Chest Lateral Decubitus is a radiography positioning projection of the Thorax. Patient on side with horizontal beam. SID 72". The centering point is located level of T7. Affected side down for fluid, affected side up for free air. The central ray is horizontal, perpendicular to the vertical IR. Image-quality criteria include fluid layers along the dependent side; free air collects along the side up, affected lung field clearly visible without superimposition. A sample technique is 120 kVp with DR 2 or CR 4 mAs, at an SID of 72 inches (183 cm).
Anatomy demonstrated
- Entire lungs, including the apices.
- Fluid layered dependently, or free air along the non-dependent side.
- Both costophrenic angles and both lateral rib borders.
Patient preparation
- Verify patient identity using two identifiers.
- Remove all radiopaque objects from the chest area.
- Affected side down for suspected fluid; affected side up for suspected free air.
- Place the IR vertical perpendicular to the table.
- Maintain the position for at least 5 minutes to allow layering.
Position & centering point
Level of T7. Affected side down for fluid, affected side up for free air.
Central ray
Horizontal, perpendicular to the vertical IR.
Exposure / technique
- kVp
- 120
- mAs
- DR 2 / CR 4
- SID
- 72" (183 cm)
- Notes
- Horizontal beam. Fluid layers to the dependent side; free air rises to the side up. Sample technique: DR 2 / CR 4 mAs at 120 kVp, 72 in SID, for a 22 cm part (Merrill's sample chart). Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Fluid layers along the dependent side; free air collects along the side up.
- Affected lung field clearly visible without superimposition.
- No rotation evident from bilateral symmetry of the field.
- Sharp visualization of the costophrenic angles.
Common errors
- 1 Insufficient time in position allows fluid to settle but not fully layer.
- 2 Beam not horizontal misses the true dependent layering.
- 3 Rotation: ribs and SC joints not equidistant from the vertebral column.
- 4 Wrong side down: fluid needs the affected side down, free air the affected side up.
Clinical indications
- Detection of pleural effusion or hemothorax.
- Small amounts of air in the pleural cavity (possible pneumothorax), affected side up.