmyelographic effects in humans and other animals, newer and safer contrast agents have been developed. Problems associated with the use of metrizamide include seizures in animals and psychic changes in humans. Analytical grade metrizamide has been used in veterinary procedures with good results. The methods for diluting and filtering the analytical grade product were time-consuming and cumbersome; however, it made myelography within a cost range acceptable to veterinarians. 2. Iohexol (Omnipaque, Winthrop-Breon, Laboratories, New York, NY, and Nycomed, Oslo, Norway) comes as a sterile solution that requires no additional processing before administration. The cost of this product may limit its use in veterinary medicine. 3. Iopamidol (Isovue, E.R. Squibb & Sons Co., Princeton, NJ, and Niopam, Bracco Industria Chimica, Milan, Italy) is marketed in various concentrations and varying quantity vials. No additional processing is needed before use. Creative sterile procedure may be used to redistribute the contrast agent from large-volume containers (200 mL) to single-dose vials (10 mL) for use in dogs and cats. This provides a contrast agent that is affordable in veterinary medicine. Interpretation of Contrast Myelography
Radiographs of a normal myelogram have contrast filling of the subarachnoid space. Regardless of the projection, the total thickness of contrast will be greatest where the beam projects tangential to the spinal cord. This results ill a radiographic image that appears as "columns" of contrast. In the lateral projection, these columns will
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appear dorsal and ventral to the cord. In the ventrodorsal projection, the columns appear lateral to the cord. The changes that occur in the appearance of the contrast columns are used to describe the spinal level and the anatomic location of the tentative lesion. Normal myelograms may have elevation of the ventral column and thinning of the contrast where it passes over the annuli of the intervertebral discs; however, a wide dorsal column remains to demonstrate cord compression is not present (Fig. 2). Leakage of contrast agent into the epidural space is not unusual and occurs most often when performing a lumbar examination. Epidural contrast material combined with the subarachnoid contrast agent results in a confusing pattern (Fig. 3). Failure to make definitive findings under these circumstances usually results in an aborted study and a repeat of the myelographic examination at a later date. Displacement of the contrast (failure of opacification) of a segment of the subarachnoid space is an indication of diffuse swelling or a space-occupying mass that might compress the spinal cord. The area through which the columns are thinned or disappear gives a spinal level of the lesion. The anatomic description, however, is defined by the appearance of contrast columns as they approach and pass through an area of compression. Extradural lesions are identified when the leading edge of the contrast column tapers toward the spinal cord and away from the bony margin of the osseous canal (Fig. 4). Location and definition of such lesions may require several projections, including oblique projections, to visualize the definitive tangent. The orthogonal view presents an appearance of a widened spinal cord, a finding that may be associated with a mass located anywhere within the spinal canal. The most common example of an extradural lesion is the herniation of an
Figure 2. A 7-year-old Poodle presented with hindlimb weakness and pain over the thoracolumbar region. The lateral cervical and thoracic myelogram was considered normal. The dorsal column was visible throughout the radiograph. Indentations or elevations of the i'entral column at the intervertebral spaces C2-5 and C6-7 were opposed by a normal ::orsal column, indicating no cord compression was present. A compressing lesion was ;Jund at the T-L junction.
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Figure 3. A Rhodesian Ridgeback with sudden hindlimb paralysis was found to have a fracture of T11. A 105-mm photospot film image taken at L1-3 during image intensification studies showed leakage of contrast agent into the epidural space (arrows). This caused a confusing pattern superimposed on the normal ventral subarachnoid column.
intervertebral disc. Extradural spinal cord lesions are the most frequently diagnosed and may account for 50% of those found. 25 Intradural-extramedullary lesions are found in the subarachnoid space and are closely associated with the spinal cord. Because they occupy a space within the subarachnoid volume, they displace contrast agent as a filling defect. Therefore the leading edge of the contrast column should taper toward the spinal cord and toward the bony margin of the osseous spinal canal. When captured in the perfect tangent projection, this pattern is described as a golf tee appearance (Fig. 5). Lesions in this location have been reported to account for 35% of spinal cord compression. 25 Intramedullary lesions may result in swelling within the parenchyma of the spinal cord. A mass in this location causes the leading edge of the contrast column to taper toward the bony margin of the osseous canal (Fig. 6). The tapered edge of this pattern is exactly opposite that which occurs with the\ extradural lesion. Intramedullary lesions result in globular enlargement and present a similar radiographic sign regardless of the projection of the beam. Lesions in this location are less frequent and may account for only 15% of the diagnoses. 25 Primary tumors in the spinal canal account for the majority of the intradural-extramedullary and intramedullary lesions in dogs and cats. 8 , 18, 25 According to one report, spinal disease in the cat may have an unusually high incidence of neoplasia. 30
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Figure 4. A lateral cervicothoracic myelogram of a Doberman Pinscher suspected of having cervical vertebral malformation. There was extradural cord compression of the dorsal and ventral columns at C6-7. The contrast columns deviated away from the margins of the osseous canal toward the spinal cord. The indentation of the ventral column at C5-6 was of concern, although the opposing dorsal column was not completely compressed. Ventral slotting technique was used to decompress both spaces.
TOMOGRAPHY
Conventional radiography gives a two-dimensional image provided from one beam direction. The result is an image of soft tissues and skeletal structures superimposed. X-ray tomography uses the same electromagnetic x-irradiation to produce the image as is used with conventional x-ray imaging. A tomogram is a picture of a slice of anatomic information taken' from a section of the body at a desired orientation.
Linear Tomography
Linear tomography is obtained by moving the x-ray source (tube) in one direction and the detector (cassette with film) in the exact opposite direction. The primary beam is continuously focused on the receiver. The axis or fulcrum of rotation is at a predetermined depth and plane in the subject. The image acquired represents a narrow focused volume in one plane of the patient, and anatomic detail in planes outside of the focused volume is blurred and is presented as a relatively homogeneous summation density. Linear tomography is simple in principle but is fraught with difficulty in locating and identifying pathologic change. This technology
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Figure 5. See legend on opposite page
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was instrumental in the development of computer-assisted tomography (CAT) and CT. Computed Tomography
Using this modality, the unwanted planes or sections of anatomic detail are totally excluded and the desired plane is reconstructed using complex mathematical processing. CT uses the axial scanning procedure to acquire data. The acquisition of data and subsequent processing is done by computer application. Transverse or axial slices of the body are obtained by rotating/revolving an x-ray tube around the patient. Opposing the x-ray tube are sensitive detectors that record the amount of x-ray penetrating the patient through an arc of 180 degrees. The intensity of emerging radiation is recorded, and a digital record is made to be translated and represented as brightness (pixel/voxel density) on a cathode ray tube (CRT). Postacquisition processing allows the operator to investigate windows of radiographic detail and compare x-ray transmission (attenuation) using a standard scale called Hounsfield numbers. The reference standard for CT numbers is zero and represents the attenuation produced in pure water. Negative numbers, less than zero, represent material less attenuating than water (e.g., fat, air) and positive numbers, greater than zero, represent material more attenuating than water (e.g., soft tissue, mineral, bone). Conventional radiography uses a single projection, and an image is recorded in film emulsion according to the relative attenuation of the x-ray beam. Discrimination of densities requires 5% difference. CT uses numerous projections of the same anatomy, and as a result discriminated contrast may be 1% or less. The transmitted radiation intensity is recorded as a digital function representing small cubes (voxels). When viewed perpendicular to the beam direction, these voxels are stacked in respective location on an "X" and "Y" axis forming a cross-sectional image. This image might be compared to a crossword puzzle. Imagine a "cross-image puzzle" representing a thin slice of the patient. A series of such slices (puzzles) when stacked would contain mutual information in the third dimension, or the "Z" axis of the puzzle. This provides a simple task for the
Figure 5. A 10-year-old German Shepherd cross with progressive hindlimb ataxia presented unable to walk. A, Lateral myelogram of the cranial lumbar spine demonstrated a spaceoccupying mass at L2-3, causing peripheral displacement and narrowing of the dorsal and ventral columns. This lesion could have been confused with an intramedullary mass except for the clearly demarcated margins. This sign indicated the filling defect was extramedullaryintradural and contained within the subarachnoid space. Note the presence of contrast agent in the central spinal canal, which was considered to have no immediate pathologic significance. B, The ventrodorsal projection located the filling defect in the right lateral column at the level of L2-3. The contrast margins had a cup-shaped interlace with the tumor. The flared area of the contrast column resembled a golf tee (arrows). The histopathologic diagnosis was neurofibrosarcoma.
