DOI: 10.1148/rg.233025712
(Radiographics. 2003;23:663-685.)
© RSNA, 2003
US of the Peritoneum1
Anthony E. Hanbidge, MB, BCh, FRCPC,
Deirdre Lynch, MRCPI, FRCR2 and
Stephanie R. Wilson, MD, FRCPC
1 From the Department of Medical Imaging, Toronto General Hospital, University Health Network, University of Toronto, 200 Elizabeth St, Toronto, Ontario, Canada M5G 2C4. Received July 17, 2002; revision requested September 17 and received November 5; accepted November 6. Address correspondence to S.R.W. (e-mail: stephanie.wilson@uhn.on.ca).
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Abstract
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Familiarity with the pathophysiology of peritoneal disease is the basis of successful ultrasound (US) study of the peritoneum. The pouch of Douglas, diaphragmatic surfaces, the paracolic gutters, and the regions of the mesentery and omentum should receive careful scrutiny in the patient at risk for a peritoneal disease process. An optimal US technique requires assessment of the entire peritoneum with a transducer selected to reflect the depth of the region of interest. US may demonstrate minute quantities of free intraperitoneal fluid and is therefore capable of providing sensitive quantitative information about ascites. Qualitative information may also be inferred, as blood, pus, and neoplastic cells demonstrate correlation with particulate ascites on gray-scale US scans. Peritoneal nodules, plaques, and thickening may be detected on the visceral or parietal peritoneal surfaces, especially when high-frequency probes are used. Transvaginal study in women increases the sensitivity of US for detection of peritoneal disease. In women who have unexplained sepsis or are at risk for carcinomatosis, transvaginal scanning should routinely be added to the regular abdominal and pelvic studies regardless of the findings of those studies. Peritoneal carcinomatosis, primary peritoneal neoplasms, pseudomyxoma peritonei, and peritonitis have characteristic appearances at US.
© RSNA, 2003
Index Terms: Endometriosis, 791.318 • Mesentery, cysts, 792.3121 • Mesothelioma, 791.329 • Peritoneum, abscess, 791.21 • Peritoneum, anatomy, 791.92 • Peritoneum, fluid, 791.77 • Peritoneum, neoplasms, 791.329, 791.33 • Peritonitis, 791.295 • Pneumoperitoneum, 791.71 • Pseudomyxoma peritonei, 791.3196
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LEARNING OBJECTIVES FOR TEST 4
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After reading this article and taking the test, the reader will be able to:
- Describe the US technique for optimal detection and characterization of peritoneal disease.
- List the common disease processes, both neoplastic and nonneoplastic, that affect the peritoneum.
- Identify peritoneal abnormalities commonly encountered at US.
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Introduction
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The peritoneum and peritoneal cavity are commonly involved in many primary and secondary disease processes. Traditionally, computed tomography (CT) has been the imaging modality of choice used in assessing pathologic conditions in these areas. More recently, magnetic resonance (MR) imaging has proved effective in demonstrating primary and secondary peritoneal tumors (1). It is our experience that ultrasound (US) can be an extremely accurate, safe, relatively inexpensive, and readily accessible imaging tool for investigation of many peritoneal diseases. Review of the US literature, in general, reveals a relatively small volume of work describing scanning technique and US features of peritoneal disease compared with the vast literature pertaining to solid organ disease. It is our belief that evaluation of the peritoneum is often neglected during abdominal and pelvic US due to a combination of inadequate technical training and unfamiliarity with the common US features encountered with peritoneal disease.
Therefore, the purpose of this article is to describe a US technique for optimal detection and localization of peritoneal disease and to illustrate the US appearances of commonly encountered abnormalities. Specific topics discussed are the appearance of the normal peritoneum, mesentery, and omentum; ascites; peritoneal inclusion cysts; peritoneal tumors; inflammatory processes of the peritoneum; endometriosis; right-sided segmental infarction of the omentum; pneumoperitoneum; and mesenteric cysts.
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Technique
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The success of US detection of peritoneal disease is dependent on (a) operator awareness of the potential involvement of the peritoneum and peritoneal cavity with specific disease processes and (b) thorough US assessment of these areas. Initially, a survey of the peritoneum and peritoneal cavity is performed with a standard-frequency transducer (3.5- or 5-MHz probe) to allow assessment of the entire abdominal and pelvic contents including the solid organs. The field of view is adjusted so that the full depth of the abdominal or pelvic cavity is included in the image (Fig 1). Adjustment of the field size to just include the entire peritoneal cavity gives perspective to the image. After the initial survey, a higher-frequency probe may be used to more closely interrogate lesions found in the near field. A tailored transvaginal examination is mandatory to assess the pelvic peritoneum and cavity in female patients, as this is a common site of peritoneal disease, particularly in carcinomatosis (Fig 1) and acute conditions. In addition to standard evaluation of the uterus and ovaries, the probe should be directed to both pelvic side walls and the pouch of Douglas for evaluation of the parietal peritoneum.
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Figure 1a. Optimization of the technique in a 73-year-old woman with stage 3 papillary serous adenocarcinoma of the ovary. (a) Sagittal suprapubic US image of the right adnexa, obtained with a 5-2-MHz curvilinear transducer during the initial survey, shows ascites and a solid, lobulated, hypoechoic mass (M). The field of view includes the full depth of the peritoneal cavity but no more, and the focal zone was set to optimize visualization of the mass. (b) Sagittal transabdominal US image of the left flank obtained with a 7-4-MHz curvilinear transducer shows ascites and seeding on the serosal surface of the descending colon (arrows). A low gain setting was used, and the focal zone was positioned to optimize visualization of the seeding. The seeding is seen as a thin continuous line on the serosal surface of the intestine, which contains shadowing air. (c) Transverse transvaginal US image of the right adnexa obtained with an 8-4-MHz transvaginal probe shows the mass (M) and particulate ascites. A high gain setting was used to better characterize the ascites.
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Figure 1b. Optimization of the technique in a 73-year-old woman with stage 3 papillary serous adenocarcinoma of the ovary. (a) Sagittal suprapubic US image of the right adnexa, obtained with a 5-2-MHz curvilinear transducer during the initial survey, shows ascites and a solid, lobulated, hypoechoic mass (M). The field of view includes the full depth of the peritoneal cavity but no more, and the focal zone was set to optimize visualization of the mass. (b) Sagittal transabdominal US image of the left flank obtained with a 7-4-MHz curvilinear transducer shows ascites and seeding on the serosal surface of the descending colon (arrows). A low gain setting was used, and the focal zone was positioned to optimize visualization of the seeding. The seeding is seen as a thin continuous line on the serosal surface of the intestine, which contains shadowing air. (c) Transverse transvaginal US image of the right adnexa obtained with an 8-4-MHz transvaginal probe shows the mass (M) and particulate ascites. A high gain setting was used to better characterize the ascites.
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Figure 1c. Optimization of the technique in a 73-year-old woman with stage 3 papillary serous adenocarcinoma of the ovary. (a) Sagittal suprapubic US image of the right adnexa, obtained with a 5-2-MHz curvilinear transducer during the initial survey, shows ascites and a solid, lobulated, hypoechoic mass (M). The field of view includes the full depth of the peritoneal cavity but no more, and the focal zone was set to optimize visualization of the mass. (b) Sagittal transabdominal US image of the left flank obtained with a 7-4-MHz curvilinear transducer shows ascites and seeding on the serosal surface of the descending colon (arrows). A low gain setting was used, and the focal zone was positioned to optimize visualization of the seeding. The seeding is seen as a thin continuous line on the serosal surface of the intestine, which contains shadowing air. (c) Transverse transvaginal US image of the right adnexa obtained with an 8-4-MHz transvaginal probe shows the mass (M) and particulate ascites. A high gain setting was used to better characterize the ascites.
