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Centennial Dissertation |
1
Department of Radiology, Evanston Hospital-Northwestern University, 2650 Ridge
Ave., Evanston, IL 60201.
2
Department of Radiology, Northwestern Memorial Hospital, Northwestern
University Medical School, 675 N. St. Clair St., Chicago, IL 60611.
Address correspondence to R. M. Gore
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Introduction
The term "acute abdomen" defines a clinical syndrome characterized by the sudden onset of severe abdominal pain requiring emergency medical or surgical treatment [1]. A prompt and accurate diagnosis is essential to minimize morbidity and mortality [2]. The differential diagnosis includes an enormous spectrum of disorders ranging from benign self-limited diseases to conditions that require emergency surgery [3]. In a review of approximately 30,000 patients with acute abdomen, de Bombal [4] observed that 28% of patients had appendicitis, 9.7% had acute cholecystitis, 4.1% had small-bowel obstruction, 4% had acute gynecologic disease, 2.9% had acute pancreatitis, 2.9% had acute renal colic, 2.5% had perforated peptic ulcer, and 1.5% had diverticulitis. In one third of patients, no cause could be determined.
The clinical diagnosis of acute abdomen can be challenging because physical examination, clinical presentation, and laboratory examination are often nonspecific and nondiagnostic. Although sonography [5,6,7] has developed a niche in evaluating the gallbladder in all patients and the appendix in children and women of reproductive age, CT [8,9,10,11,12,13,14] has evolved as the premier technique for triaging most patients. CT has earned this role because it can provide a global perspective of the gut, mesenteries, omenta, peritoneum, retroperitoneum, subperitoneum, and extraperitoneum uninhibited by the presence of bowel gas and fat. Helical scanning allows thinner contiguous images to be obtained without increasing radiation exposure and without respiratory misregistration. The rapidity of scanning allows several acquisitions to be obtained during different phases of a single IV contrast bolus.
We describe the practical aspects of optimizing helical CT and emphasize the CT features of common acute abdominal disorders.
Technical Considerations
A variety of CT patient preparation and scanning protocols have been created to study the diversity of diseases that can cause acute abdomen (Table 1). The selection of an imaging technique depends on the most likely diagnosis, clinical setting, and local expertise. The following factors should be tailored to each patient: slice collimation and pitch; the use of oral, IV, and rectal contrast material; and the scope of coverage—a limited-focus examination versus a complete abdominal and pelvic study [10].
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IV contrast media is helpful in the diagnosis of splanchnic venous thrombosis, bowel ischemia, aneurysms, and active arterial extravasation. Inflammatory mural changes in appendicitis, cholecystitis, diverticulitis, Crohn's disease, and infectious enterocolitis are also better depicted with vascular enhancement. Additionally, neoplasms, abscesses, and infarcts in the liver, spleen, and kidneys are well portrayed on contrast-enhanced scans. Iodinated IV contrast material carries the risk of nephrotoxicity and potential contrast material reaction and will obscure renal and ureteral stones. However, in most patients, the information provided justifies the risk and extra expense. Between 125 and 150 ml of 60% iodinated IV contrast material should be injected at a rate of at least 3 ml/sec. Scans are obtained during the portal venous phase using a 60- to 70-sec delay. Arterial phase imaging is useful in patients with suspected hemorrhage, bowel ischemia, and arterial thrombosis. Delayed scans through the kidneys and pelvis can reveal renal masses and bladder disease that might be overlooked during earlier phases.
For most patients, we prefer to give 800-1000 ml of a 2% solution of oral diluted water-soluble contrast material at least 1 hr before scanning. Oral contrast material is administered primarily to differentiate bowel loops from abdominal and pelvic masses and abscesses. Oral contrast material may obscure the diagnosis of bowel hemorrhage or ischemia and limit the detection of ureteral stones, appendicoliths, or bile duct stones. Practical difficulties of oral contrast material include the time it takes to opacify the gut, the randomness of contrast opacification, and the inability of sick patients to consume and retain sufficient quantities of oral contrast material.
