Ebook Brant and helms’ fundamentals of diagnostic radiology (5/E): Part 2

SECTION VII
■ GASTROINTESTINAL
TRACT
SECTION EDITOR: William E. Brant


CHAPTER ■ ABDOMEN AND
40
PELVIS
WILLIAM E. BRANT AND JENNIFER POHL

Imaging Methods
Compartmental Anatomy of the Abdomen and Pelvis
Fluid in the Peritoneal Cavity
Pneumoperitoneum
Abdominal Calcifications
Acute Abdomen
Small Bowel Obstruction
Large Bowel Obstruction
Bowel Ischemia and Infarction
Abdominal Trauma
Lymphadenopathy
Abdominopelvic Tumors and Masses
Hernias of the Abdominal Wall
HIV and AIDS in the Abdomen

IMAGING METHODS
Conventional radiographs of the abdomen remain a mainstay for the
assessment of the acute abdomen. CT, US, and MR provide comprehensive
evaluation of the abdomen including the peritoneal cavity, retroperitoneal
compartments, abdominal and pelvic organs, blood vessels, and lymph nodes.

COMPARTMENTAL ANATOMY OF THE ABDOMEN
AND PELVIS
Knowledge of the complex compartmental anatomy of the abdomen and


pelvis is fundamental to understanding the effects of pathologic processes
and to correctly interpret imaging studies. Understanding the shape and
extent of anatomic compartments and their normal variations may clarify
imaging findings that would otherwise be incomprehensible or lead to
misdiagnosis. Fundamental considerations include constant anatomic
landmarks, ligaments and fascia that define compartments, and normal
variations in size and appearance of the various compartments and recesses.
Identifying the precise compartment that an abnormality is in determines to a
great extent the origin of the abnormality.
The peritoneal cavity is divided into the greater peritoneal cavity and the
lesser peritoneal cavity (the lesser sac) (Figs. 40.1 and 40.2) . Within both
portions of the peritoneal cavity are numerous recesses in which pathologic
processes tend to loculate. The right subphrenic space communicates around
the liver with the anterior subhepatic and posterior subhepatic space (Morison
pouch) . Morison pouch (the right hepatorenal fossa) is the most dependent
portion of the abdominal cavity in a supine patient and it preferentially
collects ascites, hemoperitoneum, metastases, and abscesses. The right
subphrenic and subhepatic spaces communicate freely with the pelvic
peritoneal cavity via the right paracolic gutter.
T he left subphrenic space communicates freely with the left subhepatic
space, but is separated from the right subphrenic space by the falciform
ligament and from the left paracolic gutter by the phrenicocolic ligament. The
left subphrenic (perisplenic) space distends with fluid from ascites and with
blood from splenic trauma. It is a common location for abscesses and for

disease processes of the tail of the pancreas. The left subhepatic space
(gastrohepatic recess) is affected by diseases of the duodenal bulb, lesser
curve of the stomach, gallbladder, and left lobe of the liver.
The falciform ligament consists of two closely applied layers of peritoneum
extending from the umbilicus to the diaphragm in a parasagittal plane. The
caudal free end of the falciform ligament contains the ligamentum teres,
which is the remnant of the obliterated umbilical vein. Paraumbilical veins
(portosystemic collateral vessels) that enlarge within the falciform ligament
are a specific sign of portal hypertension. The reflections of the falciform
ligament separate over the posterior dome of the liver to form the coronary
ligaments, which define the “bare area” of the liver not covered by
peritoneum. The coronary ligaments reflect between the liver and diaphragm
and prevent access of ascites and other intraperitoneal processes from
covering the bare area of the liver.


The lesser omentum, composed of the gastrohepatic and hepatoduodenal
ligaments, suspends the stomach and duodenal bulb from the inferior surface
of the liver. The lesser omentum separates the gastrohepatic recess of the
left subphrenic space from the lesser sac. The lesser omentum transmits the
coronary veins (which dilate as varices) and contains lymph nodes (which
enlarge with involvement by gastric carcinoma and lymphoma). T he lesser
sac is the isolated peritoneal compartment between the stomach and the
pancreas. It communicates with the rest of the peritoneal cavity (the greater
sac) only through the small foramen of Winslow. Pathologic processes in the
lesser sac usually occur because of disease in adjacent organs (pancreas,
stomach) rather than spread from elsewhere in the abdominal cavity. The
lesser sac is normally collapsed but can become huge when filled with fluid.
The greater omentum is a double layer of peritoneum that hangs from the
greater curvature of the stomach and descends in front of the abdominal

viscera separating bowel from the anterior abdominal wall. The greater
omentum encloses fat and a few blood vessels. It serves as fertile ground for
implantation of peritoneal metastases, and assists in loculation of
inflammatory processes of the peritoneal cavity such as abscesses and
tuberculosis.

