10.4 Pelvis & SI Joints

10.4 Pelvis & SI Joints

Quick Answer: The AP pelvis is the survey workhorse—center midway between ASIS and pubic symphysis on the MSP, include iliac crests to proximal femora, and prove no rotation with equal obturator foramina and symmetric iliac wings. Inlet and outlet views evaluate the pelvic ring after trauma. Judet 45° obliques map acetabular columns and rims. SI joints need AP/PA axial and 25–30° obliques. In trauma, stability and bleeding risk outrank textbook frog positions; check pregnancy status and shield when policy and anatomy allow.

General radiography — skeletal (pelvis and SI joints) sits at the intersection of high-weight skeletal procedures, trauma care, and radiation-protection judgment. CAMRT expects precise centering criteria and safe priorities when the pelvis is unstable.

Landmarks & Rotation Checks

AP Pelvis

Common errors: Centering too inferior clips iliac crests; too superior clips proximal femora; rotation toward one side enlarges the downside obturator foramen pattern inconsistently—use ASIS-to-table distances before exposure. Placing the CR at the iliac crest wastes inferior field and may clip the pubic rami and proximal femora.

For AP bilateral hips focused lower than a full pelvis, centering drops slightly toward the femoral necks, but trauma “pelvis series” usually means a true AP pelvis first.

Inlet and Outlet Projections

Used primarily in pelvic ring trauma protocols (often after AP pelvis) to show AP compression vs vertical shear patterns in concert with CT—still tested as positioning knowledge.

Exact angles vary with pelvic tilt and department protocol—concept to remember: inlet angles caudad to look down into the brim; outlet angles cephalad to project the rami and assess vertical shift. Evaluation requires the pelvic ring structures of interest free of extreme foreshortening and the MSP centered.

Judet Views — Acetabulum

45° posterior oblique pelvis (Judet method) evaluates acetabular fractures by showing anterior and posterior columns and rims.

CR: Perpendicular to IR, centered to the downside acetabulum for the anterior column/posterior rim emphasis, or to the upside acetabulum when that hip is of interest—departments often obtain both obliques to show each hip’s complementary column/rim pairs. Include the entire affected acetabulum, iliac wing portion, and obturator region.

Critique tip: Match the fracture line description on a stem to the correct oblique. Confusing “downside vs upside” column anatomy is a frequent exam error—draw the pairs once in study notes and rehearse them.

SI Joints

The sacroiliac joints lie oblique (~25–30°) to the MSP, so pure AP may not open them fully.

Programs differ on whether the joint nearest or farthest from the IR is open for a given oblique—memorize the method your clinical sites use and the evaluation criterion: joint space open without overlap of ilium and sacrum. Bilateral comparison may require both RPO and LPO.

Pelvic Trauma Stability Concepts

Stability rule of thumb for positioning: If the pelvis is mechanically unstable or the patient is a major trauma activation, do not perform multi-position recreational obliques without trauma-team clearance. Obtain the ordered AP (and designated trauma projections) with the least movement that still yields a diagnostic image. Horizontal-beam adaptations and careful four-person log rolls are Care Provider skills, not optional extras.

Pregnancy Screening & Shielding Considerations

Pelvic radiography can irradiate the uterus and gonads. Practice points consistent with Canadian departmental norms and CAMRT safe-work expectations:

• Screen for pregnancy (or document status) per facility protocol for patients of childbearing potential before elective pelvic imaging; urgent trauma imaging proceeds with clinical justification and documentation.
• Shield gonads when the shield will not obscure required bony anatomy (e.g., male gonadal shielding below the pubic bone on some pelvis exams; female ovarian shielding is often impractical on true AP pelvis because ovaries overlie iliac structures—follow site policy rather than inventing a universal rule).
• Collimate tightly; avoid automatic “full abdomen” fields when only pelvis is ordered.
• Optimize technique (correct kVp/mAs, grid, AEC) to avoid repeats—repeat pelvic exposures are a major unnecessary dose source.
• Do not invent numeric dose limits or national policy text on the exam; describe ALARA behaviors: justification, optimization, shielding when it does not compromise diagnosis, and pregnancy inquiry per protocol.

Technique & Image Quality

Use a grid, appropriate higher kVp for pelvic thickness, and AEC chambers selected so the central sensor is not only over bowel gas or dense hardware. Suspended respiration. Lead markers on the IR, not only digital annotation. For inlet/outlet and Judet, confirm angulation and 45° obliquity with sponges and angle sponges before exposing—geometry errors waste dose.

CAMRT Application Focus

When the stem asks for centering of AP pelvis, answer midway between ASIS and pubic symphysis on MSP with crests-to-proximal-femora inclusion and symmetric obturators. When acetabular fracture detail is needed, choose Judet 45° obliques and recall downside vs upside column/rim pairs. When pelvic ring displacement is the question, add inlet (caudad) and outlet (cephalad) concepts. When SI joint pain is chronic and ordered as such, use axial + 25–30° obliques. When the patient is a multi-trauma with unstable pelvis, prioritize safe, minimal-movement AP and team communication over complete elective series. Procedure competence (RTR.4), image critique (RTR.6), and safe work (RTR.1 / Care Provider) converge on every pelvis requisition.