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Citation, DOI & article dataCitation: Steward, A., Fahrenhorst-Jones, T. Sacroiliac joint (AP sacrum view). Reference article, Radiopaedia.org. (accessed on 08 Sep 2022) https://doi.org/10.53347/rID-49976 The AP sacrum projection is part of the sacroiliac series that includes an oblique projection (PA/AP) of the joint on both sides. Although usually taken as an AP projection it can also be taken PA with a reverse caudal central ray angulation of 30° to 35° when patients cannot assume supine position 1. On this page:
Correcting rotational errorsDetection of rotation on an AP axial sacroiliac joint image can be done by ensuring the midsagittal plane of the sacrum is aligned with the symphysis pubis. In a supine position, if the patient is rotated the mid-sagittal plane of the sacrum will move in the direction opposite from the movement of the symphysis pubis. Hence, to correct for rotation the direction of patient movement should seek to rotate the pubic symphysis toward the raised side. For example, if the patient is rotated into an RPO position, the pubic symphysis will rotate to the right of the mid-sagittal plane of the sacrum. If the patient is rotated in an LPO position, the pubic symphysis will rotate to the left of the mid-sagittal plane of the sacrum. Correcting tube angle errorsSymphysis pubis should be projected over the lower third of the sacrum 2. Over-angulation will result in the SIJs and sacrum being elongated and will project the symphysis pubis over the inferior borders of the sacrum. Under-angulation will project the pubic symphysis free from the superimposition of the sacrum 2. References
Related articles: Imaging in practicePromoted articles (advertising)What is the CR angulation for the AP axial sacrum projection in prone?The AP sacrum projection is part of the sacroiliac series that includes an oblique projection (PA/AP) of the joint on both sides. Although usually taken as an AP projection it can also be taken PA with a reverse caudal central ray angulation of 30° to 35° when patients cannot assume supine position 1.
Where is the CR centered for an AP axial projection of the sacrum?The CR is centered 1.5 inches inferior to the iliac crest and 2 inches posterior to the ASIS.
How many degrees should the patient be rotated to demonstrate the sacroiliac joints?Internally rotate feet ~10 degrees. CR: 2 inches superior to pubic symphysis.
What is the correct angulation for an AP axial sacroiliac image on a male patient?For an AP axial inlet projection, the CR is angulated caudad 40° and is centered to a midline point at the level of both ASIS.
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