EDUCATION EXHIBIT 707

Greater and LesserOmenta: NormalAnatomy and Patho-logic Processes1

ONLINE-ONLYCME

See www.rsna.org/education/rg_cme.html.

LEARNINGOBJECTIVESAfter reading thisarticle and takingthe test, the reader

will be able to:

� Describe the nor-mal anatomy of theomenta as seen onCT scans.

� List the diseasesthat may manifest asdiffuse omental infil-tration at CT.

� Discuss the useful-ness of multidetectorCT with multiplanarreformation in evalu-ation of omental dis-eases.

Eunhye Yoo, MD ● Joo Hee Kim, MD ● Myeong-Jin Kim, MD ● Jeong-SikYu, MD ● Jae-Joon Chung, MD ● Hyung-Sik Yoo, MD ● Ki Whang Kim,MD

The peritoneum is the largest serous membrane in the body and theone with the most complex structure. The omentum is a double-lay-ered extension of the peritoneum that connects the stomach to adja-cent organs. The peritoneal reflections form the greater and lesseromenta, and the natural flow of peritoneal fluid determines the route ofspread of intraperitoneal fluid and consequently of disease processeswithin the abdominal cavity. The omenta serve both as boundaries fordisease processes and as conduits for disease spread. The omenta arefrequently involved by infectious, inflammatory, neoplastic, vascular,and traumatic processes. Computed tomography (CT) is a primarydiagnostic method for evaluation of omental diseases, most of whichmay manifest with nonspecific clinical features. Multidetector CT withmultiplanar reformation allows accurate examination of the complexanatomy of the peritoneal cavity, knowledge of which is the key to un-derstanding the pathologic processes affecting the greater and lesseromenta.©RSNA, 2007

RadioGraphics 2007; 27:707–720 ● Published online 10.1148/rg.273065085 ● Content Codes:

1From the Department of Diagnostic Radiology (E.Y., J.H.K., M.J.K., J.S.Y., J.J.C., H.S.Y., K.W.K.) and Institute of Gastroenterology (M.J.K.),Yonsei University College of Medicine, Seodaemun-ku, Shinchon-dong 134, Seoul 120-752, Republic of Korea. Presented as an education exhibit atthe 2005 RSNA Annual Meeting. Received May 2, 2006; revision requested July 24 and received September 11; accepted September 18. All authorshave no financial relationships to disclose. Address correspondence to J.H.K. (e-mail: pavane@yumc.yonsei.ac.kr).

©RSNA, 2007

Note: This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, use the RadioGraphics Reprints form at the end of this article.

See last page

TEACHING POINTS

IntroductionComputed tomography (CT) is a major diagnos-tic tool for evaluating omental lesions, especiallythose that may appear with nonspecific clinicalmanifestations. The greater and lesser omenta areanatomically complicated areas to fully assess atCT. As such, omental pathologic conditions mayappear as various and nonspecific findings, rang-ing from a fluid collection to diffuse omental infil-tration. The omenta serve not only as boundariesfor certain disease processes but also as conduitsfor disease spread. Thus, the omenta can be affec-ted by a variety of diseases, including infection,inflammation, neoplasms, trauma, and infarction.High-resolution multidetector CT with multipla-

nar reformation improves demonstration of theomental anatomy and detection of omental patho-logic conditions. Knowledge of the omental anat-omy, the disease spectrum involving the greaterand lesser omenta, and the disease-specific CTfindings is essential for proper diagnosis andtreatment.

Figure 1. Drawing of the anatomy of the greater and lesser omenta (a) and axial (b), coro-nal (c), and sagittal (d) diagrams of the upper abdomen. The greater omentum (GO)is composed of a double layer of peritoneum that extends from the greater curvature of thestomach (S) inferiorly. Its descending and ascending portions usually fuse to form a four-layer vascular fatty apron; the resulting space is contiguous with the lesser sac (LS). Thelesser omentum (LO) connects the lesser curvature of the stomach and proximal duodenumwith the liver (L) and contains blood vessels, nerves, and lymph nodes. The lesser sac isempty and collapsed so that only parts of its boundaries, such as the posterior gastric walland pancreatic body, are observed on axial CT scans. Ao � aorta, C � colon, K � kidney,P � pancreas, Sp � spleen, 1 � falciform ligament, 2 � gastrohepatic ligament, 3 � gastro-splenic ligament.

708 May-June 2007 RG f Volume 27 ● Number 3

In this article, we review the normal omentalanatomy, common disease processes of theomenta, and their characteristic CT features. Wealso discuss the role of multidetector CT withmultiplanar reformation in evaluation of omentaldisease.

CT Protocol andReformation Technique

Most omental pathologic conditions manifest asnonspecific symptoms and signs. As such, diagno-sis is based on CT findings, which are very impor-tant in patient treatment. CT is the optimal imag-ing technique for demonstrating the presence ofomental disease and its cause. Furthermore, coro-nal and sagittal reformatted CT images help de-lineate the exact location, origin, or spread pat-tern of omental disease, as well as clarify the com-plex anatomy of the omenta.

CT examinations were performed by using a16-section CT scanner (Somatom Sensation 16;Siemens Medical Solutions, Erlangen, Germany).Contrast-enhanced CT images with or withoutnonenhanced images were obtained with the fol-lowing parameters: 0.5-second rotation time, 0.75-mm collimation, 3-mm section thickness, 35-cmfield of view, 3-mm reconstruction thickness, 12-mm feed per rotation, 120 kV, and 140 mA. Two-dimensional axial reconstruction images with3-mm section thickness are routinely obtained atour institution by using the standard software ofthe scanner (Somaris/5; Siemens Medical Solu-tions). On a case-by-case basis, additional sagit-tal, coronal, or oblique multiplanar reformattedimages can be obtained by using personal compu-ter–based software (Advantage workstation; GEHealthcare, Munich, Germany).

