PA Patella is a radiography positioning projection of the Lower Extremity. Prone, leg extended with the patella flat against the IR. SID 40". The centering point is located center of the patella with the patient prone and the leg extended.. 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.
- Extend the leg and rotate the heel 5-10 degrees laterally so the patella lies parallel to the IR.
- Ensure pelvis is level and not rotated.
Position & centering point
Center of the patella with the patient prone and the leg extended.
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.
- Patella seen through the distal femur; the patellofemoral joint is not open on a PA.
- Patella superimposed on the femoral condyles, as expected on a PA.
- Collimation 1 inch on all sides around patella.
Common errors / ARRT traps
- 1 Medial rotation of leg rotates patella away from true PA.
- 2 Failing to rotate the heel 5-10 degrees laterally, or over-rotating, leaves the patella oblique.
- 3 Flexing the knee lifts the patella off the IR and distorts it.
- 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 current ARRT Radiography Content Specifications.