Sunday, September 30, 2012

AP Weight Bearing Projection - Bilateral (Both Knee)

ap weight bearing knee
AP projection
Weight bearing projection is usually taken in AP caudal central ray angulation, because AP projection may be easier for patient who are unable to fully straighten the knee joints, such as those with arthritic condition or with certain neuromascular disorder involving the knees.  PA may also use with a cephalic central ray angle if requested. The femorotibial joint spaces of the knees are demonstrated for possible cartilage degeneration or other knee joint pathologies. Both knees are included on same exposure for comparison.

Alternative PA View:

If requested, an alternative PA may be performed with patient facing the table of IR holder, knees flexed at approximately 20 degree, feet straight ahead, and thigh against tabletop or IR holder.
Direct CR to 10 degree caudad (parallel to tibial plateaus) to level of knee joints for PA projection.

Note: The CR angle should be parallel to articular facets (tibial plateau) for best visualization of "open" knee joint spaces.

Technical Factors:

IR size - 30 x 35 cm (11 x 14 inches) or 35 x 43 cm (14 x 17 inches), crosswise
Moving or stationary grid
70 +- 5 kV range
mAs 6
Shield gonads

Patient and Part Position

Take radiograph with patient erect and standing on attached step or on step stool to place patient high enough for horizontal beam x-ray tube.
Position feet straight ahead with weight evenly distributed on both feet; provide support handles for patient stability
Align and center bilateral legs and knees to CR and to midline of table and IR; IR height adjusted to CR.

Central Ray and Collimation

CR perpendicular to IR (average-sized patient), or 5 to 10 degree caudad on thin patient, directed to midpoint between knee joints at a level 1/2 inch (1.25 cm) below apex of patellae.
Minimum SID of 40 inches (100 cm) 
Collimate to bilateral knee joint region, including some distal femora and proximal tibia for alignment purposes.

Radiographic Criteria fot both AP and PA View / Projection

Proper patient positioning with no rotation of both knees a symmetric appearance of femoral and tibial condyles are demonstrated and approximately 1/2 of proximal fibula is superimposed to be tibia.

Exposure Criteria and Structure Shown

Optimal exposure should visualized faint outlines of patellae through femora.
Soft tissue should be visible, and trabecular markings of all bones should appear clear and sharp, indicating no motion.
The distal femur, proximal tibia, and fibula femorotibial joint spaces are demonstrated bilaterally.

Collimation and CR:

Knee joint spaces should appear open if CR angle was correct (parallel to tibia plateau).
Collimation field should be centered to knee joint spaces and should include sufficient femur and tibia to determine long axes of these long bones for alignment determinations.

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