CT Scanogram (Leg Length)

Indications

• Limb-length discrepancy quantification (congenital, post-traumatic, post-arthroplasty)
• Pre-operative planning for lengthening/epiphysiodesis or revision arthroplasty
• Suitable for children (~age 2+) and adults

Patient prep

• Supine, legs fully extended, patellae pointing up, feet secured neutral (footboard/tape) — no rotation
• Both legs in the field, positioned symmetrically
• No contrast; no breath-hold needs

Contrast

Technique

• TOPOGRAM/SCOUT-ONLY technique — an AP (± lateral) CT localizer from above the iliac crests through the bottoms of the feet; no helical acquisition at all
• Very tall patients: split into two overlapping localizers (crests → knees, knees → feet) and measure per segment
• Electronic cursor measurements: femoral head → medial malleolus (total), femoral head → medial femoral condyle (femur), tibial plateau → tibial plafond (tibia)
• Dose is comparable to a radiograph — a fraction of any helical CT

Series / Sequences

Key points

• No cross-sectional images are acquired — the entire exam is localizer radiographs with cursor measurements, which eliminates the magnification error of conventional radiographic scanograms
• Rotation is the enemy of accuracy: symmetric, secured, extended legs matter more than anything else
• Report per-segment (femur, tibia) and total discrepancies
• Flexion contracture shortens the apparent limb on an AP — add the lateral localizer when extension is limited

References