Seminars

Emergency Radiology of the Abdomen: The Acute Abdomen

B. Marincek1, J.P. Heiken2

11nstitute of Diagnostic Radiology, University Hospital Zurich, Zurich, Switzerland 2 Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, USA

Introduction

The term 'acute abdomen' defines a clinical syndrome characterized by a history of hitherto undiagnosed abdominal pain lasting less than one week. A large number of disorders, ranging from benign, self-limited diseases to conditions that require immediate surgery, can cause acute abdominal pain. Eight conditions account for over 90% of patients who are referred to hospital and are seen on surgical wards with acute abdominal pain: acute appendicitis, acute cholecystitis, small bowel obstruction, urinary colic, perforated peptic ulcer, acute pancreatitis, acute diverticular disease, and non-specific, non-surgical abdominal pain ('dyspepsia', 'constipation').

Imaging Techniques

Clinical assessment of acute abdomen is often difficult because of the often non-specific findings of physical examination and laboratory investigations. In many centers plain radiographs of the abdomen, despite significant diagnostic limitations, serve as the initial radiological approach. Two views are usually taken, one supine and one erect. If the patient is unable to stand, a left lateral decubitus view is performed. For a systematic film analysis it is helpful to follow the mnemonic 'gas, mass, stones and bones' for the detection of (1) signs of mechanical bowel obstruction or paralytic ileus; (2) gas outside the bowel lumen in the peritoneal cavity (pneumoperitoneum), retroperitoneum, bowel wall, portal veins, or biliary tract; (3) mass or fluid collections, displacement of organs or bowel loops; (4) abnormal calcifications and/or calculi; (5) skeletal pathology.

The need for plain abdominal radiographs has declined due to the impact of cross-sectional imaging. The traditional indications for plain abdominal radiography - pneumoperitoneum, bowel obstruction, and the search for ureteral calculi - are better evaluated by un-enhanced helical computed tomography (CT). A number of authors have shown that CT is clearly superior to plain radiography for diagnosing pneumoperitoneum, detecting a bowel obstruction, and for identifying ureteral calculi. The major obstacles to replacing plain abdominal radiography with unenhanced CT are its higher cost, more limited availability, and higher radiation dose.

Although ultrasonography (US) has gained widespread acceptance for evaluating the gallbladder in affected patients and the pelvis in children and women of reproductive age, CT is considered to be one of the most valued tools for triaging patients with acute abdominal pain. This is because it can provide a global perspective of the gastrointestinal (GI) tract, mesenteries, peritoneum, and retroperitoneum, inhibited by the presence of bowel gas and fat. Over recent years, most emergency centers have been equipped with newer helical CT scanners that permit imaging procedures to be performed in less time, with greater accuracy, and with less patient discomfort. The introduction of multidetector CT (MD-CT) technology, with advances in contrast dynamics and high-resolution volumetric data acqusition, has further enhanced the utility of CT in abdominal imaging. Image interpretation with helical CT and particularly with MD-CT is primarily performed at a workstation by manually paging or continuously scrolling up and down through the stack of reconstructed images. Additionally, multiplanar reformation (MPR) using coronal, sagittal, and curved planes, has evolved as a routine supplement to the axial images.

Three-dimensional volume rendered and maximum intensity projection (MIP) images are also easily produced from MDCT data sets. Inquiry about the site of abdominal pain facilitates the choice of imaging technique. For practical reasons, it is helpful to discuss the imaging strategies for acute pain localized in an abdominal quadrant separately from acute abdomen with diffuse pain and acute abdomen with flank or epigastric pain.

Acute Pain in an Abdominal Quadrant

Acute abdomen with pain localized in an abdominal quadrant can be classified as pain in the right upper, left upper, right lower, and left lower abdominal quadrant.

Right Upper Quadrant

Acute cholecystitis is by far the most common disease in the right upper quadrant. Other important diseases that resemble acute cholecystitis are pyogenic or amebic liver abscess, spontaneous rupture of a hepatic neoplasm (usually hepatocellular adenoma or carcinoma), hepatitis, and myocardial infarction.

US is the preferred imaging method for evaluating patients with acute right upper abdominal pain. It is a reliable technique for establishing the diagnosis of acute calculous cholecystitis. The primary criterion is the detection of gallstones. Secondary signs include the sonographic Murphy sign, gallbladder wall thickening by 3 mm or more, and pericholecystic fluid. Typically, a calculus obstructs the cystic duct in acute calculous cholecystitis. The trapped concentrated bile irritates the gallbladder wall, causing increased secretion, which in turn leads to distension and edema of the wall. Rising intraluminal pressure compresses the vessels, resulting in thrombosis, ischemia, and subsequent necrosis and perforation of the wall. Gallbladder perforation and complicating pericholecystic abscesses typically occur adjacent to the gallbladder fundus because of the sparse blood supply. CT may be useful for confirmation of the sonographic diagnosis. Emphysematous cholecystitis is a rare complication of acute cholecystitis and is associated with diabetes mellitus. US or CT demon-stratation of gas in the wall and/or lumen of the gallbladder imply underlying gangrenous changes (Fig. 1). Acalculous acute cholecystitis accounts for approximately only 5% of cases of acute cholecystitis. It is especially common in intensive care unit patients. Prolonged bile stasis results in increased viscosity of the bile that ultimately leads to functional cystic duct obstruction.

US and CT are both accurate techniques for diagnosing liver abscesses. US usually reveals a round or oval hy-poechoic mass with low-level internal echoes. Although the lesion may mimic a solid hepatic mass, the presence of through transmission is a clue to its cystic nature. Normally, pyogenic liver abscesses are the result of seeding from appendicitis or diverticulitis, or direct extension from cholecystitis or cholangitis. Amebic abscesses result from primary colonic involvement with seeding through the portal vein. In most cases, pyogenic and amebic abscesses are indistinguishable by US appearance. The CT appearances of pyogenic and amebic abscesses also show substantial overlap. Amebic abscesses are low attenuation cystic masses. An enhancing wall and a peripheral zone of edema surrounding the abscess are common but not universally present. Extrahepatic extension of the amebic abscess with involvement of chest wall, pleura, or adjacent viscera is a frequent finding. Whereas amebic abscesses are usually solitary and unilocular, pyogenic abscesses may be multiple or multiloculated and may demonstrate an irregular contour.

Spontaneous rupture of a hepatocellular carcinoma and subsequent hemoperitoneum represent a frequent complication found in countries with a high incidence of

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