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76 Section I: Diagnostics and Planning

In cases of suspected intervertebral disc herniation there may be a Interpretation
temptation to omit the T1‐weighted images, but hydrated disc The general principles of interpretation apply to projection and
material has a similar signal intensity to epidural fat on T2‐ cross‐sectional imaging modalities. Normally, the contrast columns
weighted images and noncompressive material (type III inter­ or subarachnoid space should be thin, sharply margined, and paral­
vertebral disc herniation) may be overlooked if T1 images are not lel to the vertebral canal on all projections (Figure 7.7A). The ventral
obtained (Figure 7.6). T2 images with fat suppression (T2 Fat Sat subarachnoid space can undulate and thin over the intervertebral
or STIR) are useful for evaluation of the vertebral canal for disc spaces. Spinal disease is classified as extradural, intramedul­
infection or neoplasia. T2* images are useful for assessing for lary, and intradural–extramedullary based on changes in the size
hemorrhage [41]. (attenuation or enlargement of) and displacement of the subarach­
Signal characteristics of tissues vary with pulse sequences. noid space and spinal cord. Some diseases, such as ischemic mye­
Relative signal intensities are given in Table 7.3. The majority of lopathy, degenerative myelopathy, myelitis, and meningitis, can
tumors and inflammatory lesions have relatively high T2 signal have normal imaging.
intensity and intermediate to low T1 signal intensity with variable Signs of an extradural lesion include attenuation and devia­
degrees of contrast enhancement. Degenerative discs have low sig­ tion of the subarachnoid space toward the center of the vertebral
nal intensity on both T1 and T2 sequences. Hemorrhage has differ­ canal (Figure 7.7B). Differentiating normal undulation of the
ent imaging characteristics depending on the age of the hemorrhage ventral subarachnoid space from clinically important lesions is
and the imaging sequence [41]. dependent on attenuation of the dorsal subarachnoid space.
With myelography, if a lesion is directly ventral or lateral,
Advantages deviation of the contrast column centrally will be noted on one
• Superior soft tissue resolution provides information on spinal view and the orthogonal projection will show signs of an
cord integrity. intramedullary lesion (deviation of the contrast columns toward
the vertebral canal). With MRI, intervertebral disc protrusions
Disadvantages can often be differentiated from extrusions by their broad‐based
• Cost. contact with the intervertebral disc and a fusiform shape, while
• Availability. extrusions tend to have less contact with the intervertebral disc,
• Contraindicated in dogs with pacemakers or with metallic are longer dorsally compared to ventrally, and a have a rounded
implants near the area of interest. shape (Figure 7.8).

A B

Figure 7.6 Transverse T2‐weighted (A) and T1‐weighted (B) MRI of herniation of hydrated intervertebral disc material into the left ventral vertebral canal.
Although slight displacement of the spinal cord is evident on the T2‐weighted images, the disc material is best identified on the T1‐weighted images.

Table 7.3 Relative signal intensities of vertebral column tissues for the common pulse sequences.
T1 T2 STIR
Nucleus of the intervertebral disc Intermediate High High
Annulus of the intervertebral disc Low Low Low
Cortical bone Low Low Low
Cancellous bone Intermediate to high Intermediate to high Low
Spinal cord Intermediate Intermediate Intermediate
Subarachnoid space Low High High
Muscle Intermediate Intermediate Intermediate
Fat High High Low