**3.1 Pneumoperitoneum**

Air is usually cited to be the enemy of ultrasonography as it obscures the view and affects image acquisition. However, this can be used to the advantage of the observer

when examining pneumoperitoneum, that is, free air in the peritoneal cavity. The patient is usually examined in a supine position with the bed elevated by 30 degrees at the head end or semilateral position with the elevation of the right flank. This is to allow the collection of free air in the peritoneal space anterior to the liver surface. Linear array probe is used in the sagittal plane with a pointer toward head end with right intercostal scanning.

The probe is placed gently over the right upper quadrant and scanned for air. Free air is visualized as a hyperechoic line with reverberation artifacts similar to that of the pleural line of the lung (**Figure 8**). When the caudal end of the probe is pressed gently over the area, the air can be displaced and the hyperechoic line will disappear and with release of pressure, the reverberation artifact will reappear. In some cases, the pleural line above the diaphragm is seen along with the pneumoperitoneum over the liver producing the pleuro-peritoneal step-off sign.
