Peritoneum, Retroperitoneum, and Mesentery

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). The bowel is now at risk of ischemia at the hiatal margin as the tight orifice further constricts the arterial and venous blood supply. Ultimately, if hernias are left untreated, bowel infarction (see Chapter 4) and sometimes death occur. All external hernias are at risk of this dynamic.

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Figure 10-1 Coronal contrast-enhanced CT in an 85-year-old woman with an obstructed right inguinal hernia (large arrow), dilated proximal bowel (curved arrows), and collapsed distal small bowel (arrowhead).

Diaphragmatic Hernia

Diaphragmatic hernias can be congenital or develop as a result of trauma (blunt injury or iatrogenic).

Bochdalek Hernia

Bochdalek hernia accounts for approximately 95% of congenital diaphragmatic hernias and is situated in a posterolateral position, mostly on the left. Sometimes the hiatus is large enough for stomach, small or large bowel, and rarely the spleen to freely enter the chest, which can cause compression of the left lung and mediastinal displacement. Many patients are asymptomatic, however, and the hernia is detected at CT performed for incidental reasons (Fig. 10-2).
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Figure 10-2 Posteroanterior (A) and lateral plain chest radiograph (B) and axial (C), coronal (D), and sagittal (E) contrast-enhanced CT in a 39-year-old woman with a Bochdalek hernia. A posterior soft tissue supradiaphragmatic density (large arrow) and gas lucency (small arrow) are identifiable on chest radiograph, and represent stomach and pancreas, identified on axial, coronal, and sagittal CT images (arrowheads).

Morgagni Hernia

Morgagni hernia is uncommon, comprising approximately 2% of congenital diaphragmatic hernias. It is situated anteriorly and occurs when the colon and omentum (less commonly stomach, small bowel, and liver) herniate through the foramen of Morgagni, situated adjacent to the sternal xiphoid process. Most are asymptomatic, but the heart may be compressed by the herniated bowel (Fig. 10-3).
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Figure 10-3 Lateral plain radiograph of the chest (A) and axial (B) and sagittal (C) contrast-enhanced CT in a 77-year-old man with a Morgagni hernia and the hepatic colonic flexure passing into the chest anteriorly (large arrows). There is also a small left subphrenic collection (small arrows).

Diaphragmatic Eventration

Diaphragmatic eventration is not actually a hernia, but rather a congenital elevation of one side of an intact diaphragm, creating a space above the normally situated diaphragm that becomes filled by the bowel. It is quite common, usually small (Fig. 10-4) but occasionally large (Fig. 10-5), and asymptomatic in most cases. In the newborn, however, it can cause respiratory distress.
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Figure 10-4 Plain posteroanterior (A) and lateral chest radiograph (B) in a 66-year-old woman with right hemidiaphragmatic elevation caused by slight eventration (arrows).
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Figure 10-5 Posteroanterior chest radiograph in an 83-year-old woman with a large left diaphragmatic eventration with elevation of the splenic flexure (arrow), which remains in the abdomen.

Diaphragmatic Rupture

Diaphragmatic rupture is a tear of the diaphragm and is usually traumatic. Given that abdominal pressure is higher than chest pressure, the bowel often herniates through the diaphragmatic rent, which may sometimes be sufficient to cause symptomatic compression of the lungs or heart. The diagnosis is usually made by multiplanar CT or MRI (Figs. 10-6 and 10-7), although oral contrast material injected through a nasogastric tube should demonstrate herniated bowel contents in the chest. Most traumatic diaphragmatic hernias require surgical repair.
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Figure 10-6 Axial (A and B) and coronal (C) noncontrast CT in a 78-year-old woman who was recently involved in a motor vehicle accident. She has left diaphragmatic rupture (large arrows) and herniation of the splenic flexure (small arrows) into the chest, causing lung compression (arrowheads).
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Figure 10-7 Axial (A) and coronal (B) contrast-enhanced CT in a 29-year-old man who was recently involved in a motor vehicle accident and has left diaphragmatic rupture and splenic herniation into the chest (arrows).