Normal AnatomyThe greater omentum is composed of a doublelayer of peritoneum that hangs down like anapron from the greater curvature of the stomachand the proximal part of the duodenum, coveringthe small bowel. Its descending and ascendingportions fuse to form a four-layer vascular fattyapron (the gastrocolic ligament), with a spacecontiguous with the lesser sac (Fig 1) (1,2). Thegreater omentum has considerable mobility andmoves around the peritoneal cavity. It functionsas a visceral fixation and serves to shield an ab-

normality and limit its spread (1,2). However, itis also a common location for neoplastic intraperi-toneal seeding and infectious processes because itis bathed in the peritoneal fluid.

The greater omentum is composed mainly offatty tissue, with some thin serpentine gastroepi-ploic vessels. At CT, it appears as a band of fattytissue with a variable width, just beneath the ante-rior abdominal wall and anterior to the stomach,transverse colon, and small bowel. Ascites be-tween the greater omentum and the adjacent softtissues makes the omentum appear as a simplefatty layer, and soft-tissue deposits in the omen-tum can create an amorphous hazy stranding or anodular or masslike appearance at CT (3).

The lesser omentum, which is a combinationof the gastrohepatic and hepatoduodenal liga-ments, connects the lesser curvature of the stom-ach and proximal duodenum with the liver andcovers the lesser sac anteriorly (Fig 1) (1,4–6).The gastrohepatic ligament contains the left gas-tric vessels and left gastric lymph nodes. Thehepatoduodenal ligament, the thickened edge ofthe lesser omentum, contains the portal vein, he-patic artery, extrahepatic bile duct, and hepaticnodal group.

As a result of rotation and growth of the stom-ach during fetal development, the lesser sac is aunique peritoneal space that extends behind thestomach, anterior to the pancreas. The superiorrecess of the lesser sac surrounds the caudate lobeof the liver, and it communicates with the perito-neal cavity through the epiploic foramen, com-monly called the foramen of Winslow (Fig 1)(7,8). As the stomach rotates and the greateromentum elongates, the lesser sac also expandsand acquires an inferior recess between the layersof the greater omentum. Later, the inferior recessalmost disappears as the layers of the greateromentum fuse (1). The gastrohepatic ligament isidentified at CT as a triangular fat-containingarea between the stomach and liver. The lessersac is collapsed at normal times, so only parts ofits boundaries, such as the posterior gastric walland pancreatic body, are observed on CT scans.

RG f Volume 27 ● Number 3 Yoo et al 709

TeachingPoint

TeachingPoint

Abnormalities Involv-ing the Greater Omentum

The CT appearances of abnormalities involvingthe greater omentum are as follows: (a) multifo-cal, ill-defined infiltrative lesions, including peri-toneal carcinomatosis, tuberculous peritonitis,malignant peritoneal mesothelioma, pseudo-myxoma peritonei, lymphomatosis, and theconditions of cirrhosis and portal hypertension;(b) solid or cystic mass-forming lesions includingprimary and secondary neoplasms and infectiousprocesses; and (c) miscellaneous conditions in-cluding omental infarction, foreign-body granu-loma, hematoma, and hernia.

Multifocal, Ill-defined, Infiltrating LesionsWhen there is diffuse infiltration of the perito-neum, omentum, or mesentery at CT, a variety ofconditions—including infiltrative edema fromliver cirrhosis, diffuse peritoneal tumors, and in-fectious peritonitis—should be considered. Dis-tinguishing between diffuse peritoneal tumors,such as peritoneal carcinomatosis, malignant me-sothelioma, or lymphomatosis, and tuberculousperitonitis is difficult because of nonspecificsymptoms and overlapping imaging features. TheCT patterns of omental abnormalities, such asfatty stranding, nodular infiltration, large masses,or omental caking, are not significantly differentin these diseases (9).

Liver cirrhosis with portal hypertension is oneof the most common causes of diffuse omentalinfiltrative lesions. Patients with cirrhosis fre-quently present with mesenteric, omental, or ret-roperitoneal edema identifiable at CT. The radio-logic features of omental edema vary from a mildinfiltrative haze to the presence of masslike lesionswith discrete margins and are similar to those ofother omental pathologic conditions (10).

Metastatic peritoneal tumors most often origi-nate from the ovary, stomach, pancreas, colon,uterus, and bladder. Hematogenous metastasesfrom malignant melanoma, as well as breast and

Figure 2. Peritoneal carcinomatosis in a 22-year-old man with epigastric pain. Axial (a) and coronal (b) CTscans show large amounts of ascites, diffuse nodular omental infiltration (omental cake) (arrows), and abnor-mal gastric wall thickening (arrowhead in b), findings compatible with carcinomatosis from the stomach.

Figure 3. Peritoneal carcinomatosis in a 30-year-oldwoman with malignant melanoma. CT scan shows he-matogenous dissemination of malignant nodules in theperitoneal space including the omentum (arrows), ret-roperitoneal spaces, and the subcutaneous fat layer ofthe abdomen.

710 May-June 2007 RG f Volume 27 ● Number 3

TeachingPoint

TeachingPoint

lung carcinoma, are also common. Patients withperitoneal carcinomatosis may demonstrate as-cites, peritoneal thickening, seeding nodules, andomental infiltration (Figs 2, 3) (3,11). However,these findings are not specific for peritoneal carci-nomatosis and can be seen with other entities thatseed the peritoneum, including mesothelioma,tuberculosis, and lymphomatosis. Therefore, theradiologist should make an effort to look for aprimary tumor, especially in the gastrointestinaland genitourinary tracts. Although omental cak-ing is commonly seen in patients with peritonealcarcinomatosis, it is not diagnostic for this dis-ease. Irregular thickening of the outer contour ofthe infiltrated omentum favors the diagnosis ofperitoneal carcinomatosis (11).

