Anatomy
The peritoneum is a thin serosal membrane that lines both the inner surface of the peritoneal cavity (parietal peritoneum) and the abdominopelvic organs (visceral peritoneum).
It is composed by a single layer of mesothelial cells supported by connective tissue.
Its main functions are to provide a frictionless,
lubricated surface over which the viscera can move, act as local bacterial defense as well as to serve as a site of fluid transport.
The potential space between the parietal and visceral peritoneum is called peritoneal cavity.
Peritoneal ligaments,
mesenteries and omenta divide the peritoneal cavity into two major spaces - the greater and lesser sac,
which communicate through the epiploic foramen.
They serve as anatomic boundaries that divide the peritoneum into compartments,
as well as conduits for the dissemination of disease processes.
The peritoneal ligaments are double layers of peritoneum that support a structure within the peritoneal cavity.
They are composed of adipose and connective tissue,
blood vessels,
nerves and lymphatics.
The omenta and mesenteries are specifically named peritoneal ligaments.
The mesenteries are double layers of peritoneum that connect a portion of the bowel to the posterior abdominal wall.
They include the small bowel and sigmoid mesenteries and the transverse mesocolon.
The omenta are specialized mesenteries that extend from the stomach and gastroduodenal bulb to adjacent organs.
The differential diagnosis of mesenteric and peritoneal diseases can be challenging.
It is useful to have an organized diagnostic approach based on the categorization of pathologic processes into three main categories: 1)Non-neoplastic (inflammatory,
infectious,
reactive); 2)Tumor mimickers and 3)Neoplastic (primary and secondary).
Non-neoplastic disease processes
Sclerosing mesenteritis
Sclerosing meseneritis represents an idiopathic inflammatory disorder of the mesentery,
characterized by a variable degree of inflammation,
fat necrosis and fibrosis.
It is more common in men around the seventh decade.
Its exact etiology has not yet been determined,
but an association with IgG4-related diseases has recently been described.
It can be further subcategorized into three subtypes or stages - mesenteric lipodystrophy,
mesenteric panniculitis and retractile mesenteritis - based on the histologic features and predominant tissue type.
Panniculitis is the milder subtype in which inflammation predominates.
On CT,
it appears as a focal area of increased attenuation within the mesenteric fat,
that can be surrounded by a pseudocapsule (“misty mesentery” appearance).
It may be associated with multiple non-enlarged lymph nodes (Fig.1).
An important imaging finding is the “Fat halo” sign which refers to the preservation of normal fat density in the perivascular fat.
It has an estimated sensitivity of 75% for the diagnosis (Fig.2).
Retractile mesenteritis refers to the chronic form in which fibrosis predominates.
The typical CT appearance is of one or more irregular,
fibrotic,
soft-tissue mesenteric masses,
that may contain calcifications (Fig.3).
The mesenteric masses can cause retraction of the small bowel mesentery and may present with partial small bowel obstruction.
Although the disease process is usually indolent it cannot be confidently distinguished from neoplastic processes such as carcinoids or desmoid tumors without a biopsy.
Sclerosing encapsulating peritonitis or Cocoon syndrome is an inflammatory condition of the peritoneum believed to result from chronic low-grade or subclinical peritonitis.
It is commonly seen in patients on long-term ambulatory peritoneal dialysis,
but can also occur as reactive process to foreign material.
On CT,
there is ascites associated with diffuse peritoneal thickening and enhancement.
The small bowel loops are often collected centrally by the fibrotic peritoneum.
The fibrous capsule may encase the small bowel and lead to obstruction (Fig.4).
In advanced stages it can lead to diffuse peritoneal calcifications which are apparent on abdominal radiographs.
Epiploic appendagitis and omental infarction
Epiploic appendages are small outpouches of visceral peritoneum that arise from the serosal surface of the large bowel.
Occasionally,
they may undergo torsion or ischemia.
The clinical presentation of epiploic appendagitis may resemble that of acute appendicitis and the diagnosis is primarily based on imaging findings.
On CT,
it appears as a small,
oval area of fat surrounded by a ring of soft tissue that represents the inflamed visceral peritoneum (Fig.5).
A “central dot” sign may be present,
representing the internal thrombosed vein.
Omental infarction is another rare cause of acute abdominal pain that is generally attributable to vascular compromise caused by torsion,
trauma or previous surgery.
Other factors such as congestive heart failure or strenuous physical activity can also predispose to infarction.