Pneumoperitoneum

Although pneumoperitoneum is sometimes referred to as “free air,” the gas is generally not “air,” but rather carbon dioxide inserted from laparoscopic procedures or bowel gas that is present in an extraluminal location. The gas may resemble air, however, for a short while after open laparotomy procedures. The presence of pneumoperitoneum may therefore be a benign postsurgical finding or may represent more sinister causes secondary to bowel ischemia or perforation (Box 10-1). The detection of pneumoperitoneum is important and sometimes critical because it may be the only imaging sign of bowel perforation. Its detection, however, can be challenging (especially on plain radiograph) but is more likely when the x-ray beam is tangential to the location of the gas, which rises to the most nondependent part of the abdomen. Therefore, on an upright view the gas is most likely to be identified under the diaphragm (Fig. 10-8), but on an anteroposterior view the gas may not be appreciated because the x-ray beam does not pass tangential to it. Pneumoperitoneum in the lateral or sagittal plane is better appreciated with computed tomography (CT), which demonstrates the extraluminal gas against the anterior abdominal wall (Fig. 10-9). Smaller volumes of pneumoperitoneum can be quite subtle to detect on plain radiograph (Fig. 10-10), and a lateral chest view may be required before small volumes of gas are identified (Fig. 10-11). The most sensitive plain radiograph procedure is the left decubitus (right side up) view of the right upper quadrant, where as little as 1 mL of extraluminal gas can be detected between the liver margin and diaphragm. This procedure is rarely performed, primarily because CT is a far more sensitive tool for the detection of pneumoperitoneum, especially for small volumes of gas. However, even larger volumes of extraluminal gas may be missed unless viewed using lung window contrast settings, since the gas might otherwise be confused with intraluminal gas (Fig. 10-12). Other plain radiograph findings can be demonstrated with larger volumes of gas. These include the visualization of the falciform ligament (Fig. 10-13), which is also better visualized by CT (Fig. 10-14) or recognized as a “football” sign, representing a large ovoid lucency in the center of the abdomen on supine radiographs. Gas can also sometimes be visualized in the Morison pouch or may outline the lateral umbilical ligaments, but more often both sides of the bowel wall are outlined, usually referred to as the Rigler sign (less commonly, double-wall sign). This sign is generally identified on the supine view, and large volumes of extraluminal gas are usually present (Fig. 10-15). The sign may be quite subtle or obvious (Fig. 10-16).
 
Box 10-1   Causes of Pneumoperitoneum
Iatrogenic

Postsurgery
Postlaparoscopy
Endoscopy
Peritoneal hemodialysis
Barium enema
Perforated Viscus

Bowel obstruction (e.g., volvulus)
Ischemia (e.g., obstructed hernia)
Peptic ulcer disease
Diverticulitis
Appendicitis
Colitis (e.g., Crohn, infectious)
Malignancy
Steroids
Pneumatosis
Intrathoracic

Pneumomediastinum
Pneumothorax
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Figure 10-8 Upright plain abdominal radiograph in a 51-year-old woman who recently underwent abdominal surgery. A large pneumoperitoneum is best appreciated in the most nondependent part under the diaphragms (arrows).

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Figure 10-9 Sagittal reconstruction CT on lung window settings in a 41-year-old man with extraluminal gas against the anterior abdominal wall (arrow).
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Figure 10-10 Upright abdominal radiograph in an 83-year-old man who recently underwent abdominal and chest surgery. A “sliver” of gas (arrow) under the right hemidiaphragm represents pneumoperitoneum.

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Figure 10-11 Posteroanterior (A) and lateral chest radiograph (B) in a 40-year-old woman who recently underwent abdominal surgery. Subtle pneumoperitoneum is detected only on the lateral view (arrow).
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Figure 10-12 Axial contrast-enhanced CT on soft tissue (A) and lung window (B) contrast settings in a 51-year-old man. The differentiation of intraluminal (large arrows) from extraluminal (small arrows) gas is far better appreciated on lung window settings.
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Figure 10-13 Magnified view of supine abdominal radiograph in a patient with pneumoperitoneum that outlines the falciform ligament (arrows) and a Rigler sign (arrowheads).
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Figure 10-14 Coronal contrast-enhanced CT on lung windows in a 53-year-old man with pneumoperitoneum (arrow) with the falciform ligament outlined (small arrow).
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Figure 10-15 Upright abdominal radiograph in a 78-year-old woman with Rigler sign (arrow) and gas under both diaphragms (small arrows).
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Figure 10-16 Supine plain abdominal radiograph in a 63-year-old woman with obvious Rigler sign (arrows) caused by pneumoperitoneum.

