Page 366 - Atlas of Small Animal CT and MRI

356 Atlas of Small Animal CT and MRI

not extruded, and the dorsal longitudinal ligament material is sometimes present (Figures 3.5.2, 3.5.3,
remains intact. Hansen’s type II disk protrusion results 3.5.4, 3.5.5). 10,17
2,3
from fibrous degeneration and is most common in Similar features are seen on MR images, with disk
nonchondrodystrophoid breeds. 2,3 material appearing T1 and T2 hypointense. Attenuation
Extrusion of apparently normal disk material can of the T2 hyperintense cerebrospinal fluid layer occurs
also occur as a result of physical activity or overt at the site of cord compression, and T2 hyperintensity of
trauma. These are sometimes referred to as high‐ velocity cord parenchyma may also be seen as a result of edema.
extrusions because of the force of extrusion and the When present, hemorrhage appears as variable, mixed
predominately liquid composition of normal nucleus T1 and T2 intensity (Figure 3.5.6, 3.5.7, 3.5.8, 3.5.9,
pulposus. A description of traumatic intervertebral disk 3.5.10). Other uninvolved disks will appear T2 hypoin­
18
disease can be found in Chapter 3.2. Acute spontaneous tense because of disk dehydration.
extrusion of hydrated disk material seemingly unrelated
to activity or trauma can also occasionally occur. In Imaging features of Hansen’s type II disk protrusion
8
one canine study, a variety of nonchondrodystrophic CT may be less accurate for detecting type II disk pro­
and chondrodystrophic breeds were represented with a trusions according to one report. CT features include
9
median age of 9 years at the time of diagnosis. Clinical a variable decrease in intervertebral disk space width
signs include acute onset tetraparesis or tetraplegia, and and a mildly hyperattenuating mass arising from the
the mid to caudal cervical intervertebral disks are most dorsal aspect of the affected disk and extending into
commonly affected. 8 the ventral or ventrolateral vertebral canal. The bulg­
ing annulus cannot be distinguished from the overly­
Imaging features of Hansen’s type I disk extrusion ing dorsal longitudinal ligament. The spinal cord is
There are several studies that have compared the displaced, and its shape is often distorted by impinge­
accuracy of unenhanced CT, contrast‐enhanced CT, ment of the disk even when overt compression is
MRI, and conventional myelography for detection of absent. Contrast columns are attenuated at the site of
Hansen’s type I disk herniation. 9–15 Unenhanced CT impingement or compression on CT myelographic
has been reported to be 89–100% accurate for lesion images.
localization, and CT myelography is slightly bet­ MR features of type II disk protrusions are similar to
ter. 7,9,10,15 CT has been shown to be better than conven­ those seen on CT images. Protruding disk material is T1
tional myelography for detecting disk herniation in and T2 hypointense and appears contiguous with in situ
large dogs, but myelography was found to be better in disk material and the overlying longitudinal ligament.
dogs weighing less than 5 kg. Authors of one report The spinal cord can be displaced, distorted, and
16
found similar detection accuracy for CT myelography compressed, and the T2 hyperintense cerebrospinal fluid
and contrast‐enhanced CT following intravenous columns are attenuated at the site of protrusion. It is
contrast administration. MRI is thought to be the common to see multiple sites of involvement with
14
most accurate imaging method, but the degree of varying degrees of disk protrusion, and in these patients
improvement compared to CT myelography is it can be useful to use a single‐shot turbo spin‐echo
minor. 10,13 sequence as a “rich man’s myelogram” to localize the
CT features of type I disk extrusion include clinically relevant site (Figure 3.5.11). In patients with
19
the presence of hyperattenuating disk material in the chronic disease, the spinal cord can be focally atrophic,
epidural space, with the density depending on the with syringohydromyelia and T2 parenchymal intensity
degree of mineralization. Disk material can migrate suggesting gliosis. Uninvolved disks are often T2 hypoin­
horizontally along the floor of the vertebral canal and tense because of dehydration.
circumferentially around the spinal cord. Material can
also be dorsolaterally extruded into the intervertebral Imaging features of hydrated nucleus pulposus
foramina. Depending on the volume and distribution extrusion
of extruded disk material, the spinal cord is displaced MR imaging features include narrowing of the interver­
and compressed. Subarachnoid contrast columns are tebral disk, T2 hyperintensity of extruded disk material
attenuated at the site of compression on CT myelogra­ that is difficult to distinguish from epidural fat, and a
phy. Diffuse alterations with mixed attenuation in the characteristic “seagull sign” on T2 transverse images
epidural space can be seen in acute disease associated representing the dorsal margin of the extruded mate­
with hemorrhage, and edema can cause an increase in rial. Extrusion results in spinal cord compression, and
cord diameter. The affected intervertebral disk space is many dogs have intrinsic T2 hyperintensity at the site of
often narrowed, and residual mineralized in situ disk compression (Figure 3.5.12). 8

356