Abdomen Lateral Decubitus is a radiography positioning projection of the Abdomen. Left side down, horizontal CR at 2 inches above iliac crest. SID 40". The centering point is located 2 inches above iliac crest at midline (Bontrager). Merrill's centers at the iliac crests and goes 2 inches higher when needed to include the diaphragm. Follow your program's source. The central ray is horizontal (cross-table), perpendicular to the vertical IR at 2 inches above iliac crest at midline. Image-quality criteria include both hemidiaphragms and the up side of the abdomen included, patient on a radiolucent pad and firm surface, so the dependent side does not sag and get cut off. A sample technique is 85 kVp with DR 11 or CR 28 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Entire abdomen in an AP projection.
- Air-fluid interfaces visible laterally.
- Free air lucency if pneumoperitoneum present.
- Air-filled stomach and loops of bowel.
- Liver silhouette along the non-dependent side, where free air collects.
- Flank structures.
Patient preparation
- Verify patient identity using two identifiers.
- Remove all radiopaque objects from abdomen.
- Position in the left lateral decubitus (left side down), the standard for free air.
- Ensure no rotation of the pelvis or shoulders, with the knees partially flexed, one on top of the other.
- Raise the arms up near the head.
- Keep the patient on the side at least 5 minutes (10-20 preferred) before exposure.
- Expose at the end of expiration.
Position & centering point
2 inches above iliac crest at midline (Bontrager). Merrill's centers at the iliac crests and goes 2 inches higher when needed to include the diaphragm. Follow your program's source.
Central ray
Horizontal (cross-table), perpendicular to the vertical IR at 2 inches above iliac crest at midline.
Exposure / technique
- kVp
- 85
- mAs
- DR 11 / CR 28
- SID
- 40" (102 cm)
- Notes
- Grid required. Patient lateral recumbent with a vertical IR against the back and a horizontal beam. Sample technique: DR 11 / CR 28 mAs at 85 kVp, 40 in SID, for a 24 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
- Both hemidiaphragms and the up side of the abdomen included.
- Patient on a radiolucent pad and firm surface, so the dependent side does not sag and get cut off.
- Free air visible as lucency along the non-dependent (upper) side if present.
- Air-fluid levels shown on this AP projection taken with a horizontal beam.
- Spine seen in the midline without rotation.
- No motion: ribs and all gas bubble margins sharp.
Common errors
- 1 Patient rotated: iliac wings not symmetric and outer rib margins at unequal distances from the spine.
- 2 No firm surface under the patient: sagging cuts off anatomy.
- 3 CR too high excludes lower abdomen and pelvis.
- 4 Patient motion blurs anatomy; use the shortest exposure time possible.
Clinical indications
- Free air detection (pneumoperitoneum evaluation).
- Bowel obstruction assessment with lateral air-fluid levels.
- Abdominal pain with patient unable to stand.
- Abdominal masses.