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.

The Ligaments of the Atlas and Axis

The axis and atlas are anchored to the skull base by several layers of strong ligaments, the apical and lateral alar ligaments. These ligaments are covered by the broad tectorial membrane, an anterior layer of the posterior longitudinal ligament. Posteriorly, the occiput and the atlas are connected by the thin, wide, elastic posterior atlantooccipital membrane, which is pierced by the vertebral artery and the first cervical nerve. Anteriorly, the broad atlantooccipital and atlantoepistrophic ligaments are partially hidden by the anterior longitudinal ligament. The alar ligaments and the longitudinal bundles of the cruciate ligament of the atlas are not discernible from the cortical bone on MRI. On T1-weighted MR images, the transverse ligament is represented by a band of low signal intensity that joins the medial aspect of the lateral masses of the atlas.

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.

Figure 3–1 T1-weighted coronal MR image shows the outline of the cervical spine. The cervical vertebral bodies are stepladder-like in this view. The bodies are broader in the transverse diameter than superoinferior dimension.

Joints of von Luschka

The upper and lateral edges of the superior surface project upward and have sagittal ridges that form the uncinate processes. Together with corresponding notches in lower end plates of the vertebra above, they form the uncovertebral joints of von Luschka (Fig. 3-2).

Figure 3–2 T2-weighted coronal MR image presents the uncovertebral joints of von Luschka (large arrows). The uncinate process from the upper lateral edge of the superior surface of the lower vertebral body faces the lower lateral notch in the lower end plate of upper vertebral body. Small arrows indicate the vertebral arteries.

Transverse Foramen

The transverse foramen, a characteristic feature of the cervical spine, houses the vertebral artery, veins, and sympathetic nerves in the deep cranial groove of the transverse process. The spinal nerves and ganglia cross the dorsal border of the artery along with segmental vessels. The transverse foramen of the seventh cervical vertebra contains only vertebral veins, not the vertebral artery (Figs. 3-3 and 3-4).

Figure 3–3 Axial CT scan of the seventh cervical vertebra. The transverse foramen of the seventh cervical vertebra contains only vertebral veins (thin arrow), not the vertebral artery (thick arrows). However, the other cervical vertebrae contain the vertebral artery within their transverse foramina. See also Fig. 3-4.

Figure 3–4 T1-weighted axial MR image corresponding to the CT scan in Fig. 3-3. The vertebral arteries are revealed as signal voids (arrows) anterior to the transverse foramina.

Articular Processes

The superior and inferior articular processes of adjacent neural arches form the facet joints. These structures are more oblique in the cervical level than in other levels of the spine. Each joint surface is lined with articular cartilage, and menisci cushion the cervical facets. Surrounding each joint is a fibrous capsule with a synovial membrane on its inner aspect; this capsule is relaxed to allow a gliding motion. Anteriorly, the superior articular facet is rounded in its anterior aspect; its articular surface faces posteriorly and is flat. Posteriorly, the inferior articular facet of the vertebra above has a flat anterior articular surface and a convex posterior aspect (Fig. 3-5).

Figure 3–5 The superior articular process of the lower vertebral body, located at the anterior aspect of the facet joint, is shown facing posteriorly, and its flat articular surface can be seen. The inferior articular process of the vertebra has a flat anterior articular surface and a convex posterior aspect.

Spinal Canals

The spinal canals are relatively large in order to house the cervical enlargement of the spinal cord. The spinal canal is triangular with the apex of the triangle posterior. The canal decreases in size from vertebra C1 through vertebra C3 and has a fairly uniform dimension from C3 through C7. C7 is a transitional vertebra, whose spinous process is longer and thicker and has a more inferior tilt than the more rostral cervical spinous processes of C3 to C6. On a T1-weighted MR image, the exact size of the subarachnoid space is difficult to assess because of its low signal intensity, and it may be confused with the posterior longitudinal ligament and the cortical bone (Fig. 3-6).

Figure 3–6 The subarachnoid space, the posterior longitudinal ligament, and the cortical bone reveal low signal intensity on this T1-weighted MR image. They cannot be defined individually.

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.

Figure 3–7 There is little epidural fat tissue in the cervical spine. Wide venous plexuses surround roots and nerves in the lateral aspect of the epidural space and the vertebral artery within the transverse foramen on this contrast-enhanced axial CT scan.

Cervical Discs

The cervical intervertebral discs are smaller than the discs of other regions of the spine. The uncinate processes on the upper end plate limit lateral extension of the discs. These discs are wedge-shaped, the greater width being anterior, corresponding to the cervical lordosis [2]. The intervertebral discs do not extend anteriorly or posteriorly beyond the level of the vertebral body in younger people. The nucleus pulposus cannot be differentiated from the anulus fibrosus, but the periphery of the disc is less intense than its central portion. (Figs. 3-6 and 3-8).

Figure 3–8 On T2-weighted sagittal MR images, high signal intensity can be seen in the nucleus pulposus and the anulus pulposus, which thus cannot be differentiated. The periphery of the disc shows a slightly lower signal intensity than the center.

Nerves of the Cervical Cord

Spinal nerves arise from the cervical cord. Each nerve consists of a dorsal sensory root and a ventral motor root. The nerve roots join just lateral to the dural sheath to form the spinal nerves. The dorsal root ganglion is located in the neural foramen just proximal to the point of union of the dorsal and ventral roots. The roots of each spinal nerve from C1 through C7 leave the spinal canal through the intervertebral foramina above the corresponding vertebra. The eighth cervical nerve passes through the foramen between C7 and T1. The spinal ganglion, located outside and below the neural foramen, is clearly seen posterior to the vertebral artery on MRI as a structure of intermediate signal intensity. On contrast-enhanced MR images, the spinal ganglion appears as a mildly enhancing ovoid structure posterior to the vertebral artery (Fig. 3-9).

Figure 3–9 On a contrast-enhanced T1-weighted axial MR image, the spinal ganglion shows intermediate signal intensity posterior to the vertebral artery (arrows).

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