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 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 .
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
- 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
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
- 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
- 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