PA PROJECTION : PATELLA

Pathology Demonstrated:

• Patellar fracture are evaluated before the knee joint is flexed for other projections.

Technical Factors:

• IR size - 18 x 24 cm ( 8 x 10 inches), lengthwise.
• Moving or stationary grid (or screen < 10 cm)
• 75 +- 5 kV range (increase by 4 to 6 kV from PA knee technique for better patellar visualization)
• mAs 5

Shielding:

• Place shield over gonadal area.

Patient Position:

• Take radiograph with patient in the prone position, legs extended; give pillow for head; place support under ankle and leg with smaller support under femur above knee to prevent direct pressure on patella.

Part Position:

• Align and center long axis of leg and knee to midline of table or IR.
• True PA: Align intercondylar line parallel to plane of IR. ( This usually requires about 5 degree internal rotation of anterior knee.)

Central Ray:

• CR is perpendicular to IR.
• Direct CR to midpatella area (which is usually at approximately the mid-popliteal crease).
• Minimum SID is 40 inches (100 cm)

Collimation:

• Collimate closely on four sides to include just the area of the patella and knee joint.

Note: With potential fracture of the patella, extra care should be taken to not flex knee and provide support under thigh (femur) so as not to put direct pressure on patella area.
The projection also may be taken as an AP projection positioned like an AP knee if patient cannot assume a prone position.

Radiographic Criteria:

Structure Shown:

PA Patella

• Knee joint and patella are shown, with optimal recorded detail of patella because of decreased OID if taken as a PA projection.

Position:

• No rotation is present as evidenced by symmetric appearance of the condyles.
• The patella will be centered to the femur with correct slight internal rotation of the anterior knee.

Collimation and CR:

• Centering and angulation are correct if the knee joint is open and the patella is in the center of the collimated field.

Exposure Criteria:

• Optimal exposure without motion will visualized soft tissue in joint area and also will clearly visualize sharp bony trabecular markings and outline of the patella as seen through the distal femur.