Skull PA is a radiography positioning projection of the Skull. Forehead and nose on IR. OML perpendicular. CR exits glabella or nasion by source. The centering point is located glabella (Bontrager) or nasion (Merrill's), where the CR exits. OML and MSP perpendicular to IR. The central ray is perpendicular to IR (parallel to the OML), exiting at the glabella (Bontrager) or nasion (Merrill's). Image-quality criteria include entire cranium with no rotation or tilt, equal distance from lateral orbital rims to lateral skull margins. A sample technique is 85 kVp with DR 6.3 or CR 12.5 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Frontal bone and frontal sinuses.
- Petrous ridges filling the orbits.
- Crista galli in the midline.
- Dorsum sellae and internal auditory canals.
- Frontal and ethmoid sinuses.
- Greater wings of the sphenoid.
Patient preparation
- Trauma patient who cannot be positioned for a PA: use the AP, after ruling out cervical spine injury.
- Verify patient identity using two identifiers.
- Remove glasses, earrings, dentures, and hairpins.
- Position patient prone or upright facing the IR.
- Rest forehead and nose against the receptor.
- OML perpendicular to the IR.
- MSP perpendicular to the IR (no rotation or tilt).
- Suspend respiration during exposure.
Position & centering point
Glabella (Bontrager) or nasion (Merrill's), where the CR exits. OML and MSP perpendicular to IR.
Central ray
Perpendicular to IR (parallel to the OML), exiting at the glabella (Bontrager) or nasion (Merrill's).
Exposure / technique
- kVp
- 85
- mAs
- DR 6.3 / CR 12.5
- SID
- 40" (102 cm)
- Notes
- Grid required. 10"x12" IR lengthwise. Sample technique: DR 6.3 / CR 12.5 mAs at 85 kVp, 40 in SID, for a 20 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Entire cranium with no rotation or tilt.
- Equal distance from lateral orbital rims to lateral skull margins.
- Petrous ridges fill the orbits, level with the supraorbital margins.
- Anterior and posterior clinoids seen just above the ethmoid sinuses.
- Crista galli centered along the midline.
- No tilt: MSP perpendicular to the IR, with the EAMs the same distance from the surface.
Common errors
- 1 Rotation of the head: unequal distance from orbital rims to skull.
- 2 Tilt of the head: MSP not perpendicular to the IR.
- 3 OML not perpendicular: petrous ridges not at the level of the supraorbital margins.
Clinical indications
- Skull fractures (medial and lateral displacement).
- Neoplastic processes.
- Paget disease.
- Showing the frontal bone with minimal distortion.