Skull PA axial (Caldwell) is a radiography positioning projection of the Skull. PA axial / Caldwell. Frontal bone, frontal & ethmoid sinuses. CR 15° caudad to OML. The centering point is located CR exits at the nasion. Forehead and nose touching the IR; OML perpendicular to IR. The central ray is CR 15° caudad to OML, centered to exit at the nasion. (Alternative: 25° to 30° caudad, exiting at mid-orbit, for the superior orbital fissures and inferior orbital rims.). Image-quality criteria include petrous ridges fill the lower one-third of the orbits (15° caudad CR), frontal bone clearly demonstrated. A sample technique is 85 kVp with DR 7.1 or CR 14 mAs, at an SID of 40 inches (102 cm).
Watch the setup
All steps (5)
- Position patient prone or upright facing the IR.
- Forehead and nose resting against the IR.
- Flex the neck as needed to bring the OML perpendicular to the IR.
- MSP perpendicular to the midline of the grid, with no rotation or tilt.
- Center the IR to the CR.
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
- Frontal bone.
- Greater and lesser wings of the sphenoid.
- Superior orbital fissures within the orbits.
- Frontal and anterior ethmoid sinuses.
- Supraorbital margins.
- Crista galli.
Patient preparation
- Verify patient identity.
- Remove all metallic or plastic objects from the head and neck.
- Position patient prone or upright facing the IR.
- Forehead and nose resting against the IR.
- Flex the neck as needed to bring the OML perpendicular to the IR.
- MSP perpendicular to the midline of the grid, with no rotation or tilt.
- Center the IR to the CR.
- Suspend respiration.
Position & centering point
CR exits at the nasion. Forehead and nose touching the IR; OML perpendicular to IR.
Central ray
CR 15° caudad to OML, centered to exit at the nasion. (Alternative: 25° to 30° caudad, exiting at mid-orbit, for the superior orbital fissures and inferior orbital rims.)
Exposure / technique
- kVp
- 85
- mAs
- DR 7.1 / CR 14
- SID
- 40" (102 cm)
- Notes
- For a sinus exam, Bontrager requires an erect patient and a horizontal CR, and ARRT lists the sinus Caldwell as horizontal beam in both the 2022 and 2027 outlines. Sample technique: DR 7.1 / CR 14 mAs at 85 kVp, 40 in SID, for a 20 cm part. Adjust for your equipment. Sources differ on the 25° to 30° alternative: Bontrager gives the exit as both the nasion and mid-orbit; Merrill's aims 25° to 30° at the nasion and uses 20° to 25° through mid-orbit for the superior orbital fissures.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Petrous ridges fill the lower one-third of the orbits (15° caudad CR).
- Frontal bone clearly demonstrated.
- Frontal and anterior ethmoid sinuses shown.
- No rotation: equal distance from the midlateral orbital margins to the lateral cranial cortex on each side.
- Superior orbital fissures symmetric within the orbits.
- Supraorbital margins seen without superimposition; crista galli shown.
Common errors
- 1 Rotation: unequal distance from each midlateral orbital margin to the lateral cranial cortex.
- 2 Tilt, where the MSP is not perpendicular to the image receptor.
- 3 Too little caudal angle or too much neck flexion (chin down) projects petrous ridges too high.
- 4 Too much caudal angle projects the petrous ridges at or just below the inferior orbital margins.
Clinical indications
- Skull fractures.
- Neoplastic processes.
- Paget disease.