Dr. Curry, Dr. Urban, and Dr. Fishman are with The Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins Medical Institutions, Baltimore, MD.

A variety of processes involving the gastrointestinal tract can cause acute abdominal pain. There is often overlap in the clinical presentation of many entities that may not be distinguishable based on clinical findings or laboratory results. Additionally, even when the diagnosis is certain, the underlying etiology and associated complications which will affect patient management may remain unknown. Therefore, a rapid and accurate diagnosis in patients presenting with an acute abdomen is essential for appropriate triage and care.

Computed tomography has gained widespread acceptance as a rapid, reliable, highly accurate, and cost-effective modality in the evaluation of patients presenting with an acute abdomen. It is often used in the work-up of many gastrointestinal causes of abdominal pain, including bowel obstruction, inflammation, appendicitis, diverticulitis, and ischemia. ^1-5 With recent advances in volumetric data acquisition provided by helical (spiral) computed tomography, CT continues to provide an increasingly important role in the evaluation of the acute abdomen. ^5-7 This article reviews the application of CT for acute GI conditions which may present with acute abdominal pain.

CT technique

Helical CT is the technique of choice when performing computed tomography. When possible, it is important to tailor the helical CT protocol to the working clinical diagnosis. ⁸ For most GI conditions, both oral and intravenous contrast are utilized; 750 to 1000 cc of a 3% iodinated, water-soluble oral contrast is typically given by mouth in divided doses over 30 to 45 minutes. In cases of suspected appendicitis, at least 1 hour of transit time is needed for opacification of the distal small bowel and right colon. Alternatively, contrast can be administered per rectum for colonic opacification. In patients with high-grade small-bowel obstruction, studies can be performed without oral contrast, as the obstructed, fluid-filled loops provide "natural" contrast when combined with an intravenous contrast injection. Similarly, water can be ingested and utilized as a negative contrast agent in evaluation of patients with suspected gastric pathology, including peptic ulcer disease. ⁹ We recommend IV contrast injections for most patients, although in many cases of appendicitis and diverticulitis the diagnosis can be made accurately without it. The rate and amount of IV contrast administration varies, but generally 120 cc of iodinated contrast is administered at a rate of 2 to 3 cc per second via a peripheral vein. Usually, images are obtained with a single-phase helical acquisition 70 seconds after the initiation of injection; 5 mm collimation with a pitch of 1.6 is adequate in most patients. Images are reconstructed every 5 to 8 mm.

Appendicitis

Acute appendicitis is a very common cause of acute abdominal pain. Patients classically present with pain in the periumbilical region, which typically localizes over several hours to the right lower quadrant. Nausea, vomiting, and an elevated white blood cell count are often seen. Symptoms are sometimes nonspecific and can be mimicked by a variety of inflammatory processes including pelvic inflammatory disease, diverticulitis, and inflammatory bowel disease. CT is often utilized to confirm diagnosis, to quantify the extent of inflammation, and identify complications, such as perforation and abscess formation. ^10,11

Studies have demonstrated that helical CT utilizing 5 mm collimation, with or without oral or IV contrast, is very accurate (94% to 100%) for the diagnosis of appendicitis. ^10,11 A noncontrast CT exam is the study of choice in patients with renal impairment, a history of prior reaction, or increased risk of adverse reaction to iodinated contrast. A noncontrast exam is quick and easy to perform. However, a noncontrast exam can be more difficult to interpret as subtle findings may be obscured, particularly in patients with minimal intra-abdominal fat. In addition, other inflammatory processes that clinically mimic appendicitis are often better evaluated with the use of IV and oral contrast. Therefore, we routinely use both IV and oral contrast if at all possible.