Peritoneal Disease

As a potential space, the peritoneal cavity can fill with fluid (ascites) and is susceptible to a number of inflammatory conditions. There are also a number of rare primary neoplastic lesions involving the mesentery, although metastatic deposits from intraabdominal malignancies are more common, particularly given its larger surface area.

Peritonitis

At imaging, the presence of ascites, loculated or otherwise (Fig. 10-17), can be confirmed by ultrasound (US) or contrast-enhanced CT. The features are generally nonspecific, and the diagnosis is based on clinical symptoms and signs (e.g., history of recent endoscopic procedure). Chronic peritonitis (which is sometimes recognized in patients undergoing peritoneal hemodialysis) may heal by peritoneal calcification that envelops the intraabdominal organs (Fig. 10-18). Tuberculous peritonitis often demonstrates regional adenopathy, terminal ileitis, diffuse ascites, and omental thickening (Fig. 10-19). Sometimes there is a characteristic “cocoon” appearance as the fibrotic mesenteric process encapsulates the small bowel (also known as sclerosing encapsulating peritonitis) (Fig. 10-20).
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Figure 10-17 Axial contrast-enhanced CT in a 35-year-old man with tuberculous peritonitis and enhancing peritoneal lining (arrows) and a cocoon-like appearance to the small bowel (arrowheads).
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Figure 10-18 Plain abdominal radiograph (A) and axial contrast-enhanced CT (B) in a 25-year-old woman with diffuse peritoneal calcification (arrows) from multiple prior episodes of peritonitis resulting from hemodialysis.
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Figure 10-19 Axial contrast-enhanced CT in a 20-year-old woman with tuberculous peritonitis and diffuse ascites, terminal ileal thickening (A; arrow), and omental thickening (B; small arrows). There are also retroperitoneal nodes (arrowhead).
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Figure 10-20 Axial (A) and coronal (B) contrast-enhanced CT in a 29-year-old man with diffuse tuberculous peritonitis with peritoneal fibrosis and confinement of the small bowel mesentery and bowel centrally (arrows) in a cocoon-like appearance. There is also colonic tuberculous disease (small arrow).

Peritoneal Abscess

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Figure 10-21 Axial contrast-enhanced CT in a 67-year-old man with intraabdominal abscess (arrows) and multiple small gas bubbles.
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Figure 10-22 Axial contrast-enhanced CT in a 75-year-old man with colonic perforation and intraabdominal abscess (A; arrowheads) with a gas-pus fluid level (A; arrow) and anterior extraluminal gas (B; arrow) as seen on lung window settings.
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Figure 10-23 Axial (A) and coronal (B) contrast-enhanced CT in a 59-year-old man with a sigmoid diverticular abscess (arrowhead) and an enhancing wall (arrows).
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Figure 10-24 Plain abdominal radiograph (A) and axial contrast-enhanced CT (B) in a 59-year-old woman with a large necrotic pelvic sarcoma (arrows).

Mesenteritis

Mesenteritis is a benign process of unknown cause and is also known as fibrosing mesenteritis, sclerosing mesenteritis, retractile mesenteritis, or mesenteric panniculitis. It represents inflammation of the mesenteric fat that is identified at CT as a “hazy” mesentery produced by inflammatory change (Fig. 10-25). It often heals by fibrosis as a simple mesenteric mass (with or without associated calcification) (Fig. 10-26) or with constriction or retraction of the mesentery, often with calcification (Fig. 10-27). It should be differentiated from mesenteric metastases from carcinoid, which can have similar appearances (Fig. 10-28).
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Figure 10-25 Axial contrast-enhanced CT in a 58-year-old woman with “hazy” mesentery (arrow) resulting from mesenteritis.
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Figure 10-26 Axial contrast-enhanced CT in a 56-year-old woman with an irregular calcified mesenteric mass (arrow) caused by prior mesenteritis.

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Figure 10-27 Axial (A) and coronal (B) contrast-enhanced CT in a 73-year-old woman with a calcified mesenteric mass (large arrows) with mesenteric retraction (small arrow) caused by retractile mesenteritis.
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Figure 10-28 Axial contrast-enhanced CT with a right lower quadrant mass representing carcinoid (large arrow) and a mesenteric desmoplastic reaction and associated colonic thickening (small arrows).

