Sacroiliac joint (PA oblique view)

Last revised by Andrew Murphy on 19 Sep 2021

The posteroanterior (PA) oblique sacroiliac joint view demonstrates the sacroiliac joints (SIJs) in an open profile. It is commonly used in conjunction with the sacroiliac AP view

This projection examines both left and right sacroiliac joints for comparison purposes in the evaluation of sacroiliitis and ankylosing spondylitis 1. It is also used to determine any dislocation or subluxation to the joint.

  • patient is positioned in an oblique position, ~25-30° RPO or LPO 2
    • a flexed knee can usually support this position
  • the patient's up side is the side of interest
    • LAO position examines the right SI joint
    • RAO position examines the left SI joint
  • the degree of obliquity is approximately 25-30°
  • posteroanterior oblique projection
  • centering point
    • perpendicular to the image receptor, centering 1 cm medial to the posterior inferior iliac spine closest to the image receptor.
  • collimation
    • laterally to include the entire sacroiliac joint of interest
    • superiorly and inferiorly to include the entire joint
  • orientation
    • portrait
  • detector size
    • 24 x 30 cm
  • exposure
    • 75 kVp
    • 25-30 mAs
  • SID
    • 100 cm
  • grid
    • yes                                                                  
  • the sacroiliac joint closest to the image receptor should be shown open or with minimal superimposition of the ilium and the sacrum
  • the sacroiliac joint is centered and demonstrated clearly through adequate penetration
  • the degree of obliquity required to show the sacroiliac joint in a PA oblique position is usually shallow (25-30°)
  • a radiolucent support such as a sponge can be used to help elevate the hip and thorax to maintain position

  • a 20-25° caudal angle, centered to posterior inferior iliac spine will project the central ray perpendicular to the long axis of the sacroiliac joint
     
  • attempt similar positioning for both sides, in order for accurate comparative assessment

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