PLAIN FILM OF THE ABDOMEN: INTRODUCTION

In recent years, new techniques such as ultrasonography, computerized tomography (CT), and MR imaging have been used widely and have altered the use of plain films of the abdomen in the evaluation of abdominal diseases. Plain films of the abdomen are still used primarily to assess calcifications and intestinal perforation or obstruction. The plain radiograph is commonly used as a preliminary radiograph before other studies such as CT and barium enema. Plain abdominal radiography is routinely employed before intravenous urography (IVU) because stones in the urinary tract can be obscured by iodinated contrast material but may be shown on plain abdominal radiographs. The yield of plain radiographs is higher in patients with moderate or severe abdominal symptoms and signs than in those with minor symptoms.

TECHNIQUE AND NORMAL IMAGING

Technique

The most common plain radiograph of the abdomen is an anteroposterior (AP) view with the patient in the supine position. The AP view of the abdomen is also called a KUB film because it includes the kidneys, ureters, and bladder. When acute abdominal disease is suspected clinically, an erect film of the abdomen and a posteroanterior (PA) view of the chest are also required. Digital imaging is becoming more common, and abdominal images may be viewed on computer monitor rather than on films.

Normal Imaging

SOFT TISSUE

The abdomen is composed primarily of soft tissue. The density of soft tissue is similar to the density of water, and the difference in density between solid and liquid is not distinguishable on a plain radiograph. The liver is a homogeneous structure located in the right upper quadrant; the hepatic angle delineates the lower margin of the posterior portion of the liver (Fig. 8–1). In the left upper quadrant a similar angular structure, the splenic angle, can be identified by the fat shadow around the spleen (see Fig. 8–1).

Organ enlargement can be recognized by the effect of displacement on nearby bowel loops or by obliteration of the adjacent normal fat or gas pattern. Hepatomegaly may compress the proximal transverse colon below the right kidney. Splenomegaly may push the splenic flexure of the colon downward. A large fused renal shadow across the psoas muscle and lumbar spine suggests a horseshoe kidney.

FAT SHADOW

Fat density, which is between that of soft tissue and that of gas, outlines the contour of solid organs or muscles. In obese patients, fat may not be distinguishable from ascitic fluid on plain abdominal film. The flank stripe, also called the properitoneal fat stripe, is a line of fat next to the muscle of the lateral abdominal wall (see Fig. 8–1). The flank stripes are symmetrically concave or slightly convex in obese people located along the side of the abdominal wall. The normal properitoneal fat stripe is in proximity to the gas pattern seen in the ascending or descending colon. Widening of the distance between the properitoneal fat stripe and the ascending or descending colon suggests fluid, such as abscess, ascitic fluid, or blood within the paracolic gutter.

Fat is present in the retroperitoneal space adjacent to the psoas muscle (see Fig. 8–1). The psoas muscle shadow may be absent unilaterally or bilaterally as a normal variant or as a result of inflammation, hemorrhage, or neoplasms of the retroperitoneum. Unilateral convexity of the psoas muscle contour suggests an intramuscular mass or abscess. The quadratus lumborum muscles may be delineated by fat located lateral to the psoas shadow (see Fig. 8–1). In the pelvis, the fatty envelope of the obturator internus muscle is seen on the inner aspect of the pelvic inlet (see Fig. 8–1). The dome of the urinary bladder may be delineated by fat.

GAS PATTERN

Gas has the lowest density (radiolucency) in the abdomen. It is seen in the stomach and colon, but it is rarely seen in the normal small bowel because the air rapidly traverses the organ. The presence of more than a minimal amount of gas in the small bowel should be considered abnormal and is indicative of a functional ileus or mechanical obstruction. Identification of the differences between the gas shadows of the jejunum, ileum, or colon helps to assess the location of bowel obstruction (Fig. 8–2). A gas pattern in distended intestinal loops is usually limited above the point of mechanical obstruction, but functional ileus has a more diffuse distribution in both the small intestine and the colon. If the gas shadow in the intestine is displaced to an unusual location, a soft-tissue mass, either inflammatory or neoplastic, may be suspected. The presence of air-fluid levels in a distended small intestine on upright films suggests either functional ileus or mechanical obstruction. Fluid levels within the stomach or colon are ordinarily of no pathologic importance, because fluid may be introduced by oral agents or by cleansing enemas. The presence of solid material with a mottled appearance and small bubbles of gas surrounded by the colonic contour suggests feces in the colon.

