Sacroiliac joint (PA sacrum view)

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.

Indications

This view is useful in visualising any fractures, sacroiliac joint dislocations or subluxations, and possible inflammation (i.e. sacroiliitis) to 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 visualisation of the joint compared to the AP projection ¹.

Patient position

• 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

Technical factors

• posteroanterior projection
• centring 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°- 35-35°
• 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

Image technical evaluation

• 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

Practical points

• 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 visualising 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