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Chapter 7: Advanced Imaging: Spinal Surgery 79

material (which is hyperdense to the spinal cord) (Figure 7.12). CT
is very sensitive to small amounts of mineralization but if the herni­
ated material is not mineralized it may not be detected. Additionally,
if there is little epidural fat (high spinal cord to vertebral canal
ratio), as seen in small breed dogs, there may be insufficient con­
trast to detect nonmineralized material (Figure 7.13).
Another confounding issue is when there are multiple herniated
discs, as it may not be possible to determine which is the active
lesion when there is no contrast ring. Attenuation of the contrast
ring indicates spinal cord swelling that can point to the acute lesion.
As spinal cord swelling can also be related to prognosis, this addi­
tional information may justify the use of CT myelography over
unenhanced CT [30,50].
Several studies have concluded that plain CT is as effective as
myelography in detecting intervertebral disc herniation, with a
sensitivity of 81–97% [25,26,31,51,52]. Unfortunately, although

Figure 7.10 A ventrodorsal post‐myelogram radiograph shows attenuation
of the right contrast column over a greater length than the left contrast col­ Figure 7.12 Transverse CT at the cranial aspect of L2 shows hyperdense
umn. Based on the paradoxical contrast obstruction, this indicates that the (mineralized) intervertebral disc material in the left vertebral canal causing
extradural lesion (confirmed extradural on the lateral view) is on the left compression of the spinal cord. This lesion is clearly evident without the
side. This finding was confirmed at surgery. need for intravenous or intrathecal contrast medium.

A B C

Figure 7.11 Ventrodorsal (A), left ventral–right dorsal
oblique (B), and right ventral–left dorsal oblique (C)
radiographs of a lumbar myelogram show a right‐sided
extradural lesion at L3–4. The ventrodorsal view
shows a slight deviation of the right contrast column
medially but this lesion is more evident on the oblique
projections.