A large amount of gas seen in the peritoneal cavity indicates postoperative status or bowel perforation. Air bubbles in the peritoneal cavity indicate a perforated viscus, abscess, or necrotic tumor. In the right upper quadrant, air that is seen in the biliary tree or around the gallbladder suggests cholecystoenteric fistula or emphysematous cholecystitis. A finely arborizing gas pattern over the right upper quadrant that extends peripherally to the edge of the liver is characteristic of hepatic portal vein gas. In the bowel wall, multiple air bubbles may indicate pneumatosis cystoides intestinalis. Extraluminal gas also may appear within the retroperitoneal structures, including the lesser omental bursa, a subhepatic site, the paraduodenal fossa, and the pericecal or periappendiceal areas. A gas pattern seen below the bony pelvis indicates an inguinal or femoral hernia.

BONY STRUCTURE OR CALCIFICATION

Bony structures or calcifications have the highest density (radiopacity) that is seen on plain films. Bony structures comprise the ribs superiorly, the lumbar spine, and the pelvis. Calcifications in the abdomen include calcified arteries, calculi in the urinary or biliary tract, prostatic calculi, pancreatic calcifications (which are usually indicative of chronic pancreatitis, with or without carcinoma), appendicolith, or ectopic gallstone in the small bowel associated with mechanical obstruction from gallstone ileus. Some foreign bodies, including ingested foreign bodies, bullets, or surgical clips, may be seen in the abdomen. Other rare structures, such as parasitic, metastatic, or heterotopic bone formations, also may be seen in the abdomen.

Suspicion of urinary calculi is a common indication for abdominal radiography. About one-half of calculi in the urinary tract that are visible via unenhanced helical CT can be detected on plain abdominal films. On the other hand, about 15% of gallstones are radiopaque and are seen on abdominal plain radiographs. Ultrasonography is the better choice for evaluating gallstones. Oral cholecystography (OCG) is occasionally used to demonstrate radiolucent gallstones.

TECHNIQUE SELECTION

The routine abdominal films consist of supine and upright views. If the patient cannot stand for an erect abdominal film and a PA view of the chest, the cross-table lateral projection with the right side elevated may be used to assess pneumoperitoneum and air-fluid levels. As little as 1 to 2 mL of free air in the peritoneal space may be identified if the films are appropriately obtained. The PA view of the chest is usually obtained as part of an acute abdominal series because an abnormality in the chest may have symptoms referred to the abdomen.

Oblique views and conventional tomography of the abdomen are obtained, especially in examination of the urinary tract, when full delineation of the ureters and depiction of the renal collecting system are desirable. Likewise, oblique studies of the ureter and urinary bladder are helpful in delineating abnormalities of that organ.

Plain abdominal radiography is less sensitive in evaluating solid organs or metastases. In recent years, increased use of cross-sectional techniques, such as ultrasonography and CT, has shown them to be more sensitive in assessing disorders of the abdominal solid organs and metastatic diseases. Acute cholecystitis is better assessed by ultrasonography or nuclear medicine studies.

EXERCISE 8-1: UPPER ABDOMINAL CALCIFICATIONS

Clinical Histories:

Case 8-1. A 44-year-old woman presents with right upper quadrant pain (Fig. 8–3).

Case 8-2. A 36-year-old woman presents with flank pain (Fig. 8–4).

Case 8-3. A 48-year-old man who is an alcoholic presents with epigastric pain (Fig. 8–5).