Rectal contrast material is advocated by some authors to optimize the detection of appendicitis, diverticulitis, and epiploic appendagitis [15,16,17,18,19]. With the patient in the left decubitus position, 400-600 ml of a 3% solution of water-soluble contrast material is administered by gravity through a soft rubber rectal catheter without using a balloon. The patient is then turned to the supine position for scanning. Other authors (with whom we agree) question whether rectal contrast material is of sufficient benefit to warrant its routine use [20].
An alternate approach to the patient with acute abdomen is to perform CT without oral, IV, or rectal contrast material. This technique is fast, is virtually risk free, and causes no patient discomfort [8,9,10, 21]. However, these scans are the most difficult to interpret.
Appendicitis
Acute appendicitis is the most common abdominal surgical emergency, affecting approximately 250,000 people annually in the United States. Although the correct diagnosis can be made in most patients on the basis of history, physical examination, and laboratory tests, diagnosis is uncertain in 20-33% of patients who present with atypical symptoms [21, 22]. The diagnosis is most difficult for infants, young children, elderly patients, and women of reproductive age. In the past, an average negative laparotomy rate of 20% was acceptable [22]. The widespread use of helical CT in patients with suspected appendicitis positively affects patient outcome and increases the number of laparotomies with positive results [23, 24].
CT findings of acute appendicitis (Fig. 1A,1B) reflect the extent and severity of inflammation [25,26,27,28,29,30,31]. In mild disease, the appendix appears as a slightly distended (diameter, 5-15 mm) fluid-filled structure that shows circumferential symmetric mural thickening. Homogeneous dense contrast enhancement of the wall is typical, but a target sign may be seen on axial images. Periappendiceal inflammation manifests as slight haziness of mesoappendix fat. A calcified appendicolith is more reliably revealed on CT than on radiography.
With disease progression and perforation, the appendix becomes fragmented, destroyed, and replaced by a phlegmon or abscess. Associated mural thickening of the adjacent distal ileum and cecum may also occur. In patients with these symptoms, the specific diagnosis of appendicitis can be made if an appendicolith is seen in the abscess or phlegmon.
When rectal contrast material is administered, other CT features of appendicitis (Fig. 2) appear. Inflammatory changes that involve only the tip of the cecum produce the focal cecal apical thickening sign [32]. Intraluminal contrast material can funnel between each side of the cecal apical thickening—the arrowhead sign of appendicitis [33]. In some patients, the inflammatory process may separate the cecal lumen from the base of the appendix or an appendicolith, the so-called cecal bar sign [25]. Studies evaluating the efficacy of high-resolution CT show sensitivities of 90-100%, specificities of 83-97%, and accuracies of 93-98% for the diagnosis of acute appendicitis [16, 21,22,23,24,25,26,27,28,29,30,31,32,33].
Diverticulitis
Diverticulitis occurs in 10-25% of patients with known diverticulosis [34]. These patients typically present with left lower quadrant pain, fever, and leukocytosis. Clinical misdiagnosis rates range from 34% to 67% [35]. The role of CT in these patients is to confirm the diagnosis, establish the presence of complications (e.g., abscess), provide a "road map" for percutaneous [36, 37] or surgical therapy, and suggest alternative diagnoses for patients in whom diverticulitis has been excluded [38].
The CT hallmark of diverticulitis is inflammatory change in the pericolic fat (Fig. 3A,3B), seen in 98% of patients [39]. Minimal haziness of adjacent fat occurs in mild cases. Small fluid collections, fine linear strands, and extraluminal gas bubbles may also occur [40]. In more severe cases, phlegmon or frank abscess formation can occur. Diverticula are evident in more than 80% of patients, and symmetric mural thickening greater than 4 mm is seen in about 70% of patients [41,42,43,44]. In some patients, contrast material collects in an arrowhead shape adjacent to the inflamed colonic diverticulum—the arrowhead sign of diverticulitis [40]. The offending inflamed diverticulum may appear as a rounded paracolic outpouching centered in the paracolic inflammation with soft-tissue, calcium, barium, or air attenuation (Fig. 4).
A perforated carcinoma is the major differential diagnostic consideration in patients with sigmoid diverticulitis. Although the colon wall is usually less than 1-cm thick in acute diverticulitis, in patients with severe muscular hypertrophy, the wall may be 2-3 cm thick, simulating carcinoma [43]. CT findings favoring the diagnosis of acute diverticulitis include a tethered or sawtooth luminal configuration and engorged vasa recta [45]. An abrupt zone of transition with normal bowel, enlarged local lymph nodes, and mural thickness greater than 1.5 cm favors carcinoma [46].