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Figure 6. A 1.5-year-old male Brittany with a 7-month history of hindlimb lameness and ataxia. Image intensification studies at L4-5 following myelography were recorded on 105mm photospot film. A, Lateral, B, right dorsal oblique, and C, left dorsal oblique projections all showed medullary spinal cord expansion with contrast columns compressed and deviated toward. the osseous margins of the spinal canal. Histopathologic diagnosis was focal medullary granulomatous myelitis.
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computer to locate the appropriate liZ" axis voxels and reconstruct an image in any desired longitudinal plane. Images created by CT are the product of diagnostic x-rays; thus the interpretive skills require the same fundamentals as with conventional radiography. Differences are found in the improved detail and increased information that is acquired when using digital acquisition of images. A knowledge of cross-sectional anatomy is required, and knowledge of anatomy of soft tissues and bone is essential to accurate interpretation. Locations that provide CT services will have adequate expertise to assist with interpretation of the images. One might also solicit the assistance of a veterinary radiologist, most of whom are qualified to interpret CT images. CT is routinely used for examining small animals, although it is also used for examination of large patients. 4 This technology can demonstrate differences in soft tissue anatomy and has been used in conjunction with myelography to define lesions in the spinal canal (Fig. 7). Despite its contribution to soft tissue imaging, the greatest value of this modality has been to define changes in bony tissues (Fig. 7C,D).
MAGNETIC RESONANCE IMAGING
MRI is a relatively new imaging modality with considerable potential for visualizing soft tissues. This has enhanced our ability to study the anatomy and pathologic changes of the brain and spinal cord in vivo. MRI has been used by chemists for many years, and during its medical development it was hoped that MRI signals would allow for differentiation of specific pathology change. Although more abnormalities could be detected, unfortunately characterizing abnormalities was less than hoped. Contrast-enhanced MRI is generally accepted as the study of choice for numerous central nervous system diseases in humans. 5 Protocols for spinal imaging in humans have been established.12, 19 Survey and enhanced MRI is routinely used for detection and study of canine brain tumors. 14, 15,22 Anatomy of the canine brain on MRI has been published. 16 The canine spine has been the subject of MRI imaging. 27 Diagnostic clinical studies at this institution have included the use of MRI. 20 MRI using low field magnets in mobile configuration or MRI centers willing to image animals are being used by private practitioners in the United States and in other parts of the world. 13 An example of MRI images produced, in a rural area in southern Idaho, of a dog with a transmedullary lesion created by traumatic disc protrusion serves to emphasize availability of sophisticated imaging modalities to the practicing veterinarian regardless of their geographic constraints (Fig. 8). Factors that limit the use of any procedure usually stem from a lack of familiarity with the fundamentals and intimidation by the technology. The following discussion is designed to simplify the phys-
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Figure 7. An 11-year-old Weimaraner with a history of 3 weeks of progressive ataxia and 7 weeks of tetraparesis. A, Lateral survey radiographs showed extensive remodeling of vertebral bodies and end plates with osseous proliferation at T5-9. These findings were compatible with spondylitis and discospondylitis but were not consistent with the clinical signs. B, Lateral projection on the cervicothoracic myelography showed disruption of the contrast columns and obstruction of subarachnoid filling between C5-T3. Computed tomography was performed following myelography.
ical foundations of MRI so that one can understand how the image is created and why s'oft tissues, especially those of the central nervous system, can be visualized. Other simplifications have been published.3, 29, 33 MRI requires a strong magnetic field (minimum 17,000 times that of earth's gravity) within which the patient is placed. Available protons (hydrogen atoms) in the patient have a positive charge and are spinning (precession about an axis). A moving electrical charge is a current, and an electric current induces a magnetic field. Therefore the patient has numerous magnetic fields (H+). Within the external magnetic field, the patient's protons align parallel or antiparallel to the external field. Parallel forces cancel antiparallel forces, and the sum represents a
Figure 7 Continued. C, The section through T1 documented destruction of the left body and lamina of the vertebrae with bony displacement. No contrast agent was present, and soft-tissue detail was disrupted by a mass with soft-tissue density surrounded by a bony margin. 0, The left parasagittal image reconstructed through C6,7 and T1,2 recorded the cranial and caudal limits of the bony remodeling. E, The computed tomography section through the caudal aspect of C4 of the same patient was normal and is presented for comparison.
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Figure 8. See legend on opposite page
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magnetic field within the patient. The patient's magnetic field is now aligned with the external field and cannot be measured. A highfrequency radio wave, having the same frequency as the precessing protons, is introduced into the patient. Protons can acquire energy from the radio wave, a phenomenon called resonance, causing the protons to precess in phase. The sum of their vectors is now transverse to the external magnetic field. The radiofrequency pulse is discontinued, and the transverse magnetic field of the patient begins to realign with the external field. A moving magnetic field induces an electrical current in an antenna, and thus an MRI signal is captured from the patient. This signal is translated to form the image recorded on a CRT similar to other digital image systems. Increased brightness on the CRT represents stronger signal intensity. MRI is said to provide a proton map, and it is this map that charts anatomic information and pathologic changes. Altering the manner by which the signal is acquired, one is able to observe longitudinal relaxation time (T1-weighted image) and transverse relaxation time (T2-weighted image). T1-weighted images are dependent on tissue composition, structure, and surroundings and are best thought of as a reflection of the degree of organization of a substance. T2-weighted images depend on the physical state of a substance and are best thought of as the degree of fluidity (the opposite of viscosity) of a medium. Postacquisition manipulation of the data allows one to reconstruct images of sections of the patient made through virtually_ any plane (i.e., sagittal, parasagittal, coronal, axial). Interpretation of MRI images requires considerable knowledge of the physics of data acquisition using this modality. Experience is needed to become competent and ultimately confident in one's interpretive skills. Fortunately the anatomic similarities among human and animal tissues are such that anyone familiar with the technology can provide diagnostic interpretation. Interpretation and recognition of gross anatomy are simple and present no obstacle to the veterinary diagnostician. Any provider of MRI services will have the expertise to interpret the scans and explain the methods of data acquisition.