Tuberculous peritonitis is caused by hematog-enous spread of pulmonary tuberculosis or byrupture of a mesenteric node. CT findings favor-ing a diagnosis of tuberculous peritonitis overother disease processes are as follows: smoothperitoneum with minimal thickening and pro-nounced enhancement, mesenteric involvementwith macronodules (5 mm in diameter), a thinomental line (fibrous wall covering the infiltratedomentum), mesenteric adenopathy with low-at-tenuation centers (caseous necrosis), and calcifi-cations (Fig 4) (11–13). The fibrotic type of tu-berculous peritonitis, although not common, is

characterized by loculated ascites, large omentalmasses, and separation or fixation of bowel loops.

Malignant peritoneal mesothelioma is a rarecondition and accounts for 12%–33% of all me-sotheliomas. Malignant peritoneal mesotheliomamay have a variable appearance at CT. It is com-monly associated with ascites, irregular or nodu-lar peritoneal thickening, a “stellate” pattern ofthe mesentery, bowel wall thickening, and omen-tal involvement ranging from finely infiltrated fatwith a “smudged” appearance to discrete omentalnodules or omental caking (Fig 5) (13). It some-times manifests as a large quantifiable mass in theupper abdomen with minimal ascites and discretenodules scattered over the peritoneum.

Pseudomyxoma peritonei is characterized bythe gradual accumulation of large volumes of mu-cinous ascites, which arise from a ruptured be-nign or malignant mucin-producing tumor of theappendix, ovary, pancreas, stomach, colorectum,or urachus. At CT, pseudomyxoma peritonei ap-pears as a low-attenuation, frequently loculatedfluid collection in the peritoneal cavity, omentum,and mesentery. Scalloping of visceral surfaces,especially the liver, is the diagnostic characteristicthat distinguishes mucinous from serous ascites atCT (Fig 6) (14). Curvilinear or punctate calcifi-cations in the mucinous materials are frequentlyidentified (15).

Figure 4. Tuberculous peritonitis in a 38-year-oldwoman with abdominal distention for 1 week. CT scanshows a large amount of ascites with even peritonealthickening (arrowhead) and diffuse omental infiltration(arrow) without associated lymphadenopathy. The ini-tial impression was carcinomatosis. When the primarymalignancy is unclear, the differential diagnosis shouldinclude tuberculous peritonitis, particularly in endemicareas. The final diagnosis was tuberculous peritonitis.

Figure 5. Malignant peritoneal mesothelioma in a47-year-old man with dyspnea for 1 month. CT scanshows a diffuse, platelike mass in the greater omentum(arrows), massive ascites, and peritoneal thickening.Malignant mesothelioma was confirmed with pleuralbiopsy and cytologic analysis of peritoneal fluid.

RG f Volume 27 ● Number 3 Yoo et al 711

Peritoneal lymphomatosis is curable withoutsurgery, unlike other peritoneal malignant dis-eases. Diagnosis of peritoneal lymphomatosiswith CT is difficult because it closely mimics peri-toneal carcinomatosis and tuberculous peritonitis.However, ascites without any loculation or septa-tions and a diffuse distribution of enlarged lymphnodes are promising prognosticators (16). Retro-peritoneal and mesenteric lymphadenopathy isdemonstrable, and the enlarged lymph nodes ap-pear as homogeneous attenuation or central lowattenuation with peripheral rim enhancement(Fig 7). CT findings of omental involvement in-clude omental “smudging” and omental cakingrather than a discrete nodular pattern.

Solid or Cystic Mass-forming LesionsSecondary neoplasms involving the greater omen-tum are far more common than primary tumors.Many neoplasms have been shown to involve thegreater omentum by direct spread, peritonealseeding, or hematologic spread (Fig 8). Meta-static peritoneal tumors most often originate fromcarcinomas of the ovary, stomach, pancreas, andcolon (Fig 9) (9).

Primary neoplasms of the omentum are un-common and include mesotheliomas, hemangio-pericytomas, stromal tumors, leiomyomas, lipo-mas, neurofibromas, fibromas, leiomyosarcomas,liposarcomas, and fibrosarcomas. Imaging find-ings of primary omental tumors are nonspecific.Benign tumors are usually well circumscribed and

localized in the omentum. Malignant tumors fre-quently have indistinct margins and invade intosurrounding structures. Both malignant and be-nign tumors can appear complex, with cystic andsolid elements. Some cystic lesions may involvethe greater omentum, including cystic lym-phangioma, enteric duplication cyst, enteric cyst,mesothelial cyst, and nonpancreatic pseudocyst(17). Abdominal lymphangioma is characterized

Figure 6. Pseudomyxoma peritonei in a 47-year-old man with abdominal discomfort. Axial (a) and coro-nal (b) CT scans show multiple low-attenuation nodules and masses in the omentum and peritoneal cavity.Curvilinear or punctate calcifications of the seeding nodules; scalloping of the liver, spleen, and stomach; andsmall bowel adhesions from mesenteric infiltration are noted. Pseudomyxoma peritonei was proved with peri-toneoscopic biopsy.

Figure 7. Peritoneal lymphomatosis in a 71-year-oldman with abdominal distention for 15 days. CT scanshows ascites in the pelvic cavity and innumerableseeding nodules in the peritoneal cavity and omentum(white arrow). Multiple enlarged lymph nodes withconglomeration (black arrows) are seen in the retro-peritoneal spaces. Endoscopic gastric biopsy showedB-cell lymphoma, thus confirming the diagnosis of lym-phomatosis.

712 May-June 2007 RG f Volume 27 ● Number 3

by a uni- or multiloculated fluid-filled mass with athin wall and occasionally with septa (Fig 10)(17,18).

Unusual infections such as actinomycosis orparagonimiasis may manifest as solid or cysticmass lesions in the greater omentum. Actinomy-cosis has a worldwide distribution in urban andrural areas and commonly involves the cervico-facial, thoracic, and abdominopelvic regions. Ithas an infiltrative nature and a tendency to invadenormal anatomic barriers. Abdominal actinomy-cosis manifests as a solid mass with focal areas ofdecreased attenuation or a mostly cystic masswith irregularly thickened, heterogeneously en-hanced walls on CT scans (19,20). Neoplasms

and other inflammatory diseases, especially tuber-culosis, manifest in a similar manner and may beconfused with actinomycosis.

