Peritonitis results from a variety of infectious or noninfectious causes, many of which are related to bowel perforation. Trauma, complications of surgery and inflammatory bowel disease are common underlying causes.
Peritonitis appears as diffuse increased enhancement of the peritoneum and mesentery, and is most clearly defined on interstitial-phase gadolinium-enhanced fat-suppressed gradient-echo images.
Pseudocysts may develop in the setting of peritonitis as walled-off collections of fluid. In uncomplicated cases they are low in signal intensity on T1-weighted images and very high in signal intensity on T2-weighted images. Complex fluid is characterized by either increased intensity signal on T1-weighted images, decreased or heterogeneous signal intensity on T2-weighted images, or a combination of both.
Gadolinium-enhanced T1-weighted images reveal increased enhancement and occasionally increased thickness of the inflamed peritoneum, which is more conspicuous in combination with fat-suppression (Kanematsu et al. 1997).