Nontrauma Abdomen

Imaging Findings

Plain radiographs may demonstrate a markedly dilated stomach, which may be filled with either air or fluid. Infrequently, plain radiography may suggest an underlying diagnosis—for example, an irregular mass lesion identified as a filling defect in cases of malignancy such as gastric cancer. Filling defects also may be identified on plain radiographs in cases of bezoar obstruction. In patients with Bouveret syndrome, a calcified gallstone may be identified in the duodenal bulb.

In patients presenting with acute symptoms of nausea and vomiting, a CT scan may be performed for further evaluation. Similar to radiography, CT clearly demonstrates the often marked distention of the stomach. CT is also helpful in characterizing the underlying causes of obstruction. Malignant causes are seen as enhancing soft tissue mass lesions arising in the stomach in cases of gastric cancer, or slightly more distally around the pancreatic head or ampulla in cases of pancreatic cancer, cholangiocarcinoma, and ampullary and duodenal cancers. Distinction among the latter can be difficult because the area of origin contains quite a bit of overlap.

In cases in which peptic ulcer disease (PUD) obstructs the stomach, the ulcer may rarely be identified, but the secondary CT findings of gastritis are typically seen as focal thickening of the gastric wall with hyperenhancement of the mucosa related to the inflammation. In these cases, the focal area of inflammation should be scrutinized for a central ulcer seen in PUD. Often, on the basis of CT, the distinction between PUD and underlying carcinoma is difficult to ascertain. Similar to PUD, in cases of obstruction resulting from inflammatory causes such as pancreatitis or cholecystitis, the epicenter of inflammation will be seen around the respective organs, but the secondary inflammation of the stomach may be identified on CT as mural thickening, possibly with hyperenhancement of the mucosa.

Gastric outlet obstruction resulting from various benign causes is often apparent, as in cases of pancreatic pseudocyst obstruction. Hyperplastic polyps, the most common form of gastric polyp, may rarely cause gastric obstruction and may be seen in the region of the prepyloric antrum because they cause obstruction by prolapse into the pyloric channel. On CT, the appearance is that of a pedunculated, soft-tissue attenuation filling defect. In cases of Bouveret syndrome, the obstructing gallstone may be identified as a calcified filling defect in the duodenal bulb. Bezoars are seen at CT as mottled-appearing filling defects within the stomach, although they may be seen more distally within the gastrointestinal tract and have been reported to cause small bowel obstructions.

Imaging Findings

The most common causative factor of gastric volvulus in the adult population is diaphragmatic defects, and thus plain radiographs may demonstrate an intrathoracic, gas-filled viscus. In cases of mesenteroaxial volvulus, supine radiographs may demonstrate typical findings of a spherical lucency with a characteristic “beak” in the region of the distal stomach. Upright radiographs show differential air–fluid levels at different heights in cases of mesenteroaxial volvulus. Organoaxial volvulus, on the other hand, characteristically reveals a single air–fluid level in an abnormally transversely oriented stomach.

In patients presenting with a clinical suspicion of a gastric volvulus, an urgent upper gastrointestinal series can secure a prompt diagnosis. In cases of gastric volvulus, an obstruction may be identified at the site of volvulus. Mesenteroaxial volvulus demonstrates a barium-filled stomach with the GEJ below the antrum and with the typical “beak” seen in the region of the distal stomach, as on radiography. If barium gets past the GEJ, an “upside-down” stomach is seen. Transverse position of the stomach and an abnormal inferior location of the GEJ suggest organoaxial volvulus.

Because of the multiplanar capabilities of multidetector CT (MDCT) scanners, patients with suspected gastric volvulus may undergo a CT scan in the initial diagnostic evaluation. As on radiography, CT typically demonstrates the presence of a hiatal hernia, commonly the paraesophageal type, or diaphragmatic eventration, which increases the likelihood of organoaxial gastric volvulus. Multiplanar reformations are particularly useful in precisely defining the anatomy of the volvulus. Like fluoroscopy, CT demonstrates the abnormally positioned gastric antrum located superior to the GEJ—an “upside-down” stomach when the rotation is complete. Organoaxial volvulus demonstrates an abnormally transverse lie of the stomach and an inferiorly positioned GEJ given its axis of rotation. Because organoaxial volvulus is more often associated with diaphragmatic defects, the stomach is often identified in the thorax at the time of volvulus.

In cases of gastric volvulus on CT, especially the organoaxial type, complications of ischemia may be identified, including pneumatosis and portomesenteric vein gas. Mesenteroaxial volvulus, unlike organoaxial volvulus, is more frequently incomplete and less likely to cause acute complications such as necrosis.