The typical CT findings are a solitary large (>5 cm) non-enhancing omental mass with heterogeneous attenuation (Fig.6).
Most cases occur on the right lateral free edge of the omentum since it has a more tenous blood supply,
rendering it more vulnerable to infarction.
Granulomatous Peritonitis
This includes a wide range of inflammatory and infectious processes,
with the commonest being tuberculosis.
It can also develop as a reactive process in response to foreign material.
Imaging findings may resemble peritoneal carcinomatosis with omental caking,
mesenteric nodules/masses,
diffuse peritoneal thickening and ascites,
making a definitive diagnosis challenging (Fig.7).
Tuberculous peritonitis can occur in up to 38% of patients with pulmonary tuberculosis.
On imaging,
it most commonly appears as “wet type”,
characterized by significant diffuse or loculated ascites.
The “dry type” lacks ascites and is associated with diffuse fibrotic peritoneal thickening.
Additional radiological features include: enlarged and necrotic mesenteric and retroperitoneal lymph nodes with or without internal calcifications (Fig.8),
thickening of the terminal ileum and small abscesses or calcifications of the spleen and liver.
Similar findings are seen with histoplasmosis and pneumocystosis and remain indistinguishable by imaging (Fig.9).
Tumor mimickers
Splenosis
Consists in intraperitoneal dissemination of splenic tissue after trauma or surgery that disrupts the splenic capsule.
The implants function as normal splenic tissue and may proliferate in response to a number of stressors.
On imaging,
it appears as one or more well-defined,
round or lobulated masses that have density and enhancing characteristics similar to the native spleen (Fig.10).
The diagnosis can be confirmed with Tc99m-tagged heat-damaged RBC scans with autologous erythrocytes.
Endometriosis
Endometriosis is a common affection in women of childbearing age,
consisting in the presence of functional endometrial tissue outside of the uterus.
The most common locations of the implants are the ovaries,
but the peritoneum can also be involved.
CT findings are most often non-specific,
showing nodular or spiculated soft-tissue implants on the serosal surface of the bowel (Fig.11).
MRI remains the modality of choice because of its ability to detect blood products.
Peritoneal inclusion cyst - Benign cystic peritoneal mesothelioma
Represents a non-neoplastic reactive mesothelial proliferation,
resulting from the exudation of peritoneal fluid from an active ovary,
which is entrapped in a peritoneal adhesion.
It typically affects women of reproductive age in the context of prior pelvic surgery,
pelvic inflammatory disease.
The classical imaging appearance is a multi-loculated cystic mass with simple fluid attenuation,
without solid components or enhancement (Fig.12).
A “spider-web” pattern can occasionally be seen and results from the entrapment of the ovary within the peritoneal fluid,
resembling the appearance of malignant ovarian neoplasms.
Neoplasms
Primary peritoneal neoplasms are much more uncommon than secondary neoplasms.
Malignant mesothelioma
Malignant mesotheliomas are rare neoplasms that arise from various serosal membranes.
Peritoneal mesothelioma accounts for 6-10% of all cases.
It is more common in men,
with a median age of 60 years and is often associated with asbestos exposure.
Imaging findings of peritoneal mesothelioma can be divided into two main patterns:
- Diffuse involvement of the peritoneal cavity: sheet-like,
irregular or nodular thickening of the peritoneum associated with ascites.
Omental caking may also occur,
either as fine,
nodular soft-tissue studding or as coalescent mass-like soft-tissue in the omentum.
It may infiltrate the small-bowel mesentery causing adhesions and small-bowel obstruction (Fig.13;14);
- Focal intraperitoneal masses: localized moderate to large-sized,
heterogeneous soft tissues masses associated with small volume or localized ascites.
- Calcified peritoneal plaques are rare and should raise the possibility of secondary neoplasm.
Desmoid tumor
Desmoids are uncommon benign but locally aggressive tumors,
which result from local proliferation of fibrous tissue.
They typically affect young multiparous women and up to 75% occur in patients with previous abdominal surgery.
Most cases are sporadic but the association with familial adenomatosis polyposis is well documented.
Tumors can occur anywhere along the abdomen,
although the small bowel mesentery is most commonly affected,
and they can be solitary or multiple.
On CT,
they appear as soft-tissue masses typically isoattenuating to muscle and homogeneous (Fig.15).
Areas of low attenuation can also be present due to necrosis.
Desmoids can have well-defined or poorly demarcated margins,
with strands radiating to adjacent mesenteric fat or a “whorled” appearance of fibrosis growing into the mesenteric fat (Fig.16).
