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Fig. 1. A 74-year-old man with known diabetes mellitus presents with acute abdominal pain in the right upper quadrant. a Plain abdominal radiograph shows a fluid-gas level in the distended gallbladder and gas in the gallbladder wall (open arrow). b MDCT depicts the dilated gallbladder with intramural gas (open arrow), indicative of emphysematous cholecystitis. In addition, extraluminal/extramural gas is present due to walled-off gallbladder perforation (arrow). Hydronephrosis and parenchymal atrophy of the left kidney

Fig. 1. A 74-year-old man with known diabetes mellitus presents with acute abdominal pain in the right upper quadrant. a Plain abdominal radiograph shows a fluid-gas level in the distended gallbladder and gas in the gallbladder wall (open arrow). b MDCT depicts the dilated gallbladder with intramural gas (open arrow), indicative of emphysematous cholecystitis. In addition, extraluminal/extramural gas is present due to walled-off gallbladder perforation (arrow). Hydronephrosis and parenchymal atrophy of the left kidney this tumor, but uncommon in Western countries. Subcapsular localization and tumor necrosis have been implicated in pathogenesis. US, and especially CT, are the most useful techniques for diagnosing a ruptured he-

patocellular carcinoma, which appears as a peripheral or subcapsular mass. Spontaneous hemorrhage within a he-patocellular adenoma occurs most commonly in women taking oral contraceptives. Capsular rupture with subsequent hemoperitoneum is an uncommon complication. On CT, high-density intraperitoneal fluid confirms the diagnosis of hemoperitoneum, and extravasation of contrast material is indicative of active bleeding.

Left Upper Quadrant

Acute abdomen with left upper quadrant pain is not frequent. Splenic infarction, splenic abscess, gastritis, and gastric or duodenal ulcer are the most important causes. US is usually used for screening, and CT enables accurate further evaluation. The diagnosis of gastric pathology is established by endoscopy, with imaging playing a minor role.

Common causes of splenic infarction include bacterial endocarditis, portal hypertension, and underlying splenomegaly. Pancreatitis that extends into the splenic hilum can also result in infarction. Splenic infarction may be focal or global. Typical focal splenic infarcts appear as peripheral wedge-shaped defects, hypoechoic at US and hypodense at CT, respectively. Most splenic abscesses are associated with hematogenous dissemination of infection, such as bacterial endocarditis or tuberculosis. Intravenous drug abusers are predominantly affected. Both US and CT are sensitive, but specificity is low. On US, most abscesses appear as hypo- or anechoic, poorly defined lesions; on CT, they typically appear as rounded, low-density lesions with rim enhancement.

Right Lower Quadrant

Acute appendicitis is not only the most frequent cause of acute right lower quadrant pain, but also the most commonly encountered cause of acute abdomen. Other diseases manifesting acute right lower quadrant pain include acute terminal ileitis (Crohn's disease), acute typhlitis, right sided colonic diverticulitis and, in women, pelvic inflammatory disease, complications of ovarian cyst (hemorrhage, torsion and leak), endometriosis, or ectopic pregnancy.

Most patients with typical clinical findings of acute appendicitis undergo immediate surgery without preoperative imaging. Since diagnosis is uncertain in up to one third of patients because of atypical symptoms, many centers today request appendiceal imaging for clinically equivocal patients. Although plain radiography continues to play a role in evaluating patients with acute right lower quadrant pain, its role is quite limited. Less than 50% of patients with appendicitis show an abnormality on plain radiographs. The most specific finding is the presence of an appendicolith, which is usually calcified, solitary, and rounded.

US has become an important imaging option in the evaluation of suspected acute appendicitis, particularly in children, pregnant women, and women of reproductive age. The prime sonographic criterion is the demonstration of a swollen, non-compressible appendix greater than 7 mm in diameter with a target configuration (Fig. 2). Generally, the normal appendix cannot be defined with US, thus, clear visualization of the appendix is suggestive of inflammation.

Advantages of US include lack of ionizing radiation, relatively low cost, and widespread availability. On the other hand, US requires considerable skill and is difficult to perform in obese patients, patients with severe pain, and patients likely to have a complicating periappen-diceal abscess. When the sonographic findings are unclear, CT can provide a rapid and definitive diagnosis.