The primary site of a paragonimiasis infectionis the lung, but other organs may be involved.Common CT features of abdominal paragonimi-asis include multiple, densely calcified, small nod-ules scattered in the peritoneal cavity (Fig 11)(20). Omental involvement with paragonimiasismay not be significant clinically, but knowledge ofthis imaging feature is important in establishingan early diagnosis and avoiding unnecessary sur-gery.

Figure 8. Secondary neoplasm of the greater omen-tum in an 80-year-old woman with dyspepsia. CoronalCT scan shows a large lobulated mass (arrows) in theleft upper quadrant of the abdomen. The mass repre-sents an exophytic tumor from the greater curvature ofthe stomach, a finding suggestive of an exophytic gas-tric carcinoma.

Figure 9. Metastatic peritoneal tumor in a 73-year-old woman with a palpable abdominal mass for 2months. CT scan shows a large, lobulated, heteroge-neous mass in the midabdomen, inferolateral to thestomach. Thickened peritoneum (arrow) adjacent tothe mass is suggestive of a malignant lesion. Metastaticcarcinoma was confirmed at surgical excision. The pa-tient had a history of ovarian carcinoma.

Figure 10. Abdominal lymphangioma in a 38-year-old woman with a gastric ulcer, which was an incidental find-ing at CT. Axial unenhanced (a) and coronal contrast-enhanced (b) CT scans show a lobulated cystic mass in thegreater omentum inferior to the gastric antrum. The mass is most likely a cystic lymphangioma.

RG f Volume 27 ● Number 3 Yoo et al 713

Miscellaneous LesionsSegmental omental infarctions are rare and cancause acute abdomen. Primary torsion has noknown cause. Secondary torsion is more com-mon, and the causes include a hernia, a focus ofinflammation, previous laparotomy, or a tumor.Preoperative diagnosis is difficult, as right omen-tal involvement is much more common and clini-

cal signs and symptoms are usually nonspecific;they may thus mimic acute appendicitis or chole-cystitis. CT findings range from subtle, focal,hazy soft-tissue infiltration of the omentum to amore extensive, masslike fullness that can re-semble pathologic infiltration from more ominouscauses. Whirling fatty tissue around a vascularstructure may be a specific finding for omentaltorsion (Fig 12) (21,22). The greater omentum is

Figure 11. Abdominal paragonimiasis in a 49-year-old man with hepatic lesions incidentally found during laparo-scopic cholecystectomy. (a) CT scan shows multilocular cystic lesions in the right lobe of the liver. (b) CT scanshows multifocal ill-defined cystic lesions and several nodules (arrow) in the omentum on the right side of the abdo-men. These appearances are suggestive of multilobulate parasitic abscesses in the liver with peritoneal seeding ofparasitic granulomas. Biopsy of the liver and omentum demonstrated paragonimiasis.

Figure 12. Omental infarction in a 47-year-old man with abdominal pain. Axial (a) and sagittal (b)CT scans show localized fatty infiltration and congestion with a secondary mass (arrow) in the rightlower aspect of the anterior abdomen. This appearance most likely indicates an omental infarction.

714 May-June 2007 RG f Volume 27 ● Number 3

more frequently traumatized by penetrating inju-ries than by blunt injury. Injury to the omentalvasculature can cause an omental infarct (2).

Foreign-body granuloma from a retained lapa-rotomy sponge may manifest as acute or delayednonspecific symptoms. It is usually an asepticprocess that creates adhesions and a thick capsulearound the sponge (Fig 13). If an exudative re-sponse occurs, it may lead to the complications offistula or abscess formation. The typical spongi-form pattern with gas bubbles seems to be themost characteristic sign for a retained surgicalsponge (23,24).

Ventral hernias are subdivided, on the basis ofsite or cause of herniation, into the following cat-egories: epigastric, umbilical, subumbilical, spige-lian, incisional, and parastomal hernias. In gen-eral, ventral hernias contain properitoneal fat,omentum, vascular structures, and occasionallybowel (25). A subcutaneous hernia sac may beconfused with a lipoma of the abdominal wall.CT scans show the precise anatomic site and con-tent of the hernia sac, as well as the characteristicsof the hernia cuff and surrounding wall (Fig 14).CT is essential to confirm the clinical diagnosisand to identify any potential complications.

Bochdalek hernias (posterolateral hernias) arethe most common congenital diaphragmatic her-nia and frequently occur on the left side (26).Herniated organs may include the omental fat,intestine, stomach, spleen, and left lobe of theliver. Because of pulmonary hypoplasia, thesepatients are usually symptomatic at birth. A fewreported cases have identified patients who wereasymptomatic until adulthood. Morgagni hernias(retrosternal hernias) are a rare type of diaphrag-matic hernia and usually lie on the right side,slightly posterior to the xiphoid process (Fig 15).Morgagni hernias in children are usually asymp-tomatic and found incidentally (26). Traumaticdiaphragmatic hernias usually result from blunttrauma (traffic accident or fall) as well as frompenetrating injuries or iatrogenic causes (26). Theleft diaphragm is more commonly involved. Iatro-genic diaphragmatic hernias usually develop fromthoracoabdominal surgery, such as esophagogas-tric surgery for esophageal cancer. Hernia orifices,retained organs, and concomitant complicationsare clearly visualized in most cases, especially atmultidetector CT with sagittal and coronal refor-mation.

Figure 13. Foreign-body granuloma in a 39-year-oldwoman with a palpable mass for 10 years and abdomi-nal pain for 1 week. Unenhanced CT scan shows alarge, well-circumscribed mass with dense calcificationin the anterior midabdomen, an appearance suggestiveof a foreign-body granuloma or organizing hematoma.After injection of contrast material, the mass showedno enhancement. A foreign-body granuloma with sur-gical gauze was found at surgical excision. The patienthad a history of cesarean section 10 years earlier.