Complications of acute diverticulitis include large- and small-bowel obstruction [47], secondary inflammation of the appendix [48], fistula, sinus tracts, and frank intraperitoneal perforation.
Right-sided diverticulitis is usually difficult to diagnose clinically. When compared with patients with appendicitis, individuals with right-sided diverticulitis have a more protracted history, milder pain, and a higher point of maximum tenderness, which may clinically simulate acute cholecystitis. A palpable mass is present in up to one third of patients and can mimic an appendiceal or cecal tumor [48].
The CT findings of right-sided diverticulitis consist of focal pericolic inflammatory change, slight mural thickening, and visualization of a diverticulum as an outpouching of the right colon at the level of maximum wall thickness. The offending diverticulum contains gas, fluid, contrast material, or calcified material [49,50,51]. The normal appendix should be seen. If the appendix is not visualized, appendicitis, epiploic appendagitis, typhlitis, or perforated cecal carcinoma must be considered in the differential diagnosis.
Bowel Obstruction
Small- and large-bowel obstruction accounts for approximately 20% of acute abdominal surgical conditions. Helical CT has replaced conventional contrast studies because it can more reliably answer the following questions: is obstruction present? what is the level of obstruction? what is the cause of obstruction? what is the severity of obstruction? is the obstruction simple or closed loop? and is styrangulation or ischemia present? It is important to differentiate between simple- and closed-loop obstruction because the former can be treated conservatively, whereas the latter requires prompt surgical intervention. For patients with bowel obstruction, scans are best obtained without oral contrast material because intraluminal fluid and gas serve as natural contrast agents. IV contrast material is important in assessing intestinal perfusion and ischemia and delineating the size, configuration, and patency of the mesenteric vessels [52,53,54,55].
The CT hallmark of bowel obstruction is the delineation of a transition zone between dilated and decompressed bowel. Careful inspection of the transition zone and luminal contents will often reveal the underlying causes of obstruction. CT is most helpful in patients with internal and external hernias, neoplasms (Fig. 5A), gallstone ileus (Fig. 5B), various forms of enteroenteric intussusception, and afferent-loop obstruction after a Billroth II operation. If no mass, hernia, intussusception, abscess, or inflammatory thickening is present, then adhesion is the most likely diagnosis. The typical adhesion has a beaklike narrowing, and the affected gut may be difficult to view depending on the orientation of the loop relative to the axial plane. Use of the scroll or leaf function can help establish the correct diagnosis [52,53,54,55].
An incarcerated or closed-loop obstruction manifests as a loop-shaped fluid-filled structure causing proximal segments to dilate with gas and fluid. The mesenteric vessels have a radial distribution because they become stretched and converge towards the U- or C-shaped loop. Two adjacent collapsed round, oval, or triangular segments typically represent the afferent and efferent entry points or the torsion site. The mesenteric vasculature may have an unusual course. When ischemia develops, the bowel wall may thicken and have a target appearance caused by submucosal edema. Additionally, there may be poor or delayed enhancement of the involved bowel wall. Fluid and hemorrhage may collect in the mesentery, bowel wall, and lumen of the involved segment. The mesentery becomes hazy in appearance, and ascites may develop [56,57,58,59,60,61,62,63,64,65,66,67,68,69].
In patients with high-grade small-bowel obstruction, CT has a reported sensitivity of 90-96%, a specificity of 91-96%, and an accuracy of 90-95% [52,53,54, 62, 67]. CT is less accurate in patients with low-grade obstruction.
Intestinal Ischemia
Vascular insufficiency of the gut is a differential diagnosis for elderly patients with acute abdominal pain or for any patient with a history of coronary artery disease, peripheral vascular disease, arteritis, hypotension, dehydration, or cardiac decompensation. Patients with intestinal ischemia have a broad range of symptoms that make a clinical diagnosis difficult. The major causes of intestinal ischemia include hypoperfusion and either arterial or venous occlusion or thrombosis. Typically, the predominance of one factor determines the outcome [70]. CT plays an important role in identifying the early changes of ischemia. Rapid IV contrast material administration (>3 ml/sec) is required to optimize vascular opacification and to assess the patency of the superior mesenteric artery and vein.