Figure 8. A 7.5-year-old mixed-breed dog was struck by a car and was quadriplegic but had deep pain response. A, The midsagittal MRI image confirmed the presence of a transmedullary lesion at C2-3. The intervertebral disc mass was reduced and the space collapsed, which indicated that a disc protrusion had caused the cord damage. Note the absence of signals (black) from the bone of the vertebral bodies and the intense signals (white) from the intervertebral discs and epidural fat. B, Six transverse reconstructs of sections at 1.5-mm thickness (designated by S 17.0 through S 24.5 in the upper left of each section). These sections were made through the caudal aspect of C2 and the C2-3 interspace. S 17.0 revealed a dilated central canal, but some cord parenchyma was visible peripherally. There was evidence of increased disruption of the spinal cord as the sections progressed caudally.
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SUMMARY
Various methods of documenting pathologic change in the spine and spinal cord are available to the veterinary practitioner. Intimidation caused by the imaging modality and the fear that one will not be able to recognize or diagnose a lesion are the factors that limit the use of diagnostic imaging. One needs only to be able to recognize the variations of normal anatomy to be successful. Once an abnormal area has been identified, the diagnosis is soon to follow. Therefore one should concentrate on improving the simple skills associated with image interpretation and normal anatomy. Lesion identification and definitive diagnoses will follow by natural progression. References 1. Adams WM, Stowater JL: Complications of metrizamide myelography in the dog: A summary of 107 clinical case histories. J Am Vet Radiol Soc 22:27, 1981 2. Allan GS, Wood AKW: Iohexol myelography in the dog. Vet Radio129:78, 1988 3. Anon: Magnetic resonance: New vision in medicine. Milwaukee, WI, General Electric Company, Medical Systems Group, 1985 4. Barbee DO, Allen JR, Gavin PR: Computed tomography in horses: Technique. Vet Radiol 28:144, 1987 5. Bronen RA, Sze G: Magnetic resonance imaging contrast agents: Theory and application to the central nervous system. J Neurosurg 73:820, 1990 6. Cox FH, Jakovljevic S: The use of iopamidol for myelography in dogs: A study of twenty-seven cases. J Sm Anim Pract 27:159, 1986 7. Dennis R, Herrtage ME: Low osmolar contrast media: A review. Vet Radiol 30:2, 1989 8. Fingeroth JM, Prata RG, Patnaik AK: Spinal meningiomas in dogs: 13 cases (19721987). J Am Vet Med Assoc 191:720, 1987 9. Gray PR, Indrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc 190:527, 1987 10. Gupta RC, Gupta SC, Dubey RK: An experimental study of different contrast media in the epidural space. Spine 9:778, 1984 11. Haughton VM: Intrathecal toxicity of iohexol vs. metrizamide. Survey and current state. Invest Radiol 20(suppl 1):S14, 1985 12. Hayman RA, Gorey MT: Imaging strategies for MR of the spine. Radiol Clin North Am 26:505, 1988 13. Karkkainen M, Mero M, Nummi P, et al: Low field magnetic resonance imaging of the canine central nervous system. Vet Radiol 32:71, 1991 14. Kraft SL, Gavin PR, Leathers CW, et al: Diffuse cerebral and leptomeningeal astrocytoma in dogs: MR features. J Comput Assist Tomogr 14:555, 1990 15. Kraft SL, Gavin PR, Moore MP, et al: An MR diagnosis of canine meningioma. Vet Radiol 32:5, 1990 16. Kraft SL, Gavin PR, Wendling LR, et al: Canine brain anatomy on magnetic resonance images. Vet Radiol 30:147, 1989 17. Lang J: Flexion-extension myelography of the canine cauda equina. Vet Radio129:242, 1988 18. Luttgen PJ, Braund KG, Brawner Jr WR, et al: A retrospective study of twenty-nine spinal tumours in the dog and cat. J Small Anim Pract 21:213, 1980 19. Modic MT, Masaryk T, Paushter C: Magnetic resonance imaging of the spine. Radiol Clin North Am 26:229, 1986 20. Moore MP, Gavin PR, Kraft SL, et al: MR, CT and clinical features from dogs with nasal tumors involving the rostral cerebrum and nasal passages. Vet Radiol 32:19, 1991
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21. Morgan JP, Atilola M, Bailey CS: Vertebral canal and spinal cord mensuration: A comparative study of its effect on lumbosacral myelography in the Dachshund and German Shepherd Dog. J Am Vet Med Assoc 191:951, 1987 22. Panciera DL, Duncan 10, Messing A, et al: Magnetic resonance imaging in two dogs with central nervous system disease. J Small Anim Pract 28:587, 1987 23. Pardo AD, Morgan JP: Myelography in the cat: A comparison of cisternal versus lumbar puncture, using metrizamide. Vet Radiol 29:89, 1988 24. Pasaoglu A, Gok A, Patiroglu TE: An experimental evaluation of response to contrast media: Pantopaque, iopamidol, and iohexol in the subarachnoid space. Invest Radiol 23:762, 1988 25. Prata RG: Diagnosis of spinal cord tumors in the dog. Vet Clin North Am [Sm Anim Pract] 7:165, 1977 26. Puglisi TA, Green RW, Hall CL, et al: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863, 1986 27. Sether LA, Nguyen C, Yu SN, et al: Canine intervertebral disks: Correlation of anatomy and MR imaging. Radiology 175:207, 1990 28. Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Compend Contin Educ Pract Vet 9:361, 1987 29. Sochurek H, Miller P: Medicine's new vision. National Geographic 171:2, 1987 30. Wheeler SJ: Spinal tumours in cats. Vet Annual 29:270, 1989 31. Wheeler SJ, Clayton-Jones DG, Wright JA: Myelography in the cat. J Small Anim Pract 26:143, 1985 32. Wheeler SJ, Davies JV: Iohexol myelography in the dog and cat: A series of one hundred cases, and a comparison with metrizamide and iopamidol. J Small Anim Pract 26:247, 1985 33. Widmer WR, Buckwalter KA, Braunstein EM, et al: Principles of magnetic resonance imaging and application to the stifle joint in dogs. J Am Vet Med Assoc 198:1914, 1991 34. Widmer WR: Iohexol and iopamidol: New contrast media for veterinary myelography. J Am Vet Med Assoc 194:1714, 1989
Address reprint requests to Ronald D. Sande, DVM, MS, PhD 146 McCoy Hall Washington State University College of Veterinary Medicine Pullman, WA 99164-6610
0195-5616/92 $0.00 + .20
RADIOGRAPHY, MYELOGRAPHY, COMPUTED TOMOGRAPHY, AND MAGNETIC RESONANCE IMAGING OF THE SPINE Ronald D. Sande, DVM, MS, PhD
Study of the axial skeleton and spinal cord has traditionally been performed using radiography to record the images. Conventional radiology still remains the most convenient, cost-effective, and familiar modality available to practitioners. Regardless of its tenure as the preferred method of imaging, radiography remains a sophisticated procedure that requires exacting technique and application. Alternate methods of imaging are currently available to veterinary practitioners, the availability and cost-effectiveness of which are improving. Computed tomography (CT) is commonly used in veterinary diagnostics, and magnetic resonance imaging (MRI) is rapidly becoming a practical alternative to x-ray imaging.
