AP standing knees X-ray positioning (weight-bearing)
An extra knee view, used when a routine one is not enough. The patient stands, the beam passes front to back (AP), straight on to the receptor, and the centring point is between the knees at the joint line.
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
Erect. Stand the patient with weight equal on both legs, back to the bucky.
Centring & collimation
Central ray
APCentre: between the knees at the joint line.
Beam: AP, perpendicular to the receptor.
Collimate: both knees
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 110 cm | |
| Receptor | 35 × 43 cm | landscape |
| Placement | Erect (wall) bucky | Grid in |
| Angulation | None | Perpendicular to the receptor |
Marker & breathing
- Marker: R marker
- 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 | 1.6 | 0.63 |
| S | 70 | 2.5 | 1 |
| M | 70 | 3.2 | 1.25 |
| L | 70 | 5 | 2 |
These start from a classic film-screen chart (70 kV, 5 mAs at 140 cm) and are adjusted for a digital detector at 110 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
- Both joint spaces under load.
- 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 |
|---|---|---|
| Weight on one leg | Patient leaning | Equal weight on both feet. |
Common questions
- Where do you centre for a knees AP standing X-ray?
- The centring point is between the knees at the joint line.
- What distance and beam angle does a knees AP standing need?
- 110 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 a 35 × 43 cm receptor, landscape, in the wall bucky. A grid is needed.
- Where does the lead marker go?
- R marker.
- What goes wrong most often on a knees AP standing?
- Weight on one leg. Usual cause: patient leaning. The fix: equal weight on both feet.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a knees ap standing in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.