FIGURE 40.1. Anatomy of the Peritoneal Cavity. A: Diagram of an axial cross-section of the
abdomen illustrates the recesses of the greater peritoneal cavity and the lesser sac. B: CT scan of a
patient with a large amount of ascites nicely demonstrates the recesses of the greater peritoneal cavity
and the lesser sac. The lesser sac is bounded by the stomach (St) anteriorly, the pancreas (P) posteriorly,
and the gastrosplenic ligament (curved arrow) laterally. The falciform ligament (arrowhead) separates
the right and left subphrenic spaces. Fluid from the greater peritoneal cavity extends into Morison pouch
(arrow) between the liver and the right kidney. Fluid in the gastrohepatic recess (*) separates the
stomach from the liver (L). S, spleen; GB, gallbladder; RK, right kidney; IVC, inferior vena cava; Ao,
aorta; LK, left kidney.


The retroperitoneal space between the diaphragm and the pelvic brim is
divided into anterior pararenal, perirenal, and posterior pararenal
compartments by the anterior and posterior renal fascia (Fi g. 40.3) . The
anterior pararenal space extends between the posterior parietal peritoneum
and the anterior renal fascia. It is bounded laterally by the lateroconal fascia.
The pancreas, duodenal loop, and ascending and descending portions of the
colon are within the anterior pararenal space. Disease in the anterior
pararenal space usually originates from these organs (pancreatitis,
perforating/penetrating ulcer, diverticulitis).

FIGURE 40.2. The Lesser Sac. Sagittal plane diagrams of the medial (A) and lateral (B) aspects of
the lesser sac illustrate its position posterior to the stomach and anterior to the posterior parietal
peritoneum covering the pancreas. Note that projections of the lesser sac extend to the diaphragm,

resulting in the potential for disease processes in the lesser sac to cause pleural effusions. The coronary
ligaments reflect between the liver and the diaphragm producing a bare area of liver not covered by
peritoneum.

The anterior and posterior renal fascias encompass the kidney, adrenal
gland, and perirenal fat within the perirenal space. The anterior renal fascia is
thin and consists of one layer of connective tissue. The posterior renal fascia
is thicker, consisting of two layers of connective tissue. The anterior layer of
the posterior renal fascia is continuous with the anterior renal fascia. The
posterior layer of the renal fascia is continuous with the lateroconal fascia,
forming the lateral boundary of the anterior pararenal space. The anterior


and posterior layers of the posterior renal fascia may be separated by
inflammatory processes, such as pancreatitis, extending from the anterior
pararenal space. The renal fascia is bound to the fascia surrounding the aorta
and vena cava usually preventing spread of disease to the contralateral
perirenal space. However, disease processes arising in the perivascular
space, such as hemorrhage from aortic aneurysm rupture, may extend into
the perirenal space. Fluid collections in the perirenal space are usually renal
in origin (infection, urinoma, hemorrhage). Bridging septa extend between
the renal fascia and the renal capsule tends to cause loculations of fluid
processes in the perirenal space. The right perirenal space is open superiorly
to the bare area of the liver allowing spread of disease processes (infection,
tumor) between the kidney and liver.

FIGURE 40.3. Retroperitoneal Compartmental Anatomy. Diagrams illustrate two normal
variations of the reflections of the posterior parietal peritoneum around the descending colon. In (A) the
colon is entirely retroperitoneal and in (B) the peritoneum forms a deep pocket lateral to the colon,
allowing intraperitoneal fluid to extend far posteriorly. Fluid or disease processes in the anterior pararenal

space from the pancreas or colon may also extend posteriorly to the kidney by separating the two layers
of the posterior renal fascia.

