Air is a medium posing high resistance and impermeability to ultrasound waves, making it a strong reflector. Free air can be visualized as horizontal or vertical reverberations originating in the peritoneum and extending to the lower edge of the monitor. The reverberation echo, generated by an air structure, was well described by Linchenstein [8]. It consists in a succession of roughly horizontal (or slightly curved) lines that generate a striped pattern, alternating dark and clear lines at regular intervals. They can be large or very narrow. The reverberation echo hides the information below, as does an acoustic shadow. Larger air collections make bright, highly echogenic lines with distal reverberation and shadowing artifacts (Fig. 3.2). Smaller air bubbles can appear as bright punctuate foci with or without ring-down artifacts (Fig. 3.3).

Reverberation is not specific for pneumoperitoneum unless it is possible to demonstrate the “shifting” of air in the cavity when the patient was repositioned. It is therefore needed to move the patient on the left semilateral decubitus position observing the shifting of the air in the space between peritoneum and liver or in the hepatorenal space. Excessive gas in an abscess can mimic the shifting phenomenon. For this reason, Karahan and colleagues proposed an original method for the detection of pneumoperitoneum [9]. While the patient is in the supine position, if the reverberation artifact is visible, a slight pressure is applied to the abdominal wall using the probe. During this maneuver, the free intraperitoneal air is expelled from the region anterior to the liver to other parts of the peritoneal cavity, and consequently, the reverberation artifact became much less prominent. Conversely, when the pressure on the probe is released, maintaining the contact with the skin surface, the free gas return to the epigastric region and the artifact echo pattern became more prominent. On real-time US examination, repetition of this maneuver appears like the opening and closing of scissors, leading to the term “scissors maneuver.” The diagnostic accuracy values in the diagnosis of pneumoperitoneum obtained by the authors with the adding of this maneuver was very high, and they reported a sensitivity of 94 %, specificity of 100 %, PPV of 100 %, and NPV of 98 %.

This finding was first described by Muradali et al. [10], and it was termed the “peritoneal stripe.” Under an animal experimental model, they were able to demonstrate that tiny bubbles of free air produced focal enhancement and apparent thickening of the peritoneal stripe with or without multiple reflection artifacts, depending on the amount of the peritoneal air. In the cases of air within the intestinal bowel, overlying peritoneal stripe was normal, whereas in the cases of extraluminal (intraperitoneal) air, the peritoneal stripe was thickened. The animal model was than demonstrated in nine patients who underwent laparoscopy, and it was documented for all four quadrants of the abdomen. The enhanced peritoneal stripe indicates free intraperitoneal air and permits its differentiation from intraluminal bowel gas (Fig. 3.4). In the US imaging of the abdomen, it is possible to identify a single or a double thin echogenic line between the anterior abdominal wall and the anterior liver surface.

Intraperitoneal free fluid and decreased bowel motility are indirect signs, but in the adequate clinical context can help in the diagnosis [11]. Peritonitis caused by gastrointestinal perforation leads to a paralytic or adynamic ileus, a condition characterized by gas-fluid stasis, with a reduction in intestinal movement [12].