Mesenteric Adenitis

Inflammation of the mesenteric nodes is not uncommon. It is usually nonspecific and recognized on CT as a number of slightly enlarged mesenteric lymph nodes associated with inflammatory fat changes (fat stranding) (Fig. 10-29). It is most often idiopathic but can be bacterial (Fig. 10-30) or tuberculous (Fig. 10-31). Reactive adenitis can also occur with small or large bowel infections or inflammatory bowel disease (see Chapter 4).
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Figure 10-29 Axial contrast-enhanced CT in a 56-year-old man with several slightly enlarged mesenteric nodes (large arrow) and inflammatory fat changes resulting from mesenteric adenitis.
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Figure 10-30 Axial contrast-enhanced CT in a 61-year-old man with a number of enlarged right lower quadrant mesenteric nodes (arrows) due to bacterial mesenteric adenitis.
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Figure 10-31 Axial contrast-enhanced CT in a 29-year-old woman with intraabdominal tuberculosis and multiple mesenteric nodes (arrow). There are also tuberculous deposits on the liver capsule (small arrows).

Mesenteric Fat Necrosis

Mesenteric fat necrosis is often referred to as an omental infarct and simply represents arterial disruption of a small area of mesentery, leading to infarction. It is most commonly identified in obese elderly patients and in those who have had recent abdominal surgery. Patients present with acute abdominal pain, which can be mistaken for appendicitis, diverticulitis, or epiploic appendagitis. CT findings, which can be quite subtle, are a focal area of omentum or peritoneal fat with heterogeneous edematous change (Fig. 10-32). Larger areas of fat necrosis may show gas within the infarction (Fig. 10-33, A) and even fat/fluid levels (Fig. 10-33, B).
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Figure 10-32 Axial contrast-enhanced CT in a 39-year-old woman with omental fat necrosis (arrow).
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Figure 10-33 Axial contrast-enhanced CT in a 54-year-old man with left upper quadrant fat necrosis and gas formation (A; arrows). Slightly cephalad, there is a fat/fluid level (B; arrow).

Necrotizing Fasciitis

Necrotizing fasciitis is a rare, often fatal, infection of the skin and subcutaneous tissue that is caused by gram-positive and -negative bacteria, most commonly in patients with immunosuppression, diabetes mellitus, malignancy, or alcoholism. The infection is usually secondary to trauma (surgical or nonsurgical) and develops rapidly, along with widespread fat necrosis (Fig. 10-34).
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Figure 10-34 Axial (A) and coronal (B) noncontrast CT in a 44-year-old woman with necrotizing fasciitis. There is diffuse fat necrosis and gas formation in the abdominal wall (arrows).

Injection Fat Necrosis and Granuloma

Injection fat necrosis and granuloma are commonly identified in patients who are hospitalized and have received multiple subcutaneous injections. Their features are characteristic at CT and include rounded, soft tissue changes in the subcutaneous fat because of localized fat necrosis (Fig. 10-35, A). Sometimes they show increased fluorodeoxyglucose (FDG) activity on positron emission tomography (PET) (Fig. 10-35, B). They often heal by dystrophic calcification (Fig. 10-36).
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Figure 10-35 A, Axial contrast-enhanced CT in a 71-year-old woman with multiple subcutaneous soft tissue masses (large arrow) in the anterior abdominal wall caused by fat necrosis from subcutaneous injections. B, These can demonstrate mild fluorodeoxyglucose uptake at PET (small arrow) because of the inflammatory nature of the fat necrosis.
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Figure 10-36 Axial contrast-enhanced CT in a 73-year-old woman with multiple calcified buttock injection granulomata (arrows).

Benign Peritoneal Masses

Peritoneal Inclusion Cyst

Peritoneal inclusion cysts represent loculated simple fluid within the abdomen, usually resulting from adhesions, and are identified primarily in an adnexal location. Therefore they must be differentiated from ovarian cysts. They are seen in women, mostly those of reproductive age, who have had prior pelvic inflammatory diseases or surgery. At imaging the cysts are usually complex with fluid and thin septa but do not typically contain solid elements (Fig. 10-37). They can become large, filling almost the entire pelvis (Fig. 10-38).
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Figure 10-37 Transvaginal US (A) and axial contrast-enhanced CT (B) in a 39-year-old woman with a complex cystic lesion representing a peritoneal inclusion cyst (arrows).
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Figure 10-38 Axial T2-weighted MRI in a 37-year-old woman with a large peritoneal inclusion cyst (large arrow), distinct from the bladder (small arrow).