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Although the exact location of a lesion to the peritoneum, mesentery, or omentum may be difficult to determine at US, particularly in the absence of ascites, various maneuvers can be used in an effort to establish the site of the abnormality. In our experience, lesions of the parietal peritoneum will not move with gravity or breathing maneuvers (Fig 2), whereas lesions of the visceral peritoneum, mesentery, or omentum usually will move on pressure by the transducer.
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Figure 2. Localization of an abnormality in a 72-year-old man with squamous cell carcinoma of the lung and a peritoneal metastasis. Sagittal US image of the right upper quadrant shows a hypoechoic nodule (arrows) anterior to the liver (L). With respiration, the liver moved freely independent of the nodule, which stayed stationary. This finding correctly suggested that the nodule was located on the parietal peritoneum. K = kidney.
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Appearance of Normal Peritoneum, Mesentery, and Omentum
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The peritoneum is composed of a serous membrane lined with epithelial cells. It is divided into the parietal and visceral peritoneum. The parietal peritoneum covers the anterior and posterior abdominal walls and the pelvic cavity. The visceral peritoneum is the portion that covers the solid and hollow viscera. Between the two layers lies the peritoneal cavity, a potential space that normally contains a few milliliters of fluid, which acts as a lubricant.
The normal US appearance of the parietal peritoneum is a discrete, thin, smooth, single echogenic line in the deepest layer of the abdominal wall. The peritoneal line usually represents a combination of peritoneum and deep abdominal fascia. When there is abundant extraperitoneal fat, peritoneum and fat are seen as two separate lines (2). Bowel loops can usually be seen deep to the peritoneum, moving independently of it with respiration. The presence of ascites greatly enhances visualization of the peritoneum and appreciation of pathologic conditions. The visceral peritoneal covering merges with the outer layer of the bowel wall to form the tunica serosa and with the capsule of solid organs such as the liver. Therefore, it is usually imperceptible at US in its normal state and becomes visible as a separate entity only in disease.
The small bowel mesentery is a fan-shaped, specialized fold of the peritoneum that extends obliquely from the second lumbar vertebra to the right iliac fossa and connects the jejunum and ileum to the posterior abdominal wall. In addition to a double layer of peritoneum, it contains blood vessels, nerves, lacteals, lymph nodes, and a variable amount of fat.
Derchi et al (3) describe the normal leaves of the mesentery as a series of elongated structures separated from each other by specular echoes, which represent the peritoneal surfaces. Unlike the intestine attached to its distal end, the mesentery does not demonstrate peristalsis. Linear anechoic structures within the mesentery represent blood vessels. The thickness of the leaves varies from 0.7 to 1.2 cm, which corresponds to the findings of Jain et al (4), who demonstrated a mesenteric thickness in healthy individuals of 0.5–1.4 cm. Derchi et al (3) found that the mesentery was more readily seen in obese patients and was easiest to demonstrate in the left lower quadrant while scanning in an oblique plane approximately parallel to the left iliac vessels. In the presence of ascites, the normal bowel mesentery can be more easily recognized at US, as freely floating leaves separated by fluid (Fig 3). The mesenteric leaves are directed toward the center of the abdomen, away from the small bowel loops. US has the unique ability to allow visualization of the abdominal contents for evidence of motility. Clumping of the mesenteric leaves and lack of mobility of the mesentery within the ascitic fluid occurs in the presence of adhesions.
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Figure 3. Normal mesentery with gross ascites in a 58-year-old woman with cirrhosis of the liver and portal hypertension. Sagittal oblique US image of the midabdomen shows the leaves of the normal small bowel mesentery (arrows) outlined by fluid.
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The omenta are also specialized peritoneal folds. They are composed of a double layer of peritoneum, blood vessels, lymphatics, and a variable amount of fat. The lesser omentum connects the lesser curvature of the stomach and proximal duodenum with the liver. The greater omentum descends from the greater curvature of the stomach and the proximal duodenum in front of the intestine, sometimes as low as the pelvis. The greater omentum then folds upon itself to form a four-layered structure, which ascends and separates to enclose the transverse colon. A potential space, which is continuous with the lesser sac, exists between the two layers of greater omentum.
Although superficially located within the abdomen, the normal greater omentum may be difficult to visualize as a distinct entity at US. In the presence of ascites, the inferior aspect of the omentum can be seen as a moderately hyperechoic structure of variable thickness, depending on the fat content. In the absence of ascites, the superficial location of the omentum in relation to the small intestine and mesentery will allow correct localization of disease to the omentum in a significant number of cases. Its superficial location within the abdomen also allows close interrogation with high-resolution transducers. Therefore, thickening or nodularity of the omentum due to infiltration or inflammation can be more readily appreciated. However, correct localization may be more difficult when a large mesenteric mass extends anteriorly toward the abdominal wall.
The mechanisms and pathways of spread of intraabdominal disease processes have previously been described. The reader is referred to several excellent articles available in the literature (5,6).
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Ascites
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The peritoneal cavity normally contains approximately 50–75 mL of clear fluid, which functions as a lubricant. Ascites occurs with the accumulation of excess intraperitoneal fluid. Ascitic fluid is broadly categorized into transudate or exudate, depending on the protein content. Many disease processes may lead to the development of ascites. In North America and Europe, cirrhosis, malignant neoplasms, congestive cardiac failure, and tuberculous peritonitis account for up to 90% of cases. Accumulations of blood, urine, chyle, pancreatic secretions, or bile within the peritoneal cavity are more unusual causes.
Ascites can be detected at physical examination when the volume of peritoneal fluid reaches approximately 500 mL. US can readily demonstrate the presence of a large amount of ascites, and indeed this was one of its earliest uses (7) (Fig 4). It can also demonstrate much smaller quantities of fluid (Fig 5) and is more sensitive than CT in this regard. Nichols and Steinkampf (8) demonstrated as little as 0.8 mL of free intraperitoneal fluid with transvaginal US.
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Figure 4. Quantification of ascites in a 39-year-old man with cirrhosis of the liver and portal hypertension. Sagittal US image of the right upper quadrant clearly shows a large amount of ascites surrounding an enlarged, bulbous, fatty liver.
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Figure 5. Quantification of ascites in a 40-year-old woman with grade 2-3 ovarian mucinous cystadenocarcinoma. Transverse transvaginal US image of the right adnexa shows a small amount of particulate free fluid (F) and serosal seeding (arrowheads) on bowel loops in the pelvis. This appearance was visible only on transvaginal scans.
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With the patient in a supine position, free fluid tends to accumulate in the flanks, particularly the superior end of the right paracolic gutter, and in the pelvis due to the effects of gravity and capillary attraction (5). Therefore, these areas should be carefully assessed when ascites is suspected. Free intraperitoneal fluid may occasionally be confused with fluid-filled structures; the characteristic angled borders demonstrated by ascites may aid in differentiation. This feature also aids in detection of small amounts of interloop fluid.