Abdominal Quadrants

Fig. 2. A 25-year-old man presented with acute abdominal pain in the right lower abdominal quadrant. Physical examination and laboratory tests revealing elevated white blood cell counts, raised the suspicion of acute appendicitis. Longitudinal (a) and perpendicular (b) graded compression US shows an enlarged appendix (cursors, diameter > 10 mm) with edematous thickening of the appen-dical wall, confirming the diagnosis of acute appendicitis

Fig. 2. A 25-year-old man presented with acute abdominal pain in the right lower abdominal quadrant. Physical examination and laboratory tests revealing elevated white blood cell counts, raised the suspicion of acute appendicitis. Longitudinal (a) and perpendicular (b) graded compression US shows an enlarged appendix (cursors, diameter > 10 mm) with edematous thickening of the appen-dical wall, confirming the diagnosis of acute appendicitis

CT has emerged in many centers as the primary imaging modality for patients with suspected acute appendicitis due to its exceptional accuracy. In the case of mild disease, the findings include a dilated, fluid-filled appendix with a calcified appendicolith or inflammatory changes of the mesenteric fat (Fig. 3). An inflammatory mass or an abscess may develop with disease progression and perforation.

Diverticulitis rarely manifests itself as a right-sided condition. Right-sided colonic diverticula are often congenital, solitary and true diverticula, unlike sigmoid di-verticula. The normal appendix should be visible in right-sided diverticulitis. If the appendix cannot be identified, right-sided omental infarction or epiploic appendagitis must be considered in the differential diagnosis.

Left Lower Quadrant

Diverticulitis is the most common cause of acute abdominal pain in the left lower quadrant. Diverticulitis occurs in up to 25% of patients with known diverticulosis and typically involves the sigmoid colon. CT has replaced barium enema examinations because it is very sensitive and approaches 100% specificity and accuracy in the diagnosis or exclusion of diverticulitis. CT is also very useful in establishing the presence of pericolic complications and differentiating sigmoid diverticulitis from carcinoma - a major differential diagnostic consideration.

Superimposed on diverticulosis, the CT diagnosis of acute diverticulitis is based on the identification of segmental colonic wall thickening and pericolic inflammatory changes, such as fat stranding, inflammatory mass, gas bubbles, abscess, or free fluid (Fig. 4). Occasionally, patients with diverticulitis may manifest pneumaturia because of a complicating enterovesical fistula.

Fig.3. 45-year-old man with elevated white blood cells and acute pain in the right lower abdominal quadrant. US examination was unable to identify the appendix. a Sagittal MDCT depicts marked enlargement of the retrocecal appendix (arrow in a and b), with wall thickening, mural enhancement, and adjacent fat stranding. b Transverse MDCT shows two appendicoliths within the appen-dical lumen. The US-diagnosis of acute appendicitis is difficult to establish in a retrocecal appendix

Sigmoid Colon Walled Off Perforation

Fig.4. 55-year-old man with known diverticular disease and acute abdominal pain in the left lower quadrant. MDCT shows a narrowed segment of the sigmoid colon with wall thickening, pericol-ic inflammatory changes, and an adjacent fluid-collection with marked peripheral enhancement (arrow), indicating an abscess after walled-off perforation in acute diverticulitis

Fig.4. 55-year-old man with known diverticular disease and acute abdominal pain in the left lower quadrant. MDCT shows a narrowed segment of the sigmoid colon with wall thickening, pericol-ic inflammatory changes, and an adjacent fluid-collection with marked peripheral enhancement (arrow), indicating an abscess after walled-off perforation in acute diverticulitis

Acute Abdomen with Diffuse Pain

Any disorder that irritates a large portion of the GI tract and/or the peritoneum will cause diffuse abdominal pain. The most common disorder is gastroenterocolitis. Other important disorders are bowel obstruction, ischemic bowel disease, and GI tract perforation.

Bowel Obstruction

Bowel obstruction is a frequent cause of abdominal pain and accounts for approximately 20% of surgical admissions for acute abdominal conditions. The small bowel is involved in 60-80% of cases. Frequent causes of small bowel obstruction are adhesions resulting from prior surgery, hernias, and neoplasms. In the large bowel, mechanical obstruction is commonly due to diverticular disease or colorectal carcinoma. 5-10% of cases of large bowel obstruction are caused by volvulus, which is most commonly in the sigmoid, followed by the cecum.

