PA skull X-ray positioning
One of the routine skull views. The patient lies face down, the beam passes back to front (PA), tilted 20° towards the feet, and the centring point is the nasion.
By the ScanFlo Editorial Team · Updated
In this section · X-ray positioning
- X-ray positioning guides
- Skull positioning
- Face & sinuses positioning
- Soft tissue neck positioning
- C-spine positioning
- Chest positioning
- Shoulder positioning
- Humerus positioning
- Elbow positioning
- Forearm positioning
- Wrist positioning
- Hand positioning
- T-spine positioning
- Abdomen positioning
- L-spine positioning
- Sacrum & SI joints positioning
- Pelvis positioning
- Hip positioning
- Femur positioning
- Knee positioning
- Tibia & fibula positioning
- Ankle positioning
- Foot positioning
Still being checked: a radiographer is reviewing this guide. If your department does it differently, follow your department.
Patient position
Prone. Remove dentures, earrings, hair clips and anything in the hair. Lay the patient prone, forehead and nose on the table, orbitomeatal line perpendicular to the detector, hands under the chest. Not for suspected facial fracture or the unconscious patient.
Centring & collimation
Central ray
PACentre: the nasion.
Beam: PA, angled 20° caudad.
Collimate: the whole skull vault
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 100 cm | |
| Receptor | 24 × 30 cm | portrait |
| Placement | Table bucky | Grid in |
| Angulation | 20° | caudad |
Marker & breathing
- Marker: R or L marker on the side nearest the detector
- Breathing: No breathing instruction needed.
Exposure by patient size
Treat these as a starting point. Your detector, your generator and the patient in front of you will all move the numbers.
| Patient size | kV | mAs (chart) | mAs (DR default) |
|---|---|---|---|
| XS | 70 | 12.5 | 5 |
| S | 70 | 20 | 8 |
| M | 70 | 25 | 10 |
| L | 70 | 40 | 16 |
These start from a classic film-screen chart (70 kV, 50 mAs at 140 cm) and are adjusted for a digital detector at 100 cm. The DR column is what ScanFlo's console suggests; each step up or down changes the mAs by about a quarter.
Image quality checklist
- Orbits symmetric about the midline (no rotation).
- Petrous ridges projected in the lower orbits.
- The whole vault is included.
- The anatomical side marker is visible, correct and clear of the anatomy.
- Collimation borders are visible on all sides, no wider than the area requested.
Faults & how to fix them
| Fault | Cause | Fix |
|---|---|---|
| Orbits asymmetric | Head rotated | Check the mid-sagittal plane is perpendicular. |
Common questions
- Where do you centre for a skull PA X-ray?
- The centring point is the nasion.
- What distance and beam angle does a skull PA need?
- 100 cm from the tube to the receptor. The beam passes back to front (PA), tilted 20° towards the feet.
- What size receptor, and do you need a grid?
- Use a 24 × 30 cm receptor, portrait, in the table bucky. A grid is needed.
- Where does the lead marker go?
- R or L marker on the side nearest the detector.
- What goes wrong most often on a skull PA?
- Orbits asymmetric. Usual cause: head rotated. The fix: check the mid-sagittal plane is perpendicular.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a skull pa in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.