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 eam (external auditory meatus). 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 eam.. 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.. Standard exposure ranges from 80 to 90 kVp, 25 to 50 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
- 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 EAM (external auditory meatus). 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 EAM.
Exposure / technique
- kVp
- 80–90
- mAs
- 25–50
- SID
- 40" (102 cm)
- Notes
- High mAs to penetrate dense occipital bone. 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.
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 / ARRT traps
- 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.
- 5 Treating 30 degrees to OML and 37 degrees to IOML as additive rather than alternatives.
Clinical indications
- Occipital bone trauma or fracture evaluation.
- Posterior fossa / cerebellar pathology workup.
- Foramen magnum visualization (mass, herniation, congenital anomalies).
- Dorsum sellae for pituitary fossa enlargement.
Aligned to the current ARRT Radiography Content Specifications.