The CT features of intestinal ischemia vary and depend on its cause, chronicity, and severity [71, 72]. Mural thickening of the gut is the most common finding, and the wall may have a target or halo appearance caused by submucosal edema. Thickened edematous wall is best appreciated in bowel distended by fluid, air, or contrast material. This appearance is nonspecific and can be seen in infectious and inflammatory bowel disease. Mesenteric haziness reflects edema and hemorrhage. The presence of focal pneumatosis or thrombus in the superior mesenteric artery or vein permits a specific diagnosis to be made. Air in the bowel wall (Fig. 6A), mesentery, and portal venous system has grave prognostic implications for patients with ischemic bowel. CT is far more sensitive than radiography in detecting pneumatosis and portal venous gas [71,72,73].
Colonic ischemia generally results from hypoperfusion or hypotension. Accordingly, a mesenteric thrombus is rare. Helical CT reveals segmental thickening of the colon (Fig. 6B) with scalloped irregular margins caused by submucosal edema.
Peptic Ulcer Disease
Patients with peptic ulcer disease often present with nonlocalizing signs and symptoms indistinguishable from those of acute pancreatitis or cholecystitis, and CT is normally the first examination ordered. The most common CT finding is focal mural thickening, which is a nonspecific finding. Occasionally an active ulcer (Fig. 7A) or perforation (Fig. 7B) is identified, accompanied by inflammatory change of the adjacent fat, mesenteries, and omenta [74,75,76].
Miscellaneous Gastrointestinal Disorders
Epiploic Appendagitis
This unusual condition occurs when an epiploic appendage of the colon
develops inflammation, torsion, or ischemia. Epiploic appendagitis can
simulate appendicitis and right- and left-sided diverticulitis clinically and
on CT scans. The inflamed appendage presents as a small fat-attenuation mass
with a hyperattenuating rim (Fig.
8) that abuts the serosal surface of the colon. At the center of
the lesion, a small round or linear hyperdense focus may be seen, probably
representing vascular thrombosis. Epiploic appendagitis also produces mass
effect, focal thickening of the adjacent bowel, infiltration of the mesenteric
fat, and focal thickening of the surrounding peritoneum
[77,78,79].
Omental Infarction
In this disorder, portions of the omentum undergo segmental infarction
simulating acute appendicitis, pancreatitis, and epiploic appendagitis. CT
features include a well-circumscribed region of inflamed omental fat with
haziness, and streaklike areas of inflammatory stranding
[80]. The adjacent small
bowel, colon, and appendix appear normal.
Mesenteric Adenitis
Benign inflammation of the ileocolic lymph nodes can cause mesenteric
adenitis, which often simulates clinical appendicitis. Yersinia
enterocolitica, Y. pseudotuberculosis, and Helicobacter
jejuni are the most commonly implicated organisms. The appendix is normal
and there may be thickening of the adjacent ileum and cecum. On CT, the
mesenteric lymph nodes are enlarged (>5 mm) and there may be inflammatory
change in the surrounding mesentery
[81].
Small-Bowel Diverticulitis
This rare condition is caused by the inflammation of a jejunal or ileal
pseudodiverticulum or Meckel's diverticulum. CT findings are nonspecific and
include perienteric inflammation
[82,
83]. Occasionally, an air- or
enterolith-filled diverticulum can be identified in the inflammatory process
(Fig. 9).
Typhlitis
Neutropenic enterocolitis is an acute inflammatory and necrotizing process
that affects the cecum or terminal ileum and appendix of immunocompromised
patients with profound neutropenia. In this disorder, ulceration of the mucosa
is followed by bacterial and fungal invasion. CT features are nonspecific and
include segmental mural thickening of the cecum, intramural regions of edema
or necrosis, pericolic fluid, and perienteric standing
[84,
85]
(Fig. 10). In advanced cases,
pneumatosis intestinalis and frank perforation may develop.
Infectious Enterocolitides
Gastroenteritis and the infectious enterocolitides are responsible for
nearly 70% of emergency department visits prompted by abdominal pain
[2]. Most cases are
self-limited and do not require imaging. In atypical cases, colicky abdominal
pain rather than diarrhea may be the predominant symptom. CT scans may show
normal findings or may show nonspecific mural thickening in more severe cases
of infection with invasive Escherichia coli, Shigella, and
Salmonella, Yersinia, and Entamoeba organisms
[85].
