Peritoneal cavity: Potential space in abdomen between visceral and parietal peritoneum, containing only small amount of peritoneal fluid (for lubrication)
Abdominal cavity: Not synonymous with peritoneal cavity, contains all intraperitoneal and retroperitoneal abdominal viscera
GROSS ANATOMY
Peritoneum
Thin serous membrane consisting of single layer of squamous epithelium (mesothelium) and 2 discrete continuous layers (parietal and visceral peritoneum)
- Parietal peritoneum lines inner aspect of abdominal wall
- Shares somatic nervous supply with adjacent abdominal wall and is pain sensitive (with sharp localization)
- Visceral peritoneum (serosa) lines and covers abdominal organs
- Shares nervous supply with viscera and is therefore sensitive to pain due to stretching of bowel or mesentery (with poor localization)
- Pain due to stretching or irritation of visceral peritoneum may be referred to skin dermatomes
Divisions of Peritoneal Cavity
Greater sacof peritoneal cavity: Largest portion of peritoneal cavity ("proper" peritoneal cavity), which is outside lesser sac
Lesser****sac (a.k.a. omental bursa)
- Communicates with greater sac via epiploic foramen (ofWinslow, site of most common type of lesser sac hernia)
- Bounded: Anteriorly: Caudate lobe, stomach, and greater omentum; posteriorly: Pancreas, left adrenal, and kidney; left: Splenorenal and gastrosplenic ligaments; right: Epiploic foramen and lesser omentum
Mesentery
Double layer of visceral peritoneum that encloses organ and connects it to abdominal wall
- Covered on both sides by mesothelium and has core of loose connective tissue and fat, which encloses lymph nodes, blood vessels, and nerves passing to and from viscera
Most mobile parts of intestine have mesentery, while ascending and descending colon are considered retroperitoneal (covered only by peritoneum on their anterior surface)
Root of mesentery is its attached border with posterior abdominal wall
- Root of small bowel mesentery is ~ 15 cm in length and passes from left side of L2 vertebra downward and to right
- Contains superior mesenteric artery and vein, nerves, and lymphatics
Transverse mesocoloncrosses almost horizontally in front of pancreas, duodenum, and right kidney
Omentum
Multilayered fold of peritoneum that extends from stomach and proximal duodenum to adjacent organs
Lesser omentum joins lesser curve of stomach and proximal duodenum to liver
- Consists of 2 components: Hepatogastric and hepatoduodenalligaments
- Hepatoduodenal ligament contains common bile duct, hepatic artery, andportal vein
- Hepatogastric ligament contains right and left gastric arteries
Greater omentumis 4-layered fold of peritoneum hanging from greater curve of stomach, similar to apron, covering transverse colon and much of small intestine
- Contains variable amounts of fat and abundant lymph nodes
- Mobile and can fill gaps between viscera as well as act as barrier to generalized spread of intraperitoneal infection or tumor ("abdominal policeman")
Peritoneal Ligaments
All double-layered folds of peritoneum other than mesentery and omentum are referred to as peritoneal ligaments
Connect one viscus to another (e.g., splenorenal ligament) or viscus to abdominal wall (e.g., falciform ligament)
Contain blood vessels or remnants of fetal vessels
Peritoneal Folds
Reflections of peritoneum with defined borders, often lifting peritoneum off abdominal wall (e.g., median umbilical foldcovers urachus and extends from dome of urinary bladder to umbilicus)
Peritoneal Recesses
Dependent pouches formed by reflections of peritoneum
Because of clinical relevance, often have eponyms [e.g., Morison pouch for posterior subhepatic(hepatorenal)recess; pouch of Douglas for rectouterine recess]
ANATOMY IMAGING ISSUES
Imaging Pitfalls
Peritoneal cavity and its various mesenteries and recesses are usually not apparent on imaging studies unless distended or outlined by intraperitoneal fluid or air
Internal herniation of abdominal viscera within confines of peritoneal cavity may be difficult to diagnose
CLINICAL IMPLICATIONS
Clinical Importance
Peritoneum that is evident on imaging is thickened due to inflammation or tumor
- Nodular thickening is usually due to malignancy, whereas smooth, regular thickening of peritoneum can be seen either as result of infectious/inflammatory peritonitis or early tumoral involvement
