AP pelvis CR centering explained: midway between ASIS and pubic symphysis

The AP pelvis projection: full setup

An AP pelvis radiograph is the most common projection used to evaluate the entire pelvis, hip joints, and proximal femora. It requires precise positioning because the pelvis is large, has multiple anatomic landmarks, and any rotation or centering error will render the image undiagnostic.

The setup is straightforward: patient supine on the table, legs extended and internally rotated, CR aimed at a single midline point between two bony landmarks.

Patient position: Supine on the radiographic table. Shoulders and hips flat against the table (no rotation of the torso or lower extremities). Feet internally rotated 15-20 degrees so the femoral necks lie parallel to the IR. Arms at sides or folded across the chest.

CR location: Perpendicular to the IR, aimed at the midpoint between the ASIS (anterior superior iliac spine) and the pubic symphysis. Alternatively, 2 inches above the pubic symphysis. Both rules yield the same centering.

IR (image receptor): Grid film or digital detector placed lengthwise (portrait orientation) centered to the midline of the patient. Detector should extend from approximately 1-2 inches above the ASIS down to include the entire pubic region and upper femora.

SID: 40 inches (102 cm).

Technique: 75-85 kVp, mAs adjusted for body habitus (typically 6-18 mAs depending on attenuation). Breathing: quiet respiration or gentle expiration (no breath-hold needed for pelvis).

Common wrong answer (ASIS to ASIS) and why it sticks

Some study materials and regional radiography programs teach “center the CR at the level of the ASIS” or worse, “measure the distance from ASIS to ASIS and center the CR there.” Both are wrong, and both will cost you points on the ARRT.

Why the wrong answer exists:

The ASIS to ASIS distance is a real anatomic measurement. It defines the width of the pelvic inlet and is useful in obstetrics and anthropometry. Some older textbooks confused this with the centering rule. Additionally, the ASIS is a palpable landmark, so students remember it as “the centering point.” But ASIS alone is not the centering rule.

Why it matters:

If you center at the ASIS level, the CR misses the pubic symphysis and the midline pelvis falls below the field of view. You lose visualization of the pubic bones, ischial spines, and symphysis, the central anatomy of interest. The image is undiagnostic.

The correct rule per Bontrager:

CR is centered at the midpoint between ASIS and pubic symphysis. This places the central ray exactly where the pelvic inlet meets the pelvic outlet, capturing the entire pelvis in one projection.

How to find the centering point quickly in practice

On a real patient (not an exam question), you have two options:

Option 1: Anatomic landmarks

Palpate both ASIS points (the sharp bony projections at the front of the hips). Palpate the pubic symphysis (the midline bony prominence above the genitals). Mentally mark the vertical midpoint between the ASIS level and the symphysis level. The CR goes there.

Option 2: Measurement

Measure from the top of the ASIS to the pubic symphysis with a tape measure or calipers. Find the midpoint. Center the CR there.

Option 3: The 2-inch rule (fastest)

Palpate the pubic symphysis. Measure 2 inches superior (cephalad) from the top of the symphysis. The CR goes there. This is faster than finding the ASIS-to-symphysis midpoint on a moving patient.

All three methods are correct and the ARRT accepts all of them. On a written exam question, if they give you a specific rule to use, use that rule. If they ask where the CR should be and don’t specify method, “midway between ASIS and symphysis” is the safest answer.

Foot rotation: why 15-20 degrees internally

The femoral necks are not parallel to the long axis of the femoral shaft. They angle medially (inward) as they project from the proximal femur toward the acetabulum. Without correction, a femur lying flat on the table will show the femoral neck foreshortened.

Internal rotation of the feet by 15-20 degrees (toes pointed inward) rotates the femoral necks into a position parallel to the IR. The result is an unforeshortened view of the femoral neck and head.

Why this matters:

The femoral neck is a common fracture site (femoral neck fracture, or “broken hip”). On an unrotated AP pelvis, a femoral neck fracture can be hidden by foreshortening. With proper internal rotation, the fracture line becomes visible.

How to position:

Ask the patient to turn both feet inward (toes pointed toward each other). The rotation should be about 15-20 degrees from the neutral (feet straight) position. Too much rotation (40+ degrees) actually causes external foreshortening of the opposite side.

On an exam question: if they ask “what is the purpose of foot rotation for AP pelvis?”, the answer is “to place the femoral necks parallel to the IR” or “to eliminate foreshortening of the femoral necks.” Do not choose answers about reducing radiation dose or comfort; those are not the positioning reason.