US allows accurate assessment of the volume of ascites (9). This information may be used in clinical practice and in research to categorize patients prior to commencement of therapy and also to assess response to treatment. In addition, it plays an important role in guidance of diagnostic and therapeutic paracentesis and allows monitoring of ascitic volume following therapeutic intervention.
US is not only the modality of choice for detection and quantification of ascites but also enables localization and characterization of the fluid, in which regard it is superior to CT (Fig 6). Although diagnostic paracentesis is the standard of reference for distinguishing transudative from exudative ascites, US can be very helpful at suggesting this distinction. In patients with transudative ascites, the US appearances are predominantly those of anechoic fluid (10). Features associated with exudative ascites due to inflammatory or malignant causes include loculated fluid collections, particulate ascites, septa, and matted bowel loops with interposed fluid. Differentiation of benign from malignant ascites may be difficult on the basis of characterization of the ascitic fluid alone. Therefore, in addition to the features typical of exudative ascites, ancillary features such as thickening of the gallbladder wall (11), lymphadenopathy, an intraabdominal mass suggestive of peritoneal seeds or an omental "cake," and hepatic metastases should be sought (10,12).
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Figure 6a. Characterization of peritoneal fluid in a 60-year-old woman 13 days after liver transplantation. (a) Unenhanced CT image of the upper abdomen shows ascites (arrowheads) and two hypoattenuating, homogeneous, subhepatic fluid collections (arrows). Intravenous contrast material was not administered because of renal failure. (b) Transverse US image obtained through the fluid collections shows a complex appearance with thick septa and low-level echoes (arrows), which was correctly suggestive of abscesses.
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Figure 6b. Characterization of peritoneal fluid in a 60-year-old woman 13 days after liver transplantation. (a) Unenhanced CT image of the upper abdomen shows ascites (arrowheads) and two hypoattenuating, homogeneous, subhepatic fluid collections (arrows). Intravenous contrast material was not administered because of renal failure. (b) Transverse US image obtained through the fluid collections shows a complex appearance with thick septa and low-level echoes (arrows), which was correctly suggestive of abscesses.
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Chylous ascites is an unusual entity caused by interruption of lymphatic flow and is associated with lymphoma, trauma, surgery, intestinal lymphangiectasia, lymphadenitis, lymphangioma, or cystic hygroma. At US, it may appear as particulate ascites or a fluid-fluid level of differential echogenicity may occur due to layering of the chylous fluid (13,14).
Hemoperitoneum has many causes and is commonly seen in patients with a history of abdominal trauma, rupture of an aortic aneurysm, or ruptured ectopic pregnancy. Spontaneous hemoperitoneum may occur in patients receiving long-term anticoagulant therapy. US is a sensitive method of detecting hemoperitoneum and its role, particularly in screening trauma patients, has been highlighted by many authors (15–18). The US appearances of hemoperitoneum vary with the interval between the onset of bleeding and US examination. Fresh intraperitoneal blood may appear as anechoic or particulate fluid (Fig 7) with random movement of particles evident within the fluid. Fluid-debris levels may develop if the patient has maintained a static position for a period of time. Massive hemoperitoneum may result in formation of a large intraperitoneal hematoma, which appears initially as a homogeneously echogenic mass. Lysis of the hematoma results in a more heterogeneous appearance with development of cystic areas, septa, and internal debris (Fig 8).
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Figure 7a. Hemoperitoneum in a 61-year-old man with hepatoma on a background of cirrhosis and portal hypertension who became hemodynamically unstable 1 day after therapeutic paracentesis. (a) Sagittal US image of the right upper quadrant obtained at the time of therapeutic paracentesis shows anechoic ascites (A) and an ill-defined echogenic mass (M) in the cirrhotic liver (L). (b) Transverse US image obtained 1 day later shows gross particulate ascites (A). Hemoperitoneum was proved at surgery. An artifact (arrowheads) is noted in the near field.
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Figure 7b. Hemoperitoneum in a 61-year-old man with hepatoma on a background of cirrhosis and portal hypertension who became hemodynamically unstable 1 day after therapeutic paracentesis. (a) Sagittal US image of the right upper quadrant obtained at the time of therapeutic paracentesis shows anechoic ascites (A) and an ill-defined echogenic mass (M) in the cirrhotic liver (L). (b) Transverse US image obtained 1 day later shows gross particulate ascites (A). Hemoperitoneum was proved at surgery. An artifact (arrowheads) is noted in the near field.
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Figure 8. Hemoperitoneum in a 38-year-old man who underwent recent revision of a liver transplantation because of hepatic artery thrombosis. He became hemodynamically unstable in the intensive care unit. Transverse US image of the left lower quadrant shows a heterogeneous mass (arrowheads) with a highly echogenic component (arrows). At surgery, the entire mass was confirmed to be an acute blood clot, which was secondary to rupture of a pseudoaneurysm at the hepatic artery anastomosis.
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US can also help determine whether identified fluid is free or loculated, as free ascites moves when the patient changes position. Loculated fluid may on occasion be difficult to differentiate from a predominantly cystic structure such as a cyst, abscess, or hematoma. A cyst is usually round or oval with a smooth wall and tends to displace adjacent structures, whereas loculated fluid usually displays irregular borders and is surrounded by bowel loops and abdominal or pelvic organs. Bowel peristalsis within the loculated fluid may be appreciated.
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Peritoneal Inclusion Cysts
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Loculated fluid collections may occur anywhere in the abdomen or pelvis. Premenopausal female patients with pelvic adhesions are at risk of developing parovarian loculated fluid collections known as peritoneal inclusion cysts (Fig 9), also known as benign encysted fluid. The fluid produced by active ovaries is normally absorbed by the peritoneum. In the presence of adhesions and abnormal peritoneum, this fluid is not absorbed and becomes trapped around the ovary. This results in formation of a unilocular or multilocular cystic mass, which may be ovoid or irregular in contour and contain internal echoes and septa. Commonly, the ovary is seen within the cyst or in its wall. These cysts may be a source of persistent pelvic pain. Predisposing factors for development of peritoneal inclusion cysts include a history of surgery, trauma, pelvic inflammatory disease, endometriosis, and inflammatory bowel disease. The differential diagnosis includes ovarian cysts and parovarian cysts, hydrosalpinx, and frequently ovarian cancer. US may be used for guidance of cyst drainage (19–21).
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Figure 9. Peritoneal inclusion cyst in a 37-year-old woman with a long history of pelvic infection. Transverse transvaginal US image of the right adnexa shows a normal right ovary with follicles (arrow) surrounded by complicated strandy fluid.
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Peritoneal Tumors
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Peritoneal Carcinomatosis
Tumors that involve the peritoneum are commonly encountered at US. Peritoneal tumors are usually malignant, and metastatic tumors are considerably more common than primary tumors. In female patients, the ovary is the primary site of disease in the vast majority of cases. Other tumors with the potential to spread to the peritoneum include primary tumors arising from the stomach, colon, breast, pancreas, kidney, bladder, or uterus and melanoma (2).
Meyers et al (6) have described in detail the factors that influence the distribution of metastatic disease within the abdominal and pelvic cavities. The right subphrenic region, Morison pouch, and pouch of Douglas are commonly involved sites; therefore, US evaluation of the peritoneum for metastatic disease should include detailed evaluation of these areas (Fig 10).
