Sacroiliac joint (AP sacrum view)
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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.
- patient positioned supine on the imaging table with legs extended
- patient’s shoulders and anterior superior iliac spine are at equal distances from the imaging table to prevent rotation
- anteroposterior projection
- central ray midline 5 cm below the level of the anterior superior iliac spine (4 cm above the symphysis pubis)
- central ray with cephalad angle of 30° (male) to 35° (female)
- laterally to include both sacroiliac joints
- superiorly and inferiorly to include the entire sacrum
- 18 cm x 24 cm
- 75 kVp
- 20-30 mAs
- 100 cm
Image technical evaluation
- the sacroiliac joints are demonstrated closed
- the sacrum should not be foreshortened 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
- when possible, imaging the sacroiliac joints in a posterior-anterior position is preferred for better demonstration the sacroiliac joints and reduced patient dose
- the sacroiliac joint projects in an oblique coronal orientation. Therefore, when imaging it PA, it will allow the diverging x-ray beam to project through the joint space, demonstrating it to maximum effect compared to the AP projection
- compression of tissues for the PA projection significantly reduces radiation dose compared to the AP projection without compromising image quality 3
Correcting rotational errors
Detection 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 errors
Symphysis 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.