Infiltration into adjacent organs or the abdominal musculature is not uncommon.
Primary peritoneal serous carcinoma
It is a rare epithelial tumor arising from the peritoneum.
It typically affects postmenopausal women and is histologically similar to its primary ovarian counterpart.
Three imaging criteria must be met to make an accurate diagnosis: 1) normal appearance of both ovaries; 2) greater involvement of extra-ovarian sites than of the surface of either ovary; 3) ovarian involvement limited to the epithelial surface either without stromal invasion or involvement of cortical stroma with tumor size less than 5x5 mm.
Diffuse peritoneal thickening is frequently detected at presentation with extensive ascites,
calcifications and omental soft-tissue infiltration (Fig.17).
The diagnosis of papillary serous carcinoma should be suggested when these findings occur in women with no evidence of primary ovarian masses.
Secondary peritoneal neoplasms far outnumber primary peritoneal lesions.
Peritoneal carcinomatosis
Represents intraperitoneal dissemination of any tumor that does not originate from the peritoneum itself.
It is by far the most common diffuse peritoneal disease.
Primary ovarian,
colonic and gastric carcinomas tend to metastize by direct invasion and peritoneal seeding,
whereas hematogenous metastases to the peritoneum are typically caused by malignant melanoma,
breast and lung carcinomas.
CT remains the primary imaging modality for the evaluation of peritoneal metastases,
although its sensitivity significantly decreases for tumor deposits with less than 5 mm.
When evaluating a CT for peritoneal carcinomatosis the most characteristic locations of intraperitoneal deposition,
including the right paracolic gutter,
ileocecal junction,
pelvis and the superior aspect of the sigmoid mesocolon,
should always be carefully assessed.
Direct findings of peritoneal carcinomatosis include nodular thickening and enhacement of the peritoneum (Fig.18;19),
and involvement of the greater omentum,
with the typical “omental cake” appearance (Fig.21).
More uncommon appearances include palpable subcutaneous masses,
such as the Sister Mary Joseph which results from metastization to the umbilicus (Fig.20).
Indirect findings include ascites and displacement of the small-bowel against the mesenteric root.
Diffuse infiltration of the mesentery leading to a stellate appearance can also occur.
Lymphomatosis
Lymphomatous involvement of the peritoneum can either occur as a primary or secondary process.
Imaging features of secondary lymphomatosis resemble those of peritoneal carcinomatosis,
with multiple enhancing masses,
diffuse thickening of the peritoneal surface and “omental caking”.Lymphoma may appear as confluent masses that often encase abdominal vessels without causing obstruction producing a typical “sandwich” sign (Fig.22;23).
The presence of large,
bulky retroperitoneal and mesenteric lymphadenopathies with associated splenomegaly and hepatomegaly generally favors the diagnosis of lymphomatosis over that of peritoneal carcinomatosis.
Pseudomyxoma peritonei
It is a syndrome characterized by the presence of thick mucinous or gelatinous ascites produced by a low-grade primary mucinous adenocarcinoma that penetrates or ruptures into the peritoneal cavity.
It is currently accepted that the primary tumor is an adenocarcinoma of the appendix,
although an ovarian etiology has not entirely been excluded.
On CT,
mucinous ascites has a characteristic low attenuation and exerts a mass effect on adjacent structures,
leading to scalloping and indentation of the solid organs and displacement of the hollow viscera(Fig.24).
It generally does not cause direct infiltration of the parenchymal organs.
In the presence of these findings,
a careful evaluation for any primary appendiceal (Fig.25;26) or ovarian tumors should be warranted.
Carcinoid Tumors
Mesenteric carcinoids occur secondarily to direct or lymphomatous spread of gastro-intestinal (GI) carcinoids.
The primary intestinal lesion may be difficult to detect and the mesenteric component is often the first abnormality detected.
GI most commonly involve the distal ileum,
with approximately 40-80% extending into the mesentery.
Although these tumors are rare,
they are the most common malignancy of the small intestine.
On CT,
the typical appearance is that of an enhancing soft tissue mass with linear bands radiating into the mesenteric fat.
These radiating soft-tissue bands can either represent tumor infiltration or intense fibrotic proliferation and desmoplastic reaction.
Calcifications are seen in approximately 70% of cases.
Thickening of the adjacent bowel loops may occur,
either caused by direct infiltration or by ischemia resulting from sclerosis of bowel vessels (Fig.27;28).