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.. Standard exposure ranges from 110 to 125 kVp, 2 to 5 mAs, at an SID of 72 inches (180 cm).
Anatomy demonstrated
- Both lungs with the affected side partially dependent.
- Fluid layered dependently, or free air along the non-dependent side.
- Mediastinal structures in recumbent position.
- Thoracic vertebral bodies.
- Ribs and costophrenic angles.
- Heart and mediastinal contours.
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
- 110–125
- mAs
- 2–5
- SID
- 72" (180 cm)
- Notes
- Horizontal beam. Fluid layers to the dependent side; free air rises to the side up.
Image-quality criteria
- Fluid layers along the dependent side; free air collects along the side up.
- Affected lung field clearly visible without superimposition.
- Mediastinal structures visible and not obscured.
- No rotation evident from bilateral symmetry of the field.
- Sharp visualization of the costophrenic angles.
Common errors / ARRT traps
- 1 Insufficient time in position allows fluid to settle but not fully layer.
- 2 Beam not horizontal misses the true dependent layering.
- 3 Rotation of the patient twists the mediastinum.
- 4 IR not truly vertical prevents clear visualization.
- 5 Wrong side down: fluid needs the affected side down, free air the affected side up.
Clinical indications
- Detection of pleural effusion or hemothorax.
- Demonstration of free air (pneumothorax) rising to the non-dependent side up.
- Assessment of free fluid loculation or layering.
- Patients unable to sit or stand upright due to acute illness.
Aligned to the current ARRT Radiography Content Specifications.