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Figure 10a. Sagittal (a) and coronal (b) drawings of the midabdomen and pelvis show common locations for the spread of peritoneal carcinomatosis. Commonly involved sites include the right subphrenic region (arrowhead in b), omentum (straight arrows), and pouch of Douglas (curved arrow in a). (Courtesy of J. Tomash, Toronto, Canada.)
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Figure 10b. Sagittal (a) and coronal (b) drawings of the midabdomen and pelvis show common locations for the spread of peritoneal carcinomatosis. Commonly involved sites include the right subphrenic region (arrowhead in b), omentum (straight arrows), and pouch of Douglas (curved arrow in a). (Courtesy of J. Tomash, Toronto, Canada.)
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Ascites is a common finding with peritoneal carcinomatosis and may be particulate, reflecting the proteinaceous content of the fluid. Occasionally, chylous ascites or hemoperitoneum may be demonstrated. Septa within the ascitic fluid that produce interlacing bands of echoes may also be seen.
Carcinomatous seeds on the parietal or visceral peritoneum may produce hypoechoic nodules (Fig 11) or irregular masses or may infiltrate the peritoneum to form sheetlike masses (6,12). The peritoneal line may be preserved in the presence of small seeds but is often lost as the lesion increases in size. Growth of the mass usually occurs inward toward the peritoneal cavity, but the tumor may occasionally grow outward into the abdominal wall. If psammomatous calcification occurs within a peritoneal nodule, it appears echogenic (Fig 12); in the presence of dense calcification, it demonstrates posterior acoustic shadowing. Although peritoneal seeds may be seen in the absence of ascites (Fig 13), its presence greatly enhances detection of these lesions, and nodules as small as 2–3 mm may be identified on the parietal peritoneum or the serosa of the intestine (2) (Fig 14). The dependent pelvic peritoneum is commonly involved in carcinomatosis. We believe that transvaginal US in women is the optimal imaging modality for demonstrating these lesions.
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Figure 11. Peritoneal carcinomatosis in a 57-year-old man with increasing abdominal girth 1 year after surgery for colon cancer. US showed evidence of peritoneal carcinomatosis. Transverse US image of the right side of the midabdomen shows a large amount of ascites (A) with a small parietal peritoneal implant in the right paracolic gutter (arrow).
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Figure 12a. Calcified implants in a 21-year-old woman with a serous ovarian neoplasm. (a) Unenhanced CT image obtained through the inferior liver (L) shows a small calcified implant on the surface of segment VI (arrow). (b) Transverse US image obtained at the same level shows the implant (arrow). (c) Sagittal US image obtained through the Morison pouch shows multiple other calcified implants (arrows), which were not evident at CT even in retrospect. K = kidney.
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Figure 12b. Calcified implants in a 21-year-old woman with a serous ovarian neoplasm. (a) Unenhanced CT image obtained through the inferior liver (L) shows a small calcified implant on the surface of segment VI (arrow). (b) Transverse US image obtained at the same level shows the implant (arrow). (c) Sagittal US image obtained through the Morison pouch shows multiple other calcified implants (arrows), which were not evident at CT even in retrospect. K = kidney.
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Figure 12c. Calcified implants in a 21-year-old woman with a serous ovarian neoplasm. (a) Unenhanced CT image obtained through the inferior liver (L) shows a small calcified implant on the surface of segment VI (arrow). (b) Transverse US image obtained at the same level shows the implant (arrow). (c) Sagittal US image obtained through the Morison pouch shows multiple other calcified implants (arrows), which were not evident at CT even in retrospect. K = kidney.
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Figure 13. Peritoneal carcinomatosis without ascites in a 77-year-old woman with papillary serous ovarian cancer. Transverse US image of the upper midabdomen shows hypoechoic seeding (arrows) that encases the gastric antrum.
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Figure 14. Tiny peritoneal implants in a 53-year-old woman with papillary serous ovarian cancer. Transverse transvaginal US image of the right adnexa shows particulate ascites (A), a "rind" of hypoechoic seeding on the serosal surfaces of small bowel loops (arrowheads), and tiny parietal peritoneal implants in the near field (arrows), which measure 2 mm in maximum diameter.
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The omentum is commonly involved in peritoneal carcinomatosis (22). Extensive infiltration by tumor results in marked thickening with formation of an omental cake, which is usually echogenic. The echogenicity differs from that of adjacent bowel loops, and detection of bowel peristalsis will help differentiate the two structures. Omental cakes may take several different forms. The "floating" cake is seen as an echogenic plaque surrounded by ascitic fluid (Fig 15). The thickened omentum may be adherent to the parietal peritoneum with ascitic fluid or intestine deep to it. As an alternative, the omentum may adhere to the visceral peritoneum surrounding small bowel loops but be free of the overlying parietal peritoneum of the anterior abdominal wall (Fig 16).
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Figure 15. Omental cake in a 79-year-old man who underwent right nephrectomy 2 years earlier for renal cell carcinoma. Sagittal US image shows a floating omental cake (arrows) with its free inferior margin (arrowhead) surrounded by ascites, an appearance indicative of peritoneal carcinomatosis.
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Figure 16. Omental cake in a 71-year-old woman with proved metastatic adenocarcinoma. Transverse US image of the midabdomen shows a thick omental cake (arrowheads) that adheres to the visceral peritoneum and encases gas-filled bowel loops (BL).
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Thickening of the mesentery (Fig 17) and mesenteric adenopathy are other features seen in carcinomatosis. Adhesions between bowel loops, the mesentery, and the omentum (2) as well as matting of bowel loops may appear as lack of separation of the loops by fluid at US.
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Figure 17. Mesenteric tumor deposit in a 39-year-old man with known metastatic hepatocellular carcinoma. Transverse oblique US image of the midabdomen shows a tumor deposit (T) on a mesenteric fold, which is outlined by ascites. Note the normal adjacent mesenteric fold (arrow).
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Primary Tumors of the Peritoneum
Primary malignant tumors of the peritoneum are extremely rare, and almost all are either malignant mesothelioma or lymphoma. Malignant mesothelioma is an invariably fatal neoplasm that arises from the lining of the pleura and peritoneum. The prevalence is highest in middle-aged men, and there is a well-established association with exposure to asbestos. Thirty-three percent of cases of mesothelioma involve the peritoneum as a primary site (23).
Presenting symptoms of peritoneal mesothelioma include weight loss, malaise, abdominal fullness, and discomfort. There may be a delay of up to 3–6 months before the patient seeks medical advice. In up to 65% of cases, chest radiographs demonstrate evidence of asbestos exposure at the time of diagnosis. At laparotomy, the parietal and visceral peritonea are diffusely thickened or extensively involved by tumor plaques or nodules. These nodules aggregate in layers, plaques, or, less commonly, discrete masses. The viscera are often encased or invaded by tumor. Ascites is a common finding in peritoneal mesothelioma, being detected in up to 90% of patients. Some authors have reported that the amount of ascites is disproportionately small in relation to the amount of solid tumor, in contrast to metastatic peritoneal disease (24); however, others describe variable amounts of ascites (25). On occasion, the ascites is loculated and contains adhesive bands.