In pseudomembranous colitis, potent antibiotics disrupt the normal bacterial flora of the colon, resulting in the overgrowth of Clostridium difficile. The release of its enterotoxins causes mucosal inflammation and the development of pseudomembranes consisting of mucous and inflammatory debris. On CT, mural thickening averages 15-20 mm with a target or halo pattern caused by submucosal edema. Contrast material caught between thick haustra may simulate deep ulcerations (Fig. 11) and produce an accordionlike appearance. Additionally, the lumen may be completely effaced. Ascites and pericolic inflammatory changes accompany these features [86].
Helical CT is most useful in differentiating the panoply of inflammatory, infectious, and neoplastic disorders that can cause acute abdomen in AIDS patients. Infections such as cryptosporidiosis (Fig. 12) and cytomegalovirus produce thickening of the gut wall, edema of the submucosa, and increased enhancement of the mucosa [87].
Inflammatory Bowel Disease
Most patients with inflammatory bowel disease experience chronic symptoms
punctuated by periodic exacerbations. Fortunately, true emergencies are
uncommon; however, emergencies are associated with high rates of morbidity and
mortality. Bowel obstruction and abscess formation are the most common
emergencies in Crohn's disease, whereas fulminant colitis, toxic megacolon,
and perforation develop in patients with ulcerative colitis.
Abscesses (Fig. 13A) develop in nearly 25% of patients with Crohn's disease, and helical CT is the preferred means of establishing a diagnosis and guiding percutaneous drainage [88]. In patients with obstruction, the status of the diseased bowel, as depicted on CT (Fig. 13B), can significantly influence patient treatment. CT scans that reveal mural stratification (the ability to visualize distinct mucosal, submucosal, and muscularis propria layers) indicate the presence of submucosal edema. This edema may improve with steroid therapy. The reduced edema can lead to widening of lumen caliber with subsequent amelioration of the obstruction. If mural stratification is lost, then transmural fibrosis is probably present and the obstruction may require surgery. CT may also reveal other, nonemergent complications of Crohn's disease, including fibrofatty proliferation of the mesentery, fistulas, and reactive adenopathy [89]. In patients with fulminant ulcerative colitis, CT is the preferred noninvasive means of assessing the status of the bowel wall and detecting early perforation in toxic megacolon.
Perforation
Gastrointestinal perforation usually indicates a catastrophic complication
of peptic ulcer disease, diverticulitis, severe intestinal inflammation,
infarction, trauma, neoplasm, or closed-loop obstruction. Helical CT is ideal
for evaluating patients with signs of peritonitis, which is often misdiagnosed
as another acute lesion. CT can detect pneumoperitoneum that may be overlooked
on chest or abdominal radiography.
Detection of the site of perforation is often difficult but can be assisted by the oral (Fig. 7B) and IV administration of contrast material. Loculated fluid and gas, focal mesenteric or omental infiltration, and focal enhancement of the parietal peritoneum can help pinpoint the site of perforation.
Intraabdominal Sepsis
Patients with an abdominal abscess or peritonitis can present with an acute abdomen. Abdominal infections most commonly result from the contiguous spread of bacteria from the gut, biliary tract, or genitourinary system. These infections are typically polymicrobial in nature and include both aerobic and anaerobic organisms. Helical CT is the most accurate imaging examination for the diagnosis of intraabdominal abscesses. Initially, abscesses appear as a mass of soft-tissue attenuation caused by the influx of inflammatory cells. With maturation, the abscess undergoes central liquefaction necrosis, and highly vascularized peripheral connective tissue develops. As a result, this lesion has a low-attenuation center with an enhancing rim (Fig. 13B). Small gas bubbles or air-fluid levels are present in 40-50% of patients and are highly suggestive of intraabdominal sepsis. Abscesses tend to be round or oval unless they are adjacent to a solid organ. In these patients, abscesses may develop a lentiform or crescentic configuration. Abscesses also displace surrounding structures, obliterate or thicken adjacent fascial planes, and cause inflammation of the contiguous mesenteric or omental fat [90].
