16.1 Cervical Spine

Cervical Spine Anatomy and Landmarks

The cervical spine contains seven vertebrae (C1-C7) and has the greatest range of motion in the vertebral column. C1, the atlas, is a bony ring with no vertebral body and no spinous process; it cradles the skull. C2, the axis, projects the dens (odontoid process) upward, a tooth-like peg around which C1 and the head rotate. Uncinate processes rise from the superior lateral margins of the lower cervical bodies and form the joints of Luschka. C7 is the vertebra prominens, an easily palpated spinous process used for centering.

Useful surface landmarks include the mastoid tip (C1), the gonion or angle of the mandible (C3), the thyroid cartilage (C5), and the vertebra prominens (C7). Two joint sets dictate how the cervical spine is positioned. The zygapophyseal (apophyseal) joints lie at 90 degrees to the midsagittal plane (MSP), so they are demonstrated on a true lateral. The intervertebral foramina are oriented 45 degrees to the MSP and open 15 degrees to the transverse (long) axis, which is why they require a 45-degree oblique combined with a 15-20 degree central-ray (CR) angle.

Routine Projections

AP Axial (C3-C7)

The AP axial projection directs the CR 15-20 degrees cephalad, entering near C4 (the thyroid cartilage). The cephalad angle throws the mandible and skull base superior to the spine and opens the C3-C7 intervertebral disk spaces and the uncinate processes. Because the mandible superimposes C1-C2, the AP axial cannot demonstrate the upper two segments.

AP Open-Mouth (Atlantoaxial)

The upper cervical segments require the AP open-mouth projection. With the MSP perpendicular to the IR, adjust the head so that a line from the lower margin of the upper incisors to the base of the skull (mastoid tips) is perpendicular to the IR. The CR passes through the open mouth to demonstrate the dens (odontoid process), the lateral masses of the atlas (C1), and the atlantoaxial (C1-C2) joints. Two classic traps: if the upper incisors superimpose the dens, the head is flexed too much; if the base of the skull superimposes the dens, the head is extended too much. Have the patient keep the tongue on the floor of the mouth (softly phonate 'ahh') so it does not project over the dens.

Oblique Projections

Cervical obliques rotate the patient 45 degrees. AP oblique projections (RPO and LPO) demonstrate the intervertebral foramina farthest from the IR (the up-side) and use a CR 15-20 degrees cephalad. PA oblique projections (RAO and LAO) demonstrate the intervertebral foramina nearest the IR (the down-side) and use a CR 15-20 degrees caudad. Memory aid: with the beam entering posteriorly you image the far side; entering anteriorly you image the near side. Every oblique must include C7-T1.

Lateral, Swimmer's, and Functional Studies

The lateral cervical spine is performed erect at a 72-inch (180 cm) SID to offset the large object-to-image-receptor distance (OID) that magnifies the spine. Suspend respiration on full expiration to depress the shoulders so all seven vertebrae, including the C7-T1 junction, are visualized. When the shoulders obscure C7-T1, obtain the cervicothoracic (Twining) lateral, or 'swimmer's' position: depress the near shoulder and elevate the opposite arm above the head to separate the superimposed humeral heads. Flexion and extension laterals are functional studies of anteroposterior mobility and hardware stability.

Trauma Cervical Spine

On an acute trauma patient the cross-table (horizontal-beam) lateral is obtained first and must include C7-T1, because most instability and fractures occur at that junction. Do not remove the cervical collar, rotate the head, or move the patient until a physician has cleared the spine.

Evaluation Criteria and Common Errors

A well-positioned AP axial shows the C3-C7 disk spaces open and the spinous processes equidistant from the lateral borders, confirming no rotation. On the lateral, the rami of the mandible should be superimposed, the zygapophyseal joints should appear in profile, and the C4 body should be centered. On the open-mouth view the dens must sit centered within the ring of C1 with the lateral masses symmetric; if the head is rotated, the dens appears off-center between the lateral masses, and if the head is tilted the spinous process of the axis shifts to one side. A frequent error on the lateral is failing to depress the shoulders, so that C7 is cut off at the bottom of the image; the swimmer's (Twining) projection rescues this junction. On obliques, apply the 45-degree rule: too little rotation partially closes the up-side foramina, while too much rotation elongates and distorts them. Always confirm the entire C7-T1 region is included on obliques and laterals, because a missed cervicothoracic junction is one of the most common causes of a repeat exposure.