Other recognized features of peritoneal mesothelioma include sheetlike peritoneal masses, which are of lower echogenicity than the abdominal wall muscle or occasionally appear homogeneously anechoic but without posterior acoustic enhancement (26). Less commonly, large irregular focal masses occur (Fig 18). Isolated peritoneal nodules are not frequently seen (2,24). Reuter et al (24) found that omental thickening producing a mantlelike mass in the anterior abdominal cavity was the predominant finding in their patients. The mass demonstrated low-level echoes with scattered hyperechoic areas, which represented entrapped intraabdominal and omental fat. The inner surface of the mass had a lobular appearance and was separated from underlying intestine by a hypoechoic line, which represented ascites or a thickened bowel wall (24). The solid organs should be evaluated for evidence of invasion or metastases. Pleural effusions and pleural plaques are other features that may be appreciated with US.
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Figure 18a. Peritoneal mesothelioma in a 73-year-old woman with biopsy-proved malignant mesothelioma. (a) Sagittal US image of the left upper quadrant shows a lobulated, heterogeneous mass (M) that involves the greater omentum. (b) Sagittal US image of the lower abdomen shows two small, hypoechoic implants in the near field (arrows). (c) Sagittal US image of the right lower quadrant shows an omental cake (arrows). Note the absence of ascites.
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Figure 18b. Peritoneal mesothelioma in a 73-year-old woman with biopsy-proved malignant mesothelioma. (a) Sagittal US image of the left upper quadrant shows a lobulated, heterogeneous mass (M) that involves the greater omentum. (b) Sagittal US image of the lower abdomen shows two small, hypoechoic implants in the near field (arrows). (c) Sagittal US image of the right lower quadrant shows an omental cake (arrows). Note the absence of ascites.
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Figure 18c. Peritoneal mesothelioma in a 73-year-old woman with biopsy-proved malignant mesothelioma. (a) Sagittal US image of the left upper quadrant shows a lobulated, heterogeneous mass (M) that involves the greater omentum. (b) Sagittal US image of the lower abdomen shows two small, hypoechoic implants in the near field (arrows). (c) Sagittal US image of the right lower quadrant shows an omental cake (arrows). Note the absence of ascites.
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US-guided biopsy may be helpful in diagnosing peritoneal mesothelioma. Although Reuter et al (24) obtained positive results in each of their four patients with fine-needle aspiration biopsy, core biopsy is likely to yield better results. Because of the difficulty sometimes encountered in establishing the diagnosis, wide sampling in different locations is advised.
Primary lymphoma of the non-Hodgkin variety may rarely involve the peritoneum, omentum, and mesentery (27,28). Some studies have demonstrated an increased prevalence in patients with acquired immunodeficiency syndrome (AIDS) (29). Features include ascites and diffuse peritoneal and omental seeding, which is indistinguishable from diffuse carcinomatosis (Fig 19).
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Figure 19a. Non-Hodgkin lymphoma of the peritoneum in a 37-year-old man with biopsy-proved non-Hodgkin lymphoma in the right lower quadrant. (a) Transverse US image shows a hypoechoic mass (M) that encompasses a gas-containing bowel loop. An echogenic mass effect (arrows) represents infiltrated fat and omentum. (b) CT image of the lower abdomen shows a large, homogeneous peritoneal mass (M) that displaces bowel loops (arrowheads). Note infiltration of the fat anterolateral to the mass (arrows).
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Figure 19b. Non-Hodgkin lymphoma of the peritoneum in a 37-year-old man with biopsy-proved non-Hodgkin lymphoma in the right lower quadrant. (a) Transverse US image shows a hypoechoic mass (M) that encompasses a gas-containing bowel loop. An echogenic mass effect (arrows) represents infiltrated fat and omentum. (b) CT image of the lower abdomen shows a large, homogeneous peritoneal mass (M) that displaces bowel loops (arrowheads). Note infiltration of the fat anterolateral to the mass (arrows).
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Pseudomyxoma Peritonei
Pseudomyxoma peritonei is a rare condition that results from peritoneal implants of columnar epithelium associated with progressive accumulation of mucinous ascites, which is often massive. This condition is most commonly associated with benign, borderline, or malignant mucinous tumors of the ovary or appendix. Controversy surrounds the pathogenesis of pseudomyxoma peritonei (30). Some studies have demonstrated that synchronous ovarian and appendiceal tumors are present in 90% of patients (31), and some researchers believe that the primary lesion is appendiceal in origin and that the ovarian lesions represent metastatic disease (32,33). Pseudomyxoma peritonei is rarely associated with tumors of the colon, stomach, uterus, pancreas, common bile duct, urachal duct, or omphalomesenteric duct. The disease process tends to remain localized in the peritoneal cavity, and extraperitoneal spread is rare.
Clinically, patients present with abdominal pain and distention associated with weight loss. With progression of the disease, the intestine becomes encased in mucinous material and bowel obstruction eventually occurs. Repeated surgical intervention to remove the accumulated mucinous material remains the treatment of choice and is the only therapy known to prolong survival. Individual survival depends significantly on the type of underlying tumor. Patients with adenocarcinoma of the ovary or appendix have a worse prognosis than those with a benign neoplasm. An overall 5-year survival rate of 40%–50% is suggested from the literature (30).
The surgical appearances of pseudomyxoma peritonei are characteristic, the abdomen being filled with large amounts of gelatinous material with mucinous globules studding the omentum and peritoneal surfaces.
Since many of these patients will undergo US or CT as the initial investigation, the diagnosis may be made preoperatively. Several authors have described the US appearances of pseudomyxoma peritonei (34–36). Recognized features include echogenic ascites, which reflects the gelatinous nature of the fluid. Unlike particulate ascites due to the presence of pus, blood, or fibrin, the echogenic foci are nonmobile and the bowel loops, instead of floating freely, are displaced centrally and posteriorly by the surrounding mass. In our experience, a "starburst" appearance within the fluid is suggestive of this diagnosis (Fig 20). The ascites may also appear septated; the septa are thought to represent the margins of the mucinous nodules. Seshul and Coulam (34) found scalloping of the liver margin by adjacent peritoneal masses to be characteristic of pseudomyxoma peritonei (Fig 21), although scalloping of the liver can be seen with peritoneal spread from other primary malignancies such as ovarian carcinoma. Other reported appearances of pseudomyxoma peritonei include highly echogenic masses, either focal or sheetlike, due to involvement of the parietal peritoneum and omentum (35). The increased echogenicity of these masses was thought to be due to the presence of numerous tiny cysts.
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Figure 20a. Pseudomyxoma peritonei in a 70-year-old woman with increasing abdominal girth. Surgery showed pseudomyxoma peritonei, which was likely of appendiceal origin. (a) Transverse US image of the midabdomen shows bowel loops (BL) displaced centrally by echogenic ascites with a starburst appearance. At real-time imaging, the echogenic foci in the ascites were nonmobile. (b) Sagittal US image of the pelvis shows the uterus (U) surrounded by echogenic ascites.
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Figure 20b. Pseudomyxoma peritonei in a 70-year-old woman with increasing abdominal girth. Surgery showed pseudomyxoma peritonei, which was likely of appendiceal origin. (a) Transverse US image of the midabdomen shows bowel loops (BL) displaced centrally by echogenic ascites with a starburst appearance. At real-time imaging, the echogenic foci in the ascites were nonmobile. (b) Sagittal US image of the pelvis shows the uterus (U) surrounded by echogenic ascites.
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Figure 21. Pseudomyxoma peritonei secondary to a ruptured appendiceal mucocele in a 51-year-old man. Sagittal oblique US image of the right upper quadrant shows echogenic ascites (A) with scalloping of the liver.
