AP BILATERAL "FROG LEG" PROJECTION: PELVIS

Sunday, November 25, 2012

MODIFIED CLEAVES METHOD

Warning: Do not attemp this position on patient with destructive hip disease or with potential hip fracture or dislocation.

Pathology Demonstrated:

  • This projection is useful for demonstration of a nontrauma hip or developmental dysplasia of hip (DDH), also known as congential hip dislocation (CHD).


Technical Factors:

  • IR size - 35 x 43 cm (14 x 17 inches), crosswise
  • Moving or stationary grid
  • 80 +- 5 kV range or 90 +- 5 kV range
  • mAs: 12


Shielding:

  • Shield gonads for both males and females without obscuring essential anatomy.


Patient Position:

  • Align patient to midline of table and/or IR and to CR.
  • Ensure pelvis is not rotated (equal distance of ASISs to tabletop).
  • Center IR to CR, at level of femoral heads, with top of IR approximately at level of illiac crest.
  • Flex both knees approximately 90 degree, as demonstrated.
  • Place the plantar surfaces of feet together and abduct both femora 40 to 45 degree from vertical. Ensure that both femora are abducted the same amount and that pelvis is not rotated.
  • Place supports under each leg for stabilization if needed.


Central Ray:

  • CR is perpendicular to IR, directed to a point 3 inches (7.5 cm) below level of ASIS ( 1 inch [2.5 cm] above symphysis pubis).
  • minimum SID is 40 inches (100 cm).


Collimation:

  • Collimate to IR borders on four sides.


Respiration:

  • Suspend respiration during exposure.


Note 1: This projection frequently is performed for periodic follow-up exams on younger patients, thus correct placement of gonadal shielding is important for both male and female patients, ensuring that hips joints are not covered.

Note 2: Less abduction of femora such as only 20 to 30 degree from vertical provides for the foreshortening of femoral necks, but this placement foreshortens the entire proximal femora, which may not be desirable.

Radiographic Criteria:

Structure Shown:

  • Femoral heads and necks, accetabulum, and tronchanteric areas are visible on one radiograph.


Position:

  • No rotation is evidenced by symmetric appearance of the pelvic bones, especially the ala of the ilium, two obturator foramina, and ischial spines, if visible.
  • The femoral heads and necks and greater and lesser trochanters should appear symmetric if both thighs were abducted equally.
  • The lesser trochanters should appear equal in size, as projected beyond the lower or medial margin of the femora.
  • Most of the area of the greater tronchanter appears superimposed over the femoral neck, which appear foreshortened.


Collimation and CR:

  • The pelvic girdle should be centered to the collimation field from right to left, with the midpoint being about 2.5 cm (1 inch) superior to the symphysis pubis.


Exposure Criteria:

  • Optimal exposure visualizes the margins of the femoral head and the acetabulum through overlying pelvic structures, without overexposing the proximal femora.
  • Trabecular markings appear sharp, indicating no motion.

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