AP Dens (Fuchs) is a radiography positioning projection of the Spine. Supine, chin raised so the MML is near perpendicular, CR parallel to the MML. Alternate dens visualization. The centering point is located inferior tip of the mandible, with the chin raised so the MML is near perpendicular to the IR. The central ray is parallel to the MML (near perpendicular to the IR), directed to the inferior tip of the mandible. Image-quality criteria include tip of the mandible clears the upper dens and foramen magnum, dens centered within the foramen magnum. A sample technique is 85 kVp with DR 7.1 or CR 12.5 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Odontoid (dens) of axis (C2).
- Dens projected within the foramen magnum.
- Lateral masses of atlas (C1).
- C1-C2 articulation relationship.
- Axis vertebral body.
Patient preparation
- Trauma: do not raise the chin until a physician has cleared the cross-table lateral or CT of the cervical spine.
- Do not attempt this position if fracture or degenerative disease of the upper cervical region is suspected.
- Verify patient identity using two identifiers.
- Remove metal objects from the neck.
- Supine on table, no rotation or tilt.
- Extend head backward gently until the mentomeatal line is near perpendicular to the IR.
Position & centering point
Inferior tip of the mandible, with the chin raised so the MML is near perpendicular to the IR.
Central ray
Parallel to the MML (near perpendicular to the IR), directed to the inferior tip of the mandible.
Exposure / technique
- kVp
- 85
- mAs
- DR 7.1 / CR 12.5
- SID
- 40" (102 cm)
- Notes
- Used when the upper dens is not clearly shown on the open-mouth projection. Do not attempt if fracture or degenerative disease of the upper cervical region is suspected. Sample technique: DR 7.1 / CR 12.5 mAs at 85 kVp, 40 in SID, for a 14 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
- Tip of the mandible clears the upper dens and foramen magnum.
- Dens centered within the foramen magnum.
- No rotation: mandible arched symmetrically over the foramen magnum.
- Body of C2 inferior to dens shown.
- Structures sharp with no motion artifact.
Common errors
- 1 Too little head extension: mandible tip does not clear the upper dens and foramen magnum.
- 2 Head rotation: mandible not arched symmetrically over the foramen magnum.
- 3 CR not parallel to the MML or not directed to the inferior tip of the mandible.
- 4 Dens not centered within the foramen magnum.
Clinical indications
- Upper dens not clearly shown on the open-mouth projection.
- Pathology of the dens and C1 ring, once fracture and degenerative disease of the upper cervical region are ruled out.