IMAGING DOGS WITH SUSPECTED DISC HERNIATION: PROS AND CONS

OF MYELOGRAPHY, COMPUTED TOMOGRAPHY, AND MAGNETIC

RESONANCE

IAN ROBERTSON, DONALD E. THRALL

Myelography, computed tomography (CT), and magnetic resonance (MR) imaging are the diagnostic

modalities currently used in the evaluation of dogs with suspected disc herniation. Where high-field MR imaging

is available, it is considered the optimal modality for any myelopathy in dogs, including those with disc disease.

CT myelography may be the next best option, particularly in nonchondrodystrophoid dogs. In

chondrodystrophoid dogs, in which extrusion of mineralized disc material is common, plain CT will enable

diagnosis in most cases. Myelography is still considered adequate for diagnosis of disc herniation when MR and

CT are unavailable. r 2011 Veterinary Radiology & Ultrasound, Vol. 52, No. 1, Supp. 1, 2011, pp S81–S84.

Key words: computed tomography, dog, intervertebral disc disease, MR imaging, myelography.

Introduction

SUSPECTED INTERVERTEBRAL DISC herniation is a common

indication for spinal imaging in dogs. Although survey

radiography is often the modality used first, particularly in

first-opinion practice, it is an inaccurate method for diag-

nosis of disc herniation and is subject to marked interob-

server variation.1 Survey radiographic signs associated with

disc herniation provide only limited, indirect evidence

about the spinal cord injury responsible for the clinical

signs. Imaging dogs suspected of having intervertebral disc

herniation with a modality other than survey radiography

is a prerequisite to surgical intervention. Accurate assess-

ment of the location, laterality, and degree of spinal cord

compression facilitates surgical treatment and should en-

hance patient outcomes.

Myelography, computed tomography (CT), and mag-

netic resonance (MR) imaging are the diagnostic modal-

ities currently used in the assessment of dogs with

suspected disc herniation. Optimal imaging depends on se-

lecting a modality on the basis of various factors related to

the patient, modality attributes, and availability. Factors

not directly related to the suspected myelopathy affect the

choice of modality including presence of metal near the

region of interest2 (a contraindication to MR, but may also

adversely affect CT), presence of a cardiac pacemaker (a

contraindication to MR), history of a prior adverse reac-

tion to contrast medium (which may inhibit repeat use of

organic iodide or gadolinium-containing media), and a

clinical need for immediate imaging and treatment rather

than referral to a distant center. In patients requiring a

repeat examination (e.g., because of deterioration of signs

or to assess the effects of surgery), comparison is facilitated

by using the same imaging modality as for the original

examination.

Myelography

For many years, myelography was the principal imaging

modality used for diagnosis of disc herniation in animals,

and it is still adequate for this purpose. Nowadays, a de-

cision to use myelography for a dog with suspected disc

herniation is usually based on the lack of availability of

either CT or MR imaging (MRI).

Myelography is opacification of the subarachnoid space

around the spinal cord with the aim of identifying any

deviations that may reflect the site of a spinal cord lesion.

Problems relating to myelography include its invasiveness,

which can cause spinal cord damage,3 potential for adverse

reactions to contrast medium (e.g., exacerbation of mye-

lopathy, seizures), and the tendency for many spinal

lesions, including disc herniation, to narrow the sub-

arachnoid space, which excludes contrast medium and

hence may obscure the nature of the lesion. Myelography

can be completed within 30min, but difficulty making the

injection may greatly prolong the procedure. Similarly, any

epidural contrast deposition following inaccurate needle

placement will reduce the diagnostic quality of the study.

The two-dimensional nature of radiographs means that

superimposition by unrelated structures may make it diffi-

cult to visualize small lesions, particularly in the thoracic

spine. The superior latitude afforded by digital radiogra-

Address correspondence and reprint requests to Ian Robertson, at theabove address. E-mail: ian_robertson@ncsu.eduReceived November 10, 2010; accepted for publication November 12,

2010.doi: 10.1111/j.1740-8261.2010.01788.x

From the Department of Molecular Biomedical Sciences College ofVeterinary Medicine, North Carolina State University, Raleigh, NC 27606

S81

phy systems, particularly flat-panel detectors, enhances le-

sion conspicuity compared with film-screen radiography

and may be augmented by image manipulation at the

workstation.

CT

CT is an accurate imaging modality for dogs with sus-

pected disc herniation.4–7 The tomographic (slice) imaging

of CT eliminates the problem of superimposition that

affects radiographs and the improved contrast resolution

of CT enables detection of mineralized disc material or

hemorrhage in the vertebral canal. In chondrodystrophoid

patients, where disc mineralization is much more common,

conventional helical/multidetector CT will yield a correct

diagnosis of disc herniation in the majority of affected

dogs. Helical and multidetector scanners enable rapid,

thin-slice imaging of the entire spinal column and high-

quality multiplanar reconstructions, which can greatly en-

hance lesion conspicuity. While single slice scanners can

acquire thin slices, the acquisition time and tube heat load-

ing often make this impractical for large patient volumes.