Signs of acute appendicitis on helical CT include a dilated, fluid-filled appendix, stranding of the fat around the appendix, an enhancing appendiceal wall, and a calcified appendicolith (figure 1). ^10,11 A dilated, fluid-filled appendix, the most specific sign for appendicitis, is not always seen due to superimposed inflammation or perforation or abscess formation. Enhancement of the appendiceal wall can be quite subtle and may be the only sign of inflammation (figure 2). Other conditions can lead to inflammation in the right lower quadrant and mimic findings of acute appendicitis, including Crohn's disease and cecal diverticulitis. In fact, any inflammatory processes in the right lower quadrant can cause inflammatory stranding in the mesenteric fat, and secondary signs specific to the appendix should be seen before making the diagnosis of acute appendicitis.

US is also useful in patients with clinically suspected appendicitis, particularly in women and children. However, in most patients, CT is still the preferred modality of choice. Balthazar et al ¹² prospectively compared CT with graded-compression US and demonstrated increased sensitivity (96% vs. 76%), negative predictive value (95% vs. 76%), and accuracy (94% vs. 83%) of CT relative to US. The normal appendix, when present, was seen 48% at CT compared to only 4% at US. ¹² CT is also very helpful in evaluation of the obese patient, in whom US evaluation can be very difficult.

Diverticulitis

Diverticular disease is very common in patients over the age of 60 and most commonly involves the sigmoid colon. Typical presenting symptoms include left lower quadrant pain, fever, and elevated white blood cell count. CT is very sensitive (93%) and specific (nearly 100%) for diagnosing diverticulitis. ^13,14 Oral, rectal, and/or IV contrast are helpful for diagnosing diverticulitis, but often diagnosis is possible without contrast. However, contrast is most beneficial for the detection of complications, including perforation and abscess formation, and is therefore recommended for most patients.

CT findings of acute diverticulitis include colonic wall thickening, diverticula, and pericolonic fatty stranding. Luminal opacification is important for depiction of wall thickening, and is best obtained by contrast administration via an enema. ¹³ Helical CT obtained after enema is 99% accurate for confirming or excluding the diagnosis of diverticulitis. ¹³ CT is also very sensitive to the presence and nature of pericolic complications, including extraluminal air, enterovesical fistula, and abscess formation (figure 3). ^15,16 CT with IV contrast accentuates the rim enhancement of pericolic abscesses, and helps differentiate neighboring bowel loops from the inflammatory changes. In addition, CT is an excellent modality for detecting other causes of left lower quadrant pain that may mimic diverticulitis, and can suggest alternative diagnoses in 78% of cases when they are present. ¹³

Small bowel obstruction

Small bowel obstruction (SBO) is a common cause of acute abdominal pain in patients presenting with symptoms such as nausea and vomiting. The common causes of SBO are adhesions, hernias, and tumors. ¹⁵ Complete or high-grade SBO is detected by CT with an accuracy of 95%. ^17,18 However, when low-grade or partial obstructions are included, the accuracy decreases to 66%. ¹⁶ In these cases, a small bowel series is often indicated to define the level of obstruction. Routine helical CT evaluation in suspected SBO is used following the administration of IV contrast. In general, oral contrast is not necessary prior to CT evaluation of the abdomen when a high-grade obstruction is suspected. In these cases, the bowel is distended with fluid, providing excellent delineation of bowel loops.

The most common CT findings in a patient with a high-grade obstruction include dilated loops of bowel with a discrete transition point, followed by decompressed loops of small bowel. Careful evaluation of the bowel loops in the region of the transition point will often reveal the underlying pathologic etiology. ^17,19 Hernias are usually seen in the inguinal region or the abdominal wall (figure 4). ^18,20 Incisional hernias, parastomal hernias, and spigelian hernias constitute other types of abdominal wall hernias. ^18-20 The presence of a strangulated obstruction should be entertained when there is engorgement of the mesenteric vessels and poor enhancement of the bowel wall. ²¹ Occasionally, two limbs of a bowel loop will be obstructed producing what is called a "closed-loop" obstruction. ²² The bowel loops and associated mesentery will be oriented in a radial pattern converging toward the point of obstruction (figure 5). An obstructing mass, tumor implant, or intussusception can also present with a SBO and should be searched for and excluded in all patients presenting with a bowel obstruction (figure 6). The absence of mass, hernia, intussusception, or other pathology is usually indicative of an adhesion, particularly when the patient has a history of abdominal surgery (figure 7).