Acute Cholecystitis
Although sonography is the preferred method for diagnosing acute cholecystitis, CT is frequently the initial examination because the diagnosis is unclear. The most sensitive helical CT findings of acute cholecystitis are mural thickening greater than 3 mm (in the setting of a distended gallbladder) and enhancement of the inflamed wall [91] (Fig. 14A). Transient focal increased attenuation of the liver (Fig. 14B) may develop adjacent to the inflamed gallbladder resulting from hepatic artery hyperemia and early venous drainage [92]. Less specific signs include pericholecystic fluid, haziness of the pericholecystic fat, and increased attenuation of the gallbladder bile. Helical CT can also depict complications of acute cholecystitis including perforation and gangrene. INtramural or intraluminal gas is present in emphysematous cholecystitis.
Choledocholithiasis
Patients with choledocholithiasis typically present with acute right upper quadrant pain, fever, jaundice, and pancreatitis. Thin-collimation (3 mm) scans are needed to optimize the detection of stones on CT. A high-density nidus may be visualized in the duct, or alternating low- and high-density rings of mixed cholesterol-calcium stones may be seen (Fig. 15). Biliary dilatation may be proximally evident. Helical CT has a sensitivity of 88%, specificity of 97%, and accuracy of 94% in the detection of choledocholithiasis; however, positive intraluminal and intravascular contrast material can obscure the detection of peripherally calcified stones [93,94,95].
Pancreatitis
Helical CT plays a vital role in the clinical treatment and staging of patients with acute pancreatitis. CT can reveal hemorrhage or necrosis in the pancreas and identify the extension of inflammation in adjacent organs [96,97,98,99,100,101,102]. CT findings of acute pancreatitis reflect edema of the gland and surrounding fat and may be normal in up to 28% of mild cases [97]. The entire gland may become diffusely enlarged with a shaggy irregular contour. In mild cases, the peripancreatic fat contains wisps of high attenuation, the vascular margins are cuffed, and the fascial planes are thickened. Mild peripancreatic inflammation may be present around an otherwise normal-appearing gland. Segmental pancreatitis occurs in 10-18% of patients and is usually associated with stone disease [99]. Typically, the gland shows uniform enhancement.
In more advanced cases, intraglandular intravasation of pancreatic fluid leads to the formation of many small intrapancreatic fluid collections. In necrotizing pancreatitis, the gland becomes enlarged and is often enveloped by high-attenuation exudates. Necrotic parenchyma shows decreased or no enhancement that is sharply demarcated from normally enhancing viable tissue (Fig. 16). The body and tail are usually involved; the head is spared because of its rich collateral vascular network. Enhancing islands of viable tissue may be scattered throughout the gland. The poorly defined peripancreatic exudates obliterate the peripancreatic fat, dissect fascial planes, and penetrate through fascial and peritoneal boundaries and ligaments. These collections typically accumulate in the lesser sac, anterior pararenal space, and anterior interfascial space. Helical CT is also useful in revealing vascular complications such as pseudoaneurysms and splenic and portal vein thrombosis [98, 101].
Helical CT can help predict patient outcome by delineating necrosis. In one
study, patients with no evidence of necrosis on CT had no mortality and only
6% morbidity. Patients with small areas of necrosis (<30%) showed no
mortality and 40% morbidity, whereas patients with large areas of necrosis
(
50%) had a 75-100% morbidity and a 11-25% mortality
[100].
Aortic Aneurysm Rupture (Dissection)
The clinical triad of symptoms of a ruptured aortic aneurysm include abdominal pain, a pulsatile mass, and hypotension. Nearly one third of patients do not have this classic presentation and are misdiagnosed as having renal colic and diverticulitis. The diagnosis of ruptured aneurysm should be considered in elderly men who are smokers because they run a higher risk of rupture. Helical CT is the imaging procedure of choice in patients with suspected aneurysm dissection and rupture [103, 104]. Oral contrast material should not be administered. Unenhanced images are initially obtained to search for hyperdense blood associated with one of the following signs of impending rupture: the draped aorta sign, in which the posterior wall of the aorta cannot be identified and is closely applied to the spine; the high-attenuation crescent sign, attributed to hemorrhage in mural thrombus or in the wall of the aneurysm, which may be the first sign of aneurysm rupture; and focal discontinuity of intimal calcification [105, 106].
