AP Odontoid (Open Mouth) is a radiography positioning projection of the Spine. Supine, mouth open wide, CR perpendicular. C1-C2 visualization. The centering point is located odontoid (dens), with mouth wide open to clear upper teeth.. The central ray is perpendicular to ir, centered to odontoid.. Image-quality criteria include odontoid (dens) clearly visualized without tooth superimposition., lateral masses of c1 aligned symmetrically.. Standard exposure ranges from 70 to 80 kVp, 10 to 15 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Odontoid (dens) process of axis (C2).
- Lateral masses of atlas (C1).
- Atlantoaxial joint space.
- Atlas (C1) vertebral body.
- Axis (C2) body below dens.
Patient preparation
- Verify patient identity using two identifiers.
- Remove dentures, removable bridges, and metal objects from neck.
- Supine on table with no rotation or tilting.
- Instruct patient to open mouth wide.
- Ensure neck in neutral alignment.
Position & centering point
Odontoid (dens), with mouth wide open to clear upper teeth.
Central ray
Perpendicular to IR, centered to odontoid.
Exposure / technique
- kVp
- 70–80
- mAs
- 10–15
- SID
- 40" (102 cm)
- Notes
- Patient must open mouth wide. Supine tabletop technique.
Image-quality criteria
- Odontoid (dens) clearly visualized without tooth superimposition.
- Lateral masses of C1 aligned symmetrically.
- No rotation or lateral tilting of head.
- Upper incisors clear of odontoid.
- C1 and C2 shown in true AP relationship.
Common errors / ARRT traps
- 1 Mouth not opened wide enough obscures the odontoid.
- 2 Rotation of head tilts lateral masses off symmetry.
- 3 CR not perpendicular to IR creates distortion.
- 4 Teeth superimposed over odontoid prevents clear visualization.
Clinical indications
- Trauma evaluation for upper cervical spine fracture.
- Suspected odontoid (dens) fracture or instability.
- Post-operative follow-up of C1-C2 fixation.
- Evaluation of atlantoaxial joint alignment.
Aligned to the 2025 ARRT Content Specifications.