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Chapter 7: Advanced Imaging: Spinal Surgery 81

providing a real‐time assessment of the degree of compression so
that the procedure can be stopped if the lesion is severe. Traction
views can be obtained with any imaging modality but one study
indicated that myelography characterized more lesions as dynamic
than MRI [69]. Since a gold standard for characterizing a lesion as
dynamic is not available, it is not clear whether this represents
true‐positive or false‐positive results (i.e., which modality is
correct).
Over‐interpretation of cervical lesions on MRI has been reported
by several authors, with degenerative changes being evident in clin­
ically normal animals [68–70]. Although there is generally good
agreement on the primary site of compression, there is poor inter­
observer agreement in evaluation of the articular processes and
foramenal stenosis [71]. However, the direct multiplanar imaging of
MRI and ability to assess the spinal cord is advantageous. Chronic
compressive lesions can lead to spinal cord atrophy, gliosis, and
syringomyelia. Signal changes on MRI have been correlated with
histological changes in people [72]. A T2 hyperintensity with nor­ Figure 7.14 Sagittal reformatted images of an L2 vertebral body fracture
mal signal on T1 images is due to mild loss of nerve cells, gliosis, shows comminution of the vertebral body and moderate displacement of
edema, demyelination, and Wallerian degeneration. Lesions that the fracture with narrowing of the vertebral canal.
are hyperintense on T2‐weighted images and hypointense on T2‐
weighted images are associated with severe changes such as necrosis CT may be superior to MRI for the evaluation of spinal trauma.
and myelomalacia. In human patients with cervical spondylotic The fine anatomical detail provided by the ability to acquire 1‐mm
myelopathy, an increased T2 signal intensity only is thought to rep­ or submillimeter slices and to subsequently perform multiplanar
resent a potentially reversible change while a lesion that has and three‐dimensional reconstructions aid in the diagnosis and
increased T2 signal intensity and decreased T1 signal intensity is surgical planning for vertebral fractures [42] (Figure 7.14). An
likely irreversible and a poor prognostic indicator [73]. In one added advantage of CT is that due to the speed of acquisition, it can
study, 55% of dogs had signal intensity changes in the spinal cord be performed in sedated patients. Since acute hemorrhage is hyper­
but these changes were assessed in relation to clinical signs or prog­ dense on CT, it allows for concurrent evaluation of extradural com­
nosis [68,69]. pression by hemorrhage.

Other Extradural Lesions Intramedullary Spinal Cord Lesions
The sensitivity of CT to bony lysis in vertebral tumors is dramati­ Although all imaging modalities can detect spinal cord enlargement
cally higher than that of radiographs [30]. CT can detect a density due to intramedullary spinal cord disease, the superior contrast
change of 0.5% while radiographs require a change of at least 10% to resolution of MRI provides additional characterization of the lesion
be evident [74]. Intravenous contrast administration is required to (Figure 7.15). As most intramedullary lesions have a similar appear­
determine the extent of the soft tissue mass in order to plan therapy ance on MRI (hyperintense to spinal cord on T2‐weighted images,
appropriately. Concurrent myelography may be required to better isointense to spinal cord on T1‐weighted images, and variable
characterize the spinal cord lesion (extradural vs. intradural– degrees of contrast enhancement), there is considerable overlap in
extramedullary vs. intramedullary) [30]. the differential diagnosis. Ischemia, acute noncompressive intervertebral

A B

Figure 7.15 Transverse T2‐weighted (A) and T1‐weighted (B) MRI of the lumbar vertebral column. A focal intramedullary lesion that is hyperintense to the
spinal cord on the T2‐weighted image and isointense to the spinal cord on the T1‐weighted image is present in the mid to left spinal cord. This lesion did
not contrast enhance.