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What is the minimum amount of free air that xray abdomen can detect?
As little as 1 ml of free air can be detected in an erect abdominal radiograph provided that x-ray is done 10 minutes after positioning the patient in erect posture. This allows the slightest amount of air to layer under the dome of diaphragm.
Supine radiograph clearly illustrates the lucent liver sign, liver edge sign, doge cap sign, Rigler sign of free intraperitoneal air.
Assisted erect radiograph demonstrates the cupola sign, continuous diaphragm sign, falciform ligament sign.
A shoot-through lateral x-ray in the right decubitus position reveals large amounts of free air layering to the left side.