Rapid infusion of contrast material (3 ml/sec) and thin collimation (3
mm) are required for optimal vascular resolution
[107], depiction of intimal
flaps, and multiplanar three-dimensional vascular image creation (Fig.
17A,17B).
Although the atherosclerotic walls of aneurysms enhance and are perfused by
the vasa vasorum, necrotic areas of the aortic wall reveal nonenhancing focal
areas of low density [108].
On CT, direct signs of rupture include a retroperitoneal hematoma
(Fig. 18) or frank
extravasation of IV contrast material
[109].
Abdominal Hemorrhage
Acute hemorrhage in the gut, mesenteries, omenta, retroperitoneum, or abdominal musculature can cause acute abdomen. Patients with significant bleeding have a declining hematocrit and hypotension. First, unenhanced scans should be obtained to detect hyperdense hematoma. IV contrast material delivered at a high rate (4 ml/sec) may identify an active site of hemorrhage and provide a useful guide for subsequent angiographic embolization [110]. Bleeding may occur into the rectus sheath or the psoas muscle (Fig. 19). Most spontaneous hemorrhages are caused by anticoagulation; however, occasionally they may result from tumor hemorrhage, most commonly in cases of renal cell carcinoma.
Hepatosplenic Vascular Disease
Patients with hepatic venous (Budd-Chiari syndrome), portal venous, and hepatic arterial thrombosis can present with acute right upper quadrant pain. The severity of symptoms depends on the extent and speed of onset of the occlusion.
Budd-Chiari syndrome has many causes including coagulopathy, polycythemia vera, myeloproliferative disorders, and neoplasms. Thrombus may occur in the hepatic veins and the inferior vena cava. On early scans, the liver shows patchy enhancement, with the central portions having increased enhancement and the periphery having decreased enhancement. Delayed images show a reversal of this pattern [111].
Portal vein thrombosis develops in patients with cirrhosis, hepatic neoplasms, pancreatitis, and mesenteric pyophlebitis. Portal vein thrombosis appears as a low-density central zone surrounded by an enhanced periphery on contrast-enhanced scans. Transient inhomogeneous enhancement of the affected liver segment also occurs. Tumor thrombus may dilate the vein and show arterial phase enhancement [111].
Hepatic infarction is rare because the liver has a dual blood supply. Hepatic infarction usually results from thrombosis of the hepatic artery, which can be seen in patients with sepsis, shock, oral contraceptive use, a transplanted liver, sickle cell disease, eclampsia, bacterial endocarditis, trauma, and polyarteritis nodosa. CT reveals wedge-shaped peripheral areas of low attenuation without contrast enhancement (Fig. 20). Splenic and renal infarcts may also be present [111].
Splenic infarction manifests as acute left upper quadrant pain but may be clinically silent. Bacterial endocarditis, pancreatitis, portal hypertension, sickle cell disease, and splenomegaly are responsible for most infarcts. On CT, focal infarcts manifest as wedge-shaped zones of decreased attenuation that extend to the splenic capsule. Although some peripheral enhancement may be caused by perfusion of capsular vessels, global infarction can cause diffuse splenic hypodensity.
Conclusion
The subjective nature of pain, its complex neuroanatomic pathways, and the fact that a common symptom can arise from a broad spectrum of diseases combine to make acute abdomen difficult to diagnose. Nevertheless, two important decisions must be made: does the patient need surgery? and if so, how soon? Immediate surgery is required for patients with massive hemorrhage (e.g., abdominal aortic aneurysm rupture); other conditions (e.g., perforation and intestinal ischemia) require surgical intervention in a few hours because additional delay increases morbidity. A delay of more than 12 hr is detrimental in disorders such as appendicitis, mesenteric venous thrombosis, and strangulated small-bowel obstruction. Helical CT has become the most important noninvasive imaging tool to diagnose acute abdomen and answer the questions posed above. Helical CT has the potential to positively affect the outcome, length of stay, and overall health care expenditures of patients with acute abdomen.
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N. Ridley and S. E. M. Green Mesenteric Arterial Thrombosis Diagnosed on CT Am. J. Roentgenol., February 1, 2001; 176(2): 549 - 549. [Full Text] [PDF]
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