The pelvic ring is one of the most complex anatomical structures in the body, serving as the critical weight-bearing bridge between the axial skeleton and the lower extremities. For radiologic technologists, mastering pelvis X-ray positioning is not just about producing diagnostic images — it is about recognizing when a patient has sustained life-threatening pelvic trauma and adapting the imaging approach accordingly.
Pelvic fractures account for approximately 3–8% of all skeletal fractures, but they carry a mortality rate of up to 50% in unstable fracture patterns due to the risk of massive hemorrhage from the presacral venous plexus and iliac vessels. The AP pelvis radiograph is so critical in trauma assessment that it remains part of the Advanced Trauma Life Support (ATLS) primary survey, alongside the chest X-ray and lateral cervical spine — collectively known as the "trauma triage series."
This guide covers every standard pelvis projection you will encounter in clinical practice and on the ARRT registry examination: the AP pelvis, AP inlet and outlet views, and the Judet (oblique) views for acetabular evaluation. We will cover centering points, IR sizes, angulation, breathing instructions, trauma modifications, and evaluation criteria for each view.
Know your angulations. The ARRT frequently tests the difference between the inlet (40° caudad) and outlet (40° cephalad) projections. Remember it this way: "Inlet looks down into the bowl; Outlet looks up out of the bowl." Inlet = CR angled toward the feet (caudad). Outlet = CR angled toward the head (cephalad).
The anteroposterior (AP) projection of the pelvis is the most frequently requested pelvis examination and serves as the screening view for virtually all pelvic pathology — trauma, metastatic disease, degenerative changes, and congenital abnormalities.
The patient is placed supine on the radiographic table with the median sagittal plane aligned to the midline of the table and grid. The arms are positioned away from the body (usually crossed over the chest or placed on the upper abdomen) to prevent superimposition over the pelvis. The legs are internally rotated approximately 15–20° — this brings the femoral necks parallel to the image receptor, optimizing visualization of the femoral heads and necks. To achieve this, the feet are taped together or supported with sandbags, with the great toes touching and heels slightly separated.
Use a 14 × 17 inch (35 × 43 cm) image receptor placed crosswise (landscape orientation). The IR is positioned so that the top border is 1–2 inches above the iliac crests and the bottom border includes the proximal femurs below the lesser trochanters. The central ray is directed perpendicular to the IR, centered 2 inches (5 cm) inferior to the midpoint of a line connecting the anterior superior iliac spines (ASIS). In clinical terms, this is approximately at the level of the symphysis pubis and the greater trochanters.
For an average adult (70–80 kg), a typical technique is 75–85 kVp with AEC using the two outer chambers. If using manual technique, approximately 20–30 mAs is appropriate depending on the patient's body habitus. A grid (8:1 or 10:1) should be used for all pelvis radiographs on adult patients. For pediatric patients, reduce mAs appropriately and use gonadal shielding whenever it does not obscure the area of interest.
Why 15–20° internal rotation? The femoral neck normally projects anteriorly at an angle of about 15° from the coronal plane. Internally rotating the legs aligns the femoral necks parallel to the IR, allowing the true length and contour of the femoral neck to be demonstrated. This is critical for diagnosing occult hip fractures — a foreshortened femoral neck can hide a subtle nondisplaced fracture that might otherwise be missed, delaying treatment and increasing the risk of avascular necrosis.
The AP inlet projection is designed to demonstrate the pelvic brim, sacral ala, sacroiliac joints, and — most importantly — anterior-posterior displacement of pelvic ring disruptions. This view is essential in the workup of pelvic trauma, particularly when an anteroposterior compression injury or lateral compression injury is suspected.
The patient remains supine in the same position as for the AP pelvis. No change in body or leg position is needed. The key difference is the CR angle:
A properly positioned inlet view projects the pelvic brim as an oval ring — imagine looking down into a bowl. This en face view of the pelvic inlet allows the radiologist to directly visualize any inward or outward displacement of the pelvic ring segments. The sacral ala, sacroiliac joints, and the anterior sacral foramina should be well demonstrated. In patients with pelvic ring disruption, this view provides critical information about rotational and translational instability that guides surgical management decisions.