Imaging Findings

In patients with acute perforation related to PUD, pneumoperitoneum may be identified on initial plain radiographs. Like plain radiography, a CT scan may identify findings of pneumoperitoneum in cases of ulcer perforation. Often, the underlying ulcer or mural rent is not clearly identified. Small foci of air may be seen around the area of perforation in the stomach or duodenum and suggest the underlying cause. Focal wall thickening may be present in the region of the ulcer in the stomach or duodenum. When oral contrast material is administered in cases of unsuspected hollow viscus perforation, extravasation of contrast material into the peritoneal cavity is often identified. A CT scan may characterize underlying PUD in cases of gastric outlet obstruction, as previously detailed.

Imaging Findings

The initial imaging evaluation of patients with suspected mechanical small bowel obstruction often includes plain radiographs. The diagnosis of small bowel obstruction on plain radiography involves the visualization of distended loops of small bowel filled with air or fluid. The differentiation of small bowel loops from the colon is achieved by identifying the valvulae conniventes, also known as plicae circulares , of the small bowel, which are thin, mucosal folds extending across the entire small bowel loops. The haustral folds of the colon, on the other hand, are thicker bands alternating with the thick folds termed plicae semilunares , which do not cross the entire lumen of the colon. The small bowel loops are more central in the abdomen, with the colon seen along the periphery. Typically, 3 cm is accepted as the upper limit of normal for the diameter of the small bowel. The presence of a hernia as the cause of a small bowel obstruction may be suggested by plain radiographs based on the presence of small bowel loops in unusual locations, such as the inguinal canal, distal to dilated loops of small bowel. Alternatively, radiographs demonstrating protrusion of a segment of small bowel, as evidenced by a short segment of bowel clearly outlined by air outside the abdominal wall, again distal to dilated loops of small bowel, may indicate a hernia as the underlying cause.

CT is often used for further characterization in patients with suspected mechanical small bowel obstruction. Similar to radiography, the diagnosis on CT involves identifying distended air- and fluid-filled loops of small bowel, typically greater than 3 cm in diameter. The small bowel “feces” sign, which is the presence of air and particulate matter resembling feces within loops of small bowel, is a finding commonly seen in small bowel obstruction; its diagnosis is helpful because it suggests increased bowel transit time. Often, CT allows for the diagnosis of the exact point of transition between distended loops of small bowel and the more normal collapsed loops of small bowel and possibly for identifying the underlying cause of the small bowel obstruction ( Fig. 9-1 ).

Closed loop obstructions are a subset of mechanical small bowel obstructions that demand acute clinical intervention. The term closed loop obstruction signifies the presence of two transition points, one of which is found at the proximal extent of the closed loop and the other at its distal extent. The vascular supply of these segments of small bowel is often compromised in this setting, and ischemia and necrosis may quickly ensue. Although the diagnosis of a closed loop obstruction might be suggested on plain radiographs by the presence of a short segment of distended small bowel, as well as transition points that may or may not be visualized, this diagnosis is typically confirmed by CT. The CT imaging findings include a sharp transition point, or “beak,” at the proximal and distal extent of the closed loop. CT findings suggesting ischemia, including vascular engorgement, and ascites, as well as abnormally decreased enhancement after intravenous administration of contrast material, often require emergent intervention. More ominous CT imaging findings, including pneumatosis intestinalis, also may be seen and are discussed in the following text.

As noted, adhesions from prior surgical intervention are the most common cause of small bowel obstruction and are notoriously difficult to directly visualize with imaging; the diagnosis often remains one of exclusion. However, secondary signs such as abrupt angulation of small bowel loops and adherence of small bowel loops to nondependent surfaces such as the anterior parietal peritoneum lend credence to a diagnosis of adhesions as the underlying cause, especially when they are found at the transition between distended and normal small bowel loops.

Hernias are a second common cause of small bowel obstruction. CT is used to further characterize these hernias, which may be complex in some cases. Inguinal, umbilical, incisional, and Spigelian hernias in the location of the linea semilunaris are common types of hernias ( Fig. 9-2 ). Among less common causes of mechanical small bowel obstruction are internal hernias, including congenital or surgically acquired rents within the mesentery, as well as a myriad of other named internal hernias that have been described. CT images, including multiplanar reformations, are often used to further characterize these complex hernias.