With thicker slices (4–5mm), volume averaging can reduce

lesion conspicuity. If CT is available but MRI is not, plain

(noncontrast) CT is a logical first test to detect disc her-

niation in chondrodystrophoid dogs. For other breeds, or

for conditions other than disc herniation, myelography

may be a more suitable first choice if MRI is not available.

CT Myelography

CT imaging after opacification of the subarachnoid

space combines the benefits of CT and myelography. Con-

centrations or volumes of contrast medium in the sub-

arachnoid space that would not be visible radiographically

are readily detectable with CT. This means that a markedly

narrowed subarachnoid or uneven contrast medium dis-

tribution can be tolerated in a CT study when it might

render a myelogram nondiagnostic; however, the other

disadvantages of myelography relating to difficult injec-

tion, cord damage, and adverse reactions remain. The de-

cision to perform CT myelography often follows an

inconclusive myelogram or plain CT study. In the absence

of high-field MRI, CT myelography (þ /� intravenous

contrast medium administration) is often the next best op-

tion, particularly in nonchondrodystrophoid dogs.

CT Myelography Augmented with Intravenous ContrastMedium

Intravenous administration of contrast medium can

aid assessment of lesions affecting the epidural space and

intervertebral foraminae. This technique is more beneficial

for assessment of patients with neoplastic or inflammatory

disorders affecting the vertebrae or paravertebral tissues

than those with disc herniation.

MRI

MRI has revolutionized neural imaging and, if available,

is a suitable imaging modality for most neurologic patients.

In many veterinary institutions, MRI has largely replaced

myelography because it avoids the need for intrathecal

contrast medium administration and provides a relatively

complete anatomic depiction of the spinal cord and ver-

tebral column. Uniquely with MRI, lesions affecting the

spinal cord can be visualized directly.� Use of a heavily

T2-weighted single-shot turbo spin-echo MR sequence8

accentuates the high signal from cerebrospinal fluid

and provides a rapid, noninvasive evaluation of the sub-

arachnoid space that appears similar to a conventional

myelogram and provides similar insight into the morpho-

logy of the subarachnoid space.

There is considerable variation in MRI quality in vet-

erinary practice. It may be difficult to obtain diagnostic

studies of the spine with low-field MR, particularly in small

patients. Low-field permanent magnets have low signal to

noise ratio (SNR) and relatively small field of view, which

may necessitate repositioning the patient repeatedly to ob-

tain images of the entire region of interest.w Enlarging the

field of view, increasing slice thickness, and increasing the

number of acquisitions will increase SNR, but these ad-

justments reduce image resolution and prolong anesthesia

and magnet time, potentially exacerbating patient hypo-

thermia. If high-field MR is available, it is an excellent

first-line test to assess disc herniation. A comprehensive

high-field MR study of the spine takes approximately

40–45min, slightly longer on average than a myelogram.9

If only a low-field magnet is available, there may be an

advantage of performing myelography as the initial screen-

ing procedure, assuming that CT is not available, at least in

large dogs.

Myelography vs. CT

At veterinary facilities with routine access to both my-

elography and CT it may not be clear which test should be

performed first for a dog with suspected disc herniation.

Results of these modalities have been compared retrospec-

tively in 182 dogs with disc disease.4 Limitations of this

study arise because the choice of imaging modality was

clinician-dependant (rather than randomized), and few en-

rolled dogs were imaged using both modalities; however,

the number of dogs is large and the diagnosis was known in

each dog, thus some reasonable conclusions are possible.

�See accompanying article ‘‘Optimal Magnetic Resonance Imaging ofthe Spine,’’ pp. S72–S80.wSee accompanying article ‘‘Pros and Cons of Low Field Magnetic

Resonance Imaging in Veterinary Practice,’’ pp. S5–S14.

S82 ROBERTSON andTHRALL 2011

The overall sensitivity for disc herniation was similar for

myelography (84%) vs. CT (82%). CT was more sensitive

than myelography in chronically affected dogs, and my-

elography was more sensitive than CT in dogs weighing

o5 kg.

CT and myelography were compared prospectively in 19

chondrodystrophoid dogs with acute disc herniation.6

Dogs had CT followed by myelography and all lesions

were confirmed surgically. This study concluded that my-

elography and CT were equally sensitive for acute disc

herniation in chondrodystrophoid dogs.

CT and myelography were also compared prospectively,

along with contrast-enhanced and CT myelography, in 46

dogs with acute cervical or thoracolumbar myelopathy.4

This study included 25 chondrodystrophoid and 21

chondrodystrophoid dogs; the order of imaging was con-

sistent and the diagnosis known for all dogs. CT myelo-

graphy was the most sensitive technique for identification

of lesions compressing the spinal cord, enabling detection

of lesions in some dogs with normal myelograms. Also, in

some dogs with spinal cord swelling, lesion laterality could

be determined with CT when it was not evident myelo-

graphically. Plain CT was adequate for detection of her-

niation of mineralized disc in chondrodystrophoid dogs,

but was inadequate for this purpose in many non-

chondrodystrophoid dogs. In the latter group, the X-ray

attenuation by the herniated disc material was not suffi-

ciently different from the adjacent cord to be reliably de-

tected. Based on these results, plain CT should be the

imaging modality used first for chondrodystrophoid dogs

with suspected acute disc herniation, and myelography

should be used first for patients in which other spinal dis-

orders are considered more likely than disc disease.