Peritoneal recesses are common sites for accumulation of peritoneal fluid (ascites), pus, or peritoneal tumor implants
Recesses all potentially communicate with each other but become functionally isolated by processes that cause adherence between layers of peritoneum (e.g., abscess)
- Phrenicocolic ligament limits spread of fluid from left subphrenic space to left paracolic gutter
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Images
Lateral View of Mesenteries and Peritoneal Cavity
Sagittal graphic of the abdomen shows the peritoneal cavity artificially distended with air. Note the margins of the lesser sac in this plane, including the caudate lobe of the liver superiorly, stomach and gastrocolic ligament anteriorly, and pancreas posteriorly. The hepatogastric ligament is part of the lesser omentum and carries the hepatic artery and portal vein to the liver. The mesenteries are multilayered folds of peritoneum that enclose a layer of fat and convey blood vessels, nerves, and lymphatics to the intraperitoneal abdominal viscera. The greater omentum is a 4-layered fold of peritoneum that extends down from the stomach, covering much of the colon and small bowel. The layers are generally fused together caudal to the transverse colon. The gastrocolic ligament is part of the greater omentum.
Lesser Sac and Peritoneal Recesses
The lesser sac (omental bursa) is bordered anteriorly to the right by the lesser omentum, which conveys the common bile duct, hepatic artery, portal vein, and gastric vessels. The left borders include the gastrosplenic ligament (with short gastric vessels) and the splenorenal ligament (with splenic vessels).
The paracolic gutters are formed by reflections of peritoneum covering the ascending and descending colon and the lateral abdominal wall. Note the innumerable potential peritoneal recesses lying between the bowel loops and their mesenteric leaves.
Omentum and Peritoneal Reflections
The liver has been retracted upward in this graphic, revealing the lesser omentum. The lesser omentum, which is composed of the hepatoduodenal and hepatogastric ligaments, forms part of the anterior wall of the lesser sac and conveys the common bile duct, hepatic artery, portal vein, and gastric arteries. The aorta and celiac artery can be seen through the lesser omentum, as they lie just posterior to the lesser sac. The greater omentum is seen on this graphic as a large fatty apron extending from the stomach downward along the anterior abdomen.
Frontal graphic shows the abdomen with all of the intraperitoneal organs removed. The root of the transverse mesocolon divides the peritoneal cavity into supramesocolic and inframesocolic spaces that communicate only along the paracolic gutters. The coronary and triangular ligaments suspend the liver from the diaphragm. The superior mesenteric vessels traverse the small bowel mesentery whose root crosses obliquely from the upper left to the lower right posterior abdominal wall.
Peritoneal Spaces and Reflections
First of 4 axial CECT images of a middle-aged man with cirrhosis with delineation of the peritoneal spaces and reflections is shown. Ascites distends the peritoneal cavity in these images, allowing visualization of recesses and peritoneal reflections, which are not normally seen.
The lesser and greater sacs of the peritoneal cavity are distended with ascites. The gastrosplenic ligament and pancreas border the lesser sac, as does the lesser omentum, whose position is marked by the portal vein and celiac trunk.
The falciform ligament suspends the liver from the anterior abdominal wall. The greater omentum lies between the bowel and the anterior abdominal wall.
The intraperitoneal organs, such as the liver, transverse colon, and small bowel, are suspended within the ascites, while the position of the retroperitoneal organs, such as the kidneys and pancreas, is unaffected by the ascites.
Peritoneal Spaces and Mesenteries
First of 4 axial CECT images shows a middle-aged man with cirrhosis and ascites delineating the peritoneal spaces and mesenteries. The liver is suspended from the anterior abdominal wall by the falciform ligament and from the diaphragm by the coronary ligament, between the leaves of which lies the bare area of the liver. Notice that there is no ascites contacting the liver at this bare area of the liver.
