1  Answers: A5; B8; C6; D4; E1; F3; G2; H7.The peritoneum is the longest and most complex serous membrane in the body. The peritoneal cavity is a potential space that is surrounded by parietal peritoneum along the abdominopelvic body wall and by visceral peritoneum around the abdominopelvic organs. It is open in women at the ostia of the fallopian tubes and closed in men. The abdominal peritoneal cavity can be subdivided into the supramesocolic and inframesocolic compartments, which are located superior and inferior to the transverse mesocolon, respectively. The supramesocolic compartment can be subdivided into the subphrenic spaces, subhepatic spaces, and lesser sac, whereas the inframesocolic compartment can be subdivided into the infracolic and paracolic spaces. The subphrenic, subhepatic, infracolic, and paracolic spaces together comprise the greater sac of the abdominal peritoneal cavity.

A5: The falciform ligament is a peritoneal reflection that extends between the umbilicus and liver and contains the ligamentum teres, the obliterated remnant of the umbilical vein.

B8: The small bowel mesentery is a fold of peritoneum that suspends the small bowel from the posterior abdominal wall and extends from the ligament of Treitz to the ileocecal junction.

C6: The hepatoduodenal ligament extends from the duodenum to the hepatic hilum and contains the main portal vein, proper hepatic artery, and extrahepatic bile duct. It forms the anterior margin of the foramen of Winslow (the site of communication between the greater and lesser sacs of the peritoneal cavity) and comprises a portion of the lesser omentum.

D4: The paracolic spaces are located inferior to the transverse mesocolon. The right and left paracolic spaces are located lateral to the ascending and descending colon, respectively, and deep to the parietal peritoneum.

E1: The subphrenic spaces are separated by the falciform ligament anteromedially. The right subphrenic space surrounds the right hepatic lobe and medial segment of the liver deep to the right diaphragm/body wall. The left subphrenic space surrounds the lateral segment of the liver, proximal stomach, and spleen deep to the left diaphragm/body wall.

F3: The lesser sac is bounded by the stomach, proximal duodenum, lesser omentum (hepatoduodenal and gastrohepatic ligaments), and gastrocolic ligament anteriorly, the pancreas posteriorly, the gastrosplenic and splenorenal ligaments on the left, and the transverse mesocolon and greater omentum inferiorly. The foramen of Winslow is located along its right side.

G2: The subhepatic spaces are located inferior to the liver. The right subhepatic space includes the hepatorenal space, or Morison pouch, and is contiguous with the right subphrenic space. The left subhepatic space, also called the gastrohepatic recess, is located between the lateral segment of the liver and the stomach, to the left of the gastrohepatic ligament, and is contiguous with the left subphrenic space. The left subhepatic space is separated from the left paracolic space by the phrenicocolic ligament.

H7: The gastrocolic ligament is a portion of the greater omentum that extends between the greater curvature of the stomach and the transverse colon.

The infracolic spaces are located inferior to the transverse mesocolon. The right infracolic space is located between the ascending colon and small bowel mesentery, and the left infracolic space is located between the descending colon and small bowel mesentery.

References: Meyers MA, Charnsangavej C, Oliphant M. Intraperitoneal spread of infections and seeded metastases. In: Meyers MA, Charnsangavej C, Meyers OM (eds). Dynamic radiology of the abdomen—normal and pathologic anatomy, 6th ed. New York, NY: Springer-Verlag, 2011:69–108.

Torigian DA, Kitazono MT. Peritoneal cavity. In: Torigian DA, Kitazono MT (eds). Netter’s correlative imaging: abdominal and pelvic anatomy. Philadelphia, PA: Elsevier, 2013:109–206.

2  Answer A.Retractile mesenteritis is in the spectrum of sclerosing mesenteritis, a condition characterized by chronic mesenteric inflammation. It can be seen as part of the multisystemic IgG4-related inflammatory diseases, which includes retroperitoneal fibrosis, the correct answer choice in this question. There is a lobulated mass (arrows) with dense calcifications and spiculated margins tethering the ileal mesentery. The two primary considerations with these features are carcinoid tumor metastasis and retractile (fibrosing) mesenteritis. Both can occur with or without calcifications. The two entities are frequently indistinguishable with other malignancies also in the differential diagnosis, so further evaluation and/or biopsy is usually required. A clue to the diagnosis of carcinoid instead of retractile mesenteritis would be a hypervascular mass representing the primary neuroendocrine neoplasm within the adjacent small bowel, often a subtle finding. In this case, further evaluation was negative for neuroendocrine tumor or other malignancy, and subsequent stability for years without treatment was consistent with retractile mesenteritis.