Case 8-4. A 59-year-old woman is seen who underwent colectomy surgery for colon cancer 10 years ago (Fig. 8–6).

Questions:

Radiologic Findings:

8-1. Figure 8–3 demonstrates multiple faceted calcifications in the right upper quadrant, which are characteristic for gallstones. (B is the correct answer to Question 8-1.)

8-2. Figure 8–4 shows three separate deposits of calcified density confined to the right renal shadow. The largest one measures 2 cm in greatest diameter. (C is the correct answer to Question 8-2.)

8-3. Figure 8–5 shows multiple stippled calcifications in the upper abdomen adjacent to the lumbar spine. In a patient with a history of alcoholism, pancreatic calcification from chronic pancreatitis would be the most likely diagnosis. (C is the correct answer to Question 8-3.)

8-4. Figure 8–6 shows stippled and discrete calcifications overlying the right 12th rib, just above the renal outline. When calcification in the lung base, skin, retroperitoneum, pancreas, kidney, and adrenal glands is excluded, hepatic calcification should be considered in a patient with a history of colon cancer. (B is the correct answer to Question 8-4.)

Discussion:

About 15% to 20% of gallstones are calcified sufficiently to be seen on a plain abdominal film. Most gallstones comprise mixed components, including cholesterol, bile salts, and biliary pigments. Pure cholesterol and pure pigment stones are uncommon. Calcified gallstones vary in size and shape. Most gallstones have thin, marginal calcification with central lucency and are laminated, faceted, or irregular in shape. Some gallstones contain gas in their fissures whether calcified or noncalcified, called the Mercedes Benz sign. Milk of calcium or "limy" bile occurs in patients with long-standing cystic duct obstruction. The bile contains a high concentration of calcium carbonate and is densely radiopaque on plain radiograph (Fig. 8–7). Calcification of the gallbladder wall (porcelain gallbladder) develops in patients with chronic cholecystitis, cholelithiasis, and cystic duct obstruction. Porcelain gallbladder is characterized by curvilinear calcification in the muscular layer of the gallbladder mimicking a calcified cyst (Fig. 8–8).

Nephrolithiasis is the most common cause of calcification within the kidneys. Most renal calculi (85%) contain calcium complexed with oxalate and phosphate salts. Any process that creates urinary tract stasis may cause the development of urinary calculi. Renal calculi are usually small and lie within the pelvicalyceal system or in a calyceal diverticulum. They may remain and increase in size, or they may pass distally. When calcifications are seen projecting over the renal shadows on routine films of the abdomen, an oblique view or conventional tomography is often needed to localize the densities in relation to the kidneys. A staghorn calculus contains calcium mixed with magnesium, ammonium, and phosphate and forms in the environment of recurrent urinary tract infection with alkaline urine. In general, CT is more sensitive than plain radiography in evaluating urinary calculi.

The adrenal gland is located at the superomedial part of the adjacent kidney. The right gland is lower than the left. Normally the adrenal gland measures less than 2.5 x 3 cm. Stippled, mottled, discrete, or homogeneous calcifications may appear as a portion of the adrenal gland or may occupy the entire organ, forming a triangular clump in the adrenal glands (Fig. 8–9). Most adrenal calcifications are incidental findings in normal-sized glands. They are caused by neonatal adrenal hemorrhage, prolonged hypoxia, severe neonatal infection, or birth trauma. Less than one-fourth of patients with Addison's disease have adrenal calcifications.

In the United States, 85% to 90% of patients with pancreatic lithiasis are alcoholics. Conversely, less than half of patients with chronic pancreatitis ever develop pancreatic calcifications visible on plain radiograph. Although gallstones passing through the biliary tract can cause acute pancreatitis, chronic pancreatitis or pancreatic calcification is rarely caused by cholelithiasis.

