5.2 Proximal Femur Acquisition & ROI Selection
Key Takeaways
Use the device’s positioning aid to rotate the leg internally and obtain a reproducible projection of the femoral neck.
A prominent lesser trochanter with a foreshortened neck suggests insufficient internal rotation. Recheck setup rather than applying a fixed BMD correction.
Follow the scanner-specific neck ROI procedure. Review the bone map and exclude non-target pelvic bone from the diagnostic region.
Position for a reproducible hip projection
Dual-energy X-ray absorptiometry (DXA) estimates bone mineral density (BMD) from a two-dimensional projection. The femoral neck is angled anteriorly in relation to the distal femur. With the leg resting in external rotation, the neck can appear foreshortened and the lesser trochanter can project prominently medial to the shaft. Different positioning changes projected area and can change measured BMD. The direction and size of the error cannot be inferred from a universal percentage.
Position the patient supine with the pelvis level and the leg aligned according to the scanner instructions. Use the positioning device to rotate the whole leg internally at the hip. The international DXA practice guideline describes approximately 15 to 20 degrees of internal rotation as a usual starting point. Verify the resulting projection. The aim is a well-profiled femoral neck with a small or barely visible lesser trochanter and a femoral shaft aligned with the table edge. Adjust alignment as needed for the anatomy and equipment. Do not force the position in a patient with pain or restricted movement. Record limitations and consider the opposite hip if it can be assessed reliably.
| Observation on the image | What to check |
|---|---|
| Prominent lesser trochanter and foreshortened neck | Insufficient internal rotation is likely. Review patient setup and the prior projection if available. |
| Change in lesser trochanter profile compared with prior study | Check whether the same degree of rotation was reproduced and whether the ROIs still match. |
| A pelvis that appears tilted or a shaft that is oblique | Recheck alignment and comfort, and verify the suitability of the acquisition. |
| An overlap, prosthesis or focal abnormality at the selected site | Assess whether a valid ROI can be defined or the other hip should be scanned. |
Worked acquisition problem
A 62-year-old woman has a wide, rounded lesser trochanter on the current hip DXA and a foreshortened femoral neck. The prior image has a well-profiled neck and a small lesser trochanter. Review the current positioning before interpreting the apparent BMD change. If a technically appropriate repeat acquisition is feasible, reproduce the prior position using the scanner's positioning aid. The LSC does not correct for a systematic difference between projections. If positioning cannot be matched, document that limitation.
Check the scan field and analysis
The global ROI must include the proximal femur and adequate surrounding soft-tissue reference. Check the bone map and verify that the femoral neck ROI excludes adjacent pelvic bone and the greater trochanter. A box placed over the acetabular rim or ischium may include extra mineral that distorts the result. Correct the ROI or bone map using the validated scanner tools if the original image permits it. Otherwise determine whether another acquisition or site is appropriate.
The femoral neck box is not positioned identically on every device. The international practice guideline describes a Hologic approach that anchors it near the upper corner of the greater trochanter and a GE approach that uses the narrow neck region and a search feature. Follow the protocol for the actual instrument. The total hip uses a larger composite region incorporating the femoral neck and trochanteric and intertrochanteric areas. Review the outer boundaries and the placement of the distal edge according to the model's instructions. For follow-up, compare the old and new images and replicate the relevant ROIs.
Worked analysis problem
The software has placed the superior-medial corner of the neck box over dense acetabular cortex. Inspect the neck position, global box and bone map. If the scan shows adequate separation, correct the analysis with the scanner's tools and verify that the revised neck ROI excludes the extraneous bone. A measurement from Ward's area cannot replace an invalid diagnostic femoral neck ROI. Use another valid diagnostic site if the neck result remains unusable.
When the image is otherwise interpretable, compare the bone and soft-tissue boundaries before and after an edit. If the revised region extends into new anatomy simply to reproduce an older box, revisit the original analysis rather than accepting a numerical match.
Report the side and any technical limitation that affects interpretation. For choosing the lowest diagnostic score or a bilateral monitoring measurement, see hip DXA interpretation.
International Working Group DXA practice guideline · ISCD Adult Official Positions · Primary study of hip rotation and BMD
A hip DXA image shows a wide, prominent lesser trochanter and a foreshortened femoral neck. What should the interpreter check?
Whether a numerical correction can be added for the patient’s age.
Whether the femoral neck BMD should be replaced by Ward’s area.
Whether the T-score reference should be changed to account for the projection.
Whether the hip was insufficiently internally rotated and needs repositioning or a technical limitation.
The automated femoral neck ROI on a left hip DXA includes a portion of the acetabular rim. What is the appropriate next step?
Accept the analysis because non-femoral cortical bone is part of the neck ROI.
Review and correct the ROI or bone map according to the device protocol, if a valid neck measurement can be recovered.
Substitute Ward’s area for the neck measurement.
Widen the neck box to include the pubic ramus.
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