The fat within the root of the small-bowel mesentery (small arrows) is outlined by fluid. Imaging plays an important role in their diagnosis, with CT being the method of choice. The right paracolic gutter is continuous with the right perihepatic space and with the intraperitoneal pelvic space. They are three times more frequent on the left side than on the right. They include paraduodenal (53%), pericaecal (13%), foramen of Winslow (8%), transmesenteric and transmesocolic (8%), intersigmoid (6%) and retroanastomotic (5%) hernias. Accumulation of fluid in the peritoneal cavity is not a disease by itself, but it is the manifestation of a wide spectrum of processes that may involve intraperitoneal or extraperitoneal organs. They are more likely than other hernias to develop volvulus. This is the key difference between the omentum and the mesentery. The pelvic peritoneal cavity consists of the lateral paravesical spaces and the pouch of Douglas—the rectovesical space in men, the rectouterine space in women. When they become thickened by oedema, inflammation or neoplastic infiltration, they can be directly recognised on computed tomography (CT) or magnetic resonance imaging (MRI). The gastrocolic ligament is the segment of the greater omentum that links the stomach with the transverse colon. Ultrasound is useful for the detection of membranes, septa and hydatid sand within the cyst. Developmental Defects Blood collections may be the result of trauma, haemorrhagic diathesis or tumour rupture. The mesenteric folds are not discernible, unless they are separated by intervening fluid (Fig.

In the intact abdomen, fluid migrates to the upper abdomen due to the lower hydrostatic pressure in the subdiaphragmatic related to respiratory movements. It contains the infracolic space and the paracolic gutters (Fig. The sigmoid mesocolon attaches the sigmoid colon to the posterior pelvic wall (Fig. It consists of the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelops hollow and solid abdominal viscera. CT demonstrates marked thickening and enhancement of the parietal peritoneum (arrows). The opacified fluid delineates the right (large black arrow) and the left paracolic gutters. Gas within a loculated fluid collection is highly suggestive of abscess, but is not pathognomonic, because a necrotic non-infected tumour or a mass that communicates with the bowel may also contain air. Mesenteric Cysts The most common location of free intraperitoneal air while the patient is in the supine position is anterior to the liver. It is divided into right and left peritoneal compartments, which are arbitrarily subdivided into intercommunicating spaces.