RADIOGRAPHY
Spinal radiography may be a frustrating or a rewarding experience. Success requires that one observe fundamental rules of radiography." Survey radiographs of the spine must be performed with absolute perfect positioning and excellent diagnostic detail. The probability of From the Department of Clinical Medicine and Surgery, Washington State University College of Veterinary Medicine, Pullman, Washington
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irreversible or fatal error is significant, and one must be certain of procedures, findings, diagnosis, and treatment.
Survey Radiography
Survey radiography should be performed after a thorough clinical examination. A complete neurologic examination should result in tentative location of the disease and perhaps even an expected anatomic description or etiology. Radiography may be performed according to a series of predetermined routine projections or a selective study based on the differential diagnoses derived during the neurologic and clinical examination. This radiologist prefers the latter because it encourages the diagnostic talents of clinical personnel, reduces unnecessary manipulation of the patient, and often avoids invasive procedures in a compromised patient. An added bonus is reduced cost to the client. It is preferable to have the patient heavily sedated or anesthetized to facilitate positioning. The radiographic projections should be made with the patient in perfect lateral and ventrodorsal position, respectively. The use of sandbags or other positioning aids will reduce exposure to attendants and result in more consistent positioning. Oblique projections are misleading and difficult to interpret owing to unfamiliar edge effects and other perceptual distortions. Films with suboptimal positioning should be retaken. Adequate positioning may be obtained in the lateral projection if the dorsal and ventral midlines of the patient are equidistant from the cassette. The axial skeleton should be extended and the central x-ray beam centered directly over the area of interest. The ventrodorsal projection should be made when the ventral midline and the dorsal midline are superimposed. In both projections, it is necessary to use foam pads and wedges to prevent "sagging" of the spine and insure that the spine is parallel to the cassette and film. Errors caused by parallax are the most frequent cause of misdiagnosis. Seven radiographic projections are necessary to complete a minimum examination of the cervical spine to the sacrum. The cervical spine, thoracic spine, thoracolumbar junction, and the lumbar segments should be visualized independently. Lateral projections centered on each area and ventrodorsal projections of the cervical, thoracolumbar, and lumbar spine are most informative. Ventrodorsal projection of the thoracic spine is confusing and usually not productive owing to superimposition of the sternum and the contribution of osseous detail provided by the dorsal spinous processes of the thoracic vertebrae. The central x-ray beam should be centered at each location and the beam limited to include only the axial skeleton; otherwise beam divergence and scattered radiation will result in loss of detail. Additional projections may be selected following evaluation of the survey films. Radiographic technique should be developed to maximize detail. High detail film and screen combinations using relatively higher mil-
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liamperage and moderate kilovolt peak will result in the best film detail. This type of technique will produce better film quality during contrast studies also.
Interpretation of Survey Radiographs
Interpretation of survey radiographs of the spine requires that the diagnostician understand the form and function of all components of the axial skeleton. Each vertebra or section of the spine performs a distinct function. Development (form) of a spinal segment has been derived to perform the required function. The spine has no paired anatomic structure such as the appendicular skeleton for comparison. Therefore the radiographer must compare radiographs of the spine to an expected mental standard. The atlanto-occipital junction provides flexion and extension of the head (the "yes" function). Fulcrum for this function is the joint, and the lever and moment arm of the function are the nuchal protuberance, the squamous part of the occipital, and basilar portion of the occipital bone. The atlantoaxial joint provides for rotation of the head about the axis of the "axial" skeleton (the "no" function). Leverage for performing this function is provided by the wings of the atlas. These vertebrae perform specific functions and are quite different from other vertebrae. The junction of C-3 with the axis is a transition of function, where lateral motion, elevation, and declination of the head and neck occur in the cervical spine. Cervical vertebrae exhibit gradual change in anatomic appearance as the transverse processes become progressively larger, being largest on C-6, and disappearing on C-7. C-7 to T-l is the next transitional space as the ribs replace transverse processes and the articular facets form sliding joints. The next transition occurs at T-IOT-Il, where the articular facets change from sliding joints to rotational joints. This site is usually narrower than the adjacent spaces and may be confused as having a protruded disc. The final transition of significance occurs at L-7-S-1. This segment often has congenital anomalies, usually of no clinical significance. Interpretation of radiographic signs must be made by recognition and recall when examining the atlas, axis, and each of the transitional spaces. The remaining vertebrae and their respective intervertebral spaces are best evaluated by comparing each one to the vertebral segment immediately cranial and immediately caudal. Therefore one evaluates the radiograph by studying groups of three vertebrae. Transition in anatomic features is gradual, and the vertebrae and their respective interspaces and articulations are sufficiently similar to use groups of three for controlled r.eference. The principles of interpretation can be used in any species. One needs only to recognize basic species differences. Figure 1 has examples of well-positioned projections of a canine spine demonstrating normal anatomic features and some degenerative disc disease.
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Figure 1. See legend on opposite page
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MYELOGRAPHY Myelography should be considered only after thorough neurologic examination has indicated a profound or progressive disease of the central nervous system and survey radiographs have failed to provide substantive findings. Myelography should be considered if the neurologic signs and the survey radiographs appear to be in conflict; also myelography might be useful in providing an accurate indication of cord compression. Myelography is an invasive technique with a significant morbidity, although low mortality, and it is expensive for the client. The patient must be anesthetized for the procedure, and preanesthesia should include diazepam. Promazine derivatives are contraindicated because they decrease the convulsive threshold and contribute to unfavorable recovery and increased seizure activity. One should select the site for puncture according to the neurologic signs and the level at which the disease is expected. The cisternal or atlanto-occipital puncture is the simplest to perform, and the location is one from which cerebrospinal fluid (CSF) can be easily removed. Myelography using the cisternal tap is selected most often. It is not unusual, however, to select either the cisternal or lumbar site and fail to produce adequate opacification of the entire subarachnoid space. Failure to define a lesion may result owing to obstruction of the flow of contrast material to or around the lesion. When this occurs, one should not hesitate to perform myelography from the alternate location and introduce the contrast agent from the opposite direction. Using this method, one should be able to define the area or section of the cord with pathologic change. Myelography using the cisternal site requires surgical preparation of an area from the nuchal crest to the second cervical vertebrae and
Figure 1. A lateral survey study of the spine of a 5-year-old Dachshund with undefined back pain. A, This projection was centered at the mid-cervical spine. The atlanto-occipital and atlanto-axial junctions were difficult to interpret, and the shape of each vertebrae is unique to their respective function. The intervertebral spaces at C2-3 and at C7-T1 appeared narrow. Note the similarity and gradual change of anatomy of the articular facets, dorsal spinous processes, and transverse processes. B, Lateral projection centered on the thoracic spine. Note the gradual transition of the shape and direction of the dorsal spinous processes. Sliding articulations of T1-1 0 undergo transition at T10-11 to a rotational joint, and the intervertebral space was narrow as expected. There was degenerative mineralization of the intervertebral discs from T2-1 O. C, Lateral projection centered at the thoracolumbar junction. T10-11 appeared narrow, consistent with the transition of articular function at that site. T11-12 was narrow, and there was degenerative mineralization of the intervertebral disc at T12-13, either of which may have pathologic significance. D, Lateral projection centered on the lumbar spine. Note- the similarity but gradual change in the architecture of dorsal spinous processes and shape and size of the intervertebral foramina and vertebral bodies. There was degenerative mineralization of intervertebral discs at L3-6. Compare intervertebral foramina at L2-4 and note the subtle opacification at L3-4, where an intervertebral disc had herniated.