Hepatic calcifications are caused primarily by neoplasms, infections, or parasitic infestations. Primary hepatic tumors, both benign and malignant, may have calcifications. Colonic carcinoma and papillary serous cystadenocarcinoma of the ovary are the most frequent primary tumors causing calcified metastases in the liver. Other primary neoplasms in the thyroid gland, lung, pancreas, adrenal gland, stomach, kidney, and breast may cause calcified hepatic metastases. Inflammatory calcified granulomas related to tuberculosis or histoplasmosis are common in miliary calcifications. Calcified cystic lesions, such as echinococcus disease in the liver, are commonly seen in areas of the world where the causative organism is endemic.

EXERCISE 8-2: PELVIC CALCIFICATIONS

Clinical Histories:

Case 8-5. A 15-year-old boy presents with right lower quadrant pain and fever (Fig. 8–10).

Case 8-6. A 15-year-old boy presents with right lower quadrant pain and fever (Fig. 8–10).

Case 8-7. A 48-year-old woman presents with lower abdominal fullness (Fig. 8–12).

Case 8-8. A 14-year-old girl presents with lower abdominal pain and a palpable mass in the pelvis (Fig. 8–13).

Questions:

Radiologic Findings:

8-5. This case (Fig. 8–10) is that of a boy with acute appendicitis. (A is the correct answer to Question 8-5.) An oval calcification measuring 0.8 cm in diameter projects over the iliac bone and laterally to the right sacroiliac joint with a distended appendiceal lumen filled with gas. At surgery, gangrenous appendicitis with perforation and an obstructing appendicolith were found.

8-6. This case (Fig. 8–11) demonstrates a 5- x 5-m and a 4- x 4-mm calcified density (arrows) along the expected course of the right distal ureter. These densities were formerly identified in the right kidney and have migrated inferiorly to their current positions, indicating right ureteral calculi. With the history of hematuria, the most likely choice would be right ureteral calculi. (C is the correct answer to Question 8-6.)

8-7. Figure 8–12 shows a large, 2-cm-diameter mottled calcification and curvilinear calcifications in the midpelvis. These calcifications overlie the sacrum and are consistent with calcification in uterine fibroids. (D is the correct answer to Question 8-7.)

8-8. Figure 8–13 shows several "teeth-like" calcifications in the right side of the pelvis. With a palpable pelvic mass, the most likely diagnosis is ovarian dermoid cyst. (C is the correct answer to Question 8-8.)

Discussion:

Calcified appendiceal stones are present in only about 10% of patients with appendicitis; however, in a symptomatic child, an appendicolith indicates at least a 90% chance of acute appendicitis. Prophylactic appendectomy has been recommended in the child with an incidentally discovered appendicolith because of a high incidence of gangrene and perforation.

Ureteral calculi are always a consideration in patients with hematuria. About 50% of urinary calculi are radiographically opaque and shown on the plain abdominal radiograph. Close scrutiny of the abdominal film is crucial because ureteral calculi may be elusive when they project over the lumbar transverse processes or the sacroiliac region. To confirm a ureteral calculus, CT or intravenous urography is often needed to localize the density to the ureter. CT is more sensitive in evaluating ureteral calculi. Phleboliths are thrombi within the pelvic veins, and this location accounts for their circular shape. Calcification within these thrombi starts peripherally with a typical radiolucent center that is seen radiographically. Phleboliths have little clinical significance except that they can be confused with other pelvic densities, particularly distal ureteral calculi. In general, ureteral stones lie above and medially to the ischial spines, and they lack a radiolucent center.

Most uterine leiomyoma calcifications appear as multiple mottled or speckled calcifications or as dense, smooth, curvilinear calcifications around the mass. The real soft-tissue mass is often larger than the area of calcification. Other calcifications in the pelvis include calcified ovarian tumors (Fig. 8–14), foreign material, lymph nodes, or prostate.

Ovarian dermoid cysts account for about 10% of ovarian neoplasms. Ovarian dermoid cysts range from 6 to 15 cm in diameter and contain teeth, abortive bone, and curvilinear capsular calcification, which may be seen on plain radiographs. A dermoid cyst may contain sebaceous material simulating low-density fat compared to surrounding soft tissue.

