CT and MRI of the Peritoneum, Omentum, and Mesentery

1

What is the peritoneum?

The peritoneum is the largest and most complexly arranged serous membrane in the body. It is closed in men and open to the ends of the fallopian tubes in women. The parietal layer lines the abdominal wall, whereas the visceral layer lines the visceral organs. It contains a potential space, the peritoneal space, which normally contains a small amount of fluid. Its major function is to provide for unimpeded activity and mobility of contained viscera, although it also has absorptive and immune functions.

2

What are peritoneal ligaments, small bowel mesentery, mesocolon, and omentum?

The various “ligaments” of the peritoneum are actually peritoneal reflections that may contain fat, blood vessels, nerves, lymphatics, and lymph nodes.

The small bowel mesentery is a double-layered fold of visceral peritoneum that contains subperitoneal fat, mesenteric blood vessels, nerves, lymphatics, and lymph nodes and connects the small bowel to the posterior body wall, extending from the left upper quadrant at the ligament of Treitz to the right lower quadrant at the ileocecal valve.

The mesocolon is similarly a double-layered fold of visceral peritoneum that connects intraperitoneal portions of the large bowel (i.e., the transverse colon and sigmoid colon) to the posterior body wall.

The lesser omentum is a double-layered peritoneal fold comprised of the gastrohepatic and hepatoduodenal ligaments that extends from the lesser curvature of the stomach and first portion of the duodenum to the liver.

The greater omentum is a large peritoneal fold comprised of the gastrosplenic and gastrocolic ligaments. It hangs from the greater curvature of the stomach in the anterior abdomen, folds upon itself, and then heads superiorly and posteriorly to attach to the posterior abdominal wall just superior to the attachment of the transverse mesocolon. Its redundant portion in the anterior abdomen fuses to itself during development inferior to the level of the transverse colon and is therefore comprised of 4 layers of visceral peritoneum. A portion of the greater omentum also extends between the stomach and spleen.

The peritoneal ligaments, small bowel mesentery, mesocolon, and omentum are often involved by disease processes and may serve as either barriers to or conduits of disease spread.

3

What is the anatomy of the abdominal peritoneal cavity?

The abdominal peritoneal cavity can be divided into supramesocolic and inframesocolic compartments, where the former is located superior to the transverse mesocolon and the latter is located inferior to the transverse mesocolon. The supramesocolic component is further subdivided into the subphrenic space (located inferior to the diaphragm), the subhepatic space (located inferior to the liver), and the lesser sac (located anterior to the pancreas and posterior to the stomach). The lesser sac communicates with the greater sac (which includes the subphrenic, subhepatic, infracolic, and paracolic spaces) via the foramen of Winslow (also called the epiploic foramen), which is located posterior to the hepatoduodenal ligament and anterior to the inferior vena cava. The inframesocolic component is further subdivided into the infracolic spaces (located medial to the ascending and descending portions of the large bowel and lateral to the small bowel mesentery) and the paracolic spaces (located lateral to the ascending and descending portions of the large bowel).

4

What is the anatomy of the pelvic peritoneal cavity?

The pelvic peritoneal cavity can be divided into paravesical and rectovesical spaces. The paravesical space is further subdivided into the supravesical space (located superior to the bladder and medial to the medial umbilical folds), the medial inguinal fossae (located lateral to the medial umbilical folds and medial to the lateral umbilical folds), and the lateral inguinal fossae (located lateral to the lateral umbilical folds). The median umbilical fold of parietal peritoneum is located anteriorly in the midline adjacent to the obliterated urachus (also called the median umbilical ligament), whereas the paired medial umbilical folds of parietal peritoneum are adjacent to the obliterated umbilical arteries (also called the medial umbilical ligaments) and the paired lateral umbilical folds of parietal peritoneum are adjacent to the inferior epigastric vessels.

In men, the rectovesical space is located between the rectum and the bladder. In women, the rectovesical space is further subdivided into the vesicouterine space (located between the uterus and bladder) and the rectouterine pouch of Douglas (located between the uterus and rectum).

5

What is the differential diagnosis for nonparenchymal cystic lesions in the abdomen and pelvis?

Nonparenchymal cystic lesions may infrequently be encountered in the omentum, mesentery, mesocolon, or retroperitoneum and are most often incidentally detected. For answers, see Table 31-1 .

6

List some potential complications that may occur with nonparenchymal cystic lesions.

Potential complications include growth, hemorrhage, superinfection, torsion, rupture, and bowel obstruction. Whereas imaging surveillance may suffice for stable small asymptomatic nonaggressive-appearing cystic lesions, surgical resection may be performed to prevent potential complications, particularly for large or symptomatic lesions.

7

Are there any distinctive CT and MR imaging features of specific types of nonparenchymal cystic lesions?

Some imaging features listed below are useful to suggest specific types of nonparenchymal cystic lesions. Otherwise, there is much overlap in the CT and MR imaging appearance of nonparenchymal cystic lesions.

Lymphangiomas, benign cystic mesotheliomas, and tailgut cysts are typically multilocular, whereas simple lymphatic, simple mesothelial, enteric duplication, enteric, bronchogenic, epidermoid, and dermoid cysts are typically unilocular. Enteric duplication cysts have diffusely thickened walls because they contain all of the normal bowel wall layers, whereas enteric cysts have thin walls because they are lined only by enteric mucosa.

Enteric duplication cysts and enteric cysts are typically located adjacent to bowel loops, extrathoracic bronchogenic cysts and extralobar pulmonary sequestration are typically located in the subdiaphragmatic retroperitoneum, and tailgut cysts are typically retrorectal in location.

Mature cystic teratoma may contain fat attenuation or fat signal intensity components, whereas cystic or necrotic lymphadenopathy and cystic or necrotic neoplasms typically have residual soft tissue enhancing components.

Hemorrhagic fluid collections typically have nonenhancing high attenuation and high T1-weighted signal intensity fluid, sometimes with a hematocrit effect due to layering of cellular components of blood. Uriniferous fluid collections will characteristically be associated with leakage of excreted intravenous contrast material from the urothelium during the delayed phase of enhancement. Enteric and infectious fluid collections sometimes contain foci of very low attenuation and very low signal intensity gas.

8

What are the major causes of transudative ascites?

Hepatic failure, cirrhosis, renal failure, heart failure, and hypoproteinemia are the major causes of transudative ascites.

9

What are the major causes of exudative ascites?

Peritoneal malignancy, bowel perforation, infection, and noninfectious inflammatory disease conditions such as pancreatitis or peritonitis are the major causes of exudative ascites.