The diagnosis of bowel obstruction is established on clinical grounds and usually confirmed with plain abdominal radiographs. Because of the diagnostic limitations of plain films, CT is increasingly used to establish the diagnosis, identify the site, level, and cause of obstruction and determine the presence or absence of associated bowel ischemia. CT can be useful for differentiating between simple and closed loop obstruction. Closed loop obstruction is a form of mechanical bowel obstruction in which two points along the course of the bowel are obstructed at a single site. It is usually secondary to an adhesive band or a hernia. A closed loop tends to involve the mesentery and is prone to produce a volvulus, thus representing the most

Strangulation Bowel

common cause of strangulation. However, only volvulus of the large bowel is associated with classic features on plain abdominal radiographs. The sigmoid volvulus produces a distended loop, with the twisted mesenteric root pointing to the origin of the volvulus, i.e., to the sigmoid.

CT is particularly reliable in higher grades of bowel obstruction. It has proved useful for characterizing bowel obstruction from various causes, including adhesions, hernia, neoplasm, extrinsic compression, inflammatory bowel disease, radiation enteropathy, intussusception, gallstone ileus, or volvulus. The essential CT finding of bowel obstruction is the delineation of a transition zone between the dilated and decompressed bowel. Careful inspection of the transition zone and luminal contents usually reveals the underlying cause of obstruction. However, the presumed point of transition from dilated to non-dilated bowel can be difficult to determine in the axial plane. MDCT facilitates this task by providing the radiologist with a volumetric data set that can be viewed in the axial, sagittal, or coronal plane or any combination of the three. These MPR views centered on the anticipated transition point help to determine the site, level, and cause of obstruction.

Mechanical obstruction has to be differentiated from paralytic ileus. Numerous causes exist for both diffuse and localized paralytic ileus. Paralytic ileus is a common problem after abdominal surgery. It may be secondary to ischemic conditions, inflammatory or infectious disease, abnormal electrolyte, metabolite, drug or hormonal levels, or innervation defects. A massively dilated colon with a thickened wall ('thumbprinting') caused by wall edema and inflammation is seen with toxic megacolon in pseudomembranous colitis. Toxic megacolon is the radiological manifestation of a paralytic ileus.

Ischemic Colitis

Ischemic Bowel Disease

Arterial or venous occlusion or thrombosis and hypoper-fusion are predominant causes of bowel ischemia. Usually, a combination of these factors is observed. The predominance of one factor determines the outcome and the findings on CT. Diminished bowel wall enhancement is the only direct sign of vascular impairment of the bowel and has been reported in predominantly arterial disease, such as infarction, as well as in predominantly venous diseases, such as strangulation. Other CT findings are direct visualization of the thrombus in the superior mesenteric artery or vein. Bowel distention and bowel wall edema are nonspecific findings and can be seen with inflammatory or infectious causes. Bowel distention reflects the interruption of peristaltic activity in ischemic segments.

In closed loop bowel obstruction the closed loop can become strangulated, i.e., ischemic, although the progression of a closed loop obstruction to a strangulated one is not inevitable. The reported prevalence of strangulating obstruction ranges from 5-40%. Strangulation is a predominantly venous disease. The most frequent abnormality seen on CT is bowel wall thickening. The thickened bowel wall is sometimes associated with the target sign, alternating layers of high and low attenuation within the thickened bowel wall, which results from submu-cosal edema and hemorrhage. The bowel segment proximal to an obstruction can become ischemic due to severe bowel distention. CT findings that suggest subsequent infarction include non-enhancement of the bowel wall, gas in the bowel wall, mesenteric or portal veins, edema/hemorrhage in the mesentery adjacent to thickened and/or dilated bowel loops, and ascites (Fig. 5).