Lymphangioma (Mesenteric Cyst)

Lymphangiomas (mesenteric cysts) are cystic lesions arising within the abdomen and are usually caused by obstructed lymphatics. They are to be differentiated from mesenteric duplication cysts, which are also cystic. They are usually identified incidentally at CT as simple irregular cystic structures (Fig. 10-39) that may show punctate calcification in the cyst wall. These cysts can also occur in a retroperitoneal location (Fig. 10-40).
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Figure 10-39 Axial contrast-enhanced CT in a 53-year-old man with a lower abdominal cystic mesenteric lesion (arrow) representing lymphangioma.
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Figure 10-40 Axial contrast-enhanced CT in a 39-year-old woman with a 3-cm retroperitoneal cystic structure (arrow) representing lymphangioma.

Seroma

Seroma refers to a well-circumscribed, low-density mass representing a pocket of serous fluid that most commonly has collected as leakage from surgically damaged regional vasculature. Seromas less commonly result from trauma. Their appearances at CT are characteristic, with a well-defined smooth mass (Fig. 10-41) of uniform fluid density. The diagnosis is likely in patients with the appropriate surgical history.
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Figure 10-41 Axial contrast-enhanced CT in a 67-year-old man with a postoperative seroma (arrows).

Lymphangiectasia

Lymphangiectasia is a benign, usually congenital disease that is caused by dilated peritoneal lymphatics, usually idiopathic in nature but sometimes resulting from the lymphatic obstructive effects of granulomatous disease or malignancies. The lymphatic obstruction can lead to diarrhea, hypoproteinemia, and small bowel mucosal thickening (see Chapter 4). The disease is usually identified at CT as cystic structures (sometimes similar to lymphangioma) along the route of the mesentery (Fig. 10-42).
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Figure 10-42 Axial contrast-enhanced CT in a 49-year-old woman with congenital lymphangiectasia (arrows).

Dermoid Tumor

Dermoid tumors are most commonly present in the pelvis, but larger lesions can extend into the abdomen and are ovarian in origin. They are also known as dermoid cysts or cystic teratomas and represent a primitive tumor that contains multiple tissue elements, including fat, teeth, hair, and cartilage, among other tissues. Almost all of these tumors are benign, although a malignant teratoma is recognized. They are readily identifiable at CT by their fat content (Fig. 10-44) (and sometimes other soft tissue or calcified features) and at US by diffuse hyperechogenicity resulting from the fat content. Rarely, multiple well-circumscribed fatty dermoid masses can be identified throughout the abdomen after traumatic rupture of the pelvic neoplasm, which disseminates throughout the peritoneum. At ultrasound, dermoid tumors are thought to appear similar to falling snow or a snowstorm (Fig. 10-45), but they might also be identified by other calcified or soft tissue elements.
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Figure 10-44 Axial contrast-enhanced CT in a 41-year-old woman with a predominantly fatty mass due to a dermoid tumor (large arrow). There is also dense internal calcification (small arrow).
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Figure 10-45 Transvaginal US in a 33-year-old woman with a left adnexal dermoid tumor (arrows) and a diffuse hyperechogenicity caused by its fat content.

Malignant Peritoneal Masses

Metastases

Metastatic deposits are by far the most common malignant peritoneal mesenteric masses or retroperitoneal deposits. They arise by direct invasion or ascitic spread (Figs. 10-46 and 10-47) or via hematogenous or lymphatic routes. The deposits can be either single and focal (Fig. 10-48) or diffuse. Diffuse involvement can invade the omentum, causing it to be studded with tumor deposits (known as omental “caking”), an appearance that can be subtle (Fig. 10-49) or obvious (Fig. 10-50). This should be differentiated from the omental caking resulting from diffuse tuberculous abdominal disease (Fig. 10-51).
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Figure 10-46 Axial (A) and coronal (B) contrast-enhanced CT in a 61-year-old woman with metastatic ovarian cancer that characteristically “scallops” the liver capsule (arrows). There is widespread malignant ascites.
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Figure 10-47 Axial contrast-enhanced CT in an 83-year-old woman with diffuse peritoneal ascites and crowding of small bowel caused by metastatic ovarian cancer.
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Figure 10-48 Axial contrast-enhanced CT in a 64-year-old woman with a soft tissue metastatic deposit (arrow) in Morison pouch from ovarian cancer.
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