Figure 3b. Free perihepatic air in a 67-year-old man with perforating sigmoid diverticulitis. (a) Axial contrast-enhanced CT scan (lung window) shows free air (arrowheads) in the perihepatic space. Free air is more easily identified by using lung window settings. (b) CT scan of the pelvis shows a thickened wall of the sigmoid colon (arrow) and mild pericolonic inflammatory fat stranding, findings that represent diverticulitis.