Bladder stone is often seen in association with bladder outlet obstruction. Bladder calculi are composed of mixed calcium oxalate and phosphate salts that are radiopaque. Other calcifications in the bladder include foreign body, transitional cell carcinoma, urachal carcinoma, schistosoma infestation, tuberculosis, or alkaline encrusting cystitis. Calcifications in the same area include prostatic calculi (Fig. 8–15) and calcified vas deferens (Fig. 8–16). The prostate gland may be calcified. If enlarged, it may protrude into the bladder.

EXERCISE 8-3: INCREASED ABDOMINAL DENSITY OR MASSES

Clinical Histories:

Case 8-9. A 57-year-old man presents with history of hepatitis (Fig. 8–17).

Case 8-10. A 35-year-old woman presents with fever and anemia (Fig. 8–18).

Case 8-11. A 40-year-old man presents with back pain (Fig. 8–19).

Case 8-12. A 45-year-old woman presents with lower abdominal fullness (Fig. 8–20).

Questions:

Radiologic Findings:

8-9. In this case (Fig. 8–17), the right side of the abdomen shows increased density and is relatively free of gas. Displacement of the gas pattern in the duodenum and jejunum to the left side is indicative of hepatomegaly. (C is the correct answer to Question 8-9.) A radionuclide liver scan showed hepatic metastases from lung cancer.

8-10. In this case (Fig. 8–18), a soft-tissue mass in the left upper quadrant displaces the gas in the splenic flexure of the colon downward. Left adrenal or renal cell carcinoma rarely presents as a large mass to the left of the midline. The most likely diagnosis is splenomegaly. (D is the correct answer to Question 8-10.)

8-11. In this case (Fig. 8–19), a mass in the midabdomen delineates the lower poles of both kidneys, which are fused at the midline, consistent with horseshoe kidney. (D is the correct answer to Question 8-11.) Small renal calculi (arrowheads) are present bilaterally.

8-12. Figure 8–20 shows a soft-tissue mass in the pelvis. In a middle-aged woman, an ovarian or uterine mass would be the most likely considerations. Ultrasonography of the pelvis showed a large, fluid-filled mass, confirmed surgically as an ovarian cyst. (A is the correct answer to Question 8-12.)

Discussion:

Although abdominal plain radiographs are useful in detecting hepatomegaly or splenomegaly, they are of little use in diagnosing hepatic disease, particularly if hepatomegaly is not present. Other imaging modalities, such as ultrasonography, CT, MR imaging, and radionuclide liver scans, are more sensitive and accurate for evaluating hepatic primary diseases or metastases. In addition, barium studies of the gastrointestinal tract and intravenous urograms may be helpful in excluding gastric outlet obstruction, carcinoma, or renal cell carcinoma.

Fusion of the kidneys may occur in the embryologic stage during the second month of gestation. Most (95%) of these fusions occur at the lower poles of the kidneys. Intravenous urography shows the kidney to be vertical or even in the reverse oblique direction and its position to be lower than normal. Horseshoe kidney may be associated with other congenital anomalies, as well as a high incidence of urinary tract obstruction, infection, or stone formation. A horseshoe kidney may also deviate the upper ureters laterally.

When a plain film suggests the presence of a pelvic mass, a specific diagnosis is often not possible. Intravenous urography and barium enema are useful in excluding a mass arising from the lower urinary tract or colon and in showing extrinsic involvement of these structures. On the other hand, pelvic ultrasonography, CT, or MR imaging better demonstrates the pelvic organs and their interrelationship and will differentiate between solid or fluid content in the mass.

EXERCISE 8-4: INTESTINAL DISTENTION

Clinical Histories:

Case 8-13. A 66-year-old man presents with fever, chills, and abdominal pain (Fig. 8–21).

Case 8-14. A 65-year-old woman presents with abdominal distention and a history of abdominal surgery (Fig. 8–22).

Case 8-15. A 70-year-old man presents with abdominal distention (Fig. 8–23).