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between the ears. A 20- to 22-gauge, 1- to 1V2-inch short bevel spinal needle is adequate even for large canine patients. The patient is placed in lateral recumbency, the head should be elevated, and the head and neck should be placed in flexion. The needle is inserted percutaneously on the dorsal midline of the neck at its intersection with a line between the rostral limits of the wings of the atlas. The needle should be directed slightly rostral. Entry into the subarachnoid space is recognized by a change in the "drag" or friction as the needle penetrates the ligamentum flavum. The bevel of the needle should be inserted into the dorsal subarachnoid space and placed such that the bevel is toward the desired direction of flow of the contrast agent. Removal of the stylet is usually followed by drip or steady flow of CSF unless low pressure is encountered, in which case gentle aspiration may be required. Passing the stylet across the back of the rubber glove should leave a faint wet streak if the needle has penetrated the subarachnoid space (CSF will ascend the needle by capillary action, and the stylet will leave a wet line on the glove). Hemorrhagic CSF or frank blood indicates the needle has deviated lateral to the midline and entered a vertebral sinus. The needle should be withdrawn and discarded and the procedure begun anew. A volume of CSF equal to the intended dose of contrast agent may be removed if excessive intracranial pressure is not a problem. Injection of the contrast agent should be made slowly, and one should expect little back pressure. Volume for injection is variable depending on the area of the spinal cord to be visualized and is based on experience with normal patients. A contrast dose of 0.3 mL/kg was found to be adequate for visualization of the caudal extent of the dural sac when administered by cisternal tap.17 Personal experience supports a dosage rate of 0.25 mL/kg, not to exceed a total dose of 10 mL in larger patients. Distribution of the contrast agent requires elevation of the patient's head, neck, and thorax approximately 20 degrees, using gravity to disburse the media to the caudal end-sac. This procedure should result in successful opacification of the caudal limit of the subarachnoid space within 10 minutes. 2 Even using larger volumes, the contrast agent may displace cranial and not flow caudal without the use of gravity distribution. Unobstructed, the contrast should flow to the caudal end-sac of the subarachnoid space in the sacrum of dogs and perhaps even into the coccygeal space in cats. 31 , 32 Myelography in the cat, using the cisternal tap, is performed using the same landmarks and procedures as described for the dog. Injection volume for high-quality opacification in an adult cat requires 1.5 to 2 mL of contrast material. 31 Volume of contrast may also be based on patient weight, using 0.2, 0.45, and 0.5 mL/kg for cervical, thoracolumbar, and complete spinal studies, respectively.23 Lumbar myelography in the dog is usually performed at the intervertebral space between L-4-L-5-L-6. These spaces were selected because L-4-L-5 has been shown to be the caudal limit of the spinal cord in nonchondrodystrophic dogs and L-5-L-6 in the chondrodystrophied breeds. 21 It may be difficult to obtain CSF at this cord level. Therefore one may wish to select L-4-L-5 routinely. Surgical preparation
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of the caudal lumbar region should be made. A 20-gauge, 2- or 21/2inch needle is usually adequate depending on the size and obesity of the patient. The patient should be in lateral recumbency, and the back may be slightly flexed. The needle is placed through the skin at the intervertebral space caudal to the intended space selected for puncture (Le., at L-5-L-6 when the intended space is L-4-L-5). The needle should be advanced until it contacts the dorsal spine of the more cranial vertebrae (L-5) and then directed along the lateral surface of the spinous process at approximately 45 degrees to the topline of the patient. This procedure should direct the needle toward the articular fossa and through the dorsal intervertebral foramen. The hindlimbs and tail will "jump" or "twitch" as the needle passes through the filum terminale of the cord to the floor of the spinal canal. The bevel of the needle should be placed facing the desired direction. The stylet is removed and passed across the surgical glove to see if the needle has entered the subarachnoid space. CSF may be withdrawn for analysis. The quantity of CSF will be less, and it will be more difficult to withdraw than at the atlanto-occipital junction. The contrast material should be injected slowly with no back pressure. Volume of contrast agent required to reach the thoracolumbar junction and the rostral cervical spine is 0.3 mL/kg and 0.5 mL/kg, respectively. Even the largest dog should require less than 10 mL total volume. Lumbar myelography in the cat is performed using the same landmarks and procedures as described in the dog. Percutaneous puncture of the subarachnoid space is advised at L-5-L-6 interspace. Volume of contrast material may be based on patient weight, using 0.2, 0.35, and 0.4 mL/kg for lumbar, thoracolumbar, and complete spinal studies, respectively. 23
Contrast Agents Suitable contrast agents for use as a myelographic drug should be nontoxic to the central nervous system; water-soluble and therefore miscible with the CSF; radiopaque while remaining iso-osmolal; easily and rapidly eliminated from the subarachnoid space; and in veterinary medicine, reasonably affordable. 34 Iohexol, iopamidol, and metrizamide are nonionic, water-soluble, triiodinated benzoic acid-based contrast agents that approach the requirements outlined. Metrizamide has been reported to have adverse postmyelographic effects in humans and other animals. 1, 9,34 The concentration of iodine in the contrast agent accounts for the relative opacification of the subarachnoid space following intrathecal injection of the contrast agent. Concentration of iodine in the range of 180 mg/mL is iso-osmolal, although 240 mg/mL will provide better opacification and does not seem to increase problems following myelography.34 Personal experience favors the use of the lower concentration of iodine augmented by radiographic techniques using lower kVp, higher mAs, and high detail screen-film combination.
