PA Patella is a radiography positioning projection of the Lower Extremity. Prone, knee flexed 45 degrees with lateral rotation. SID 40". The centering point is located center of patella with patient prone and knee flexed.. The central ray is perpendicular to the patella in pa position.. Image-quality criteria include patella in true pa projection, not rotated., patellar outline sharp and distinct.. Standard exposure ranges from 60 to 70 kVp, 4 to 7 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Patella in direct PA projection.
- Femoral condyles bilaterally.
- Patellofemoral articulation.
- Patellar facets.
- Femoral groove containing patella.
- Knee joint space.
Patient preparation
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient prone on table with affected knee forward.
- Flex knee 45 degrees and apply slight lateral rotation of leg.
- Ensure pelvis is level and not rotated.
Position & centering point
Center of patella with patient prone and knee flexed.
Central ray
Perpendicular to the patella in PA position.
Exposure / technique
- kVp
- 60–70
- mAs
- 4–7
- SID
- 40" (102 cm)
- Notes
- PA position places patella in true projection on IR.
Image-quality criteria
- Patella in true PA projection, not rotated.
- Patellar outline sharp and distinct.
- Femoral condyles visible bilaterally with patella centered.
- Patellofemoral joint space open and clearly visualized.
- No superimposition of condyles over patella.
- Collimation 1 inch on all sides around patella.
Common errors / ARRT traps
- 1 Medial rotation of leg rotates patella away from true PA.
- 2 Insufficient knee flexion rotates patella more laterally.
- 3 Excessive flexion rotates patella medially.
- 4 Off-centered CR clips patella edges.
Clinical indications
- Patellar fracture evaluation and alignment assessment.
- Patellofemoral arthritis or degenerative disease.
- Patellar subluxation or maltracking assessment.
- Post-operative patellar fixation follow-up.
Aligned to the 2025 ARRT Content Specifications.