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US may play a useful role in guidance of paracentesis, as less viscous areas can be identified with a greater likelihood of successful aspiration (36). It also may be used in patient follow-up.
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Inflammatory Processes of the Peritoneum
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Peritonitis
Peritonitis is defined as diffuse inflammation of the parietal and visceral peritoneum. It may occur as a result of infectious or noninfectious causes. Infectious causes include bacterial (including tuberculosis), viral, fungal, and parasitic infections. Noninfectious peritonitis is less commonly encountered, and causes include chemical peritonitis (secondary to gastric or pancreatic juice or bile), granulomatous peritonitis (secondary to foreign bodies such as talc), and sclerosing peritonitis associated with continuous ambulatory peritoneal dialysis (CAPD) and exposure to certain drugs such as practolol.
Most cases of infective peritonitis are secondary to disease processes that involve intraabdominal organs. Common causes of bacterial peritonitis include necrosis of the intestine secondary to ischemia; perforation of a tumor; inflammatory processes such as appendicitis, Crohn disease, colitis, and diverticulitis; and perforation of a peptic ulcer. Gynecologic disease processes such as salpingitis may also cause peritonitis. The organisms most commonly encountered constitute a mixed flora with gram-negative bacilli and anaerobes predominating.
Spontaneous Bacterial Peritonitis
Spontaneous or primary bacterial peritonitis is much less common. In adults, it occurs predominantly in association with cirrhosis or the nephrotic syndrome. The cause of spontaneous bacterial peritonitis has not been definitively established but is believed to involve hematogenous spread of organisms whose growth is facilitated by a defective filtration process resulting from a diseased liver and altered portal circulation. Patients with advanced liver disease also have decreased neutrophil phagocytic properties. Characteristically, only a single organism, usually Escherichia coli, is isolated from the peritoneal fluid in contrast to secondary bacterial peritonitis. The clinical findings in spontaneous bacterial peritonitis may be subtle, and correct diagnosis requires a high index of suspicion. It should be considered in any cirrhotic patient with fever and ascites or in the setting of an unexplained clinical deterioration. The US appearances of infective peritonitis include loculated ascites or ascitic fluid that contains debris, gas, or septa (10,14). Diffuse thickening of the peritoneum, omentum, and mesentery may also be observed (Fig 22). US is superior to CT in demonstrating the complexity of the fluid in these cases.
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Figure 22. Purulent peritonitis 1 week after laparotomy in a 23-year-old man. Sagittal US image of the midabdomen from the initial survey shows diffuse peritoneal thickening between bowel loops (arrowheads). There is strandy ascites in the near field (arrows).
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Abscess
Delayed treatment of peritonitis may lead to development of an intraperitoneal abscess. Because of gravity, abscesses tend to develop in the dependent areas of the abdomen and pelvis, such as the subphrenic and subhepatic areas, and the pouch of Douglas. Abscesses may also be found interposed between bowel loops, the interloop abscess. Recognized features of intraabdominal abscesses include round or oval fluid collections with a well-defined and irregular wall (Fig 23). An abscess commonly contains internal septa and debris; however, anechoic fluid collections may also be infected. Occasionally, small pockets of gas are identified within the collection and appear as echogenic foci, which may demonstrate posterior reverberation artifact depending on the amount of gas present (37,38) (Fig 24). The presence of gas within the collection is virtually diagnostic of infection.
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Figure 24. Abscess in a 55-year-old woman with constipation and perforation of the sigmoid colon with fecal peritonitis. Sagittal oblique US image of the left lower quadrant shows a large fluid collection (arrows) with stranding and bright echogenic foci (arrowheads), which represent gas bubbles.
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The limitations of US in detecting intraabdominal fluid collections and in differentiating sterile and infected collections are well recognized, especially in the obese or postoperative patient. Unfortunately, gas within an abscess may also be problematic for detection of abscesses—particularly interloop abscesses—if present in large quantities. Differentiation of a gas-containing abscess from a gas-filled adjacent bowel loop in such cases may be difficult or even impossible, and CT is often indicated to help make this distinction. US- or CT-guided percutaneous drainage is the treatment of choice, and follow-up US examinations are helpful in assessing response to therapeutic intervention.
Peritonitis secondary to fungal or parasitic infection is rarely encountered and usually occurs in immunosuppressed patients and those undergoing CAPD. Echinococcal disease may occasionally involve the peritoneum, resulting in formation of typical hydatid cysts.
Tuberculous Peritonitis
Tuberculous peritonitis deserves a special mention, as tuberculosis is still prevalent in developing countries. A recent resurgence of tuberculosis has also been seen in the developed world, particularly among AIDS patients and among the immigrant population (39). Other patients at risk for this disease are alcoholics and those with cirrhosis.
Among all patients with tuberculosis, extrapulmonary spread occurs in approximately 10%–15%. This prevalence of extrapulmonary spread increases to 50% in those with AIDS (40). Tuberculous peritonitis is one of the more common extrapulmonary manifestations and is thought to arise in most cases from reactivation of latent tuberculous foci in the peritoneum. Other possible routes of infection include lymphohematogenous spread and direct spread from the gastrointestinal tract or fallopian tubes. Radiologic evidence of pulmonary tuberculosis is present in only 14% of patients. Tuberculous peritonitis can mimic many disease processes. Therefore, a high index of suspicion, particularly in high-risk groups, and knowledge of the US features commonly encountered allow earlier diagnosis of this potentially curable disease, thus reducing morbidity and mortality. US may also be used as guidance for paracentesis and aspiration of enlarged lymph nodes for culture and cytologic analysis, as well as in follow-up of these patients (4).
Three types of tuberculous peritonitis have been described: (a) a "wet" type with free or loculated fluid; (b) a "dry" type with caseous nodules and adhesions; and (c) a fibrotic-fixed type with mass formation consisting of omentum and loops of intestine or mesentery, sometimes with ascites (41).
Several authors have described the US features of tuberculous peritonitis (41–46). Although none of the features described are pathognomonic of tuberculous peritonitis, in the appropriate clinical setting, they may strongly suggest the diagnosis. Ascites is frequently present (60%–100% of cases) and may be free or loculated. It commonly contains fine, freely mobile septa composed of fibrin, which frequently produce a latticelike pattern. The ascites may also be particulate and occasionally may be anechoic (45) (Figs 25, 26).
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Figure 25a. Tuberculous peritonitis in a 30-year-old Vietnamese woman with drug-resistant tuberculosis. (a) Transverse midline US image shows ascites (A) with matted bowel loops (BL). (b) Sagittal US image of the left adnexa shows ascites and the left ovary (O) embedded in thickened visceral peritoneum (arrowheads).
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Figure 25b. Tuberculous peritonitis in a 30-year-old Vietnamese woman with drug-resistant tuberculosis. (a) Transverse midline US image shows ascites (A) with matted bowel loops (BL). (b) Sagittal US image of the left adnexa shows ascites and the left ovary (O) embedded in thickened visceral peritoneum (arrowheads).
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Figure 26a. Tuberculous peritonitis in a 27-year-old Vietnamese man. (a) Transverse US image of the right upper quadrant shows echogenic fluid (F) scalloping the liver (L). (b) Contrast material-enhanced CT image obtained at the same level shows fluid (arrows) scalloping the liver. Note the enhancing rind (arrowheads).
