Radiologic Anatomy of the Spine

Published on 10/03/2015 by admin

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Last modified 10/03/2015

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Chapter 3 Radiologic Anatomy of the Spine

The cervical spine

The first two cervical vertebrae, the atlas and the axis, and the last cervical vertebra are structurally special. However, the C3 to C6 vertebrae are fairly uniform and can be described together. The atlas and axis form a complex articular system for both the nodding and rotational movements of the head. These bony structures of the base of the skull and the craniocervical junction are better seen on computed tomography (CT) than on magnetic resonance imaging (MRI). The atlas and the axis are linked together and to the skull and other cervical vertebrae by several ligaments.

Atlas (C1)

The atlas supports the weight of the skull and is very appropriately named after the mythical giant who carried the earth on his shoulders. It is a bony ring consisting of an anterior arch and a posterior arch, which are connected by two lateral masses. The anterior arch forms a short bridge between the anterior aspects of the lateral masses. On the posterior surface of the anterior arch, a midline facet marks the synovial articulation of the odontoid process of the axis, and internal tubercles on the adjacent lateral masses show the attachments of the transverse atlantal ligaments that hold the odontoid against this articular area. The posterior arch consists of modified laminae that are round and a posterior tubercle that represents a rudimentary spinous process. The atlas is devoid of a body and of a full spinous process.

The lateral masses consist of superior and inferior articular facets and transverse processes. The superior articular facets are concave and ovoid, and they face upward and inward as shallow foveae for articulation with the occipital condyles. Nutatory movements of the head mainly occur at these atlantooccipital joints. The inferior articular facets are concave and face downward, slightly medially, and backward; they articulate with the superior articular facets of the axis. The relative horizontal orientation of the atlantoaxial facet joints allows rotation at the expense of bony stability. The paired alar ligaments, running from the posterolateral aspects of the odontoid process to the occipital condyles, prevent excessive rotation [1]. The transverse processes are each pierced by a foramen for the vertebral artery. On coronal CT scans, the occipitoatlas and the atlantoaxial joints resemble a capital X.

Axis (C2)

The second cervical vertebra, or axis, supports the dens, or odontoid process, which projects rostrally from the body, serving as a pivotal restraint against horizontal displacement of the atlas. Unlike the remaining portions of the cervical spine, on MRI the dens can demonstrate a decreased signal relative to other vertebral bodies, presumably because of partial volume averaging. Embryologically, the odontoid process fuses with the body by 3 to 6 years of age. A persistent remnant of the subdental synchondrosis is often recognized on sagittal MR images or on reformatted sagittal CT scans as a horizontal dark band at the base of the odontoid process; this is a normal feature and should not be mistaken for a fracture.

The space between the clivus, the anterior arch of the atlas, and the tip of the odontoid process demonstrates high signal intensity on MRI owing to its fat component. Also, the fatty marrow of the clivus, the occipital condyle, and the arch of C1 appear as high signal intensities on a T1-weighted MR image. The cortical bone and the articular surface show low signal intensity, and the vertebral artery exhibits its characteristic signal void. The inferior articulating surfaces of the axis begin the typical articular columns of the cervical vertebrae. The lateral processes of the axis are directed downward, and their posterior or noncostal elements are often quite thin. Anteriorly, the inferior aspect of the body of the axis forms a liplike process that descends over the first intervertebral disc and the body of the third cervical vertebra.

C3 to C7 Vertebrae

Vertebral Bodies

The vertebral bodies in the cervical spine are ladder-like in cross section; they are broader in the transverse diameter than in the anteroposterior (AP) dimension, and their end plates are parallel (Fig. 3-1). The cervical vertebral bodies are smaller than those of the other movable vertebrae and increase in size from C3 downward. The vertebrae are connected by the anterior and posterior longitudinal ligaments. Each ligament’s fibers diverge at each disc level and blend with the anulus fibrosus and the adjacent margins of the vertebral bodies. At the mid-vertebral level, the posterior longitudinal ligament is narrower and lies behind the body, posterior to the retrovertebral venous plexus.

Epidural Space

The epidural space that surrounds the dural sac contains neurovascular and connective tissue elements that are more clearly seen on MRI and CT after intravenous injection of a contrast agent. There is only a small amount of epidural fat tissue, and sinuses are formed in this fat tissue by the wide venous plexuses that surround roots and nerves as they leave the intervertebral foramina in the lateral parts of the epidural space (Fig. 3-7). The scarcity of revised epidural fat in the cervical canal in comparison with that in the lumbar canal makes it more difficult to differentiate between the soft tissue structures in the cervical spinal canal on a noncontrast CT scan. On MRI, the high signal intensity in the anterior lateral aspect of the cervical canal represents the epidural venous plexus. The epidural venous plexuses produce high signal intensity in the anterior epidural space, which should not be confused with epidural fat; epidural fat is virtually absent at the cervical level.

Nerves of the Cervical Cord

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