Case 8-16. A 66-year-old woman presents with abdominal distention and constipation for 3 days (Fig. 8–24).

Questions:

Radiologic Findings:

8-13. In this case (Fig. 8–21), a diffuse abnormal gas pattern with distention of the small bowel, colon, and rectum suggests functional ileus. Two days later the patient underwent laparotomy, and small bowel ischemia was found (Fig. 8–25). (A is the correct answer to Question 8-13.)

8-14. Figure 8–22 shows gaseous distention of the stomach, duodenum, and jejunum on the supine film, but no gas is seen in the colon, suggesting mechanical small bowel obstruction. Gastric outlet or duodenal obstruction is unlikely because many jejunal loops are dilated. At surgery, an obstructing jejunal adhesion was found. (C is the correct answer to Question 8-14.)

8-15. The patient in Fig. 8–23 has a huge distended and folded colonic loop in the midabdomen and pelvis (the "coffee bean" sign). The most likely consideration is a sigmoid volvulus. (D is the correct answer to Question 8-15.)

8-16. Figure 8–24 shows a distended transverse colon and descending colon and no gas in the sigmoid colon and rectum. The small bowel is not distended. Mechanical obstruction of the colon distal to the level of descending colon is likely. (C is the correct answer to Question 8-16.) Barium enema (Fig. 8–26) shows an irregular narrowing at the rectosigmoid region, indicative of sigmoid carcinoma.

Discussion:

Generalized or diffuse distribution of gas, both in the small bowel and in the colon, is more indicative of a functional ileus. The most common causes of functional ileus are postoperative status, neuromuscular diseases, ischemia, and intrinsic or extrinsic inflammations. Air-fluid levels may be seen in patients with functional ileus when plain films are obtained with the patient in an upright position.

Limited distribution of abnormal gas in the intestine favors a mechanical obstruction. Air-fluids levels may also be seen in patients with mechanical obstruction when an upright abdominal radiograph is obtained. The most common causes of mechanical obstruction in the small bowel are adhesions, internal or external hernias, neoplasms, or intussusceptions. Ileocolic intussusception is common in children.

When the small bowel is filled with a large amount of fluid, a row of small gas bubbles may be trapped between the valvulae conniventes. The row of gas bubbles is called the string of beads or string of pearls sign and is seen on the decubitus or upright view of the abdomen (Fig. 8–27). A fluid-filled, closed-loop small-bowel obstruction may appear as an oval mass in the abdomen and is known as the pseudotumor sign (Fig. 8–28). These signs suggest a mechanical obstruction and possible strangulation.

Sigmoid volvulus may twist along the mesenteric axis and the long axis of the bowel. The twisted and overdistended sigmoid colon may appear as an inverted U shape or a coffee bean shape, without haustra or septa, at the upper pelvis and abdomen crossing the transverse colon. The colon above the sigmoid may be distended; however, the small bowel is rarely distended in a patient with sigmoid volvulus. Barium enema may show a beaking sign adjacent to the twisted point. Vascular insufficiency may occur if volvulus cannot be corrected.

A small-bowel volvulus may be caused by internal hernia or adhesion similar to that of sigmoid volvulus. Small-bowel volvulus may be located outside the pelvis with no proximal colonic dilatation. Cecal volvulus is the cause of 1% to 2% of intestinal obstructions. Most often a cecal volvulus is twisted and relocated in the midabdomen or left upper quadrant (Fig. 8–29).

Mechanical obstruction of the colon is commonly caused by colonic neoplasm, volvulus, or inflammatory mass caused by diverticulitis of the left colon. All colonic segments proximal to the mechanical obstruction are distended with gas or a combination of gas and feces. When intestinal secretions and fecal matter fill the distended bowel loop, solid and liquid contents produce a mottled appearance. Whether the small bowel becomes distended from a colonic obstruction depends on its duration and severity, and also on the competency of the ileocecal valve.