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Contrast agents that are used for veterinary myelography are designated for human use. Extra-label use of contrast agents is not an unusual practice in veterinary medicine. Medicolegal aspects may be of little significance since there have been several studies documenting the use of these products in animals and describing the presence or absence of side effects and documenting their safety.2, 6,10,11,24,26,28,31,32,34 The development of iso-osmotic, nonionic contrast agents has revolutionized the practice of myelography in humans and animals. Despite the success rate using current agents, there will soon be a new generation of hypo-osmolar compounds that may be even less reactive. 7 Following is a list of contrast agents currently used in veterinary myelography. 1. Metrizamide (Amipaque, Winthrop-Breon, Laboratories, New York, NY, and Nycomed Oslo, Norway) was the first nonionic, water-soluble contrast agent used for human and veterinary myelography. This agent is no longer listed as advised for myelography and is used for urography and angiography. Because the use of metrizamide is associated with adverse postmyelographic effects in humans and other animals, newer and safer contrast agents have been developed. Problems associated with the use of metrizamide include seizures in animals and psychic changes in humans. Analytical grade metrizamide has been used in veterinary procedures with good results. The methods for diluting and filtering the analytical grade product were time-consuming and cumbersome; however, it made myelography within a cost range acceptable to veterinarians. 2. Iohexol (Omnipaque, Winthrop-Breon, Laboratories, New York, NY, and Nycomed, Oslo, Norway) comes as a sterile solution that requires no additional processing before administration. The cost of this product may limit its use in veterinary medicine. 3. Iopamidol (Isovue, E.R. Squibb & Sons Co., Princeton, NJ, and Niopam, Bracco Industria Chimica, Milan, Italy) is marketed in various concentrations and varying quantity vials. No additional processing is needed before use. Creative sterile procedure may be used to redistribute the contrast agent from large-volume containers (200 mL) to single-dose vials (10 mL) for use in dogs and cats. This provides a contrast agent that is affordable in veterinary medicine. Interpretation of Contrast Myelography
Radiographs of a normal myelogram have contrast filling of the subarachnoid space. Regardless of the projection, the total thickness of contrast will be greatest where the beam projects tangential to the spinal cord. This results ill a radiographic image that appears as "columns" of contrast. In the lateral projection, these columns will
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appear dorsal and ventral to the cord. In the ventrodorsal projection, the columns appear lateral to the cord. The changes that occur in the appearance of the contrast columns are used to describe the spinal level and the anatomic location of the tentative lesion. Normal myelograms may have elevation of the ventral column and thinning of the contrast where it passes over the annuli of the intervertebral discs; however, a wide dorsal column remains to demonstrate cord compression is not present (Fig. 2). Leakage of contrast agent into the epidural space is not unusual and occurs most often when performing a lumbar examination. Epidural contrast material combined with the subarachnoid contrast agent results in a confusing pattern (Fig. 3). Failure to make definitive findings under these circumstances usually results in an aborted study and a repeat of the myelographic examination at a later date. Displacement of the contrast (failure of opacification) of a segment of the subarachnoid space is an indication of diffuse swelling or a space-occupying mass that might compress the spinal cord. The area through which the columns are thinned or disappear gives a spinal level of the lesion. The anatomic description, however, is defined by the appearance of contrast columns as they approach and pass through an area of compression. Extradural lesions are identified when the leading edge of the contrast column tapers toward the spinal cord and away from the bony margin of the osseous canal (Fig. 4). Location and definition of such lesions may require several projections, including oblique projections, to visualize the definitive tangent. The orthogonal view presents an appearance of a widened spinal cord, a finding that may be associated with a mass located anywhere within the spinal canal. The most common example of an extradural lesion is the herniation of an
Figure 2. A 7-year-old Poodle presented with hindlimb weakness and pain over the thoracolumbar region. The lateral cervical and thoracic myelogram was considered normal. The dorsal column was visible throughout the radiograph. Indentations or elevations of the i'entral column at the intervertebral spaces C2-5 and C6-7 were opposed by a normal ::orsal column, indicating no cord compression was present. A compressing lesion was ;Jund at the T-L junction.
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Figure 3. A Rhodesian Ridgeback with sudden hindlimb paralysis was found to have a fracture of T11. A 105-mm photospot film image taken at L1-3 during image intensification studies showed leakage of contrast agent into the epidural space (arrows). This caused a confusing pattern superimposed on the normal ventral subarachnoid column.
intervertebral disc. Extradural spinal cord lesions are the most frequently diagnosed and may account for 50% of those found. 25 Intradural-extramedullary lesions are found in the subarachnoid space and are closely associated with the spinal cord. Because they occupy a space within the subarachnoid volume, they displace contrast agent as a filling defect. Therefore the leading edge of the contrast column should taper toward the spinal cord and toward the bony margin of the osseous spinal canal. When captured in the perfect tangent projection, this pattern is described as a golf tee appearance (Fig. 5). Lesions in this location have been reported to account for 35% of spinal cord compression. 25 Intramedullary lesions may result in swelling within the parenchyma of the spinal cord. A mass in this location causes the leading edge of the contrast column to taper toward the bony margin of the osseous canal (Fig. 6). The tapered edge of this pattern is exactly opposite that which occurs with the\ extradural lesion. Intramedullary lesions result in globular enlargement and present a similar radiographic sign regardless of the projection of the beam. Lesions in this location are less frequent and may account for only 15% of the diagnoses. 25 Primary tumors in the spinal canal account for the majority of the intradural-extramedullary and intramedullary lesions in dogs and cats. 8 , 18, 25 According to one report, spinal disease in the cat may have an unusually high incidence of neoplasia. 30
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Figure 4. A lateral cervicothoracic myelogram of a Doberman Pinscher suspected of having cervical vertebral malformation. There was extradural cord compression of the dorsal and ventral columns at C6-7. The contrast columns deviated away from the margins of the osseous canal toward the spinal cord. The indentation of the ventral column at C5-6 was of concern, although the opposing dorsal column was not completely compressed. Ventral slotting technique was used to decompress both spaces.
TOMOGRAPHY
Conventional radiography gives a two-dimensional image provided from one beam direction. The result is an image of soft tissues and skeletal structures superimposed. X-ray tomography uses the same electromagnetic x-irradiation to produce the image as is used with conventional x-ray imaging. A tomogram is a picture of a slice of anatomic information taken' from a section of the body at a desired orientation.
Linear Tomography
Linear tomography is obtained by moving the x-ray source (tube) in one direction and the detector (cassette with film) in the exact opposite direction. The primary beam is continuously focused on the receiver. The axis or fulcrum of rotation is at a predetermined depth and plane in the subject. The image acquired represents a narrow focused volume in one plane of the patient, and anatomic detail in planes outside of the focused volume is blurred and is presented as a relatively homogeneous summation density. Linear tomography is simple in principle but is fraught with difficulty in locating and identifying pathologic change. This technology
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Figure 5. See legend on opposite page
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was instrumental in the development of computer-assisted tomography (CAT) and CT. Computed Tomography
Using this modality, the unwanted planes or sections of anatomic detail are totally excluded and the desired plane is reconstructed using complex mathematical processing. CT uses the axial scanning procedure to acquire data. The acquisition of data and subsequent processing is done by computer application. Transverse or axial slices of the body are obtained by rotating/revolving an x-ray tube around the patient. Opposing the x-ray tube are sensitive detectors that record the amount of x-ray penetrating the patient through an arc of 180 degrees. The intensity of emerging radiation is recorded, and a digital record is made to be translated and represented as brightness (pixel/voxel density) on a cathode ray tube (CRT). Postacquisition processing allows the operator to investigate windows of radiographic detail and compare x-ray transmission (attenuation) using a standard scale called Hounsfield numbers. The reference standard for CT numbers is zero and represents the attenuation produced in pure water. Negative numbers, less than zero, represent material less attenuating than water (e.g., fat, air) and positive numbers, greater than zero, represent material more attenuating than water (e.g., soft tissue, mineral, bone). Conventional radiography uses a single projection, and an image is recorded in film emulsion according to the relative attenuation of the x-ray beam. Discrimination of densities requires 5% difference. CT uses numerous projections of the same anatomy, and as a result discriminated contrast may be 1% or less. The transmitted radiation intensity is recorded as a digital function representing small cubes (voxels). When viewed perpendicular to the beam direction, these voxels are stacked in respective location on an "X" and "Y" axis forming a cross-sectional image. This image might be compared to a crossword puzzle. Imagine a "cross-image puzzle" representing a thin slice of the patient. A series of such slices (puzzles) when stacked would contain mutual information in the third dimension, or the "Z" axis of the puzzle. This provides a simple task for the
Figure 5. A 10-year-old German Shepherd cross with progressive hindlimb ataxia presented unable to walk. A, Lateral myelogram of the cranial lumbar spine demonstrated a spaceoccupying mass at L2-3, causing peripheral displacement and narrowing of the dorsal and ventral columns. This lesion could have been confused with an intramedullary mass except for the clearly demarcated margins. This sign indicated the filling defect was extramedullaryintradural and contained within the subarachnoid space. Note the presence of contrast agent in the central spinal canal, which was considered to have no immediate pathologic significance. B, The ventrodorsal projection located the filling defect in the right lateral column at the level of L2-3. The contrast margins had a cup-shaped interlace with the tumor. The flared area of the contrast column resembled a golf tee (arrows). The histopathologic diagnosis was neurofibrosarcoma.
