AP knee X-ray positioning
One of the routine knee views. The patient lies on their back, the beam passes front to back (AP), straight on to the receptor, and the centring point is 1 cm below the apex of the patella.
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
Supine. Supine, the detector on the table under the knee, the foot rotated slightly in and steadied with a sandbag.
Centring & collimation
Central ray
APCentre: 1 cm below the apex of the patella.
Beam: AP, perpendicular to the receptor.
Collimate: the joint with a quarter of femur and tibia
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 100 cm | |
| Receptor | 18 × 24 cm | portrait |
| Placement | Tabletop | 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 | 53 | 1.6 | 0.8 |
| S | 53 | 2.5 | 1.25 |
| M | 53 | 3.2 | 1.6 |
| L | 53 | 5 | 2.5 |
These start from a classic film-screen chart (53 kV, 40 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 joint space is open.
- The patella is central over the femur.
- 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 |
|---|---|---|
| Patella off-centre | Leg rotated | Rotate the foot in slightly. |
Common questions
- Where do you centre for a knee AP X-ray?
- The centring point is 1 cm below the apex of the patella.
- What distance and beam angle does a knee 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 tabletop. 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 knee AP?
- Patella off-centre. Usual cause: leg rotated. The fix: rotate the foot in slightly.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a knee ap in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.