The posterior pararenal space is a potential space, usually filled only with
fat, extending from the posterior renal fascia to the transversalis fascia. The
posterior pararenal fat continues into the flank as the properitoneal fat
“stripe” seen on plain films of the abdomen. The compartment is limited
medially by the lateral edges of the psoas and quadratus lumborum muscles.
Isolated fluid collections are rare and most commonly caused by spontaneous
hemorrhage into the psoas muscle as a result of anticoagulation therapy.


FIGURE 40.4. Compartmental Anatomy of the Pelvis. Diagram in the coronal plane illustrates the
major anatomic compartments of the pelvis.

The pelvis is divided into three major anatomic compartments: peritoneal
cavity, extraperitoneal space, and perineum ( Fig. 40.4). The peritoneal cavity
extends to the level of the vagina, forming the pouch of Douglas (cul-de-sac)
in females, or to the level of the seminal vesicles, forming the rectovesical
pouch in males. The broad ligaments reflect over the uterus, fallopian tubes,
and parametrial uterine vessels and serve as the anterior boundary of the
rectouterine pouch of Douglas. The cul-de-sac is the most dependent portion
of the peritoneal cavity and collects fluid, blood, abscesses, and
intraperitoneal drop metastases. The extraperitoneal space of the pelvis is
continuous with the retroperitoneal space of the abdomen, extends to the
pelvic diaphragm, and includes the retropubic space (of Retzius). Pathologic
processes from the pelvis spread preferentially into the retroperitoneal
compartments of the abdomen. T h e perineum lies below the pelvic
diaphragm. The ischiorectal fossa serves as its anatomic landmark (Fig.
40.5).

FIGURE 40.5. Perineal Tumor. A CT scan of a 12-year-old girl with a history of a
rhabdomyosarcoma of the right leg demonstrates a tumor metastasis (T) in the right ischiorectal fossa.
The left ischiorectal fossa (IRF) shows its normal appearance as a triangle of fat bordered by the rectum
(R), obturator internus muscle (OI), and the gluteus muscles (GM) . The ischiorectal fossa is entirely
below the levator ani and is part of the perineum. c, tip of the coccyx; IT, ischial tuberosities.

FLUID IN THE PERITONEAL CAVITY
Fluid in the peritoneal cavity originates from many different sources and
varies greatly in composition. Ascites is serous fluid in the peritoneal cavity
most commonly caused by cirrhosis, hypoproteinemia, or congestive heart
failure. Exudative ascites results from inflammatory processes such as
abscess, pancreatitis, peritonitis, or bowel perforation. Hemoperitoneum
results from trauma, surgery, or spontaneous hemorrhage. Neoplastic ascites
is associated with intraperitoneal tumors. Urine, bile, and chyle may also
spread freely within the peritoneal cavity.
Conventional radiographic diagnosis of ascites requires that at least 500 cc
of fluid be present. Findings are (a) diffuse increase in density of the


abdomen (gray abdomen), (b) indistinct margins of the liver, spleen, and
psoas muscles, (c) medial displacement of gas-filled colon, liver, and spleen
away from the properitoneal flank stripe, (d) bulging of the flanks, (e)
increased separation of gas-filled small bowel loops, and (f) “dog’s ears”
appearance of symmetric densities in the pelvis due to fluid spilling out of the
cul-de-sac on either side of the bladder. CT demonstrates fluid density in the
recesses of the peritoneal cavity. The CT density of the fluid gives a clue as
to its composition. Serous ascites has attenuation values near water (−10 to
+10 H). Exudative ascites is usually above +15 H and acute bleeding into the

peritoneal cavity averages +45 H. US is sensitive to small amounts of fluid in
the peritoneal recesses. Care must be taken with US to examine the most
gravity-dependent portions of the peritoneal cavity (Morison pouch and the
pe lvis). Simple ascites is anechoic, while exudative, hemorrhagic, or
neoplastic ascites often contains floating debris. Septations in ascites are
associated with an inflammatory or malignant process. MR shows limited
specificity for defining the type of fluid present. Serous fluid is low signal
intensity on T1WI and markedly increased in signal intensity on T2WI.
Hemorrhagic fluid shows high signal intensity on both T1WI and T2WI. Serous
ascites is commonly bright on gradient-echo images due to fluid motion.
Pseudomyxoma peritonei (“jelly belly”) refers to gelatinous ascites that
occurs as a result of intraperitoneal spread of mucin-producing cells resulting
from rupture of appendiceal mucocele, or intraperitoneal spread of benign or
malignant mucinous cysts of the ovary or mucinous adenocarcinoma of the
colon or rectum. Conventional radiographs may demonstrate punctate or ringlike calcifications scattered through the peritoneal cavity. CT demonstrates
mottled densities, septations, and calcifications within the fluid. The
mucinous fluid is typically loculated and causes mass effect on the liver and
bowel (Fig. 40.6) . US demonstrates intraperitoneal nodules that range from
hypoechoic to strongly echogenic.