That some dogs receiving myelography first would have

to undergo subsequent CT imaging for complete charac-

terization is understood and the practice of performing CT

myelography immediately after a conventional myelogram

is common, particularly if the myelographic findings are

equivocal or the study nondiagnostic for technical reasons;

however, even with CT, epidural contrast medium can be

difficult to differentiate from mineralized disc material if

there is no precontrast CT study available for comparison.

Epidural contrast medium can be avoided by performing a

cisternal injection, raising the head, and allowing the con-

trast medium to gravitate caudally. Cisternal injection is

not usually recommended for myelography in dogs with

suspected thoracolumbar disc herniation because flow of

contrast medium may be obstructed even by minimally

compressive lesions; however, the superior contrast reso-

lution of CT enables detection of the small amount of

contrast medium that may flow past the lesion. Similarly,

sufficient intrathecal contrast usually remains for CT

myelography several hours after injection, which is useful

if postoperative imaging is considered necessary.

Myelography and CT vs. MR

There have been few studies to compare MRI with my-

elography and/or CT myelography in dogs with suspected

disc herniation. When MRI and myelography were both

performed in a series of dogs with caudal cervical spondy-

lomyelopathy, MRI was more accurate in predicting the

site, severity, and nature of spinal cord compression.10

Myelographic or CT myelographic assessment of lesions

is difficult when there is circumferential attenuation of the

subarachnoid space that excludes contrast medium, for

example in dogs with acute disc extrusions and extensive

epidural hemorrhage.7 While the center of a region of sub-

arachnoid space attenuation on a myelogram often ap-

proximates the site of spinal cord injury, confirming the site

of a compressive lesion and its laterality, which is impor-

tant for surgical planning, may be impossible. The diag-

nostic accuracy of high-field MRI is not usually affected by

attenuation of the subarachnoid space. In such cases, MRI

may allow the subarachnoid attenuation due to spinal cord

swelling (e.g., as a result of edema associated with acute

disc herniation, infarct, myelitis, or a mass) to be distin-

guished from an extradural lesion.

Before MRI, it is likely many patients with acute non-

compressive nucleus pulposus extrusions were subjected to

unnecessary spinal decompression, justified on the basis

that a compressive lesion could not be ruled out without

surgical investigation. In addition to the acute manifesta-

tions of disc disease, MRI is more sensitive for chronic

spinal cord lesions such as atrophy or syringomyelia. Spi-

nal parenchymal lesions are apparent with myelography or

with CT myelography only if they cause swelling of

the cord or if contrast medium enters the cord, which is

uncommon.11

In facilities where myelography, CT, and MRI are rou-

tinely available, the image quality for each modality is

commensurate with modern expectations, and cost is not a

significant factor, we consider MRI to be first choice for

imaging canine spinal patients; however, there are many

occasions when the above prerequisites are not fulfilled or

the patient’s condition suggests that an alternative modal-

ity should be selected.

There are instances where myelography or CT myelo-

graphy provide information not readily available with

MRI. For example, it can be difficult to determine if the

origin of a large spinal lesion is dural or parenchymal using

either MRI or CT, and in such cases, myelography may be

the best modality for this determination.12,13

Dynamic instability associated with caudal cervical

spondylomyelopathy or lumbosacral disc disease is an-

other example of a condition for which myelography may

be preferred. Myelograms showing the effects of traction,

flexion, and extension are easier to obtain than corre-

sponding MRI, although such positional views can be ac-

S83IMAGING SUSPECTED DISC HERNIATIONVol. 52, No. 1, Supplement 1

quired with a dog in the bore of a magnet,14 with minimal

prolongation of magnet time.

CT and CT myelography may be preferred to MRI for

the assessment of vertebral fractures. The ability to acquire

1mm or submillimeter CT slices in a bone kernel (with

subsequent multiplanar reconstruction, including surface

and volume rendering) enables depiction of complex frac-

tures in a format that can assist in surgical planning. Thin

slice acquisition is also possible with high-field MR sys-

tems, but acquisition times may be long and/or limited to

gradient echo sequences, which have less tissue contrast. In

trauma patients with spinal fractures MRI is considered

superior to CT for assessment of soft tissue injuries asso-

ciated with fractures.

There is evidence that MRI can provide prognostic

information in spinal patients. For example, in studies

of 77 dogs,15 and 159 dogs,16 the extent of spinal cord

T2 hyperintensity (cross section and cord length) was

negatively associated with the probability of a full

functional recovery. Such studies suggest there may

be a benefit from MRI in addition to that associated with

more accurate diagnosis of spinal cord disorders. Even

so, it may be prove difficult to demonstrate improved

outcomes9 for spinal patients having MRI instead of

myelography.

ACKNOWLEDGMENT

Disclosure: The authors declare no conflict of interest.

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