The spectrum of sclerosing mesenteritis at pathology includes:

• Mesenteric panniculitis (inflammation)
  
• Mesenteric lipodystrophy (fat necrosis)
  
• Retractile mesenteritis (fibrosis), considered late stage with more invasive behavior

Patients are commonly asymptomatic, or occasionally present with abdominal pain, weight loss, intestinal obstruction/ischemia, mass, or diarrhea.

The differential diagnosis for mesenteric calcification also includes benign and malignant entities. Calcifications can be seen in ovarian cancer and mucinous tumors as well as treated lymphoma. Granulomatous infection or inflammation, such as tuberculosis or sarcoidosis, may be seen as calcified mesenteric lymph nodes.

Tuberous sclerosis, Whipple disease, and graft versus host disease have not been associated with sclerosing mesenteritis. Tuberous sclerosis is a genetic neurocutaneous disorder with thoracoabdominal findings of lymphangioleiomyomatosis (LAM, with lung cysts), renal angiomyolipomas, renal cysts, and renal cell carcinomas. Whipple disease is an infection with the Whipple bacillus Tropheryma whipplei classically presenting with arthralgias and abdominal symptoms. Patients with Whipple disease may have nodular bowel wall thickening and low-density adenopathy. Graft versus host disease is an immune response in the recipient of an allogeneic hematopoietic cell transplant to donor cells. It can involve the gastrointestinal tract with diffuse bowel wall thickening.

References: Horton KM, Lawler LP, Fishman EK. CT findings in sclerosing mesenteritis (panniculitis): spectrum of disease. Radiographics 2003;23(6):1561–1567.

Levy AD, Rimola J, Mehrotra AK, et al. From the archives of the AFIP: benign fibrous tumors and tumorlike lesions of the mesentery: radiologic–pathologic correlation. Radiographics 2006;26(1):245–264.

3a  Answer C.The abdominal radiograph shows the bowel gathered toward the center of the abdomen, suggesting a large volume of surrounding ascites. There is no evidence of small bowel obstruction with scattered gas in nondilated small bowel and colon. No extraluminal lucencies to indicate pneumoperitoneum are shown, with the caveat that a single supine AP view is insensitive for pneumoperitoneum. There is increased density and absence of bowel gas in the pelvis, which can be seen with a distended bladder. However, in this case, the increased density is generalized and related to the large volume of ascites throughout the abdomen and pelvis.

3b  Answer D.Axial contrast-enhanced CT images confirm a large volume of ascites. There is thickening and nodularity of the peritoneum (short black arrows) including nodularity at the liver surface. There is confluent enhancing soft tissue in the greater omentum (long white arrows) consistent with omental caking. In the United States, findings most likely represent peritoneal carcinomatosis.

Tumors of ovarian and gastrointestinal origin are the most common causes of carcinomatosis. CT findings of peritoneal carcinomatosis include subtle infiltration of the omental or mesenteric fat; frank peritoneal, mesenteric, or omental soft tissue nodules; diffuse peritoneal nodular thickening and enhancement; complex ascites; and abdominopelvic organ invasion. Lymphomatosis (peritoneal involvement by lymphoma) is uncommon but may have the same appearance as carcinomatosis.

CT is the study of choice for the evaluation of peritoneal and mesenteric pathology. Common sites of peritoneal implants are in the dependent aspects of the pelvis (pouch of Douglas in women or retrovesical space in men), ileocecal region, paracolic gutters, subhepatic space, subphrenic space, and root of the mesentery. Patients can develop small bowel obstruction from tumor implants, which may be multifocal. Peritoneal carcinomatosis is incurable and predominantly treated with chemotherapy. In patients with ovarian and primary peritoneal carcinomatosis, cytoreductive surgery has been shown to increase survival.

Peritoneal tuberculosis may present with similar findings as peritoneal carcinomatosis, but is less common in the United States. Malignant peritoneal mesothelioma is a rare and often rapidly fatal malignancy, with the main risk factor being prior asbestos exposure. Spontaneous bacterial peritonitis (SBP) usually occurs in patients with cirrhosis. SBP typically demonstrates smooth peritoneal thickening and enhancement (arrows on the following image) along with ascites. Clinical correlation is important as smooth enhancement does not exclude peritoneal malignancy.

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