PNEUMOPERITONEUM
Free air within the peritoneal cavity is a valuable sign of bowel perforation,
most commonly caused by duodenal or gastric ulcer perforation. However,
additional causes of pneumoperitoneum include trauma, recent surgery or
laparoscopy, and infection of the peritoneal cavity with gas-producing
organisms. Postoperative pneumoperitoneum usually resolves in 3 to 4 days.


Serial images demonstrate a progressive decrease in the amount of air
present. Failure of progressive resolution, or an increase in the amount of air

present, suggests a leak of bowel anastomosis or sepsis. Pneumoperitoneum
in the absence of a ruptured viscus may occur with air introduced through the
female genital tract by orogenital insufflation, or associated with pulmonary
emphysema, alveolar rupture, and dissection of air into the peritoneal cavity.

FIGURE 40.6. Pseudomyxoma Peritonei. A CT scan of a 60-year-old man with intraperitoneal
spread of mucinous adenocarcinoma of the colon shows loculations (arrowheads) of fluid indenting the
surface of the liver (L) giving evidence of mass effect. The attenuation of the fluid measured 32 H
indicating exudative ascites.

Conventional radiographs show pneumoperitoneum best on images
obtained with the patient in the standing or sitting position. Upright chest
radiographs are the most sensitive for free air. Small amounts of air are
clearly demonstrated beneath the domes of the diaphragm. Left lateral
decubitus and cross-table lateral views may be used with very ill patients to
demonstrate air outlining the liver. Signs of pneumoperitoneum on supine
radiographs (Fig. 40.7) include the following: (a) gas on both sides of the
bowel wall (Rigler sign), (b) gas outlining the falciform ligament, (c) gas


outlining the peritoneal cavity (the “football sign”), and (d) triangular or
linear localized extraluminal gas in the right upper quadrant. On CT, small
amounts of extraluminal gas may be confused with gas within the bowel and
can be surprisingly difficult to recognize. Images should be examined at lung
windows to detect free intraperitoneal air. The peritoneal recess between the
liver and diaphragm (Fig. 40.8) is a good place to look for pneumoperitoneum
on CT.

ABDOMINAL CALCIFICATIONS
Intra-abdominal calcifications may be an important sign of intra-abdominal

disease and should be searched for on every imaging study of the abdomen.
CT and US are more sensitive to detection of calcifications than are
conventional
radiographs. However, the high spatial resolution of
conventional radiography commonly provides characteristic findings that
allow a specific diagnosis of the nature of the calcification.
Vascular calcifications are common in the aorta and iliac vessels (see Fig.
40.12) of older individuals. Plaque-like vascular calcifications overlie the
lumbar spine and sacrum and commonly require detailed inspection to detect.
Aneurysms of the aorta are manifest by luminal diameter exceeding 3 cm as
measured between calcifications in the aortic wall (Fi g . 40.9) . Ring-like
calcified aneurysms most commonly involve the splenic or renal arteries.
Phleboliths are calcified thrombi in veins most commonly visualized in the
lateral aspects of the pelvis. They are round or oval calcifications up to 5 mm
size that commonly contain a central lucency. They may be mistaken for
urinary tract calculi.


FIGURE 40.7. Pneumoperitoneum: Conventional Radiograph. A: Supine abdominal radiograph in
a patient with a perforated gastric ulcer demonstrates visualization of both sides of the bowel wall (Rigler
sign) (arrowheads), free air outlining the falciform ligament (arrow), free air outlining the edge of the
liver (curved arrow), and free air outlining the pericolic gutters (*). B: Erect chest radiograph in a
different patient shows a crescent-shaped band of gas (arrow) between the liver (L) and the diaphragm.
Pneumoperitoneum was caused by a perforated sigmoid colon diverticulitis.