Ischemic bowel

Patients with acute bowel ischemia often present with severe abdominal pain and hemodynamic instability. These patients are diagnostically challenging from a clinical and radiographic perspective. Although CT may play a vital role in identifying bowel abnormalities associated with ischemia, the findings are often nonspecific and must be correlated with the patient's clinical history and physical exam. ²³

Rapid IV contrast administration (at least 3 cc/sec) is essential in the CT evaluation of mesenteric ischemia. Helical CT can verify confidently the patency of the celiac and superior mesenteric arteries and can depict bowel wall thickening, which typically demonstrates a low-density ring of edema in cases of ischemia ²⁴ (figures 8 and 9). Bowel wall edema is non-specific, however, and can be seen with inflammatory or infectious etiologies as well. Nonenhancement of mesenteric arteries and veins indicates thrombosis and is diagnostic of ischemic bowel when seen in the presence of bowel wall edema, pneumatosis, or other signs of ischemia (figure 10). ²² Late signs of mesenteric ischemia include pneumatosis intestinalis and air within the mesenteric vein and portal venous system (figure 11). Isolated pneumatosis intestinalis is a nonspecific radiographic sign, however, and can be seen in a variety of non-ischemic benign processes such as COPD, collagen vascular disorders, and steroid administration. When present, pneumatosis intestinalis should always be correlated with other CT findings and the patient's symptoms.

Peptic ulcer disease

Patients with peptic ulcer disease present with epigastric pain, nausea, and vomiting, although patients may lack a clinical history of ulcer disease. Symptoms can be indistinguishable from those of acute pancreatitis or cholecystitis. Adequate gastric distention with either water or oral contrast is essential, and IV contrast administration is also necessary if water is used. ^9,25

The primary findings on CT exam are nonspecific and include focal thickening of the gastric or duodenal wall, which may be obscured in patients with chronic gastritis. ²⁶ Findings can be normal or undetectable with CT. Penetrating ulcers are difficult to visualize and the only CT clue to diagnosis may be pneumoperitoneum (figure 12). Subtle stranding in the fat adjacent to the ulcer may also be seen. ²⁷ CT is often most helpful in patients with peptic ulcer disease for excluding alternative diagnoses and detecting complications.

Gastrointestinal perforation

Usually patients with perforated bowel present with obvious signs of peritonitis, often as a complication of peptic ulcer disease or diverticulitis, or following endoscopic procedures. ²⁸ Clinical symptoms are often misleading in patients who are immunosuppressed or receiving steroid therapy, who are more likely to present late in the course of their illness with symptoms related to sepsis rather than pain.

Ideally, evaluation should be performed after the administration of both oral and IV contrast. However, often this is omitted in the critically ill patient to expedite the exam, and the diagnosis can be made easily with a noncontrast study. CT is known to be more sensitive than conventional radiographs in detecting subtle pneumoperitoneum, and can detect free air despite a normal abdominal radiograph. ^29,30 Extraluminal air will accumulate in nondependent regions of the abdomen, and location of air is usually not helpful in identifying the site of perforation. The location of the pneumoperitoneum varies depending on the position of the patient, and is commonly seen along the anterior peritoneal surface of the liver (figure 13). If present, oral contrast extravasation, focal fluid, or inflammatory changes are the most helpful signs for localizing the site of perforation.

Conclusion

CT scanning, in particular helical CT, is very helpful in the evaluation of the patient presenting with acute abdominal pain, particularly in the diagnosis of acute gastrointestinal disorders including appendicitis, diverticulitis, small-bowel obstruction, and ischemia. It is often the screening modality of choice for these conditions. In most cases, CT will enable the correct diagnosis and dictate appropriate treatment decisions. AR

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