The AP outlet projection is the complementary view to the inlet. While the inlet evaluates anterior-posterior displacement, the outlet evaluates superior-inferior (vertical) displacement of pelvic fracture fragments. This information is critical for determining whether a pelvic fracture requires surgical fixation and for classifying the fracture pattern according to the Young-Burgess or Tile classification systems.
Again the patient remains supine. The key change is the direction of the CR angle:
On a properly positioned outlet view, the pubic symphysis, ischial spines, and sacral foramina are clearly demonstrated. The sacrum appears elongated, and the entire sacral curve is visible from S1 through S5. The outlet view is particularly valuable for evaluating:
Students often mix these up on exams. Here is your memory aid: Inlet = "I'm looking INTO the bowl" (CR caudad, visualizing the pelvic brim ring). Outlet = "I'm looking OUT of the bowl" (CR cephalad, visualizing the pubic arch and sacrum). The inlet evaluates AP displacement. The outlet evaluates vertical displacement. Both views together provide a three-dimensional understanding of pelvic ring integrity.
Named after the French orthopedic surgeon Robert Judet, the Judet views are a pair of 45° oblique projections that provide a detailed assessment of the acetabulum. These are essential when an acetabular fracture is suspected — a common injury resulting from high-energy trauma such as motor vehicle collisions where the femoral head drives into the acetabulum.
The acetabulum is formed by three bones: the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior). The acetabular columns (anterior and posterior) and the quadrilateral surface are critical structures that must be evaluated when planning surgical fixation.
This view profiles the anterior column and the posterior wall of the acetabulum.
What it shows: The obturator foramen on the affected side is opened up or "profiled," hence the name. The anterior column (iliopubic) is seen running from the iliac crest through the pubic bone. The posterior wall of the acetabulum is also visible as a distinct cortical line. An obturator oblique view that shows disruption of the anterior column or a displaced posterior wall fragment indicates the need for surgical fixation.
This view profiles the posterior column and the anterior wall of the acetabulum.
What it shows: The iliac wing is opened up or "profiled." The posterior column (ilioischial line) is clearly visualized running from the sciatic notch through the ischium. The anterior wall of the acetabulum is also profiled. This view is essential for detecting fractures of the posterior column, which have a high association with sciatic nerve injury.
| View | CR Angle | Patient Position | Best For | IR Size |
|---|---|---|---|---|
| AP Pelvis | Perpendicular (0°) | Supine, legs IR 15–20° | Screening — overall pelvic anatomy and fractures | 14×17" crosswise |
| AP Inlet | 40° caudad | Supine (same position) | AP pelvic ring displacement | 14×17" crosswise |
| AP Outlet | 40° cephalad | Supine (same position) | Vertical (superior-inferior) displacement | 14×17" crosswise |
| Obturator Oblique (Judet I) | Perpendicular | 45° injured side up | Anterior column and posterior wall | 14×17" lengthwise |
| Iliac Oblique (Judet II) | Perpendicular | 45° injured side down | Posterior column and anterior wall | 14×17" lengthwise |
Pelvic trauma patients present unique challenges. These patients are often hypotensive, may be in a cervical collar, and are frequently unable to cooperate with positioning. The technologist must balance image quality with speed and patient safety.
In the trauma setting, the typical pelvis series is limited to three views in the stable patient: AP pelvis, inlet, and outlet. In the unstable or polytrauma patient, only the AP pelvis is obtained during the primary survey. The inlet and outlet views, as well as Judet views, are obtained after the patient has been stabilized, often in the trauma resuscitation room with portable equipment.
For portable (bedside) pelvis radiography, the patient remains on the trauma stretcher or hospital bed. The cassette or DR detector is placed in a Bucky tray or slide tray under the backboard. If a grid is not available, an air-gap technique can be used (increase SID to 50–60 inches) to reduce scatter fog. The CR is centered to the same anatomical landmark (2 inches below the ASIS).
Never force internal rotation of the legs in a trauma patient. If the patient complains of hip or pelvic pain during attempted rotation, obtain the AP pelvis with the legs in a neutral position. Forcing rotation in an unstable pelvic fracture can exacerbate bleeding, dislodge clots, and worsen hemodynamic instability. Document the lack of rotation in the image comment and notify the radiologist.
