AP Odontoid (Open Mouth) is a radiography positioning projection of the Spine. Supine or erect, mouth open wide, CR perpendicular through the open mouth. C1-C2. The centering point is located centre of the open mouth; the CR passes through the dens (C1-C2). Centre the IR to the CR. The central ray is perpendicular to the IR, directed through the centre of the open mouth. Image-quality criteria include dens and body of C2, lateral masses and transverse processes of C1 and the atlantoaxial joints seen through the open mouth, lower margin of the upper incisors superimposed on the base of the skull; neither the teeth nor the skull base over the dens. A sample technique is 85 kVp with DR 3.6 or CR 8 mAs, at an SID of 40 inches (102 cm).
Watch the setup
All steps (4)
- Supine or erect, arms by the sides.
- Align the midsagittal plane to the CR and the midline of the IR, with no rotation of the head (mandibular angles and mastoid tips equally far from the IR) or thorax.
- With the mouth open, a line from the lower margin of the upper incisors to the base of the skull (mastoid tips) perpendicular to the IR, or angle the CR accordingly.
- Open the mouth wide as the last step and work quickly; only the lower jaw moves, and the tongue stays in the lower jaw.
3D anatomy: BodyParts3D, © The Database Center for Life Science. Posed, rendered and labelled by RadTechPrepper. These animations are licensed CC BY-SA 4.0. Credits
Anatomy demonstrated
- Odontoid (dens) process of axis (C2).
- Lateral masses of atlas (C1).
- Atlantoaxial joint space.
- Axis (C2) body below dens.
Patient preparation
- Trauma: do not remove the collar or move the head until a physician has cleared the cross-table lateral or CT.
- Verify patient identity using two identifiers.
- Remove dentures, removable bridges, and metal objects from neck.
- Supine or erect, arms by the sides.
- Align the midsagittal plane to the CR and the midline of the IR, with no rotation of the head (mandibular angles and mastoid tips equally far from the IR) or thorax.
- With the mouth open, a line from the lower margin of the upper incisors to the base of the skull (mastoid tips) perpendicular to the IR, or angle the CR accordingly.
- Open the mouth wide as the last step and work quickly; only the lower jaw moves, and the tongue stays in the lower jaw.
Position & centering point
Centre of the open mouth; the CR passes through the dens (C1-C2). Centre the IR to the CR.
Central ray
Perpendicular to the IR, directed through the centre of the open mouth.
Exposure / technique
- kVp
- 85
- mAs
- DR 3.6 / CR 8
- SID
- 40" (102 cm)
- Notes
- Patient must open mouth wide as the last step. Supine or erect, with a grid. Sample technique: DR 3.6 / CR 8 mAs at 85 kVp, 40 in SID, for an 11 cm part (Merrill's sample chart). Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Dens and body of C2, lateral masses and transverse processes of C1 and the atlantoaxial joints seen through the open mouth.
- Lower margin of the upper incisors superimposed on the base of the skull; neither the teeth nor the skull base over the dens.
- No rotation: lateral masses of C1 equally far from the mandibular condyles, and the C2 spinous process centred.
- Collimation to the area of interest.
Common errors
- 1 Mouth not opened wide enough obscures the odontoid.
- 2 Rotation of head tilts lateral masses off symmetry.
- 3 Upper incisors over the dens: extend the neck slightly or angle the CR slightly cephalad.
- 4 Skull base over the dens: flex the neck slightly or angle the CR slightly caudad.
Clinical indications
- Trauma evaluation for upper cervical spine fracture.
- Suspected odontoid (dens) or Jefferson fracture.
- Pathology of C1 and C2 and adjacent soft tissue.