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74 Section I: Diagnostics and Planning

Disadvantages with spinal cord enlargement (swelling, neoplasia, etc.) or when the
• Invasive. fat is displaced by intervertebral disc or a soft tissue mass. Evaluating
• Mild risk of side effects. the CT images in both a soft tissue and a bone window may improve
• Cannot always provide circumferential lesion localization. the ability to localize lesions [26].

Nonenhanced CT, Contrast‐enhanced CT, and CT
Computed Tomography Myelography
CT examination is noninvasive and relatively rapid. To maximize Numerous authors have compared the use of unenhanced CT,
the information the CT scan can provide requires optimization of contrast‐enhanced CT (intravenous contrast), and CT myelography
the scanning protocol. Consideration should be given to the algo­ for spinal cord disease [25–32]. The ability to detect pathology on
rithm, mode of acquisition (axial vs. helical), slice thickness, pitch unenhanced CT is dependent on the presence of epidural fat and
or interval (slice overlap), field of view, and slice orientation. changes in density of the pathological tissue (e.g., mineralized disc
Commonly used imaging parameters are included in Table 7.1 material, acute hemorrhage, contrast enhancement). When mini­
[23]. Post acquisition, appropriate window and level of the images mal epidural fat is present and the lesion is of soft tissue density,
and use of reformatted images can add to the diagnostic ability of contrast medium (either intravenous or intrathecal) may be
CT [24]. required to characterize the lesion.
It is important to review images in more than one window/level The intravenous contrast medium used for CT is iodinated
setting to obtain all the necessary information (Table 7.2). When contrast (ionic or nonionic) administered intravenously at a dose
performed without myelography, a soft tissue window is useful for of 600 mg iodine per kilogram. Nonionic contrast medium is typi­
evaluating the epidural fat and spinal cord while a bone window is cally more expensive but has a lower risk of complications.
better for evaluation of the vertebrae (Figure 7.4). When combined Contraindications to intravenous contrast administration include
with myelography a bone window is most useful for reducing the previous contrast reactions and dehydration. The rate of serious
blooming artifact from the contrast medium. contrast reactions in veterinary patients is unknown but likely
The ability to acquire thin slices and reformat CT images pro­ similar to that in humans (2–4%) [33]. Minor reactions such as
vides an advantage over MRI in evaluating the intervertebral fora­ vomiting, nausea, and a burning sensation at the injection site are
men for stenosis and nerve root compression. Intravenous contrast not typically noted as most of our patients are under anesthesia
administration can also improve the assessment of nerve roots for when iodinated contrast medium is administered. Hemodynamic
neuritis and neoplasia. changes including hypertension, tachycardia, and bradycardia can
When performing unenhanced CT a decision must be made also occur [34–36]. The serious contrast reactions that have been
whether to scan the entire region of the neuroanatomical localiza­ described are anaphylaxis and contrast‐induced nephropathy.
tion (e.g., T3–L3) or a more limited area (T9–L3). The advantage of Anaphylaxis should be treated in the routine manner. Contrast‐
a more limited scan is shorter scan times with smaller slice thick­ induced nephropathy can only be treated supportively. Patients
nesses (reducing partial volume averaging artifacts) and less wear with preexisting renal disease, diabetes mellitus, and multiple
on the X‐ray tube [25]. However, this approach should only be used myeloma may have an increased risk of contrast‐induced
if the probability of intervertebral disc herniation is very high as nephropathy [37].
other lesions may be missed [26]. An alternative approach is a sur­
vey of the entire region of neuroanatomical localization with a Advantages
larger slice thickness and a smaller group of thin sections through • Noninvasive (survey or contrast‐enhanced CT).
the lesions identified. This is less of a concern as the availability of • Typically lower cost than MRI.
multislice scanners is increasing. • Often more readily available than MRI.
Although CT is very good for detecting bony lesions, the lower
contrast resolution of CT compared with MRI makes it less sensi­ Disadvantages
tive to intramedullary spinal cord lesions. Epidural fat is hypoat­ • Intrathecal contrast medium may be required to identify some
tenuating (darker) relative to soft tissue and provides contrast to the lesions.
spinal cord and nerve roots. Inability to see the epidural fat occurs • Little information regarding the integrity of the spinal cord and
spinal cord pathology.

Table 7.1 Commonly used imaging parameters for spine CT.
Positioning Dorsal recumbency Magnetic Resonance Imaging
MRI is generally considered to be the best imaging modality for
kVp 100–120
mAs 200 spinal disease because of the high tissue contrast. Because MRI is
Slice thickness 1–2 mm (up to 5 mm for entire spine survey) highly sensitive it is imperative that a careful clinical examination
Reconstruction algorithm Soft tissue (medium frequency) be performed to localize the lesion to avoid false‐positive results
(kernel) Bone (high frequency) from subclinical disease.
Both high‐ and low‐field strength magnets can be used to obtain
diagnostic images of the spine. Low‐field magnets typically require
Table 7.2 Typical window width and level settings for bone and soft tissue. more time for each imaging sequence but are also less prone to arti­
facts and have lower operating and purchase costs.
Window width Window level
The physics of MRI will not be discussed but image acquisition
Soft tissue 300 100 requires the use of a radiofrequency coil to receive or send and
Bone 3000 500
receive the radiofrequency pulse from which magnetic resonance