Calcified lymph nodes result most commonly from granulomatous diseases
such as tuberculosis or histoplasmosis. The calcification is usually mottled and
10 to 15 mm in size. Mesenteric nodes are the most commonly calcified.

FIGURE 40.8. Pneumoperitoneum: CT. A collection of air ( arrow) is seen within the peritoneal
space between the liver (L) and the diaphragm (arrowhead). This is a prime area to search to detect
small amounts of free intraperitoneal air on CT. This patient had a torn jejunum as a result of trauma
from a motor vehicle collision.

Gallstones and Gallbladder. Only about 15% of gallstones contain
sufficient calcium to be identified on conventional radiography. Most calcified
gallstones contain calcium bilirubinate and have a laminated appearance with
a dense outer rim and more radiolucent center. When multiple gallstones are
present, they are commonly faceted. Calcifications in the gallbladder wall
(porcelain gallbladder) (Fig. 40.10) are plaque like and oval in configuration
conforming to the size and shape of the gallbladder. Milk of calcium bile is a
suspension of radiopaque crystals within gallbladder bile. Layering of the
suspension can be demonstrated on erect radiographs.


FIGURE 40.9. Abdominal Aortic Aneurysm. Conventional radiograph demonstrates an aneurysm of
the abdominal aorta evidenced by wide separation of calcifications in the aortic wall (arrowheads).
Calcification in the wall overlying the spine may be difficult to visualize. A radiograph taken with the
patient in left posterior oblique position will project the aorta away from the spine and make visualization
of aortic wall calcifications easier.

Urinary Calculi. About 85% of urinary calculi are visible on conventional
radiographs. They range in size from punctate up to several centimeters.
Most characteristic are the staghorn calculi, which assume the shape of the
renal collecting system (Fi g. 40.11) . Renal calculi are differentiated from
gallstones on radiographs by oblique projections that confirm their posterior
position, as opposed to the more anterior positions of gallstones. Ureteral
calculi may be seen anywhere along the course of the ureter, but are most
common at the areas of narrowing: the ureteropelvic junction, the pelvic

brim, and the vesicoureteral junction. Bladder calculi (Fig. 40.12) are single or
multiple, commonly laminated, may be any size, and usually lie near the
midline of the pelvis. Calculi within bladder diverticula may be eccentric to
the bladder. CT has become the imaging method of choice to document
urinary tract stones.


FIGURE 40.10. Porcelain Gallbladder. Cone-down radiograph of the right upper quadrant of the
abdomen demonstrates calcification in the wall of the gallbladder (arrow). This finding is indicative of
chronic obstruction of the cystic duct, chronic gallbladder inflammation, and an increased risk of
gallbladder carcinoma.

Liver and spleen granulomas are usually multiple, small, and dense. They
are healed foci of tuberculosis, histoplasmosis, or other granulomatous
disease.
Appendicoliths and enteroliths are concretions within the lumen of the
b o w e l . Most are round or oval and have concentric laminations.
Appendicoliths are strongly indicative of acute appendicitis in patients
presenting with acute abdominal pain. Enteroliths are most common in the
colon and often due to calcium deposition on an undigestible material such as
a fruit pit.
Epiploic appendagitis results from inflammation believed to be due to
torsion of the colonic appendages, resulting in vascular occlusion and
ischemia. The resultant fat necrosis often calcifies resulting in a mobile ovalshaped calcification.


FIGURE 40.11. Staghorn Calculus. Conventional radiograph reveals a large calculus occupying the
collecting system of the left kidney and assuming its shape. Staghorn calculi (S) are usually composed of
struvite and form in the presence of chronic urinary infection.

Calcified adrenal glands are associated with adrenal hemorrhage in the
newborn, tuberculosis, and Addison disease. The calcification is mottled and
in the location of the adrenal glands on either side of the first lumbar
vertebra (Fig. 40.13).


FIGURE 40.12. Bladder Calculi. Numerous calculi (arrows) in the bladder are evident on this
conventional radiograph of the pelvis. The large prostate (P, between arrowheads), responsible for
urinary stasis leading to stone formation, makes a mass impression on the layering stones. Also evident
are atherosclerotic calcifications in the iliac arteries (curved arrows).