Common positioning errors and how to identify them

Error 1: No foot rotation (feet neutral or externally rotated)

Effect: Femoral necks appear foreshortened and shortened. The femoral neck head and neck may be hidden by the angle of the proximal femur.

Appearance on film: Femoral necks look stumpy or thick. Femoral neck detail is lost. Pelvic rotation is also visible (one ASIS higher than the other).

Fix: Internally rotate feet 15-20 degrees before exposure.

Error 2: CR aimed too high (at ASIS level)

Effect: The pubic symphysis, ischial spines, and central pelvis drop out of the field of view.

Appearance on film: Top of image shows pelvic inlet, but symphysis is cut off. Lower femora are visible but central anatomy is missing.

Fix: Aim CR 2 inches above symphysis (or midway between ASIS and symphysis).

Error 3: CR aimed too low (at symphysis level)

Effect: The ASIS and pelvic inlet are cut off; lower pelvis and proximal femora dominate.

Appearance on film: Bottom of image shows symphysis and upper femora, but acetabular region and ASIS are cut off.

Fix: Aim CR at midpoint between landmarks, not at symphysis alone.

Error 4: Pelvic rotation

Effect: One ASIS appears higher than the other. Pelvis is tilted on the table.

Appearance on film: Obvious skew. Femoral necks appear to foreshorten on opposite sides. Alignment is diagnostic.

Fix: Level the patient’s hips and shoulders to the table before exposure. Use shoulder positioning device if available.

Error 5: Femoral rotation (internal vs external)

Effect: If feet are externally rotated (toes out), femoral necks are foreshortened. If feet are over-internally rotated (toes pointing more than 20 degrees inward), the opposite femur’s neck foreshortens.

Appearance on film: One or both femoral necks appear shortened. Patient positioning does not match clinical request.

Fix: Internal rotation 15-20 degrees only. Palpate to confirm symmetry.

Why this matters on the ARRT

The ARRT Radiography Boards test positioning knowledge in the Procedures category. For AP pelvis, the most common question patterns are:

1. Centering questions: “The CR for an AP pelvis is centered at which of the following?” Answer: midway between ASIS and symphysis (or 2 inches above symphysis). ASIS-to-ASIS is NOT correct.

2. Rotation purpose questions: “Why are the patient’s feet internally rotated 15-20 degrees for AP pelvis?” Answer: to place femoral necks parallel to IR / eliminate foreshortening.

3. Error recognition: “Which of the following positioning errors would result in foreshortened femoral necks?” Answer: no foot rotation / external foot rotation / excessive rotation.

4. SID and technique: “The standard SID for AP pelvis is…” Answer: 40 inches (102 cm). “Typical kVp for AP pelvis is…” Answer: 75-85 kVp.

If you see an ARRT question about AP pelvis centering and the answer says “at the ASIS level” or “ASIS to ASIS,” that is not a valid Bontrager-canonical answer. Choose the midway rule instead.

Quick reference table

Parameter	Standard Value / Setting
Position	Supine, no rotation of torso or pelvis
CR location	Midway between ASIS and pubic symphysis perpendicular IR
Alternate CR rule	2 inches above pubic symphysis perpendicular IR
Foot rotation	Internal 15-20 degrees (toes inward)
SID	40 inches (102 cm)
kVp	75-85 kVp (high-contrast technique)
mAs	6-18 mAs depending on body habitus
IR placement	Lengthwise (portrait), centered to midline of patient
IR coverage	1-2 inches above ASIS to include proximal femora
Grid	Yes, portable or stationary grid
Breathing	Quiet respiration or gentle expiration (no breath-hold)
Evaluation criteria	Symmetrical ASIS, no pelvic rotation, femoral necks
	parallel, entire pelvis visible, proper density/contrast

ARRT exam tip

On exam day, if you see a question about AP pelvis centering, the rule is: midway between ASIS and pubic symphysis. If that phrase is not in the options, look for “2 inches above pubic symphysis”, both are correct and equivalent. Do not choose answers about “at the ASIS level,” “ASIS to ASIS,” or “at the level of the hip joints.” Those are not Bontrager-canonical.

For foot positioning, remember: internal rotation 15-20 degrees eliminates foreshortening of the femoral necks. This is the purpose. If a question asks why, choose the foreshortening answer. If it asks what happens without rotation, choose the foreshortened-neck answer.

For a full walkthrough of lower-extremity and axial skeleton projections, see our chapter on positioning foundations and lower-extremity radiography. For a comprehensive review of all ARRT procedural positioning, visit our procedures guide.