Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Six The Spine 665
present on some vertebrae. All of the thoracic vertebrae have costal fovea. The mammillary
processes (small, knoblike bony projections) begin with T2 or T3 and continue caudally
through all of the lumbar vertebrae. The accessory processes (small, tubular, caudally
directed bony projections arising from the caudal aspect of the pedicles) begin with the
midthoracic vertebrae and continue to L5 or L6.
Between almost all vertebrae, the exceptions being C1-2 and the sacral vertebrae, is an
intervertebral disc. This is composed of the annulus fibrosis, a perimeter of fibrous and
fibrocartilaginous fibers arranged in concentric lamellae, and the nucleus pulposus, an
eccentrically placed sphere of embryonic hyaline cartilage. The width of each disc space
varies depending on which vertebrae it separates. The cervical disc spaces become gradually
wider as they progress from C2 to C6. The thoracic disc spaces are narrower than the
cervical disc spaces and are uniform in width to the level of T10-11. The disc spaces then
widen gradually to the level of T13-L1, at which point they are fairly consistent in width to
the level of L7-S1. The width of the L7-S1 disc spaces varies, but it often may be wider than
the other disc spaces. The normal spine of the dog and cat consists of seven cervical, thirteen
thoracic, seven lumbar, three sacral, and a variable number of caudal vertebrae.
C E RV I C A L S P I N E
The cervical vertebrae, except C1 and C2, have a similar shape (Fig. 6-1). C1 has a short,
ovoid, tubular shape with prominent transverse processes. On the ventrodorsal view, these
processes are relatively large and the lateral foramina, through which the spinal arteries
course, are readily apparent. On the lateral view, C1 is shorter than the other vertebrae. On
a well-positioned lateral radiograph the transverse processes are superimposed, obscuring
the odontoid process of C2. If the head is placed in an oblique position, the wings of C2
will be rotated. This position is particularly helpful when subluxation between C1 and C2
is suspected or when there is a lesion within the cranial portion of C2. The axis (C2) is the
largest of the cervical vertebrae. The most cranial aspect is the odontoid process, the dens,
which projects cranially from the body of C2 into the ventral portion of the spinal canal of
C1. In the dog, the dorsal spinous process of C2 contains an area of relative radiolucency.
There are no intervertebral discs between the skull and C1 or between C1 and C2. C3 to C7
are all similarly shaped; however, the dorsal spinous processes become progressively larger
from C3 to C7. The lateral processes become progressively larger and are projected more
ventrally going from C3 to C6. C6 has large projections from the transverse process. C7 has
smaller transverse processes than the preceding cervical vertebrae. The intervertebral disc
spaces between the cervical vertebrae gradually increase in width from C2-3 to C6-7 and
become narrow again at C7-T1. The normal cervical myelogram reveals that the cord is
oval in cross-section, appearing smaller in its dorsoventral dimension on the lateral view
than it is in the left-to-right dimension on the ventrodorsal view, in the atlantooccipital
area through C1. It becomes circular in cross-section, appearing to have equal dimensions
in all views, as it passes into C2. The ventral dura is tightly juxtaposed to the dorsal border
of the disc space at C2-3. The roots of the spinal nerves frequently are seen as faint linear
radiolucencies that sweep caudally to exit from the appropriate intervertebral foramen.
The spinal cord increases in diameter as it passes through the C4 and C5 area and the cervical
enlargement or brachial intumescence, which is that part of the cord where the nerve
roots form the brachial plexus, is formed. Beyond C6, the cord reduces slightly in diameter
as it passes into the thoracic vertebral canal.
T H O R AC I C S P I N E
The thoracic vertebrae are typified by short vertebral bodies, by absence of transverse
processes, by articulations for the rib heads, and by relatively large dorsal spinous processes
that gradually decrease in height from the cranial to the caudal thoracic vertebrae (Fig.
6-2). The intercapital ligament extends between the heads of the paired ribs and crosses
over the dorsal surface of the intervertebral discs from T2 to T11. The intervertebral disc
spaces are relatively similar in size and shape, with the exception of the disc space at the
anticlinal space. The anticlinal space is the intervertebral disc space that is between the
most caudal thoracic vertebra, with a caudally directed dorsal spinous process, and the
most cranial thoracic vertebra, with a cranially directed dorsal spinous process. This