Sacroiliac joint (PA sacrum view)

Last revised by Andrew Murphy on 19 Sep 2021

The posteroanterior sacrum projection is a useful part of the sacroiliac series and demonstrates the opened sacroiliac joint spaces and L5-S1 intervertebral joint space.

This view is useful in visualizing any fractures, sacroiliac joint dislocations or subluxations, and possible inflammation (i.e. sacroiliitis) of the sacrum and/or sacroiliac joints. Due to the shallow obliquity of the sacroiliac joints, the prone position allows the diverging x-ray beam to project through the joint space giving better visualization of the joint compared to the AP projection 1.

  • the patient is prone on the imaging table with legs extended
  • patient’s shoulders and anterior superior iliac spines are at equal distances from the imaging table
  • posteroanterior projection
  • centering point
    • central ray midline at a level 1.5 cm below crest (at the level of posterior superior iliac spine)
    • central ray with a caudal angle of 30°-35°
  • collimation
    • laterally to include both sacroiliac joints
    • superiorly and inferiorly to include the entire sacrum
  • orientation
    • portrait 
  • detector size
    • 18 cm x 24 cm
  • exposure
    • 75 kVp
    • 20-30 mAs
  • SID
    • 100 cm
  • grid
    • yes
  • the sacroiliac joints are demonstrated open without foreshortening
  • the sacrum should be free of foreshortening and the inferior segments of the sacrum should be overlapped over the symphysis pubis 2
  • the mid-sagittal plane of the sacrum should be aligned with the symphysis pubis to ensure no rotation
  • when possible, imaging the sacroiliac joints in a posterior-anterior position is preferred in terms of demonstrating the sacroiliac joints and dose 3
  • the sacroiliac joint runs in an oblique coronal orientation, PA imaging allows the diverging x-ray beam to project through the joint space, better visualizing it compared to an AP projection
  • due to tissue compression, the PA projection for sacroiliac joint imaging significantly lowers radiation dose compared to the AP projection without compromising image quality 3

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Cases and figures

  • Case 1: normal sacrum
    Drag here to reorder.
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