Thumb AP is a radiography positioning projection of the Upper Extremity. Seated, hand internally rotated so the dorsal thumb surface lies flat on the IR. SID 40". The centering point is located first mcp joint of the thumb.. The central ray is perpendicular to the first mcp joint.. Image-quality criteria include thumb shown in true ap with no rotation or foreshortening., both thumb phalanges (proximal and distal) and the metacarpal included.. Standard exposure ranges from 55 to 65 kVp, 2 to 3 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal and proximal phalanges (the thumb has only two).
- First metacarpal.
- IP, MCP, and first CMC joint spaces.
- CMC articulation.
Patient preparation
- Verify patient identity using two identifiers.
- Remove rings, bracelets, and radiopaque items from the hand.
- Seat the patient at the end of the table.
- Extend the arm and internally rotate the hand until the thumb's dorsal surface rests on the IR.
- Place the dorsal surface of the thumb flat against the IR.
Position & centering point
First MCP joint of the thumb.
Central ray
Perpendicular to the first MCP joint.
Exposure / technique
- kVp
- 55–65
- mAs
- 2–3
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. No grid required.
Image-quality criteria
- Thumb shown in true AP with no rotation or foreshortening.
- Both thumb phalanges (proximal and distal) and the metacarpal included.
- Joint spaces open at the first CMC, MCP, and IP joints.
- Soft tissue and bone trabeculae sharp with no motion blur.
- Collimation: 1 inch on all sides, 1 inch proximal to CMC joint.
Common errors / ARRT traps
- 1 Thumb rotated or pronated off true AP projection.
- 2 Incomplete internal rotation lifts the dorsal thumb surface off the IR.
- 3 CR off-centered clips the distal phalanx or metacarpal head.
- 4 Flexion of the thumb foreshortens phalanges.
Clinical indications
- Trauma evaluation for thumb fracture, dislocation, or sprain.
- Suspected benign or malignant bone lesion.
- Assessment of thumb joint arthritis or degenerative changes.
- Post-operative follow-up of thumb fixation hardware.
Aligned to the current ARRT Radiography Content Specifications.