AP PROJECTION: FEMUR - MID AND DISTAL

Note: If site of interest is in area of proximal femur, a unilateral hip routine or a pelvis is recomended, as is describe in this chapter.

Pathology Demonstrated:

• Mid- and distal femur is demonstrated including knee joint for detection and avaluation of fractures and/or bone lessions.

Technical factors:

• IR size- 35 x 43 cm (14 x 17 inches). lengthwise
• Moving or stationary grid
• 75 +- 5 kV range
• Because of anode heel effect, place hip or head end of patient at cathode end of x-ray beam.
• mAs: 12

Shielding:

• Place lead shield over pelvic area to ensure correct gonadal shielding because of proximity to primary beam.

Patient Position:

• Take radiograph with patient in the supine position, with femur centered to midline of table; give pillow for head. (This projection also may be done on a stretcher with a portable grid placed under the femur.)

Part Position:

• Align femur to CR and to midline of table or IR.
• Rotate leg internally about 5 degree for a true AP, as for an AP knee. ( for proximal femur, 15 to 20 degree intenal leg rotation is required, as for an AP hip.)
• Ensure that knee joint is included on IR, considering the divergence of the x-ray beam. (Lower cassette margin should be approximately 2 inches [5 cm] below knee joint.)

Central Ray:

• CR is perpendicular to femur and IR.
• Direct CR to midpoint of IR.
• Minimum SID is 40 inches (100 cm)

Collimation:

• Collimate closely on both sides to femur with end collimation to film borders.
• Routine to include both joints: Common departmental routines include both joints on all initial femur exams. For a large adult, a second smaller IR then should be used for an AP of either the knee or the hip, ensuring that both hip and knee joints are included. If the hip is included, the leg should be rotated 15 to 20 degree internally to place the femoral neck in profile.

Radiographic Criteria:

Structure Shown:

• Distal two-thirds of distal femur, including knee joints, is shown.
• Knee joint space will not appear fully open because of divergent x-ray beam.

Position:

• No rotation is evidenced; femoral and tibial condyles should appear symmetric in size and shape with the outline of patella slightly toward medial side of femur.
• The approximate medial half of fibula head should be superimposed by tibia.

Collimation and CR:

• Femur should be centered to collimation field and aligned with long axis of IR with knee joint space a minimum of 1 inch (2.5 cm) from distal IR margins.
• Minimal collimation borders should be visible on proximal and distal margins of IR.

Exposure Criteria:

• Optimal exposure with correct use of anode heel effect will result in near uniform density of entire femur.
• No motion should occur; fine trabecular markings should be clear and sharp throughout length of femur.