Figure 14. Ventral hernia in a 66-year-old womanwith a palpable mass in the abdomen. Sagittal CT scanshows herniation of omental fat through a defect (ar-row) in the anterior abdominal wall. Focal ill-definedlesions with increased attenuation (arrowheads) in theomental fat adjacent to the abdominal wall defect aresuggestive of omental fat infarction secondary to vascu-lar compromise.

RG f Volume 27 ● Number 3 Yoo et al 715

Abnormalities Involving theLesser Omentum and Lesser Sac

CT abnormalities involving the lesser omentumand lesser sac include the following: (a) fluid col-lections of ascitic transudate, inflammatory exu-date, bile, or blood; (b) solid or cystic mass lesionsincluding inflammatory processes and primary orsecondary neoplasms; and (c) internal hernias.

Fluid Collection in the Lesser SacUnder normal circumstances, the lesser sac isempty and collapsed. Only certain parts of itsboundaries, such as the posterior gastric wall andpancreatic body, are observed at CT. Fluid col-lections in the lesser sac include ascites, exudate,bile, and blood (4,27).

The most common type of fluid in the lessersac is ascitic transudate in patients with hepaticfailure or renal failure. However, ascites in onlythe lesser sac is unusual. Large amounts of as-cites in the peritoneal cavity flow to the lesser sacthrough the epiploic foramen rather than via the

lesser omentum directly. A fluid collection withinonly the lesser sac should be considered postop-erative fluid after gastric or hepatobiliary surgeryor an inflammatory exudate from pancreatitis,cholecystitis, or gastric perforation (Fig 16) (8).

Inflammatory infiltrates in the lesser sac arecommonly secondary to acute pancreatitis (Fig17). Because the pancreas does not have a well-defined fibrous capsule, the inflammatory processmay spread into the adjacent tissue through a thinlayer of the surrounding connective tissue (28).The inflammatory fluid initially accumulates inthe lesser sac. A perforated gastric ulcer, a leftperinephric abscess, or rarely an ascending pelvicinflammation (eg, appendicitis or diverticulitis)may cause exudate in the lesser sac (7). Bile col-lection in the lesser sac is caused by bile duct sur-gery or a penetrating abdominal wound with tran-section of the common bile duct. Causes of alesser sac hematoma include traumatic injury ofthe liver and spleen, hemorrhagic pancreatitis, orbleeding from neoplasms such as a hepatocellularcarcinoma (Fig 18).

Figure 15. Morgagni hernia in a 70-year-old woman with an abnormality at chest radi-ography. Axial (a), coronal (b), and sagit-tal (c) CT scans show focal upward displace-ment of the transverse colon and omental fat(arrows in b and c) in the right anterior car-diophrenic area, an appearance suggestive of aMorgagni hernia.

716 May-June 2007 RG f Volume 27 ● Number 3

TeachingPoint

Figure 16. Fluid collection in the lesser sac in a 36-year-old man 2 days after subtotal gastrectomy with gas-trojejunostomy for stomach cancer. Axial (a) and coronal (b) CT scans show a collection of meglumine dia-trizoate (Gastrografin; Bracco Diagnostics, Princeton, NJ) in the lesser sac (black arrow). This finding was sug-gestive of leakage (white arrow in b) from the anastomosis of the gastrojejunostomy; such leakage was visual-ized during an upper gastrointestinal study with meglumine diatrizoate 2 days later.

Figure 17. Inflammatory infiltrate in the lesser sac in a 68-year-old man with a history of heavy alcohol usewho had epigastric pain for 2 days. CT scans show infiltration of peripancreatic fat (a) and spread of an inflam-matory exudate to the lesser sac (arrow in b) and retroperitoneal space, findings suggestive of acute pancreati-tis.

Figure 18. Lesser sac hematoma 1 day after ab-dominal blunt trauma in a 40-year-old man withacute abdominal pain. Contrast-enhanced CT scanshows a large acute hematoma in the lesser sac be-tween the stomach and pancreas. Emergent lapa-rotomy with hematoma evacuation and “bleeder”ligation was performed.

RG f Volume 27 ● Number 3 Yoo et al 717

Discrete Mass in theLesser Omentum or Lesser SacSpace-occupying processes in the superior recessof the lesser sac include pancreatic pseudocysts orabscesses, enlarged lymph nodes along the lessercurvature of the stomach, and primary or second-ary neoplasms (4,7). Pancreatic pseudocysts arecharacterized by a unilocular cystic mass with asmooth and thin wall, almost always occurringafter pancreatitis (Fig 19). Pathologic lymph node

enlargement in this region is commonly due togastric or esophageal cancer or tuberculosis (Fig20).

Secondary neoplasms are more common thanprimary neoplasms. Neoplasms invading the les-ser omentum usually originate from adjacentstructures such as the stomach, liver, or pancreas(Fig 21). Primary neoplasms of the lesser omen-tum are rare and include benign tumors (lymphan-gioma, neurogenic tumor, teratoma) and malig-nant neoplasms (liposarcoma, malignant gastro-

Figure 19. Pancreatic pseudocysts in a 31-year-old man who had acute pancreatitis 3 weeks earlier. Axial (a) andcoronal (b) CT scans show multiple cystic lesions in the lesser sac (arrow) and left subphrenic space, an appearancesuggestive of pancreatic pseudocysts.

Figure 20. Lymph node metastasis in a 46-year-oldwoman with an increased tumor marker level. CT scanshows a small nodular lesion (arrow) in the lesseromentum. The radiologic impression was metastaticlymphadenopathy. Early gastric cancer was detected atendoscopy, and a perigastric lymph node metastasiswas confirmed at surgery.

Figure 21. Gastrointestinal stromal tumor in a 43-year-old woman with abdominal discomfort for 2months. CT scan shows a well-circumscribed heteroge-neous mass between the left lobe of the liver, stomach,and pancreas. The differential diagnosis included anexophytic hepatic neoplasm, a gastric submucosal tu-mor, and a primary neoplasm of the lesser omentum.At surgery, the patient was found to have a malignantgastrointestinal stromal tumor that originated from thestomach.

