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 knee joint space open and clearly visualized., ankle joint space open and included on ir.. Standard exposure ranges from 60 to 70 kVp, 4 to 7 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
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient supine with leg extended on the IR.
- Align foot in true AP position with no rotation.
- Ensure full leg extension from knee to ankle.
Position & centering point
Mid-point between knee and ankle joints on the IR.
Central ray
Perpendicular to mid-leg.
Exposure / technique
- kVp
- 60–70
- mAs
- 4–7
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. Both knee and ankle joints must be included.
Image-quality criteria
- Knee joint space open and clearly visualized.
- Ankle joint space open and included on IR.
- Tibia and fibula aligned in true AP without rotation.
- Proximal tibia relative to fibula shows slight overlap at proximal joint.
- Interosseous space visible between tibia and fibula.
- Collimation 1 inch on sides, 1.5 inches above knee and beyond ankle.
Common errors / ARRT traps
- 1 Leg rotation medially or laterally distorts tibiofibular relationship.
- 2 Foot plantarflexion or dorsiflexion changes ankle angle.
- 3 Off-centered CR omits knee or ankle joint.
- 4 Inadequate leg extension cuts off ankle joint space.
Clinical indications
- Traumatic injury evaluation for tibia or fibula fracture.
- Suspected compartment syndrome or soft tissue injury.
- Arthritis or degenerative disease of the leg.
- Post-operative follow-up of leg fixation hardware.
Aligned to the 2025 ARRT Content Specifications.