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Figure 6. A 1.5-year-old male Brittany with a 7-month history of hindlimb lameness and ataxia. Image intensification studies at L4-5 following myelography were recorded on 105mm photospot film. A, Lateral, B, right dorsal oblique, and C, left dorsal oblique projections all showed medullary spinal cord expansion with contrast columns compressed and deviated toward. the osseous margins of the spinal canal. Histopathologic diagnosis was focal medullary granulomatous myelitis.
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computer to locate the appropriate liZ" axis voxels and reconstruct an image in any desired longitudinal plane. Images created by CT are the product of diagnostic x-rays; thus the interpretive skills require the same fundamentals as with conventional radiography. Differences are found in the improved detail and increased information that is acquired when using digital acquisition of images. A knowledge of cross-sectional anatomy is required, and knowledge of anatomy of soft tissues and bone is essential to accurate interpretation. Locations that provide CT services will have adequate expertise to assist with interpretation of the images. One might also solicit the assistance of a veterinary radiologist, most of whom are qualified to interpret CT images. CT is routinely used for examining small animals, although it is also used for examination of large patients. 4 This technology can demonstrate differences in soft tissue anatomy and has been used in conjunction with myelography to define lesions in the spinal canal (Fig. 7). Despite its contribution to soft tissue imaging, the greatest value of this modality has been to define changes in bony tissues (Fig. 7C,D).
MAGNETIC RESONANCE IMAGING
MRI is a relatively new imaging modality with considerable potential for visualizing soft tissues. This has enhanced our ability to study the anatomy and pathologic changes of the brain and spinal cord in vivo. MRI has been used by chemists for many years, and during its medical development it was hoped that MRI signals would allow for differentiation of specific pathology change. Although more abnormalities could be detected, unfortunately characterizing abnormalities was less than hoped. Contrast-enhanced MRI is generally accepted as the study of choice for numerous central nervous system diseases in humans. 5 Protocols for spinal imaging in humans have been established.12, 19 Survey and enhanced MRI is routinely used for detection and study of canine brain tumors. 14, 15,22 Anatomy of the canine brain on MRI has been published. 16 The canine spine has been the subject of MRI imaging. 27 Diagnostic clinical studies at this institution have included the use of MRI. 20 MRI using low field magnets in mobile configuration or MRI centers willing to image animals are being used by private practitioners in the United States and in other parts of the world. 13 An example of MRI images produced, in a rural area in southern Idaho, of a dog with a transmedullary lesion created by traumatic disc protrusion serves to emphasize availability of sophisticated imaging modalities to the practicing veterinarian regardless of their geographic constraints (Fig. 8). Factors that limit the use of any procedure usually stem from a lack of familiarity with the fundamentals and intimidation by the technology. The following discussion is designed to simplify the phys-
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Figure 7. An 11-year-old Weimaraner with a history of 3 weeks of progressive ataxia and 7 weeks of tetraparesis. A, Lateral survey radiographs showed extensive remodeling of vertebral bodies and end plates with osseous proliferation at T5-9. These findings were compatible with spondylitis and discospondylitis but were not consistent with the clinical signs. B, Lateral projection on the cervicothoracic myelography showed disruption of the contrast columns and obstruction of subarachnoid filling between C5-T3. Computed tomography was performed following myelography.
ical foundations of MRI so that one can understand how the image is created and why s'oft tissues, especially those of the central nervous system, can be visualized. Other simplifications have been published.3, 29, 33 MRI requires a strong magnetic field (minimum 17,000 times that of earth's gravity) within which the patient is placed. Available protons (hydrogen atoms) in the patient have a positive charge and are spinning (precession about an axis). A moving electrical charge is a current, and an electric current induces a magnetic field. Therefore the patient has numerous magnetic fields (H+). Within the external magnetic field, the patient's protons align parallel or antiparallel to the external field. Parallel forces cancel antiparallel forces, and the sum represents a
Figure 7 Continued. C, The section through T1 documented destruction of the left body and lamina of the vertebrae with bony displacement. No contrast agent was present, and soft-tissue detail was disrupted by a mass with soft-tissue density surrounded by a bony margin. 0, The left parasagittal image reconstructed through C6,7 and T1,2 recorded the cranial and caudal limits of the bony remodeling. E, The computed tomography section through the caudal aspect of C4 of the same patient was normal and is presented for comparison.
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Figure 8. See legend on opposite page
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magnetic field within the patient. The patient's magnetic field is now aligned with the external field and cannot be measured. A highfrequency radio wave, having the same frequency as the precessing protons, is introduced into the patient. Protons can acquire energy from the radio wave, a phenomenon called resonance, causing the protons to precess in phase. The sum of their vectors is now transverse to the external magnetic field. The radiofrequency pulse is discontinued, and the transverse magnetic field of the patient begins to realign with the external field. A moving magnetic field induces an electrical current in an antenna, and thus an MRI signal is captured from the patient. This signal is translated to form the image recorded on a CRT similar to other digital image systems. Increased brightness on the CRT represents stronger signal intensity. MRI is said to provide a proton map, and it is this map that charts anatomic information and pathologic changes. Altering the manner by which the signal is acquired, one is able to observe longitudinal relaxation time (T1-weighted image) and transverse relaxation time (T2-weighted image). T1-weighted images are dependent on tissue composition, structure, and surroundings and are best thought of as a reflection of the degree of organization of a substance. T2-weighted images depend on the physical state of a substance and are best thought of as the degree of fluidity (the opposite of viscosity) of a medium. Postacquisition manipulation of the data allows one to reconstruct images of sections of the patient made through virtually_ any plane (i.e., sagittal, parasagittal, coronal, axial). Interpretation of MRI images requires considerable knowledge of the physics of data acquisition using this modality. Experience is needed to become competent and ultimately confident in one's interpretive skills. Fortunately the anatomic similarities among human and animal tissues are such that anyone familiar with the technology can provide diagnostic interpretation. Interpretation and recognition of gross anatomy are simple and present no obstacle to the veterinary diagnostician. Any provider of MRI services will have the expertise to interpret the scans and explain the methods of data acquisition.