The lesser omentum and gastrosplenic ligament compose 2 of the walls of the lesser sac. The lesser omentum is marked by the portal vein and hepatic artery in this image.
The mesenteries are easily identified by their internal fat content and blood vessels. Retroperitoneal organs, such as the pancreas and kidneys, remain in normal position surrounded by retroperitoneal fat that conveys their blood supply.
Note the retroperitoneal position of the duodenum. The 3rd portion of the duodenum crosses behind the superior mesenteric vessels, which supply the small bowel. The branches of the mesenteric vessels lie within the leaves of the mesentery, surrounded by fat.
Distended Peritoneal Cavity
First of 6 axial CECT images of an older man with renal failure treated with peritoneal dialysis is shown. Contrast medium was added to the dialysate to identify potential sites of loculated fluid and accounts for the dense appearance of the fluid. Note the intraperitoneal fluid collecting in the subphrenic spaces.
Note how the diaphragm suspends the liver and spleen away from the chest wall. The bare area of the liver is in direct contact with the diaphragm but not with the peritoneal cavity, and, accordingly, lavage fluid is not in contact with the bare area.
Fluid invaginates into the fissure for the falciform ligament. Note that there is no fluid within the lesser sac. As a general rule, unless the ascites is tense or of a "local" source (such as a perforated gastric ulcer or pancreatitis), it remains confined to the greater peritoneal cavity and does not pass through the epiploic foramen. The greater omentum "floats" on top of the ascites and is normally of fat density with small internal vessels.
The peritoneal fluid is mostly confined to the paracolic gutters at this level.
The peritoneal fluid is somewhat loculated in this patient, typical in the setting of chronic peritoneal dialysis, which results in inflammation and scarring of the peritoneum over time. Note the transplanted kidney in the left iliac fossa, which had stopped functioning due to rejection.
The most dependent recess of the peritoneal cavity is in the pelvis, which is distended by dialysis fluid in this patient.
Peritoneal Mesenteries
Axial CECT in a patient with ascites due to cirrhosis nicely demonstrates the small bowel mesentery. The leaves of the small bowel mesentery are separated and accentuated by the ascites. Each leaf of the mesentery carries blood vessels, nerves, and lymphatics to a bowel segment.
Coronal CECT demonstrates the normal appearance of the mesentery. This image nicely demonstrates the small vessels carried by the mesentery to the small bowel.
Axial CECT demonstrates the sigmoid colon and its mesentery well defined by the adjacent ascites.
Peritoneal Recesses (Morison and Douglas)
The most dependent peritoneal recess in the upper abdomen is the hepatorenal recess, also known as the posterior subhepatic space and Morison pouch. It communicates superiorly with the right subphrenic space and inferiorly with the right paracolic gutter.
The pouch of Douglas, also known as the rectouterine recess, is the most dependent recess of the entire peritoneal cavity in either the upright or supine position and is a common site for inflammatory, neoplastic, or traumatic fluid collections.
Lesser Sac (Omental Bursa)
First of 3 axial CECT images focusing on the lesser sac is shown. The gastrosplenic ligament connects the stomach to the spleen and carries the short gastric vessels. Abdominal "ligaments" are double-layered folds of peritoneum that connect one viscus to another. They contain fat and transmit vessels, nerves, and lymphatics between the retroperitoneum and the abdominal viscera.
The gastrosplenic and splenorenal ligaments form the left anterior and posterior walls of the lesser sac, respectively.
Note the structures abutting the lesser sac, including the stomach anteriorly and the pancreas posteriorly.
Umbilical Ligaments, Delineated by Ascites
First of 3 axial CECT images focuses on the umbilical ligaments. Ascites outlines the sigmoid mesocolon and small bowel loops in this image.
The umbilical ligaments are outlined by ascites in this more caudal image. These are the remnants of the fetal umbilical arteries that had connected the internal iliac arteries to the umbilical cord. The peritoneal reflections covering these ligaments are the lateral umbilical folds.
