9.4 Lumbar Spine Positioning: The Leg Block, Centering, and Common Challenges

9.4 Lumbar Spine Positioning: The Leg Block, Centering, and Common Challenges

Quick Answer: Supine, midline centered, arms out of the field, and hips and knees flexed over a leg block to flatten lumbar lordosis. The spine should appear straight and centered with roughly equal soft tissue on each side, and the field should show T12 with ribs at the top and part of the iliac crest at the bottom so levels can be identified. Every deviation forced by anatomy is documented and reproduced.

What the Leg Block Actually Does

In a supine patient lying flat, the lumbar spine arches forward in physiologic lordosis. The vertebral bodies tilt relative to the tabletop, the intervertebral disc spaces close anteriorly, and the beam passes obliquely through each body.

Flexing the hips and knees over a leg block (also called a positioning cushion, spine block, or leg elevator) rotates the pelvis posteriorly and flattens the lordotic curve. The consequences are all favorable:

• Disc spaces open and lie perpendicular to the beam, so intervertebral markers can be placed accurately and vertebral levels separate cleanly.
• Vertebral bodies present squarely, reducing projected overlap between adjacent bodies.
• Edge detection improves because bone margins are better defined.
• Reproducibility improves, because the block imposes a standardized position rather than depending on how the patient happened to lie.

Typical hip and knee flexion is in the range of 60 to 90 degrees, with the manufacturer's block sized to produce it. What matters more than the exact angle is that the same block, at the same setting, is used at every visit — which means documenting the block and the setting.

The Standard Position, Step by Step

1. Patient supine, head on a thin radiolucent pad or directly on the table per protocol.
2. Longitudinal alignment: the patient's midsagittal plane aligned to the table centerline, verified with the laser or light localizer.
3. Leg block placed under both lower legs so hips and knees flex symmetrically. Both legs must rest on the block — asymmetric leg elevation tilts the pelvis and rotates the spine.
4. Arms out of the scan field: across the chest or at the sides away from the trunk, per manufacturer instruction. Arms lying within the baseline region contaminate the soft-tissue estimate.
5. Shoulders and hips square to the table; no rotation. Check by comparing the two anterior superior iliac spines for equal height.
6. Scan field set to begin below the level of the iliac crest and extend above T12, so the acquisition includes part of L5 or the crest inferiorly and T12 with ribs superiorly.
7. Instruct: breathe quietly and normally, remain still, do not talk.
8. Verify the localizer image before analysis: spine straight, centered, equal soft tissue bilaterally, correct levels included, no artifacts.

What a Correctly Positioned Spine Looks Like

If the spine appears angled or curved in a patient without scoliosis, the usual cause is patient rotation or asymmetric leg elevation — reposition rather than accept it.

Common Challenges

Scoliosis

A scoliotic spine cannot be made straight. The goals shift to capturing the whole spine and being reproducible.

• Center on the curve, not the table midline, so the entire spine stays within the field with soft tissue on both sides.
• Widen the scan field enough to include the full lateral excursion; a truncated spine is unanalyzable.
• Expect rotational deformity. Scoliosis includes vertebral rotation, so bodies project asymmetrically, which degrades edge detection and precision.
• Document the centering used, in enough detail to repeat it.
• Recognize when the spine is not usable. Severe scoliosis with rotation and secondary degenerative change may make the spine uninterpretable; the hip then governs, and the report should say why.

Kyphosis

Severe thoracic kyphosis prevents the upper back and head from resting on the table, leaving the patient arched and unstable.

• Support the head and upper thoracic region with radiolucent padding, positioned outside the measured lumbar field.
• Do not place padding under the lumbar spine itself; on fan-beam systems that changes object height and therefore projected area.
• Record the padding used and its placement.

Hip or Knee Contracture and Joint Replacement

• Use the largest hip and knee flexion the patient tolerates, even if it is far short of the standard block position.
• If the block cannot be used at all, support the legs with sponges and record the achieved position: "leg block not tolerated; knees supported on small sponge, hips flexed approximately 25 degrees."
• Reproduce that exact arrangement at follow-up. A patient scanned flat at baseline and over a full block at follow-up will show a BMD change that is purely geometric.

Obesity

• Verify table weight and field-width limits first.
• Displace a pendulous panniculus symmetrically where possible so the soft-tissue baseline is not lopsided.
• Use a slower acquisition mode for photon statistics.
• Confirm the arms are clear of the trunk, which is harder in large patients and more important because the baseline region is already compromised.

Pain, Tremor, and Inability to Cooperate

• Position gently and efficiently; a patient held in a painful position will move.
• Shorten table time by acquiring only ordered sites.
• Use the manufacturer's faster mode if motion risk outweighs noise, and document that choice.
• Consider whether the study should be deferred rather than repeated three times.

The Positioning–Precision Link

Positioning is the single largest contributor to in vivo precision error, and the lumbar spine is where standardization pays most. Three habits carry most of the benefit: use the same block at the same setting, center the same way, and record any deviation the moment you make it rather than at the end of the day.