AP pelvis CR centering: midway between the level of the ASIS and the pubic symphysis
Key takeaways
- CR: perpendicular, midline, midway between the ASIS level and the pubic symphysis.
- On an average adult, that point is about 5 cm (2 in) below the ASIS.
- Use a minimum 102 cm (40 in) SID with a grid and a crosswise 35 x 43 cm receptor.
- Rotate the legs in 15 to 20 degrees, but never if a fracture or dislocation is suspected.
- Suspend respiration for the exposure.
The CR for an AP pelvis is perpendicular to the image receptor, at the midline, midway between the level of the anterior superior iliac spine (ASIS) and the pubic symphysis. Bontrager’s and Merrill’s both give this rule.
How do you find the centering point?
On an average adult, the point is about 5 cm (2 in) below the ASIS. Merrill’s adds that it is also about 5 cm above the symphysis, and about 3.8 cm (1.5 in) above the palpable greater trochanter. These are averages, so they shift with body size. Merrill’s advises against palpating the symphysis, and Bontrager’s says institutional policy may not permit it, so use the other landmarks.
If the pelvis is deep, Merrill’s says to check that the top of the receptor sits 2.5 to 3.8 cm (1 to 1.5 in) above the iliac crest.
If the AP pelvis is part of a hip routine, Bontrager’s centers about 5 cm lower, at the femoral heads or necks, to show more of the proximal femora.
What is the full setup?
| Setting | AP pelvis |
|---|---|
| Patient | Supine, no pelvic rotation, equal distance from table to each ASIS |
| Legs | Internally rotated 15 to 20 degrees, unless fracture or dislocation is suspected |
| SID | Minimum 102 cm (40 in) |
| Receptor | 35 x 43 cm (14 x 17 in), crosswise, with grid |
| CR | Perpendicular, midline, midway between ASIS level and symphysis |
| Respiration | Suspended |
See the full AP pelvis projection page for the criteria and common errors.
Why rotate the legs, and when must you not?
The rotation brings the femoral necks parallel to the receptor, so they show in profile. Don’t rotate the legs if a hip fracture or dislocation is suspected. Bontrager’s says to take the pelvis “as is”, with minimal movement of the affected leg.
For the lateral in that patient, see the axiolateral hip answer.
How do you check the image?
Bontrager’s looks for symmetric iliac wings, ischial spines and obturator foramina. A narrowed obturator foramen means rotation toward that side. The lesser trochanters should be barely visible or hidden if the legs were rotated correctly.
AP pelvis is on both ARRT® outlines under Head, Spine and Pelvis. The 2027 outline also adds a weight-bearing AP pelvis. Our femur, pelvis and hip chapter and Procedures guide go further, and you can test yourself with the head, spine and pelvis questions.
Frequently asked questions
Where is the CR for an AP pelvis?
Perpendicular to the receptor at the midline, midway between the level of the ASIS and the pubic symphysis. On an average adult that is about 5 cm (2 in) below the ASIS.
Is 2 inches above the symphysis the same point?
On an average-sized patient, yes, according to Merrill's. Bontrager's gives only the measurement from the ASIS. Patients vary, so the midway landmark is the rule to learn first.
Should you rotate the legs if a hip fracture is suspected?
No. Bontrager's warns against rotating the legs if a hip fracture or dislocation is suspected, and Merrill's says trauma or pathology can rule out medial rotation. Take the image with minimal movement of the affected leg.
What SID is used for an AP pelvis?
A minimum of 102 cm (40 in). Chest radiography uses a different distance, 183 cm (72 in).
Sources
Found an error? Email [email protected] with the page and the source.
Know the answer. Now drill it.
Free tier includes 3 chapters, 50 practice questions and 1 full timed mock. No credit card.