10.3 Proximal Femur Scan Parameters and Compensation

10.3 Proximal Femur Scan Parameters and Compensation

Quick Answer: A standard proximal femur acquisition includes the femoral shaft distal to the lesser trochanter, the entire femoral neck, the greater trochanter, and enough adjacent soft tissue for the baseline estimate. Compensation is needed for large habitus (slower mode), low bone density (low-density analysis, manual review), and variant anatomy or pathology (documented achievable positioning). The femur tolerates error less well than the spine because the neck ROI sits between the ischium and the greater trochanter.

The Standard Scan Field

Getting the field right is the first parameter decision, and it constrains everything downstream.

A field cropped too tightly is the most common avoidable femur error. If the shaft is truncated, the software cannot establish the femoral axis and the neck box is placed at the wrong angle. If soft tissue is missing medially, the baseline estimate is unreliable in exactly the region where the neck ROI sits.

Standard Technique

1. Confirm site selection and side, and check for prior hip surgery on the intended side.
2. Position per the positioning protocol — supine, hip positioner, internal rotation — covered in the following section.
3. Set the scan field per the boundaries above, using the localizer.
4. Select the acquisition mode appropriate to the patient's habitus and bone density.
5. Instruct the patient: breathe quietly, remain still, do not move the foot.
6. Acquire, then review the image before releasing the patient — check rotation, field coverage, and motion.

Reviewing before the patient leaves the table is the single highest-value habit in femur scanning. A rotation error discovered at analysis, after the patient has dressed and gone, becomes either a bad measurement or a return visit.

Compensating for Body Habitus

The proximal femur lies deeper than the lumbar spine in most patients, surrounded by gluteal and thigh soft tissue that varies enormously.

• Large habitus: select a slower mode. Insufficient photons produce a noisy image where the low-contrast boundary between the femoral neck and the overlying ischium becomes indistinguishable.
• Soft tissue asymmetry: a large panniculus or asymmetric gluteal bulk distorts the baseline. Position symmetrically where possible and document.
• Table and field limits: very large patients may exceed the table weight limit or the lateral field width. When both hip and spine are unmeasurable for this reason, the 33% radius is the accepted alternative site.
• Very thin patients: less attenuation permits a faster mode, but verify there is adequate soft tissue in the baseline regions.

Compensating for Low Bone Density

The femoral neck in severe osteoporosis is a low-contrast structure sitting immediately over the ischium, which is itself bone. When neck density falls, the boundary between the two fades.

Compensating for Variant Anatomy

• Short femoral neck. A short neck leaves little room between the femoral head and the greater trochanter, and the standard neck ROI box may not fit cleanly. The software's automatic placement may overlap adjacent structures; verify and adjust per the manufacturer's method, then reproduce it.
• Inadequate space between the ischium and the femur. This is named explicitly in the ARRT outline. In some patients — particularly those with a wide pelvis, marked hip adduction, or limited internal rotation — the ischium projects directly under the femoral neck. It cannot always be corrected by positioning. Document it, and consider whether the contralateral hip is better.
• Coxa vara or coxa valga. Altered neck-shaft angle changes the appearance and the relationship of the neck to the shaft axis. The software derives the neck angle from the shaft, so accurate shaft inclusion in the field matters more here.
• Developmental dysplasia, prior slipped epiphysis, Perthes deformity. Distorted proximal femoral anatomy may make one hip unusable; use the other.

Compensating for Pathology

• Severe osteoarthritis. Joint space narrowing, subchondral sclerosis, and osteophytes at the femoral head and acetabulum raise apparent BMD, particularly in the total hip region. Severe arthritis may also physically prevent internal rotation.
• Prior fracture of the proximal femur. A healed or healing fracture, with or without hardware, invalidates that hip. Scan the contralateral side.
• Hip arthroplasty. An implanted prosthesis makes that femur unmeasurable. Scan the other side; if both are replaced, the spine or forearm governs.
• Paget disease of the femur. Markedly elevated and unreliable values; exclude the site.
• Metastatic lesions. Lytic lesions lower and blastic lesions raise measured density; either invalidates the site.
• Heterotopic ossification around the hip after surgery or trauma adds dense material within the field.

Documentation Requirements

For the femur specifically, record:

• Which side was scanned, and why if it was not the default
• Acquisition mode, and any low-density option used
• Achieved internal rotation if standard rotation could not be obtained
• Positioning aids used and their settings
• Any manual bone-map or ROI editing
• Non-removable artifacts and their location
• Any anatomical limitation such as ischium underlying the neck

At follow-up, all of it is reproduced. A femur scanned on the right at baseline and the left at follow-up produces a comparison that is meaningless regardless of how carefully each individual scan was performed.