EXERCISE 8-5: INCREASED OR DECREASED DENSITY IN THE ABDOMEN

Clinical Histories:

Case 8-17. A 35-year-old man is seen who underwent laparotomy 2 days earlier (Fig. 8–30).

Case 8-18. A 49-year-old woman presents with acute abdominal pain (Fig. 8–31).

Case 8-19. A 40-year-old woman presents with abdominal distention (Fig. 8–32).

Case 8-20. A 45-year-old woman is seen who had a motor vehicle accident (Fig. 8–33).

Questions:

Radiologic Findings:

8-17. Figure 8–30 shows crescent-shaped lucencies beneath both hemidiaphragms outlining the liver on the right and the spleen on the left in the PA chest film, suggesting pneumoperitoneum. (C is the correct answer to Question 8-17.)

8-18. Figure 8–34 shows both the inner and outer walls of the transverse colon (arrows). This double-wall sign is seen on the supine film of the abdomen because there is air within the intestinal lumen and in the peritoneal cavity, caused by rupture of a viscus. (D is the correct answer to Question 8-18.)

8-19. In Fig. 8–35, the hepatic angle (arrowheads) and the descending colon (D) are displaced medially, the small bowel (S) is located centrally in the abdomen, and there is increased density in the pelvis, suggesting ascites. (A is the correct answer to Question 8-19.)

8-20. In this case (Fig. 8–33), the soft-tissue density with no gas pattern in the pelvis is consistent with hemoperitoneum in this motor vehicle accident victim. CT better evaluates hemoperitoneum (Fig. 8–36). (B is the correct answer to Question 8-20.)

Discussion:

In adults, the most common causes of pneumoperitoneum are postoperative status, ruptured abdominal viscus, and peritoneal dialysis. Residual air in the abdomen after surgery may persist for 1 to 2 weeks. Serial abdominal films, however, should show a gradual reduction in the amount of free peritoneal air. A persistent or increasing amount of air on postoperative serial films suggests a perforated viscus or ruptured surgical anastomosis. Spontaneous pneumoperitoneum is commonly caused by the perforation of a duodenal ulcer. Less common causes include pneumomediastinum, pulmonary emphysema, pneumatosis intestinalis, and entrance of air per vagina.

Pneumoperitoneum is most readily detected on the upright film of the chest, even if only a small amount of air is present. The left lateral decubitus view is useful and may show a small amount of free air accumulating between the right lateral margin of the liver and the peritoneal surface. Normally, the interface between the air and inner intestinal wall is visible, but the serosal surface is not appreciated because its density is similar to that of the adjacent peritoneal contents. When gas is present in the peritoneal cavity, however, both inner and outer walls will be delineated; this is called the double-wall sign or Rigler's sign. A visible serosal margin of bowel can also be simulated by normal adjacent omental fat or adjacent contiguous loops of small or large bowel (Fig. 8–37). If in doubt, the upright or left lateral decubitus film may confirm pneumoperitoneum. CT is more sensitive than plain radiograph in assessing pneumoperitoneum. Colon interposition occurs on the right between the liver and hemidiaphragm, and haustrations are usually recognized that aid differentiation from pneumoperitoneum.

Although plain film of the abdomen is not sensitive in assessing small amounts of intraperitoneal fluid, the plain film can demonstrate moderate and large amounts of fluid collections. In ascites, the hepatic angle may be obscured or displaced medially (Hellmer's sign). The ascending or descending colon may be displaced medially by fluid in the paracolic gutter. A large amount of fluid may accumulate in the pelvis, causing increased density and symmetrical bulges (dog ears sign). Other signs, such as separation of the small bowel loops and overall higher density in the abdomen, are also seen, but not often. CT and ultrasonography more accurately assess intraperitoneal fluid and coexistent masses.

Blood and pus have a similar density to that of ascitic fluid in the peritoneal cavity; therefore, hemoperitoneum may produce signs similar to those found in ascites. High density in the pelvis is a sign of hemoperitoneum in patients with a history of trauma. CT better evaluates hemoperitoneum.