Figure 8. A 7.5-year-old mixed-breed dog was struck by a car and was quadriplegic but had deep pain response. A, The midsagittal MRI image confirmed the presence of a transmedullary lesion at C2-3. The intervertebral disc mass was reduced and the space collapsed, which indicated that a disc protrusion had caused the cord damage. Note the absence of signals (black) from the bone of the vertebral bodies and the intense signals (white) from the intervertebral discs and epidural fat. B, Six transverse reconstructs of sections at 1.5-mm thickness (designated by S 17.0 through S 24.5 in the upper left of each section). These sections were made through the caudal aspect of C2 and the C2-3 interspace. S 17.0 revealed a dilated central canal, but some cord parenchyma was visible peripherally. There was evidence of increased disruption of the spinal cord as the sections progressed caudally.
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SUMMARY
Various methods of documenting pathologic change in the spine and spinal cord are available to the veterinary practitioner. Intimidation caused by the imaging modality and the fear that one will not be able to recognize or diagnose a lesion are the factors that limit the use of diagnostic imaging. One needs only to be able to recognize the variations of normal anatomy to be successful. Once an abnormal area has been identified, the diagnosis is soon to follow. Therefore one should concentrate on improving the simple skills associated with image interpretation and normal anatomy. Lesion identification and definitive diagnoses will follow by natural progression. References 1. Adams WM, Stowater JL: Complications of metrizamide myelography in the dog: A summary of 107 clinical case histories. J Am Vet Radiol Soc 22:27, 1981 2. Allan GS, Wood AKW: Iohexol myelography in the dog. Vet Radio129:78, 1988 3. Anon: Magnetic resonance: New vision in medicine. Milwaukee, WI, General Electric Company, Medical Systems Group, 1985 4. Barbee DO, Allen JR, Gavin PR: Computed tomography in horses: Technique. Vet Radiol 28:144, 1987 5. Bronen RA, Sze G: Magnetic resonance imaging contrast agents: Theory and application to the central nervous system. J Neurosurg 73:820, 1990 6. Cox FH, Jakovljevic S: The use of iopamidol for myelography in dogs: A study of twenty-seven cases. J Sm Anim Pract 27:159, 1986 7. Dennis R, Herrtage ME: Low osmolar contrast media: A review. Vet Radiol 30:2, 1989 8. Fingeroth JM, Prata RG, Patnaik AK: Spinal meningiomas in dogs: 13 cases (19721987). J Am Vet Med Assoc 191:720, 1987 9. Gray PR, Indrieri RJ, Lippert AC: Influence of anesthetic regimen on the frequency of seizures after cervical myelography in the dog. J Am Vet Med Assoc 190:527, 1987 10. Gupta RC, Gupta SC, Dubey RK: An experimental study of different contrast media in the epidural space. Spine 9:778, 1984 11. Haughton VM: Intrathecal toxicity of iohexol vs. metrizamide. Survey and current state. Invest Radiol 20(suppl 1):S14, 1985 12. Hayman RA, Gorey MT: Imaging strategies for MR of the spine. Radiol Clin North Am 26:505, 1988 13. Karkkainen M, Mero M, Nummi P, et al: Low field magnetic resonance imaging of the canine central nervous system. Vet Radiol 32:71, 1991 14. Kraft SL, Gavin PR, Leathers CW, et al: Diffuse cerebral and leptomeningeal astrocytoma in dogs: MR features. J Comput Assist Tomogr 14:555, 1990 15. Kraft SL, Gavin PR, Moore MP, et al: An MR diagnosis of canine meningioma. Vet Radiol 32:5, 1990 16. Kraft SL, Gavin PR, Wendling LR, et al: Canine brain anatomy on magnetic resonance images. Vet Radiol 30:147, 1989 17. Lang J: Flexion-extension myelography of the canine cauda equina. Vet Radio129:242, 1988 18. Luttgen PJ, Braund KG, Brawner Jr WR, et al: A retrospective study of twenty-nine spinal tumours in the dog and cat. J Small Anim Pract 21:213, 1980 19. Modic MT, Masaryk T, Paushter C: Magnetic resonance imaging of the spine. Radiol Clin North Am 26:229, 1986 20. Moore MP, Gavin PR, Kraft SL, et al: MR, CT and clinical features from dogs with nasal tumors involving the rostral cerebrum and nasal passages. Vet Radiol 32:19, 1991
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21. Morgan JP, Atilola M, Bailey CS: Vertebral canal and spinal cord mensuration: A comparative study of its effect on lumbosacral myelography in the Dachshund and German Shepherd Dog. J Am Vet Med Assoc 191:951, 1987 22. Panciera DL, Duncan 10, Messing A, et al: Magnetic resonance imaging in two dogs with central nervous system disease. J Small Anim Pract 28:587, 1987 23. Pardo AD, Morgan JP: Myelography in the cat: A comparison of cisternal versus lumbar puncture, using metrizamide. Vet Radiol 29:89, 1988 24. Pasaoglu A, Gok A, Patiroglu TE: An experimental evaluation of response to contrast media: Pantopaque, iopamidol, and iohexol in the subarachnoid space. Invest Radiol 23:762, 1988 25. Prata RG: Diagnosis of spinal cord tumors in the dog. Vet Clin North Am [Sm Anim Pract] 7:165, 1977 26. Puglisi TA, Green RW, Hall CL, et al: Comparison of metrizamide and iohexol for cisternal myelographic examination of dogs. Am J Vet Res 47:1863, 1986 27. Sether LA, Nguyen C, Yu SN, et al: Canine intervertebral disks: Correlation of anatomy and MR imaging. Radiology 175:207, 1990 28. Shores A, Burns J: Technique and indications for metrizamide myelography in small animals. Compend Contin Educ Pract Vet 9:361, 1987 29. Sochurek H, Miller P: Medicine's new vision. National Geographic 171:2, 1987 30. Wheeler SJ: Spinal tumours in cats. Vet Annual 29:270, 1989 31. Wheeler SJ, Clayton-Jones DG, Wright JA: Myelography in the cat. J Small Anim Pract 26:143, 1985 32. Wheeler SJ, Davies JV: Iohexol myelography in the dog and cat: A series of one hundred cases, and a comparison with metrizamide and iopamidol. J Small Anim Pract 26:247, 1985 33. Widmer WR, Buckwalter KA, Braunstein EM, et al: Principles of magnetic resonance imaging and application to the stifle joint in dogs. J Am Vet Med Assoc 198:1914, 1991 34. Widmer WR: Iohexol and iopamidol: New contrast media for veterinary myelography. J Am Vet Med Assoc 194:1714, 1989
Address reprint requests to Ronald D. Sande, DVM, MS, PhD 146 McCoy Hall Washington State University College of Veterinary Medicine Pullman, WA 99164-6610