Pediatric pelvic radiography requires special attention. The pediatric pelvis contains numerous ossification centers (the triradiate cartilage, iliac crest apophyses, ischiopubic synchondrosis) that can simulate fractures. The gonadal shielding protocol should be followed rigorously. For the AP pelvis in a child, reduce the technique significantly — approximately 60–70 kVp at 3–8 mAs depending on age and size. Immobilization techniques (compression bands, sandbags) may be necessary for younger children to prevent motion.
Osteoporotic bone in elderly patients presents a different challenge. The trabecular pattern is diminished, making nondisplaced fractures more difficult to see. If a hip fracture is clinically suspected but the AP pelvis appears negative, an AP hip (affected side) and cross-table lateral hip should be obtained. The cross-table lateral hip view requires the opposite leg to be elevated to prevent superimposition, and a horizontal beam technique is used.
A solid understanding of pelvic anatomy is essential for evaluating your images before sending them to PACS. Here are the key anatomical landmarks to check on every pelvis radiograph:
The superior-most margin of the ilium. Should be symmetric bilaterally. Asymmetric crests suggest rotation or a fracture with displacement.
Synovial joints between the sacrum and ilium. Should be symmetric and well-visualized. Widening suggests SI joint disruption or diastasis.
Cartilaginous joint at the anterior midline. Normal width is ≤ 5 mm. Diastasis > 1 cm suggests pelvic ring disruption.
An imaginary curved line from the medial femoral neck to the inferior border of the superior pubic ramus. Disruption indicates hip dislocation or femoral neck fracture.
Even experienced technologists make positioning errors on pelvis radiographs. Here are the most common ones you will encounter (and that the ARRT loves to test):
Sign: The coccyx is displaced to one side of the pubic symphysis, and the obturator foramina are unequal in size. The foramen on the side away from the IR appears larger. Fix: Re-center the patient's median sagittal plane to the midline of the table. Ensure the ASIS are equidistant from the tabletop.
Sign: The femoral necks appear short or the lesser trochanters are visible in profile (they should be hidden medially when the leg is internally rotated). Fix: Increase internal rotation of the legs. The great toes should be touching, and the heels separated by approximately 6–8 inches.
Sign (Inlet): If the CR angle is less than 40° caudad, the brim does not appear as an oval ring. If greater than 40°, the oval becomes too elongated. Sign (Outlet): If less than 40° cephalad, the sacral foramina are not fully opened. Fix: Use a goniometer or angle indicator. Mark the 40° angle on the tube housing as a visual reference.
Sign: Excessive scatter radiation reduces contrast and increases patient dose. Fix: Collimate tightly to the pelvic rim on all four sides. The collimation borders should be visible on the image but just outside the anatomical region of interest.
Sign: The pubic symphysis or sacrum is cut off. Because the angled CR shifts the anatomical projection, the IR must be shifted accordingly. Fix: For the inlet (caudad angle), shift the IR cephalad (upward) because the beam is angling downward. For the outlet (cephalad angle), shift the IR caudad (downward) because the beam is angling upward.
Before sending a pelvis series to the radiologist, run through this quick checklist to ensure every image is diagnostically acceptable:
The ARRT registry examination frequently tests pelvis positioning in both the computed and clinical categories. Expect questions that ask you to: (1) identify the correct CR angle for inlet vs. outlet views, (2) select the appropriate centering point for the AP pelvis, (3) determine which Judet view profiles which acetabular column, and (4) recognize positioning errors on a given image. Use the comparison table in this article as your study reference.
Mastering pelvis positioning takes practice, but the principles are consistent. Every radiograph starts with proper patient positioning, correct centering, and appropriate exposure factors. By understanding the anatomy you are trying to demonstrate and why each view is ordered, you elevate yourself from a technologist who simply "takes pictures" to one who produces diagnostically valuable images — and that is what the ARRT exam (and your clinical instructors) are looking for.
Try these ARRT-style multiple choice questions based on this article. Click an option to check your answer — correct answers turn green, wrong ones turn red.