718 May-June 2007 RG f Volume 27 ● Number 3

intestinal stromal tumor) (29). Although almostall of these tumors have variable and nonspecificCT features, some manifest with characteristicappearances. For example, lymphangioma ap-pears as a multiloculated low-attenuation masswith a smooth thin wall on CT scans (Fig 22)(18).

Internal Hernia into the Lesser SacLesser sac hernias make up only about 1%–4% ofall internal hernias. The herniated organs includethe small intestine, cecum, proximal colon, trans-verse colon, omentum, and gallbladder. The en-trance of herniation into the lesser sac is usuallythrough the foramen of Winslow, and less com-monly, via the transverse mesocolon or trans-omental (often iatrogenic).

Lesser sac hernias manifest at CT as a clusterof gas-distended or fluid-filled bowel loops lo-cated between the liver, stomach, and pancreas.The stomach is usually displaced anteriorly andlaterally. Bowel caliber change and radiating vas-cular markings in the mesentery of protrudedbowel loops across the gastroduodenal or gastro-colic ligament is helpful in diagnosing lesser sachernias (30).

ConclusionsConditions involving the greater and lesseromenta include infectious, inflammatory, neo-plastic, vascular, and traumatic processes. Diag-nosis of omental pathologic conditions is difficultbecause of their nonspecific and overlapping clini-cal and imaging features. Correlation with the CT

pattern of omental involvement, associated CTfindings in the abdomen, and clinical informationare essential for proper diagnosis and treatment.Some diseases, such as omental fat infarction,omental herniation, or hemorrhage, can be accu-rately diagnosed only on the basis of characteristicCT features. Multidetector CT with coronal andsagittal reformation improves the resolution ofomental anatomy and the detection of omentalpathologic conditions. Knowledge of the omentalanatomy, the disease spectrum involving thegreater and lesser omenta, and the characteristicCT appearances of each disease is essential foraccurate diagnosis and proper treatment.

References1. Moore KL. The developing human: clinically ori-

ented embryology. 3rd ed. Philadelphia, Pa: Saun-ders, 1982; 227–229.

2. Sompayrac SW, Mindelzun RE, Silverman PM,Sze R. The greater omentum. AJR Am J Roentge-nol 1997;168:683–687.

3. Raptopoulos V, Gourtsoyiannis N. Peritoneal car-cinomatosis. Eur Radiol 2001;11:2195–2206.

4. Jeffrey RB, Federle MP, Goodman PC. Computedtomography of the lesser peritoneal sac. Radiology1981;141:117–122.

5. Healy JC, Reznek RH. The peritoneum, mesenter-ies and omenta: normal anatomy and pathologicalprocesses. Eur Radiol 1998;8:886–900.

6. Zhao Z, Liu S, Li Z, et al. Sectional anatomy ofthe peritoneal reflections of the upper abdomen inthe coronal plane. J Comput Assist Tomogr 2005;29:430–437.

Figure 22. Cystic lymphangioma in a 48-year-old woman with abdominal discomfort. Axial (a) and coro-nal (b) CT scans show a large multiloculated cystic mass in the lesser sac. The attenuation of the lesion wasabout 16 HU (range, �14 to 40 HU) on unenhanced scans. The diagnosis of cystic lymphangioma was con-firmed at surgical excision.

RG f Volume 27 ● Number 3 Yoo et al 719

7. Dodds WJ, Foley WD, Lawson TL, Stewart ET,Taylor A. Anatomy and imaging of the lesser peri-toneal sac. AJR Am J Roentgenol 1985;144:567–575.

8. DeMeo JH, Fulcher AS, Austin RF Jr. AnatomicCT demonstration of the peritoneal spaces, liga-ments, and mesenteries: normal and pathologicprocesses. RadioGraphics 1995;15:755–770.

9. Hamrick-Turner JE, Chiechi MV, Abbitt PL, RosPR. Neoplastic and inflammatory processes of theperitoneum, omentum, and mesentery: diagnosiswith CT. RadioGraphics 1992;12:1051–1068.

10. Chopra S, Dodd GD 3rd, Chintapalli KN, EsolaCC, Ghiatas AA. Mesenteric, omental, and retro-peritoneal edema in cirrhosis: frequency and spec-trum of CT findings. Radiology 1999;211:737–742.

11. Ha HK, Jung JI, Lee MS, et al. CT differentiationof tuberculous peritonitis and peritoneal carcino-matosis. AJR Am J Roentgenol 1996;167:743–748.

12. Jadvar H, Mindelzun RE, Olcott EW, Levitt DB.Still the great mimicker: abdominal tuberculosis.AJR Am J Roentgenol 1997;168:1455–1460.

13. Kebapci M, Vardareli E, Adapinar B, Acikalin M.CT findings and serum CA 125 levels in malig-nant peritoneal mesothelioma: report of 11 newcases and review of the literature. Eur Radiol2003;13:2620–2626.

14. Sulkin TV, O’Neill H, Amin AI, Moran B. CT inpseudomyxoma peritonei: a review of 17 cases.Clin Radiol 2002;57:608–613.

15. Walensky RP, Venbrux AC, Prescott CA, Oster-man FA Jr. Pseudomyxoma peritonei. AJR Am JRoentgenol 1996;167:471–474.

16. Kim Y, Cho O, Song S, Lee H, Rhim H, Koh B.Peritoneal lymphomatosis: CT findings. AbdomImaging 1998;23:87–90.

17. Ros PR, Olmsted WW, Moser RP Jr, DachmanAH, Hjermstad BH, Sobin LH. Mesenteric andomental cysts: histologic classification with imag-ing correlation. Radiology 1987;164:327–332.

18. Mar CR, Pushpanathan C, Price D, Cramer B.Omental lymphangioma with small-bowel volvu-lus. RadioGraphics 2003;23:847–851.

