Skull X-Ray PA
Cranial vault, frontal sinuses, orbits, petrous ridges. First projection of the standard skull series — general orientation view.
Patient seated or prone.
Forehead and nose against the detector.
Orbitomeatal line (OML) perpendicular to the detector plane.
Midsagittal plane perpendicular to the detector (avoid rotation).
Central ray: perpendicular, centred at the nasion.
Nasion
70–80 kV · 10–15 mAs
SID ~100 cm · grid
70–80 kV · 15–25 mAs · SID ~100 cm
Petrous ridges projected in the lower third of the orbits (confirms no tilt).
Frontal sinuses and anterior ethmoid air cells visible.
Parietal and frontal bones included.
Orbits symmetrical (confirms no rotation).
Calvaria included bilaterally.
Head rotation (orbital and petrous asymmetry).
OML not perpendicular (alters petrous projection).
Incorrect centring.
Movement.
Difference from Caldwell (15° caudal tilt): the PA 0° projects the petrous ridges in the lower third of the orbit; the Caldwell projects them below the orbital floor, freeing the frontoethmoidal sinuses. The PA standard is the general orientation view; the Caldwell is specific for frontoethmoidal sinuses. In head trauma the PA is often the first projection before Caldwell and Towne.
Paediatric patient: reduce kVp by 10–15% proportional to thickness. mAs ÷2–3 for age. Immobilisation is essential. Fontanelles and non-ossified sutures in infants must not be confused with fracture lines.