Pancreatic calcification is associated with chronic alcohol-induced
pancreatitis and hereditary pancreatitis. The calcifications are due to
pancreatic calculi and are usually coarse and of varying size (Fig. 40.14).
Calcified cysts may be found in the kidneys, spleen, liver, appendix, and the
peritoneal cavity. Calcification in the wall of a cyst is curvilinear or ring
shaped (Fig. 40.15) . Echinococcus cysts commonly calcify and may be found
in any intra-abdominal organ as well as within the peritoneal cavity.
Tumor Calcification. A wide variety of different tumors of abdominal organs
may contain calcifications. The coarse “popcorn” calcifications of uterine
leiomyomas are most characteristic. Benign cystic teratomas may form teeth
or bone. Calcified peritoneal metastases of ovarian or colon mucinous
cystadenocarcinoma may outline the peritoneal cavity (Fig. 40.16). Renal cell
carcinoma calcifies in up to 25% of cases.


FIGURE 40.13. Adrenal Calcifications. Conventional radiograph of the abdomen in a 4 year old
demonstrates calcification of both adrenal glands (arrows) resulting from bilateral adrenal hemorrhage as
an infant.

Soft tissue calcifications may be seen with hypercalcemic states,
idiopathic calcinosis, and old hematomas. Calcified injection granuloma from
quinine, bismuth, and calcium salts of penicillin are commonly evident in the
buttocks. Cysticercosis causes characteristic “rice-grain” calcifications in
muscles.

FIGURE 40.14. Pancreatic Calcifications. Coarse and punctate calcifications (arrow) extend upward
across the left upper quadrant in this patient with chronic alcoholic pancreatitis. Calcifications in the
pancreatic head (arrowhead) are obscured by the spine.


Bowel contents may include bone, pits, seeds, birdshot, or medications
containing iron or other heavy metals that result in abdominal opacities.
Peritoneal calcifications may be nodular or sheet like and result most
commonly from peritoneal dialysis, previous peritonitis, or peritoneal
carcinomatosis (Fig. 40.16).

ACUTE ABDOMEN
The differential diagnosis of patients presenting with acute abdominal pain is
extremely broad (Table 40.1 ) . Accurate and efficient diagnosis requires
cooperation between the referring physician and the radiologist to select the
imaging method most likely to provide the correct diagnosis. Routine
assessment of the acute abdomen commonly includes the “acute abdomen
series,” which consists of an erect posterior–anterior chest radiograph, and
supine and erect or decubitus radiographs of the abdomen. The chest
radiograph provides optimal detection of pneumoperitoneum and
intrathoracic diseases that may present with abdominal complaints. The
supine abdominal film permits diagnosis of many acute abdominal conditions,
and the horizontal-beam abdominal film adds confidence to the diagnosis. CT
or US is routinely obtained to provide a definitive diagnosis.

FIGURE 40.15. Calcified Renal Cyst. Conventional radiograph shows the rim calcification (arrow)
characteristic of wall calcification in a renal cyst.


Normal Abdominal Gas Pattern. Interpretation of conventional abdominal
radiographs routinely includes assessment of gas, fluid, soft tissue, fat, and
calcium densities. Normal gas in the abdomen is predominantly swallowed air
(Fig. 40.17) . Air–fluid levels are seen in normal patients commonly in the
stomach, often in the small bowel, but never in the colon distal to the hepatic
flexure. Normal air–fluid levels in the small bowel should not exceed 2.5 cm
in width. Small bowel gas usually appears as multiple small, random gas
collections scattered throughout the abdomen. Small bowel gas is increased
in patients who chronically swallow air or drink carbonated beverages. A
normal intestinal gas pattern varies from no intestinal gas to gas within three
to four variably shaped small intestinal loops measuring less than 2.5 to 3 cm
in diameter. The normal colon contains some gas and fecal material and
varies in diameter from 3 to 8 cm, with the cecum having the largest
diameter. Complete absence of gas in the small bowel may be seen in
patients with bowel obstruction with fluid rather than air filling the dilated
bowel loops. The term “nonspecific abdominal gas pattern” has no precise
meaning and should not be used.