19. Ha HK, Lee HJ, Kim H, et al. Abdominal actino-mycosis: CT findings in 10 patients. AJR Am JRoentgenol 1993;161:791–794.

20. Jeong WK, Kim Y, Kim YS, et al. Heterotopicparagonimiasis in the omentum. J Comput AssistTomogr 2002;26:1019–1021.

21. Maeda T, Mori H, Cyujo M, Kikuchi N, Hori Y,Takaki H. CT and MR findings of torsion ofgreater omentum: a case report. Abdom Imaging1997;22:45–46.

22. Paroz A, Halkic N, Pezzetta E, Martinet O. Idio-pathic segmental infarction of the greater omen-tum: a rare cause of acute abdomen. J GastrointestSurg 2003;7:805–808.

23. Kopka L, Fischer U, Gross AJ, Funke M, Oest-mann JW, Grabbe E. CT of retained surgicalsponges (textilomas): pitfalls in detection andevaluation. J Comput Assist Tomogr 1996;20:919–923.

24. Rajput A, Loud PA, Gibbs JF, Kraybill WG.Diagnostic challenges in patients with tumors:case 1—gossypiboma (foreign body) manifesting30 years after laparotomy. J Clin Oncol 2003;21:3700–3701.

25. Ianora AA, Midiri M, Vinci R, Rotondo A, An-gelelli G. Abdominal wall hernias: imaging withspiral CT. Eur Radiol 2000;10:914–919.

26. Eren S, Ciris F. Diaphragmatic hernia: diagnosticapproaches with review of the literature. Eur J Ra-diol 2005;54:448–459.

27. Mueller PR, Ferrucci JT Jr, Simeone JF, et al.Lesser sac abscesses and fluid collections: drainageby transhepatic approach. Radiology 1985;155:615–618.

28. King LR, Siegel MJ, Balfe DM. Acute pancreatitisin children: CT findings of intra- and extrapancre-atic fluid collections. Radiology 1995;195:196–200.

29. Coulier B, Van Hoof M. Intraperitoneal fat focalinfarction of the lesser omentum: case report. Ab-dom Imaging 2004;29:498–501.

30. Inoue Y, Nakamura H, Mizumoto S, Akashi H.Lesser sac hernia through the gastrocolic ligament:CT diagnosis. Abdom Imaging 1996;21:145–147.

This article meets the criteria for 1.0 AMA PRA Category 1 Credit TM. To obtain credit, see www.rsna.org/education/rg_cme.html.

720 May-June 2007 RG f Volume 27 ● Number 3

RG Volume 27 • Volume 3 • May-June 2007 Yoo et al

Greater and Lesser Omenta: Normal Anatomy and Pathologic Processes

Eunhye Yoo, MD et al

Page 709 Furthermore, coronal and sagittal reformatted CT images help delineate the exact location, origin, or spread pattern of omental disease, as well as clarify the complex anatomy of the omenta. Page 709 At CT, it appears as a band of fatty tissue with a variable width, just beneath the anterior abdominal wall and anterior to the stomach, transverse colon, and small bowel. Ascites between the greater omentum and the adjacent soft tissues makes the omentum appear as a simple fatty layer, and soft-tissue deposits in the omentum can create an amorphous hazy stranding or a nodular or masslike appearance at CT (3). Page 710 The CT appearances of abnormalities involving the greater omentum are as follows: (a) multifocal, ill-defined infiltrative lesions, including peritoneal carcinomatosis, tuberculous peritonitis, malignant peritoneal mesothelioma, pseudomyxoma peritonei, lymphomatosis, and the conditions of cirrhosis and portal hypertension; (b) solid or cystic mass-forming lesions including primary and secondary neoplasms and infectious processes; and (c) miscellaneous conditions including omental infarction, foreign-body granuloma, hematoma, and hernia. Page 710 When there is diffuse infiltration of the peritoneum, omentum, or mesentery at CT, a variety of condition—including infiltrative edema from liver cirrhosis, diffuse peritoneal tumors, and infectious peritonitis—should be considered. Distinguishing between diffuse peritoneal tumors, such as peritoneal carcinomatosis, malignant mesothelioma, or lymphomatosis, and tuberculous peritonitis is difficult because of nonspecific symptoms and overlapping imaging features.. Page 716 A fluid collection within only the lesser sac should be considered postoperative fluid after gastric or hepatobiliary surgery or an inflammatory exudate from pancreatitis, cholecystitis, or gastric perforation (Fig 16) (8).

RadioGraphics 2007; 27:707–720 ● Published online 10.1148/rg.273065085 ● Content Codes:

RadioGraphics 2007 This is your reprint order form or pro forma invoice

(Please keep a copy of this document for your records.)

Author Name _______________________________________________________________________________________________ Title of Article _______________________________________________________________________________________________ Issue of Journal_______________________________ Reprint # _____________ Publication Date ________________ Number of Pages_______________________________ KB # _____________ Symbol RadioGraphics Color in Article? Yes / No (Please Circle) Please include the journal name and reprint number or manuscript number on your purchase order or other correspondence. Order and Shipping Information Reprint Costs (Please see page 2 of 2 for reprint costs/fees.) ________ Number of reprints ordered $_________

________ Number of color reprints ordered $_________

________ Number of covers ordered $_________

Subtotal $_________

Taxes $_________ (Add appropriate sales tax for Virginia, Maryland, Pennsylvania, and the District of Columbia or Canadian GST to the reprints if your order is to be shipped to these locations.)