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Figure 26b. Tuberculous peritonitis in a 27-year-old Vietnamese man. (a) Transverse US image of the right upper quadrant shows echogenic fluid (F) scalloping the liver (L). (b) Contrast material-enhanced CT image obtained at the same level shows fluid (arrows) scalloping the liver. Note the enhancing rind (arrowheads).
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Irregular hypoechoic or nodular thickening of the peritoneum is another commonly encountered feature of tuberculous peritonitis (42,43), as well as omental and mesenteric thickening (44). The omentum is typically hyperechoic and heterogeneous, presumably due to fat deposition secondary to lymphatic obstruction. Jain et al (4) found that mesenteric thickening of over 15 mm in addition to mesenteric adenopathy were early signs of tuberculous peritonitis in their patient population.
Lymphadenopathy involving the mesenteric, periportal, peripancreatic, and retroperitoneal nodes is another common feature of tuberculous peritonitis (43,44). Lymphadenopathy may be discrete or conglomerated due to periadenitis. Caseation may give rise to a hypoechoic center within the nodal mass; although a similar appearance may occur with necrotizing metastatic nodes, in the appropriate clinical setting a diagnosis of tuberculous peritonitis should be considered. Fatty nodes, a characteristic feature of tuberculous peritonitis at CT, may show increased echogenicity relative to other enlarged nodes at US. Focal hypoechoic masses are also frequently seen in the spleen.
Sclerosing Peritonitis
One of the major complications of CAPD is sclerosing peritonitis, a condition characterized by formation of a connective tissue membrane covering the peritoneum and eventually encasing the bowel loops (47,48). Patients experience abdominal pain, progressive loss of ultrafiltration, and symptoms of acute or subacute bowel obstruction. Surgery is required to release the small bowel loops from the encasing membrane, but this may be extremely difficult. The prognosis is poor, with many patients dying in the weeks or months following surgery. In the series of patients reviewed by Hollman et al (47), the fact that the surviving patients received renal transplants soon after diagnosis suggests an associated beneficial effect of immunosuppressive therapy. The causative factors in development of sclerosing peritonitis associated with CAPD have not been fully elucidated. Sclerosing peritonitis may also be idiopathic and has also been associated with exposure to ß-adrenergic blockers (49) and with infection in young girls in tropical or subtropical regions (abdominal cocoon) (50).
US is very useful in diagnosis of sclerosing peritonitis (47,51). Hollman et al (47) found increased peristalsis affecting several bowel loops to be the earliest finding. Fluid, both free and loculated, is a common feature. Thin echogenic strands are seen within the ascitic fluid and are found mainly in a perihepatic location in early disease. With progression of the disease, the bowel wall thickens and may calcify. The loops are matted and become tethered to the posterior abdominal wall (Fig 27). In the later stages of the disease, the characteristic membrane develops. This is seen anterior to the bowel loops and appears as a uniformly echogenic layer varying in thickness from 1 to 4 mm.
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Figure 27. Sclerosing peritonitis in a 62-year-old man who discontinued peritoneal dialysis 2 months earlier because of loss of ultrafiltration. Transverse US image of the midabdomen shows extensive, complex, septated ascites (A).
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Localized Inflammatory Process of the Peritoneal Cavity
Inflammation of any part of the gastrointestinal tract may be accompanied by inflammatory change of the regional fatty tissue, with progression to phlegmon and ultimately abscess or generalized peritonitis. Appendicitis and diverticulitis are the most common acute processes in which these changes are visualized at cross-sectional imaging. Further, localized perforation is more common in both than development of frank peritonitis. Therefore, recognition of the localized manifestation of inflammatory disease at US is essential if US is to be used in assessment of these patients. In addition to appendicitis and diverticulitis, inflammatory bowel disease (52), gangrenous or perforated cholecystitis (53), and pancreatitis are other underlying processes in which recognition of inflamed regional fatty tissue may be the most striking abnormality at cross-sectional imaging.
Inflamed perienteric fat has an echogenic mass effect at US, frequently displacing gas-filled loops of intestine out of the scanning plane (Fig 28). Progression to a phlegmon typically appears as a focal hypoechoic region within the echogenic mass of inflamed fat without fluid content (Fig 29). Abscess development typically appears as fluid with particles, debris, septa, and frequently gas bubbles, surrounded by a wall of varying thickness.
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Figure 28. Localized inflamed fat in a 26-year-old woman with Crohn disease. Sagittal US image of the right lower quadrant shows the thickened terminal ileum in transverse section (arrows). Note the surrounding echogenic inflamed fat (arrowheads).
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Figure 29. Localized inflamed fat and phlegmon in a 19-year-old woman with Crohn disease. Sagittal oblique US image of the right lower quadrant shows the thickened terminal ileum (TI) with echogenic inflamed fat (arrowheads) and a hypoechoic perienteric phlegmon (arrows).
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We believe that the CT appearance of infiltrated fat is very familiar to imagers. In contrast, we think that many sonologists do not recognize this abnormality with US because images recorded remotely do not include the peritoneal cavity or because of lack of familiarity with the US appearance. As with many types of gastrointestinal tract disease, its detection is frequently enhanced by compression US. Further color Doppler imaging often shows increased blood flow in the area of inflamed fat.
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Endometriosis
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Endometriosis occurs when functional endometrium is located outside the uterus and primarily affects women during their reproductive years. The physical manifestations are varied, with some patients being asymptomatic whereas others have disabling pelvic pain, infertility, or adnexal masses (endometriomas). The ovaries are the most common sites affected, but endometriosis can also involve the gastrointestinal tract, urinary bladder, peritoneum, chest, and soft tissues (54). US is the initial imaging modality of choice for detecting and characterizing endometriomas, but small implants and adhesions are often not well demonstrated with US. However, endometriotic plaques may be seen involving the peritoneum and serosal surface of the intestine, particularly with a transvaginal probe (Fig 30).
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Figure 30a. Endometriotic plaques in a 33-year-old woman with endometriosis. Sagittal (a) and corresponding transverse transvaginal (b) US images of a pelvic bowel loop show hypoechoic endometriotic plaque along one serosal surface (arrowheads).
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Figure 30b. Endometriotic plaques in a 33-year-old woman with endometriosis. Sagittal (a) and corresponding transverse transvaginal (b) US images of a pelvic bowel loop show hypoechoic endometriotic plaque along one serosal surface (arrowheads).
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Right-sided Segmental Infarction of the Omentum
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Right-sided segmental omental infarction is a rare clinical entity that usually resolves spontaneously (55). It is thought to occur because of an embryologic variant that results in an anomalous and fragile blood supply to the right inferior portion of the omentum, leaving it susceptible to infarction. Precipitating factors are thought to include kinking of the vessels produced by variation in the patient’s position, vascular congestion due to large meals, and an abrupt increase in intraabdominal pressure. Patients present with acute or subacute right upper or lower quadrant pain occasionally associated with nausea, vomiting, a mildly elevated temperature, mild leukocytosis, and an elevated sedimentation rate. Physical examination reveals localized tenderness and signs of a moderate degree of peritonitis. A right lower quadrant mass may be palpable.
