AP Leg (Tibia & Fibula) is a radiography positioning projection of the Lower Extremity. Supine, leg extended on IR, foot true AP. SID 40". The centering point is located mid-point between knee and ankle joints on the IR. The central ray is perpendicular to mid-leg. Image-quality criteria include no rotation: femoral and tibial condyles in profile, intercondylar eminence centered in the fossa, both knee and ankle joints included on the IR. A sample technique is 70 kVp with DR 2.5 or CR 3.6 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal femur and proximal tibia at knee joint.
- Entire length of tibia and fibula.
- Tibiofibular joints at proximal and distal articulations.
- Interosseous space between tibia and fibula.
- Distal tibia and fibula at ankle.
- Tibiotalar joint.
Patient preparation
- WARNING: with possible spinal injury or severe trauma, do not attempt to move the patient; do not rotate a severely injured part or remove splints (Bontrager). Merrill's: do not attempt true AP or lateral positioning of a severely injured limb; take two projections 90 degrees apart, moving it as little as possible, and move the IR and tube if the patient cannot reach the position.
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient supine with leg extended on the IR.
- Keep the pelvis, knee and leg unrotated in a true AP; dorsiflex the foot to 90 degrees if possible.
- Keep both joints 1-2 inches from the IR ends. A diagonal leg needs at least 44 inches SID.
Position & centering point
Mid-point between knee and ankle joints on the IR.
Central ray
Perpendicular to mid-leg.
Exposure / technique
- kVp
- 70
- mAs
- DR 2.5 / CR 3.6
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. Both knee and ankle joints must be included. Minimum SID is 40 inches; 44 to 48 inches reduces beam divergence and includes more of the part. Sample technique: DR 2.5 / CR 3.6 mAs at 70 kVp, 40 in SID, for a 12 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- No rotation: femoral and tibial condyles in profile, intercondylar eminence centered in the fossa.
- Both knee and ankle joints included on the IR.
- Tibia and fibula aligned in true AP without rotation.
- Some overlap of the fibula and tibia at both the proximal and distal ends.
- Interosseous space visible between tibia and fibula.
- Collimation 1 inch on sides, 1.5 inches above knee and beyond ankle.
Common errors
- 1 Rotation: condyles not in profile, or the eminence off center in the fossa.
- 2 Off-centered CR omits knee or ankle joint.
- 3 A joint nearer than 1-2 inches to an IR end can be projected off.
Clinical indications
- Traumatic injury evaluation for tibia or fibula fracture.
- Foreign bodies or lesions of the bone.