First address included, add $32 for each additional shipping address $_________

TOTAL $_________

Shipping Address (cannot ship to a P.O. Box) Please Print Clearly Name ___________________________________________ Institution _________________________________________ Street ___________________________________________ City ____________________ State _____ Zip ___________ Country ___________________________________________ Quantity___________________ Fax ___________________ Phone: Day _________________ Evening _______________ E-mail Address _____________________________________ Additional Shipping Address* (cannot ship to a P.O. Box)

Name ___________________________________________ Institution _________________________________________ Street ___________________________________________ City ________________ State ______ Zip ___________

Country _________________________________________ Quantity __________________ Fax __________________ Phone: Day ________________ Evening ______________ E-mail Address ____________________________________ * Add $32 for each additional shipping address

Payment and Credit Card Details Enclosed: Personal Check ___________ Credit Card Payment Details _________ Checks must be paid in U.S. dollars and drawn on a U.S. Bank. Credit Card: __ VISA __ Am. Exp. __ MasterCard Card Number __________________________________ Expiration Date_________________________________ Signature: _____________________________________ Please send your order form and prepayment made payable to: Cadmus Reprints P.O. Box 751903 Charlotte, NC 28275-1903 Note: Do not send express packages to this location, PO Box.

FEIN #:541274108

Invoice or Credit Card Information Invoice Address Please Print Clearly Please complete Invoice address as it appears on credit card statement Name ____________________________________________ Institution ________________________________________ Department _______________________________________ Street ____________________________________________ City ________________________ State _____ Zip _______ Country ___________________________________________ Phone _____________________ Fax _________________ E-mail Address _____________________________________ Cadmus will process credit cards and Cadmus Journal

Services will appear on the credit card statement. If you don’t mail your order form, you may fax it to 410-820-9765 with

your credit card information. Signature __________________________________________ Date _______________________________________ Signature is required. By signing this form, the author agrees to accept the responsibility for the payment of reprints and/or all charges described in this document.

Reprint order forms and purchase orders or prepayments must be received 72 hours after receipt of form either by mail or by fax at 410-820-9765. It is the policy of Cadmus Reprints to issue one invoice per order.

Please print clearly.

Page 1 of 2 RB-9/22/06

RadioGraphics 2007 Black and White Reprint Prices

Domestic (USA only) # of

Pages 50 100 200 300 400 500

1-4 $213 $228 $260 $278 $295 $313 5-8 $338 $373 $420 $453 $495 $530

9-12 $450 $500 $575 $635 $693 $755 13-16 $555 $623 $728 $805 $888 $965 17-20 $673 $753 $883 $990 $1,085 $1,185 21-24 $785 $880 $1,040 $1,165 $1,285 $1,413 25-28 $895 $1,010 $1,208 $1,350 $1,498 $1,638 29-32 $1,008 $1,143 $1,363 $1,525 $1,698 $1,865

Covers $95 $118 $218 $320 $428 $530

International (includes Canada and Mexico) # of

Pages 50 100 200 300 400 500

1-4 $263 $275 $330 $385 $430 $485 5-8 $415 $443 $555 $650 $753 $850

9-12 $563 $608 $773 $930 $1,070 $1,228 13-16 $698 $760 $988 $1,185 $1,388 $1,585 17-20 $848 $925 $1,203 $1,463 $1,705 $1,950 21-24 $985 $1,080 $1,420 $1,725 $2,025 $2,325 25-28 $1,135 $1,248 $1,640 $1,990 $2,350 $2,698 29-32 $1,273 $1,403 $1,863 $2,265 $2,673 $3,075

Covers $148 $168 $308 $463 $615 $768 Minimum order is 50 copies. For orders larger than 500 copies, please consult Cadmus Reprints at 800-407-9190. Reprint Cover Cover prices are listed above. The cover will include the publication title, article title, and author name in black. Shipping Shipping costs are included in the reprint prices. Domestic orders are shipped via UPS Ground service. Foreign orders are shipped via a proof of delivery air service. Multiple Shipments Orders can be shipped to more than one location. Please be aware that it will cost $32 for each additional location. Delivery Your order will be shipped within 2 weeks of the journal print date. Allow extra time for delivery.

Color Reprint Prices

Domestic (USA only) # of

Pages 50 100 200 300 400 500

1-4 $218 $233 $343 $460 $579 $697 5-8 $343 $388 $584 $825 $1,069 $1,311 9-12 $471 $503 $828 $1,196 $1,563 $1,935 13-16 $601 $633 $1,073 $1,562 $2,058 $2,547 17-20 $738 $767 $1,319 $1,940 $2,550 $3,164 21-24 $872 $899 $1,564 $2,308 $3,045 $3,790 25-28 $1,004 $1,035 $1,820 $2,678 $3,545 $4,403 29-32 $1,140 $1,173 $2,063 $3,048 $4,040 $5,028

Covers $95 $118 $218 $320 $428 $530

International (includes Canada and Mexico)) # of

Pages 50 100 200 300 400 500

1-4 $268 $280 $412 $568 $715 $871 5-8 $419 $457 $720 $1,022 $1,328 $1,633 9-12 $583 $610 $1,025 $1,492 $1,941 $2,407 13-16 $742 $770 $1,333 $1,943 $2,556 $3,167 17-20 $913 $941 $1,641 $2,412 $3,169 $3,929 21-24 $1,072 $1,100 $1,946 $2,867 $3,785 $4,703 25-28 $1,246 $1,274 $2,254 $3,318 $4,398 $5,463 29-32 $1,405 $1,433 $2,561 $3,788 $5,014 $6,237

Covers $148 $168 $308 $463 $615 $768 Tax Due Residents of Virginia, Maryland, Pennsylvania, and the District of Columbia are required to add the appropriate sales tax to each reprint order. For orders shipped to Canada, please add 7% Canadian GST unless exemption is claimed. Ordering Reprint order forms and purchase order or prepayment is required to process your order. Please reference journal name and reprint number or manuscript number on any correspondence. You may use the reverse side of this form as a proforma invoice. Please return your order form and prepayment to: Cadmus Reprints P.O. Box 751903 Charlotte, NC 28275-1903 Note: Do not send express packages to this location, PO Box. FEIN #:541274108 Please direct all inquiries to:

Rose A. Baynard 800-407-9190 (toll free number) 410-819-3966 (direct number) 410-820-9765 (FAX number)

baynardr@cadmus.com (e-mail)

Reprint Order Forms and purchase order or prepayments must be received 72 hours after receipt of form.

Page 2 of 2