Because the initial clinical differential diagnosis includes acute appendicitis or cholecystitis, US is commonly performed. Puylaert (56) described the US appearances of right-sided segmental infarction in seven patients. Typically, a moderately hyperechoic noncompressible ovoid or cakelike mass is seen at the point of maximum tenderness with no evidence of associated bowel disease (Fig 31). This corresponds to a circumscribed lesion consisting of fat interspersed with hyperattenuating streaks at CT. The usual site is in the right side of the abdomen between the anterior abdominal wall and the hepatic flexure of the colon, at the level of the umbilicus anterolaterally. The mass is adherent to the parietal peritoneum, an observation that can easily be demonstrated at US by scanning in the sagittal plane while the patient breathes deeply. Unlike normal intraabdominal contents, which move in relation to the abdominal wall with respiration, the mass of inflamed fat remains fixed. Follow-up US demonstrates gradual resolution of the US findings, corresponding to the patient’s clinical improvement.
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Figure 31. Right-sided segmental infarction of the omentum in a 38-year-old woman with pain in the right side of the middle to lower abdomen. Transverse US image of the right side of the midabdomen shows an ovoid echogenic mass (arrows), which was noted to be adherent to the parietal peritoneum during respiration. This region was the site of maximal tenderness.
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Pneumoperitoneum
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The role of plain radiography in diagnosis of pneumoperitoneum is well established (57). As little as 1 mL of free air can be detected on an erect chest radiograph. CT is regarded as the standard of reference for evaluating the presence, location, volume, and configuration of free air (58). However, many patients will undergo US as their initial investigation. Although US should not be used to exclude the diagnosis of pneumoperitoneum, its identification is an extremely important finding. US may also allow identification of the underlying cause in a significant number of cases (59).
When assessing the abdomen for evidence of pneumoperitoneum, attention must be paid to technique. The patient should be scanned in the supine and left lateral decubitus positions, with attention concentrated on the epigastrium and right upper quadrant (59,60). The anterior and right lateral aspects of the liver are scanned in each position. Linear-array and sector transducers may be used. Linear-array transducers are thought to be more sensitive to the presence of pneumoperitoneum because of the broader and higher resolution in the near field, where air tends to accumulate.
In the right upper quadrant, pneumoperitoneum is seen as echogenic lines or spots with posterior ring-down or comet-tail artifacts between the anterior abdominal wall and the liver. Ring-down artifact produces multiple parallel lines of similar length, whereas these lines gradually decrease in length in comet-tail artifact. An important observation is the "shifting phenomenon," whereby free air shifts from the anterior to the lateral aspect of the liver as the patient switches from the supine to the left lateral decubitus position (Fig 32).
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Figure 32a. Pneumoperitoneum in a 28-year-old woman with a perforated duodenal ulcer. (a) Sagittal upper midline US image obtained with the patient supine shows a small area of enhancement of the peritoneal stripe (arrow) with ring-down artifact (arrowheads). (b) Sagittal US image of the right upper quadrant, obtained with the patient in the left decubitus oblique position, shows that the area of enhancement of the peritoneal stripe (arrow) has moved anterior to the liver.
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Figure 32b. Pneumoperitoneum in a 28-year-old woman with a perforated duodenal ulcer. (a) Sagittal upper midline US image obtained with the patient supine shows a small area of enhancement of the peritoneal stripe (arrow) with ring-down artifact (arrowheads). (b) Sagittal US image of the right upper quadrant, obtained with the patient in the left decubitus oblique position, shows that the area of enhancement of the peritoneal stripe (arrow) has moved anterior to the liver.
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In some patients, it may be difficult to differentiate pulmonary air from free intraabdominal air in the right upper quadrant, as the appearance produced may be that of a continuous line of reverberation. Lee et al (59) described a technique whereby the patient is evaluated during inspiration and expiration. The pulmonary and pneumoperitoneal reverberation artifacts overlap during inspiration but appear separate at expiration.
In animal and human studies, we have shown that free air may be detected anywhere in the abdomen as obvious enhancement of the peritoneal line and thus can be distinguished from adjacent intraluminal gas (61). Another US sign of pneumoperitoneum in patients with ascites is the presence of air bubbles within the ascitic fluid. They appear as floating echogenic foci (59). These bubbles have a high association with both bowel perforation and infection of the peritoneal fluid.
US may allow detection of the underlying cause of pneumoperitoneum; therefore, the remainder of the abdomen, particularly the stomach and duodenum, should be carefully evaluated for evidence of inflammation or tumor. Lee et al (59) identified the site of perforation in four of five patients. Three had a perforated duodenal ulcer, and one had a perforated gastric neoplasm. The remaining patient had perforation of the ileum, which was not seen at US.
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Mesenteric Cysts
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Mesenteric cysts are rare intraabdominal masses that are often discovered incidentally at imaging. However, they may manifest clinically as abdominal distention because of their size or acutely as pain because of a complication such as hemorrhage, rupture, or torsion. These cysts are most often of lymphatic (lymphangioma) or mesothelial origin, but cysts of enteric (enteric duplication cyst) or urogenital origin may also be encountered. Dermoid cysts and pseudocysts (infectious, inflammatory, or traumatic) are also seen (62). Mesenteric cysts vary in diameter from less than 1 cm to greater than 25 cm, filling the entire peritoneal cavity. They may be entirely simple to highly complex with extensive internal septa, as is sometimes seen with lymphangiomas (63,64) (Fig 33). Smaller mesenteric cysts are frequently mobile, changing position with palpation or with changes in the patient’s position. Asymptomatic cysts are frequently managed conservatively, particularly if they are simple or if they have a typical appearance of a lymphangioma. Surgery is usually reserved to alleviate pressure symptoms or to address acute complications.
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Figure 33a. Lymphangioma in a 58-year-old asymptomatic man. (a) Transverse US image obtained along the midline of the abdomen shows the inferior vena cava (I) and aorta (A). Anterior to these vessels, in the expected location of the mesentery, there is a multicystic mass with thin septa and a lobulated border. (b) Portal venous phase CT image shows that the mass is lobulated, of low attenuation, and nonenhancing.
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Figure 33b. Lymphangioma in a 58-year-old asymptomatic man. (a) Transverse US image obtained along the midline of the abdomen shows the inferior vena cava (I) and aorta (A). Anterior to these vessels, in the expected location of the mesentery, there is a multicystic mass with thin septa and a lobulated border. (b) Portal venous phase CT image shows that the mass is lobulated, of low attenuation, and nonenhancing.
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Conclusions
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Abnormalities of the peritoneum are commonly encountered in everyday practice. We believe that careful US evaluation of the peritoneal surfaces with knowledge of the frequently encountered appearances of pathologic conditions allow recognition of all but the most subtle of abnormalities. Assessment of the peritoneum is easily performed with US, requiring only a slight alteration in scanning technique without significant increase in scanning time. We acknowledge the limitations of US, particularly in obtaining diagnostic images in obese and postoperative patients. Many of the patterns of peritoneal disease are nonspecific, and US findings should be interpreted in light of the patient’s symptoms and clinical findings. However, US remains one of the safest, least expensive, and most available imaging modalities for detection of peritoneal disease and we believe it is complementary to CT and MR imaging.
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Footnotes
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2 Current address:Louth/Meath Group Hospital, Our Lady’s Hospital, Navan, County Meath, Ireland. 
See the commentary by Kruskal
following this article.
Abbreviations: AIDS = acquired immunodeficiency syndrome,
CAPD = continuous ambulatory peritoneal dialysis
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LEARNING OBJECTIVES FOR TEST...