EXERCISE 8-6: EXTRALUMINAL GAS PATTERN

Clinical Histories:

Case 8-21. A 42-year-old man presents with mild abdominal pain (Fig. 8–38).

Case 8-22. A 66-year-old woman is admitted with vague abdominal pain and vomiting (Fig. 8–39).

Case 8-23. A 77-year-old woman presents with fever and a 1-week history of abdominal pain (Fig. 8–40).

Case 8-24. A 64-year-old man presents with fever, abdominal pain, and distention (Fig. 8–41).

Questions:

Radiologic Findings:

8-21. Figure 8–38 shows linear air streaks along the descending and sigmoid colon in a patient with mild abdominal pain. These streaks indicate pneumotosis cystoides intestinalis. (C is the correct answer to Question 8-21.)

8-22. Figure 8–39 shows a distended proximal jejunum and a few air bubbles in the right upper quadrant, indicating gallstone ileus with mechanical obstruction and air in the biliary tree (Fig. 8–42A ). An upper gastrointestinal study demonstrates a distended proximal small bowel, a fistula (Fig. 8–42B ) between the biliary tree and the duodenum, and three gallstones in the small bowel (Fig. 8–42C ). CT shows air in the biliary tree (Fig. 8–42D ). (C is the correct answer to Question 8-22.)

8-23. In this case (Fig. 8–40), multiple air bubbles in the right upper quadrant in a patient with fever are consistent with a subdiaphragmatic abscess (Fig. 8–43). Bilateral linear rib calcifications and right hip replacement are also seen. (D is the correct answer to Question 8-23.)

8-24. Figure 8–41 shows a fine arborizing linear gas pattern in the right upper quadrant that extends to the periphery of the liver, indicating portal venous gas. (B is the correct answer to Question 8-24.)

Discussion:

Pneumatosis cystoides intestinalis appears as linear streaks of gas or intramural cystic collections of gas in the small bowel or colon. The cysts range in size from 0.5 to 3 cm and may extend into the adjacent mesentery. Pneumatosis intestinalis is an incidental finding in most patients, usually with a self-limited benign course; simple bowel obstruction, volvulus, and air from the mediastinum or retroperitoneum are commonly associated. Pneumatosis intestinalis may be caused by ischemic and necrotizing enterocolitis in patients with leukemia or non-Hodgkin lymphoma and in those who have had bone marrow transplantation.

Gallstone ileus, the mechanical obstruction of the small bowel by an impacted gallstone, is commonly seen in elderly women. Clinical presentation in gallstone ileus is nonspecific, and the mortality rate is high (15%). A gallstone enters the intestinal lumen via a cholecystoenteric fistula. Major radiographic signs include small-bowel obstruction, air in the biliary tree, and an ectopic gallstone seen on plain abdominal film, upper gastrointestinal series, or CT study.

Abscess in the subphrenic and subhepatic spaces is a serious problem, with a mortality rate of 30%. Subphrenic abscess may arise spontaneously or as a complication of abdominal surgery, pancreatitis, diverticulitis, or appendicitis. A cluster of gas may be seen on plain film in 70% of abscesses. Left-sided abscess is difficult to discern because gas in the splenic flexure, stomach, or jejunum may mimic gas within the abscess. Other radiographic findings include elevation of the adjacent hemidiaphgram, pleural effusion, and basilar atelectasis.

In the right upper quadrant, when multiple tubular lucencies are seen reaching the lateral hepatic margins, portal venous gas is a likely consideration. Biliary tree gas is located in the central hepatic zone near the porta hepatis. Benign portal venous gas has been noted in sigmoid diverticulosis, nonobstructed splenic flexure carcinoma, ulcerative colitis, and bronchopneumonia. Mesenteric vascular insufficiency and necrotizing intestinal infection are common causes of hepatic portal venous gas. In children, necrotizing enterocolitis produces intramural gas within mesenteric veins to the liver; the mortality rate in patients with the sign of hepatic portal venous gas is higher than in those without portal venous gas.

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