Best Pictures

Fig. 5. 76-year-old man with acute diffuse abdominal pain and with a history of abdominal surgery performed five weeks previously. a Transverse MDCT demonstrates mesenteric edema and infarction of the distal ileum (arrowhead) with intramural gas. b Transverse MDCT at a level below depicts thickening of the bowel wall with irregular mucosal enhancement (arrow) indicative of bowel ischemia. Intraoperatively, a strangulating small bowel volvulus secondary to adhesion of the distal ileum was identified

GI Tract Perforation

Pneumoperitoneum usually starts with localized pain and culminates in diffuse pain after peritonitis has developed. It may result from a variety of causes. Gastroduodenal perforation associated with peptic ulcer or necrotic neoplasm has become less frequent in the last few decades due to earlier diagnosis and improved therapy. At the same time, the incidence of gastroduodenal perforations resulting from endoscopic instrumentation has increased. Perforation of the small bowel is relatively uncommon. Spontaneous rupture of the large bowel is more frequent and can occur in a markedly dilated colon proximal to an obstructing lesion (tumor, volvulus) or when the bowel wall is friable (ischemic or ulcerative colitis, necrotic neoplasm). Over the last decades, fiberoptic endoscopy has been increasingly performed for evaluation and biopsy of colorectal lesions, as well as for polypectomy; these procedures cause perforation in 0.5-3% of patients.

Pneumoperitoneum can be recognized by the presence of subdiaphragmatic gas on an erect chest radiograph or an erect or left lateral decubitus radiograph of the abdomen. An abundant pneumoperitoneum is indicative of a perforation complicating large bowel obstruction, and moderate quantities of free gas are seen in the perforation of the stomach. Only small quantities of gas escape with perforation of the small bowel, because the small bowel does not usually contain gas. Detection of subtle pneumoperitoneum is often difficult. CT is far more sensitive than conventional radiography for the detection of a small pneumoperitoneum, and it has thus become the modality of choice in cases that are unclear on a conventional radiograph. To enhance the sensitivity of CT for extraluminal gas, the images are also viewed at 'lung window' settings. On CT, small amounts of gas around the stomach and the liver are seen mainly after gastroduodenal or small bowel perforation.

Retroperitoneal perforations (duodenal loop beyond the bulbar segment, appendix, posterior aspect of ascending and descending colon, rectum below the peritoneal reflection) tend to be contained locally and remain clinically silent for several hours or days. Retroperitoneal gas has a mottled appearance and may extend along the psoas muscles. In contrast to intraperitoneal gas, retroperitoneal gas does not move freely when the patient's position is changed from supine to upright for plain abdominal radiographs.

Acute Abdomen with Flank or Epigastric Pain

Acute flank or epigastric pain is commonly a manifestation of retroperitoneal pathology, especially urinary colic, acute pancreatitis, or leaking abdominal aortic aneurysm.

Urinary Colic

For several decades, intravenous urography has been the primary imaging technique used in patients with flank pain thought to be caused by urinary colic. Abdominal radiograph and US are considered useful for those patients with contraindications to irradiation or contrast media. However, because of the low sensitivity of abdominal radiographs and US for urinary tract calculi, the role of un-enhanced CT has grown rapidly. On CT, virtually all ureteral stones are radiopaque, regardless of their chemical composition. Uric acid stones have attenuation values of 300-500 Hounsfield units (HU), and calcium-based stones have attenuation values > 1,000 HU. In addition to the direct demonstration of a ureteral stone, secondary signs of ureterolithiasis may be seen, including hy-droureter, hydronephrosis, perinephric stranding, and renal enlargement (Fig. 6). Perinephric stranding and edema result from reabsorbed urine infiltrating the perinephric space along the bridging septa of Kunin. The more extensive the perinephric edema as shown on unen-hanced CT, the higher the degree of urinary tract obstruction. Focal periureteral stranding resulting from local inflammatory reaction or irritation and induced by the passage of a stone helps localize subtle calculi. MDCT is

Fig. 6. 46-year-old man with right-sided flank pain. Coronal MDCT reconstruction (a) and transverse MDCT after intravenous contrast administration (b) demonstrate an obstructing stone at the uretero-pelvic junction (arrow) with dilatation of the renal pelvis and of the calices

b favored over single-detector CT because it provides coronal MPRs, which often portray the urinary tract more effectively than axial images.

