The pelvic ring is one of the strongest skeletal structures in the human body, protecting vital organs and transmitting weight from the axial skeleton to the lower extremities. When trauma occurs — whether from a motor vehicle collision, a fall from height, or athletic injury — the pelvis is frequently involved. As a radiologic technologist, your ability to produce diagnostic-quality pelvis radiographs directly impacts patient management decisions, especially in the trauma setting where every second counts.
Pelvis X-rays are among the most commonly ordered radiographic examinations in emergency departments, and they are a staple of the ARRT registry exam. Mastering the various projections — from the standard AP pelvis to specialized views like the Judet obliques and pelvic inlet/outlet series — requires a thorough understanding of anatomy, centering points, IR size selection, and patient positioning adaptations for trauma and non-trauma patients alike.
Pelvic fractures account for approximately 3% of all skeletal fractures, but they carry a mortality rate of up to 15% in unstable fracture patterns due to associated hemorrhage. The AP pelvis is part of the standard ATLS (Advanced Trauma Life Support) trauma series alongside the cross-table cervical spine and supine chest X-ray.
In this comprehensive guide, we will walk through every major pelvis projection you need to know for clinical practice and the ARRT exam, complete with centering points, IR sizes, evaluation criteria, and expert tips drawn from Clark's positioning standards and current clinical protocols.
Before you can position correctly, you must understand what you're trying to image. The pelvis is composed of three paired bones that fuse during development:
These three bones meet at the acetabulum, the cup-shaped socket for the femoral head. The sacrum articulates with the ilia at the sacroiliac (SI) joints bilaterally, forming the posterior pelvic ring — the most critical component for stability.
The ARRT frequently tests the concept that the posterior pelvic ring (sacrum and SI joints) bears the majority of weight transmission. Disruption of the posterior ring indicates an unstable fracture pattern that may require surgical fixation, whereas isolated pubic ramus fractures are often managed conservatively.
The AP pelvis is the foundational projection and the starting point for every pelvis series. It is performed on virtually every patient presenting with pelvic trauma, hip pain, or suspected pelvic pathology.
If the lesser trochanters are clearly visible on an AP pelvis, the femurs are externally rotated (or not sufficiently internally rotated). This means the femoral necks are foreshortened and an occult fracture may be missed. Reposition and repeat — this is a common ARRT exam scenario question. The rule: great toes together, lesser trochanters invisible.
The pelvic inlet view (also called the AP axial inlet projection) is used to evaluate the integrity of the pelvic ring, particularly in trauma patients with suspected anteroposterior compression fractures or pelvic ring disruptions. It projects the pelvic brim in profile, allowing assessment of the true pelvic inlet.
The pelvic outlet view complements the inlet view in the trauma pelvis series. It projects the pubic rami and ischial regions optimally and is used to evaluate vertical shear fractures and cephalad displacement of hemipelvis.
Memorize the direction: Inlet = Caudad (CR angled downward toward the feet), Outlet = Cephalad (CR angled upward toward the head). A common trick question asks you to identify which projection uses a cephalad angle (outlet) versus a caudad angle (inlet). Both use the same centering point as the AP pelvis.
Named after French orthopedic surgeon Robert Judet, the Judet views are 45° oblique projections of the pelvis specifically designed to evaluate acetabular fractures. They are indispensable for pre-surgical planning and classification of acetabular fracture patterns. Two projections are obtained — one for each side.
Here is a simple way to remember which oblique demonstrates which acetabulum: The side down is the side demonstrated. In an RPO position (right side down toward the table), the left (up) acetabulum is seen in the iliac oblique. In an LPO (left side down), the right (up) acetabulum is seen. For the obturator oblique (down-side acetabulum), LPO shows the left (down) side, and RPO shows the right (down) side. Practice this with a partner — it is guaranteed ARRT material.
| Projection | CR Angle | IR Size | Centering Point | Primary Clinical Indication |
|---|---|---|---|---|
| AP Pelvis | None (perpendicular) | 35 × 43 cm, crosswise | 2" inferior to ASIS-symphysis midpoint | Standard screening, trauma series, hip evaluation |
| AP Inlet | 30-40° caudad | 35 × 43 cm, crosswise | Same as AP pelvis | Pelvic ring disruption, AP compression fractures |
| AP Outlet | 30-45° cephalad | 35 × 43 cm, crosswise | Same as AP pelvis | Vertical shear fractures, pubic rami evaluation |
| Iliac Oblique (Judet) | None (patient rotated 45°) | 35 × 43 cm | 2" below ASIS of examined side | Acetabular fracture classification, posterior column |
| Obturator Oblique (Judet) | None (patient rotated 45° opposite) | 35 × 43 cm | 2" below ASIS of examined side | Acetabular fracture, anterior column, obturator ring |
| AP Bilateral Hips (if ordered separately) | None (perpendicular) | 35 × 43 cm, crosswise | Same as AP pelvis | Hip arthritis, avascular necrosis, bilateral comparison |
This is a frequent source of confusion for students. The AP pelvis and AP bilateral hips use the same positioning, centering point, and IR. The difference is purely clinical: when a radiologist orders "AP pelvis," the focus is on the pelvic ring, SI joints, and general survey. When they order "AP bilateral hips," they want optimized visualization of both femoral heads, femoral necks, and acetabular regions specifically. In practice, the two are often identical projections, though some departments add a slight (5-10°) cephalad angle for dedicated hip views.
When unilateral hip pathology is suspected, the frog-leg lateral (also called the Modified Cleaves method or Lauenstein projection) is performed in addition to the AP.
Do NOT use the frog-leg lateral on a suspected hip fracture or dislocation. Abducting the hip can displace a fracture or dislocate a joint. In trauma, perform a cross-table lateral (Danelius-Miller method) instead, where the unaffected leg is raised and the CR is directed horizontally through the affected hip while the patient remains supine.
Pelvis radiography in the trauma setting demands special attention to patient safety, immobilization, and technique adaptation. Here are the key considerations:
When the patient cannot be moved to the radiographic table, a portable supine AP pelvis is obtained. The cassette (or DR detector) is placed under the trauma board or directly beneath the patient. The CR is directed perpendicular to the IR, centered as for a standard AP pelvis. Use the highest mA station and shortest exposure time available to minimize motion artifact from patient breathing or involuntary muscle spasm.
The pelvis and hip region exposes gonadal tissue to significant radiation. For male patients, a shaped gonadal shield (placed over the pubic symphysis) should be used whenever it does not obscure the anatomy of interest. In female patients, ovarian shielding is more complex because the ovaries are not in a fixed position; careful collimation is the primary dose-reduction strategy. Per the ALARA principle, always collimate to the skin margins and use the lowest exposure factors that produce a diagnostic image.
Geriatric patients with hip fractures, patients in traction, or patients with severe arthritis may not tolerate the standard supine position. In these cases:
In pediatric patients, the pelvis is not fully ossified, and the triradiate cartilage (where the ilium, ischium, and pubis meet at the acetabulum) is open until approximately 14-16 years of age. Gonadal shielding is especially important. Use lower kVp (65-75) and lower mAs techniques. The AP pelvis centering point is slightly more superior in young children — center at the level of the iliac crests for infants and adjust downward as the child grows.
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.
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.