For an anteroposterior (AP) cervical spine radiograph, where should the central ray be directed, including the appropriate cephalic angulation, source‑to‑image distance, and patient positioning?

AP Cervical Spine Radiograph: Central Ray Direction

For an anteroposterior (AP) cervical spine radiograph, the central ray should be directed at a 15-20 degree cephalic angle, centered at the level of the fourth cervical vertebra (C4), which corresponds approximately to the collar level or thyroid cartilage.

Technical Parameters

The standard AP cervical spine projection requires:

• Angulation: 15-20 degrees cephalic (angled toward the head)
• Centering point: Fourth cervical vertebra (C4), approximately at the collar/thyroid cartilage level
• Source-to-image distance (SID): 40 inches is standard for cervical spine radiography 1

Rationale for Cephalic Angulation

The cephalic angle is essential to:

• Project the mandible superiorly, preventing overlap with upper cervical vertebrae
• Optimize visualization of the cervical vertebral bodies
• Reduce anatomical superimposition from the jaw and skull base

Complete Cervical Spine Series

While you asked specifically about the AP projection, it's critical to understand that the AP view alone has limited diagnostic value. The evidence demonstrates that the AP cervical spine radiograph rarely identifies injuries not already visible on lateral or odontoid views 2. In fact, one study found no cases of cervical spine injury evident on the AP view without corresponding abnormality on the lateral or open-mouth odontoid views 2.

Standard Three-View Series Includes:

1. Lateral view (most important - must visualize C7-T1 junction)
2. Open-mouth odontoid view (anteroposterior projection of C1-C2)
3. Anteroposterior view (15-20° cephalic, centered at C4)

The lateral view is the single most critical projection and must include all seven cervical vertebrae plus the C7-T1 interspace 3, 4.

Important Clinical Caveats

The most common reason for missed cervical spine injuries is technically inadequate radiographs 4. The AP view is frequently compromised by:

• Anterior artifacts (cervical collars, tracheal tubes, gastric tubes) 5
• Poor visualization of the craniocervical junction 5
• Limited sensitivity compared to CT imaging

Modern Practice Note

CT has largely supplanted plain radiography for cervical spine trauma evaluation, with sensitivity approaching 100% versus only 36% for plain radiographs 6. Plain radiographs detect only about one-third of fractures visible on CT 7. In trauma settings where imaging is indicated by NEXUS or Canadian C-Spine Rule criteria, CT without contrast is now the reference standard 6.