Skull AP axial (Towne) is a radiography positioning projection of the Skull. AP axial / Towne. Occipital bone, dorsum sellae. SID 40", CR 30° caudad. The centering point is located enter ~2.5" superior to the glabella; exit at the level of the foramen magnum (base of the skull). OML perpendicular to the IR. The central ray is CR 30° caudad to OML (or 37° caudad to IOML). Centered ~2.5" above the glabella, exiting at the level of the foramen magnum. Image-quality criteria include symmetrical petrous pyramids; equal distance from lateral skull margin to lateral foramen magnum bilaterally, dorsum sellae and posterior clinoid processes projected within the foramen magnum. A sample technique is 85 kVp with DR 10 or CR 20 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Occipital bone.
- Petrous pyramids, symmetric on both sides.
- Foramen magnum.
- Dorsum sellae and posterior clinoids within the foramen magnum shadow.
Patient preparation
- Trauma patient: rule out cervical spine fracture, subluxation or dislocation before moving or manipulating the head or neck.
- Verify patient identity.
- Remove all radiopaque objects (glasses, earrings, hairpins).
- Position patient supine or upright with back to the IR.
- Tuck chin to bring OML perpendicular to IR.
- No rotation: shoulders equidistant from the table.
- Suspend respiration.
Position & centering point
Enter ~2.5" superior to the glabella; exit at the level of the foramen magnum (base of the skull). OML perpendicular to the IR.
Central ray
CR 30° caudad to OML (or 37° caudad to IOML). Centered ~2.5" above the glabella, exiting at the level of the foramen magnum.
Exposure / technique
- kVp
- 85
- mAs
- DR 10 / CR 20
- SID
- 40" (102 cm)
- Notes
- mAs sets the quantity of radiation, not its penetrability (Bushong). With the OML perpendicular to the IR, 30° to the OML and 30° from vertical are the same thing; the real alternative is 37° to the IOML. Sample technique: DR 10 / CR 20 mAs at 85 kVp, 40 in SID, for a 22 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Symmetrical petrous pyramids; equal distance from lateral skull margin to lateral foramen magnum bilaterally.
- Dorsum sellae and posterior clinoid processes projected within the foramen magnum.
- Occipital bone clearly demonstrated.
- No rotation or tilt of the cranium.
- Penetration adequate for occipital bone evaluation.
Common errors
- 1 Underangling the CR or under-flexing the neck projects the dorsum sellae above the foramen magnum.
- 2 Overangling or over-flexing superimposes the posterior arch of C1 on the dorsum sellae.
- 3 Rotation narrows the petrous ridge on the side the head is turned toward.
- 4 Tilt shifts the clinoid processes laterally within the foramen magnum.
Clinical indications
- Occipital bone trauma or fracture evaluation.
- Skull fractures (medial and lateral displacement).
- Neoplastic processes.
- Paget disease.