Skull AP is a radiography positioning projection of the Skull. AP cranium. SID 40". CR perpendicular to IR. The centering point is located nasion (junction of nasal bone and frontal bone). oml perpendicular to ir.. The central ray is perpendicular to ir, centered to nasion.. Image-quality criteria include midsagittal plane perpendicular to ir (no tilt or rotation)., petrous ridges projected inferior to orbits.. Standard exposure ranges from 75 to 85 kVp, 25 to 40 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Frontal bones and frontal sinuses.
- Orbital cavities and orbital margins.
- Nasal septum and nasal bones.
- Zygomatic bones.
- Petrous portions of temporal bones.
- Parietal bones bilaterally.
Patient preparation
- Verify patient identity.
- Remove glasses, earrings, hairpins, dentures, hearing aids.
- Position patient supine with head on table.
- Tuck chin so OML perpendicular to IR.
- Align midsagittal plane perpendicular to IR.
- Position nasion centered to IR.
- Suspend respiration during exposure.
Position & centering point
Nasion (junction of nasal bone and frontal bone). OML perpendicular to IR.
Central ray
Perpendicular to IR, centered to nasion.
Exposure / technique
- kVp
- 75–85
- mAs
- 25–40
- SID
- 40" (102 cm)
- Notes
- Chin tuck critical for OML positioning. Use small focal spot for cranial suture detail.
Image-quality criteria
- Midsagittal plane perpendicular to IR (no tilt or rotation).
- Petrous ridges projected inferior to orbits.
- Orbits symmetric without rotation.
- Frontal sinuses shown clearly.
- Nasal septum midline and undistorted.
- Cranium centered on IR with 1" margin all around.
Common errors / ARRT traps
- 1 Excessive chin extension puts OM line at angle.
- 2 Insufficient chin tuck superimposes petrous over orbits.
- 3 Head rotation asymmetrically demonstrates orbits.
- 4 Midsagittal plane not perpendicular distorts anatomy.
- 5 CR not centered to nasion causes anatomy off-axis.
Clinical indications
- Trauma evaluation for frontal bone fractures.
- Assessment of frontal sinus pathology.
- Orbital fracture evaluation.
- Sinus disease evaluation.
Aligned to the 2025 ARRT Content Specifications.