- As little as 1mm of free gas can be detected on radiography in an upright position with a horizontal x-ray beam
- If the patient cannot stay upright, a lateral decubitus (preferably patient on the left side) can be performed. Free gas will collect between lateral liver margin and abdominal wall
- Best chance of detection of free gas is when the radiograph is taken after the patient remains in an upright (or lateral decubitus) position for 10 minutes
- More difficult to detect
- Large free gas can be seen indirect as gas collect in different locations
- Right upper quadrant features include 1) linear gas collection running in an inferolateral to superomedial orientation (representing gas in subhepatic space, as in our patient), 2) triangular gas collection right to the spine above the kidney shadow (gas in most posterior recess of the Morrison pouch)
- Visualization of the outer wall of intestine (Rigler's sign)
- Visualization of the falciform ligament of the liver
1. Eisenberg RL. Gastrointestinal Radiology: a Pattern Approach, 4th edition, 2003.
2. Menuck L, Siemers. Pneumoperitoneum: importance of right upper quadrant features. AJR 1976;127:753-756.