AP thumb X-ray positioning
One of the routine hand views. The patient stands, the beam passes front to back (AP), straight on to the receptor, and the centring point is the first metacarpophalangeal joint.
By the ScanFlo Editorial Team · Updated
In this section · X-ray positioning
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Still being checked: a radiographer is reviewing this guide. If your department does it differently, follow your department.
Patient position
Erect. The patient stands, arm internally rotated, the back of the thumb on the detector. Half the detector can be used for each view with a lead strip over the other half.
Centring & collimation
Central ray
APCentre: the first metacarpophalangeal joint.
Beam: AP, perpendicular to the receptor.
Collimate: the thumb and first carpometacarpal joint
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 100 cm | |
| Receptor | 18 × 24 cm | portrait |
| Placement | On the holder | No grid |
| Angulation | None | Perpendicular to the receptor |
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 | 46 | 0.5 | 0.5 |
| S | 46 | 0.8 | 0.5 |
| M | 46 | 1 | 0.5 |
| L | 46 | 1.6 | 0.63 |
These start from a classic film-screen chart (46 kV, 16 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
- The whole thumb and its carpometacarpal joint shown.
- 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 |
|---|---|---|
| Base cut off | Field too tight | Include the carpometacarpal joint. |
Common questions
- Where do you centre for a thumb AP X-ray?
- The centring point is the first metacarpophalangeal joint.
- What distance and beam angle does a thumb AP need?
- 100 cm from the tube to the receptor. The beam passes front to back (AP), straight on to the receptor.
- What size receptor, and do you need a grid?
- Use an 18 × 24 cm receptor, portrait, on the holder. No 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 thumb AP?
- Base cut off. Usual cause: field too tight. The fix: include the carpometacarpal joint.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a thumb ap in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.