The ascites is not loculated in this patient, but normal structures, such as the umbilical ligaments and the wall of the urinary bladder, may be mistaken for septations within the fluid collection.
Umbilical Folds (Ligaments)
Sagittal T1 C+ FS MR of the pelvis shows a linear structure extending from the umbilicus to the dome of the urinary bladder, representing the urachus (median umbilical ligament). This is the fibrous remnant of the allantois and should be completely obliterated after birth. In some individuals, parts of the tract may remain patent leading to a urachal diverticulum or a urachal cyst, as in this patient.
The median and lateral umbilical ligaments are evident in this patient on this axial T1 C+ FS MR. Recall that these ligaments are covered with peritoneal reflections called the median and lateral umbilical folds, respectively.
The urachal cyst is evident within the median umbilical fold. The enhancement around the cyst indicates inflammation (infection) of the cyst, which ultimately brought the patient to clinical attention.
Peritonitis With Loculated Ascites
Axial T1 C+ FS MR in a patient with perforated appendicitis demonstrates findings of peritonitis with extensive thickening and enhancement of the peritoneum with large loculated ascites and fluid collections. The peritoneal lining should not be visible in normal patients and is typically only visible when thickened by infection, inflammation, or tumor.
Axial CECT in a septic patient after surgery demonstrates prominent peritoneal enhancement and thickening with an associated loculated fluid collection along the margin of the liver.
Axial CECT in the same patient demonstrates multiple loculated intraperitoneal fluid collections throughout the abdomen and pelvis, including 2 large collections extending down the paracolic gutters. The peritoneum adjacent to the these collections is thickened and enhancing. This constellation of findings is typical for peritonitis with abscess formation.
Peritoneal Reflections
Sagittal T2 MR shows the peritoneal reflection. In men, the apex of the seminal vesicle serves as a landmark to identify the peritoneal reflection. The identification of rectal cancer in relation to the peritoneal reflection is important, as the tumor is staged as T4a when it involves the peritoneal reflection.
Sagittal T2 MR shows the peritoneal reflection. In women, the equivalent to the seminal vesicles is the cervix.
Fibrosing Peritonitis Due to Peritoneal Dialysis
Axial CECT in a patient undergoing chronic peritoneal dialysis demonstrates large ascites with extensive calcification and thickening of the peritoneal lining. The small bowel is clumped together in a "cocoon" of thickened visceral peritoneum (serosa) that compresses the bowel loops together. This is a classic example of fibrosing peritonitis, most often seen in the setting of chronic peritoneal dialysis.
Axial CECT in the same patient demonstrates more extensive calcification surrounding the small bowel. The small bowel is clustered in the central abdomen, often described as an abdominal "cocoon" in this disorder.
Abdominal radiograph taken 2 hours after ingestion of barium shows slow transit of the barium and dilated small bowel. The small bowel loops are also crowded together and fixed in position instead of being freely mobile on their mesentery, as is normal. This is an example of severe fibrosing peritonitis, a rare complication of peritoneal dialysis.
Peritoneal Carcinomatosis
First of 2 axial CECT images of a middle-aged woman with ovarian carcinoma is shown. Subtle soft tissue density nodules are present in the omental fat overlying the colon and small bowel. These are characteristic of peritoneal tumor deposits (i.e., peritoneal carcinomatosis).
The nodular tumor deposits in the omentum are more evident on this axial CECT. Peritoneal spread of tumor is often, but not always, accompanied by malignant ascites, which is absent in this case.
Axial CECT in a patient with peritoneal carcinomatosis demonstrates nodular soft tissue density tumor implants present within the lesser sac and along the parietal peritoneum. A mass of tumor is noted in the omentum, a so-called omental cake. Note the presence of loculated fluid within the lesser sac. Loculated ascites is most often caused by adhesions (usually from prior surgery), peritonitis, or peritoneal carcinomatosis.