FIGURE 40.16. Tumoral Calcifications. Radiograph of the abdomen demonstrates cloud-like
calcifications (arrowheads) in the distribution of peritoneal recesses. These calcifications were caused by
intraperitoneal spread of a papillary serous cystadenocarcinomas of the ovary.

TABLE 40.1
COMMON CAUSES OF ACUTE ABDOMEN

Dilated Bowel. Small bowel is dilated when it exceeds 2.5 to 3.0 cm in
diameter. The colon is dilated when it exceeds 5 cm in diameter, and the
cecum is dilated when it exceeds 8 cm diameter. In adults, dilated small
bowel can usually be differentiated from dilated large bowel by assessment of
location and anatomic features. Small bowel is more central in the abdomen
and is characterized by valvulae conniventes, which cross the entire diameter
of the lumen. Dilated small bowel rarely exceeds 5 cm in diameter although
large bowel is not considered dilated until it exceeds 5 cm diameter. Large
bowel is more peripheral in the abdomen and is characterized by haustra that
extend only partway across the lumen. Large bowel contains fecal material
that has a characteristic mottled appearance. The cecum, which has the
largest normal diameter of the large bowel, always dilates to the greatest
extent irrespective of the site of obstruction.


FIGURE 40.17. Normal Bowel Gas Pattern. Supine radiograph shows the normal distribution of gas
in the stomach (large arrow) and duodenum (small arrow). The normal mottled pattern of stool is seen in
the distribution of the right colon (arrowhead). A few gas collections within small bowel (curved arrow)
are seen in the pelvis.

TABLE 40.2
COMMON CAUSES OF ADYNAMIC ILEUS


FIGURE 40.18. Sentinel Loop. Daily serial radiographs on this patient demonstrated a persistent loop
of dilated small bowel (arrow) in the same location. This sentinel loop was caused by acute pancreatitis.
Normal gas pattern is present in the right colon (arrowhead) . The abdomen was otherwise devoid of
intestinal gas.

Adynamic Ileus. The word “ileus” means stasis and does not differentiate
mechanical obstruction from nonmechanical stasis. The terms “adynamic
ileus,” “paralytic ileus,” and “nonobstructive ileus” are used interchangeably
and refer to stasis of bowel contents because of decreased or absent


peristalsis. Common causes of adynamic ileus are listed in Table 40.2 .
Adynamic ileus typically demonstrates diffuse symmetric, predominantly
gaseous, distention of bowel. The small bowel, stomach, and colon are
proportionally dilated without an abrupt transition. More bowel loops are
dilated than with obstruction. Occasionally adynamic ileus may result in a
gasless abdomen with dilated loops of bowel that are filled only with fluid. US
is useful in confirming decreased or absent peristalsis, although examination
may be difficult if large amounts of gas are present.
Sentinel loop refers to a segment of intestine that becomes paralyzed and
dilated as it lies next to an inflamed intra-abdominal organ. In essence, it is a
short segment of adynamic ileus that appears as an isolated loop of
distended intestine that remains in the same general position on serial
images (Fig. 40.18). A sentinel loop alerts one to the presence of an adjacent
inflammatory process. A sentinel loop in the right upper quadrant suggests
acute cholecystitis, hepatitis, or pyelonephritis. In the left upper quadrant,
pancreatitis, pyelonephritis, or splenic injury may be suspected. In the lower
quadrants, diverticulitis, appendicitis, salpingitis, cystitis, or Crohn disease is
the cause of a sentinel loop.
TABLE 40.3
CAUSES OF TOXIC MEGACOLON

Toxic megacolon is a manifestation of fulminant colitis characterized by
extreme dilation of all or a portion of the colon. In this state, peristalsis is

absent and the large bowel loses all tone and contractility. The patient has
progressive abdominal distention and is toxic, febrile, and obtunded. Bowel
sounds and bowel movements are absent. The bowel wall becomes like “wet
blotting paper,” and the risk of perforation is extreme. Mortality approaches
20% in toxic megacolon. Acute ulcerative colitis is the most common cause of
toxic megacolon (Table 40.3 ) . Conventional radiographs demonstrate
distention of the colon with absent haustra. Dilation of the transverse colon
up to 15 cm diameter is often the most striking finding. The diagnosis is