Sacrum (AP view)

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The sacrum anteroposterior (AP) view is used to demonstrate the sacrum and its articulations. The efficacy of this radiographic projection is debatable, with radiographers encouraged to follow department protocol when imaging this region .

Indications

This view can be utilized in the event of trauma, or for evaluating degenerative changes  of the sacrum and/or sacroiliac joints.

Patient position

  • the radiograph is performed with the patient in a supine position, hands placed comfortably by the patient's side, with the legs extended 
  • ensure the patient is not rotated - anterior superior iliac spine must be equidistant from the IR/table.

Technical factors

  • AP view
  • centering point
    • at the MSP, midway between the ASIS and the symphysis pubis
  • central ray
    • angled 15° cephalic
    • this angle allows the CR to be perpendicular to the sacrum as it is curved with an anterior concavity and convex posteriorly 
  • collimation
    • must adhere to the ALARA principle given the region exposed via the primary beam 
    • superior to include the L5/S1 articulation
    • inferior to include the S5 (the articulation of the sacrum and coccyx will not be demonstrated clearly given the orientation of the coccyx in this projection) 
    • lateral to include the SI joints
  • orientation  
    • portrait 
  • detector size
    • 24x30 cm 
  • exposure
    • 70 kVp
    • 20-30 mAs 
  • SID
    • 110 cm
  • grid
    • yes (ensure the correct grid is selected if using focused grids)
    • when using table bucky, ensure the CR and the IR are correctly aligned, given the use of an angled tube 

Image technical evaluation

  • the entire sacrum should be visible from S1-S5 
  • not patient rotation as demonstrated by the symmetrical appearance of the SI joints and the iliac crests 
  • adequate penetration as evident by the clear visualization of the sacral foramen 
  • the sacral foramen should appear open, which is achieved using the correct tube angle

Practical points

  • always consider department protocol before undertaking in imaging of the sacrum 
  • often overlying bowel gas can obscure the bony anatomy of the sacrum, making interpretation difficult
  • the sacrum is an area which should be reviewed on trauma pelvis radiographs, paying particular awareness to the arcuate lines