Peritoneal Carcinomatosis
Axial CECT in a patient with colon cancer demonstrates the classic findings of peritoneal carcinomatosis, including thickening and enhancement of the peritoneum, a "pleated" thickened mesentery due to tumor infiltration, and frank omental caking with tumor.
Axial T2 FS MR demonstrates extensive tumor cake throughout the omentum, which in this case is relatively T2 isointense to the bowel. Note the presence of adjacent ascites, which should always prompt a careful search for carcinomatosis in any cancer patient.
Axial T1 C+ FS MR in the same patient demonstrates enhancing tumor throughout the omentum, as well as thickening of the mesentery, in this patient with extensive carcinomatosis.
Peritoneal Disease
MR of the left upper quadrant region in this patient with laparoscopic distal pancreatectomy and splenectomy for pancreatic tail ductal adenocarcinoma shows an omental infarct as an area of heterogeneity on T2 with absence of enhancement on T1 C+ imaging. The laparoscopic approach to entering the lesser sac during surgery involves dividing the anterior leaf of the greater omentum to avoid thermal injury to the colon. This approach results in the division of the short gastric arteries and the removal of only the portion of the omentum that loses its blood supply. In contrast, the classic open technique involves dividing the greater omentum along an avascular margin.
CT example of an omental infarct in a patient with distal pancreatectomy is shown. Note mixed areas of soft tissue and fat attenuation.
Patient from India with tuberculosis peritonitis is shown. The history and the clinical picture are the key to differentiate from carcinomatosis.
Pseudomyxoma Peritonei
Axial CECT demonstrates the classic appearance of pseudomyxoma peritonei in which peritoneal metastases from a mucin-secreting appendiceal tumor result in profuse accumulation of gelatinous material within the peritoneal cavity. The loculations and quantity of the material produce the typical mass effect, or indentations, on abdominal viscera and often result in bowel obstruction.
Axial CECT in the same patient demonstrates the complex, septated appearance of the "ascites," which is actually semisolid gelatinous material. The implants are seen throughout the peritoneal cavity, including implants extending along the surface of the liver, omentum, and paracolic gutters. Note that the kidneys are unaffected due to their retroperitoneal location.
Coronal CECT with volume-rendered reconstruction demonstrates the classic "scalloping" of the liver surface by mucinous implants along with extensive implants elsewhere in the peritoneal cavity in this patient with pseudomyxoma peritonei.
Ross
The lesser sac (omental bursa) is bordered anteriorly to the right by the lesser omentum, which conveys the common bile duct, hepatic artery, portal vein, and gastric vessels. The left borders include the gastrosplenic ligament (with short gastric vessels) and the splenorenal ligament (with splenic vessels).
The liver has been retracted upward in this graphic, revealing the lesser omentum. The lesser omentum, which is composed of the hepatoduodenal and hepatogastric ligaments, forms part of the anterior wall of the lesser sac and conveys the common bile duct, hepatic artery, portal vein, and gastric arteries. The aorta and celiac artery can be seen through the lesser omentum, as they lie just posterior to the lesser sac. The greater omentum is seen on this graphic as a large fatty apron extending from the stomach downward along the anterior abdomen.
Frontal graphic shows the abdomen with all of the intraperitoneal organs removed. The root of the transverse mesocolon divides the peritoneal cavity into supramesocolic and inframesocolic spaces that communicate only along the paracolic gutters. The coronary and triangular ligaments suspend the liver from the diaphragm. The superior mesenteric vessels traverse the small bowel mesentery whose root crosses obliquely from the upper left to the lower right posterior abdominal wall.
First of 4 axial CECT images of a middle-aged man with cirrhosis with delineation of the peritoneal spaces and reflections is shown. Ascites distends the peritoneal cavity in these images, allowing visualization of recesses and peritoneal reflections, which are not normally seen.
The lesser and greater sacs of the peritoneal cavity are distended with ascites. The gastrosplenic ligament and pancreas border the lesser sac, as does the lesser omentum, whose position is marked by the portal vein and celiac trunk.