When no stone is detected, a search for an alternative diagnosis should be performed. Non-calculus urinary tract abnormalities causing symptoms of colic include acute pyelonephritis, renal cell carcinoma, acute renal vein thrombosis and renal infarction. Extraurinary diseases, such as appendicitis, diverticultis, small bowel obstruction, pancreatitis, and retroperitoneal hemorrhage may also simulate acute urinary colic. Occasionally, repeating the CT examination with intravenous, oral, or rectally administered contrast material may be required.

Acute Pancreatitis

Acute pancreatitis is an important disease causing epigastric pain. US is helpful for the demonstration of gallstones as a cause of acute pancreatitis and for the follow-up of known fluid collections. CT has become the imaging modality of choice to stage the extent of disease and to detect complications because CT findings correlate well with the clinical severity of acute pancreatitis. Pancreatic enlargement due to interstitial parenchymal edema may progress to pancreatic exudate collecting in the anterior pararenal space, the transverse mesocolon, the mesenteric root, and the lesser sac. The pancreatic parenchyma may undergo necrosis or hemorrhage. Severe pancreatitis is often complicated by thrombosis of the splenic and portal vein.

Acute pancreatic and peripancreatic fluid collections may evolve into pseudocysts. Pseudocysts exhibit defined capsules. A pseudocyst can erode peripancreatic vessels, resulting in bleeding or formation of a pseudoa-neurysm. Larger aneurysms can be diagnosed by CT or sonographically with Doppler; angiography may be necessary to diagnose small pseudoaneurysms (< 1 cm).

Leaking Aneurysm of Abdominal Aorta or Iliac Artery

One of the most life-threatening alternative diagnoses in acute flank pain is a leaking aneurysm of the abdominal aorta or iliac artery. When a patient with suspected rupture of an abdominal aortic aneurysm is hemodynami-cally unstable, US is the initial imaging technique used. The examination can be performed rapidly using portable equipment in the emergency room. However, the diagnosis of para-aortic hemorrhage by US is poor.

In hemodynamically stable patients non-contrast-enhanced CT is the initial imaging test of choice. Non-contrast CT can almost always demonstrate a para-aortic hematoma if present and may show additional findings helpful in establishing the diagnosis, such as a high-attenuating crescent sign. If the non-contrast CT findings are equivocal or if endoluminal stent graft repair of the aorta is planned, contrast-enhanced CT should be performed.

Conclusions

The practice of radiology in imaging patients with acute abdomen has changed dramatically in the last few years. The time-honored plain abdominal radiographs have been largely replaced with US and CT. In particular, helical CT and more recently, MDCT permit the examination to be performed in less time, with greater diagnostic accuracy, and with less patient discomfort. The topographic classification of pain (i.e., localized pain in one of the four abdominal quadrants, diffuse abdominal pain and flank or epigastric pain) facilitates finding the answer to specific questions. Therefore, close co-operation with the referring physician prior to imaging remains essential for rapid and accurate diagnosis.

Suggested Reading

Bhalla S, Menias CO, Heiken JP (2003) CT of acute abdominal aortic disorders. Radiol Clin N Am 41:1153-1169 Freeman AH (2001) CT and bowel disease. Br J Radiol 74:4-14 Gore RM, Miller FH, Pereles FS et al (2000) Helical CT in the evaluation of the acute abdomen. AJR Am J Roentgenol 174:901-913

Macari M, Megibow A (2001) Imaging of suspected acute small bowel obstruction. Seminars in Roentgenology XXXVI: 108117

Mindelzun RE, Jeffrey RB (1997) Unenhanced helical CT for evaluating acute abdominal pain: a little more cost, a lot more information. Radiology 205:43-47 Novelline RA, Rhea JT, Rao PM, Stuk JL (1999) Helical CT in emergency radiology. Radiology 213:321-339 Smith RC, Varanelli M (2000) Diagnosis and management of acute ureterolithiasis. AJR Am J Roentgenol 175:3-6 Taourel P, Kessler N, Lesnik A et al (2003) Helical CT of large bowel obstruction. Abdom Imaging 28:267-275 Urban BA, Fishman EK (2000) Tailored helical CT evaluation of acute abdomen. Radiographics 20:725-749 Wiesner W, Khurana B, Ji H, Ros PR (2003) CT of acute bowel ischemia. Radiology 226:635-650

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