A gallstone ileus is a rare but interesting form of mechanical small bowel obstruction. In this case, a gallstone erodes into the adjacent duodenum and proceeds to obstruct the small bowel, most commonly at the ileocecal valve. Typical imaging findings include the presence of air within the gallbladder or bile ducts, as well as dilated loops of small bowel, often with a calcified gallstone at the distal extent. These findings may be seen on plain radiographs, as well as on CT. If the gallstone erodes more proximally and obstructs the duodenal bulb, similar imaging appearances of biliary gas, obstruction, and often visualization of the calcified gallstone are seen and are termed Bouveret syndrome .

Although more common in the pediatric population, small bowel intussusception also may be seen in adults and can cause obstruction of small bowel loops proximally. In adults, one must always consider the possibility of a lead point such as a neoplasm. Other causes of small bowel intussusception in adults include Meckel diverticulum and postoperative states, especially after gastric bypass. With the increasingly widespread use of CT, transient small bowel intussusceptions are more commonly seen. The diagnosis of a transient intussusception may be suggested on the basis of location and length, as well as the absence of proximal small bowel dilatation ( Fig. 9-3 ).

Imaging Findings

Plain radiographs are also nonspecific in patients with small bowel volvulus but may demonstrate proximal small bowel obstruction. In cases of midgut volvulus related to underlying malrotation, plain radiographs may demonstrate partial duodenal obstruction with a dilated, air-filled stomach and a proximal duodenum.

Currently, CT is the imaging modality of choice in diagnosing small bowel volvulus. As on plain radiographs, secondary small bowel obstruction may be identified proximal to the volvulus. The CT “whirl” sign is often associated with volvulus of the bowel and is seen as a swirling of mesenteric vessels, strands of soft tissue, and loops of bowel. Although sensitive in the diagnosis of small bowel volvulus, the finding lacks specificity, in that most patients demonstrating this finding have been shown not to have a diagnosis of small bowel volvulus. Nevertheless, the “whirl” sign of twisting mesenteric structures, including loops of small bowel, should raise suspicion of a volvulus, and other CT findings such as infiltration of the mesentery and proximal small bowel dilatation may increase specificity. Small bowel volvulus can rapidly lead to vascular compromise, and the CT images should be scrutinized for findings of ischemia, such as abnormal hypoenhancement of the small bowel. As in cases of small bowel volvulus, the “whirl” sign, in which the superior mesenteric vein and loops of bowel are seen to rotate around the superior mesenteric artery, may be demonstrated on CT in cases of midgut volvulus.

Imaging Findings

Typically, plain radiographs demonstrate evidence of air- and fluid-filled loops of dilated colon. A diameter of 8 cm is considered the upper limit of normal for the cecum, whereas 5 cm is considered the upper limit of normal for the remainder of the colon. When acute, colonic obstruction is a medical emergency, given the potential for rapidly developing ischemia, sepsis, or perforation.

CT demonstrates dilated loops of air- or fluid-filled colon proximal to the site of obstruction and is accurate in determining the cause of the obstruction. The CT scans should be scrutinized for signs of perforation, such as free intraperitoneal fluid or air, as well as for signs of ischemia, such as abnormal hypoenhancement of the colon. Unlike in the pediatric population, colonic obstruction as a result of intussusception is generally managed surgically, as opposed to radiologically, with reduction via air enema, for example, given the high likelihood of an underlying malignancy. On CT, toxic megacolon demonstrates dilation of air- or fluid-filled colon with wall thickening and a distorted colonic contour or a lack of the expected haustral pattern. Because Ogilvie syndrome lacks the colonic inflammation, CT is unlikely to demonstrate a similar degree of wall thickening and submucosal edema as toxic megacolon. The chronic form of megacolon represents a functional failure of the colon as a result of various underlying causes, including chronic constipation. In these patients, when prior radiographs are available for comparison, it is often helpful to exclude acute colonic obstruction or megacolon.

Imaging Findings

Plain radiographs demonstrate the beaked tapering of the efferent and afferent limbs of the dilated loop of sigmoid colon. The classic plain radiograph description of the sigmoid volvulus is that of a coffee-bean–shaped collection of air-filled bowel occupying the left upper quadrant. In cases of axial torsion–type cecal volvulus, a markedly distended loop of large bowel may be identified that extends from the right lower quadrant to the epigastrium or left upper quadrant. The distended cecum may be identified anywhere in the abdomen, depending on the mobility of the right colon. The cecal bascule is generally identified as an air-filled structure occupying the mid abdomen on plain radiography.

CT demonstrates similar beaking, as well as the “whirl” sign of bowel around mesenteric vasculature common to all types of volvulus. CT has been shown to be accurate in diagnosing cecal volvulus and in differentiating between the various subtypes based on the location of the cecum. Less common forms of colonic volvulus are also encountered, including volvulus of the splenic flexure, as well as various segments of the transverse colon.

Imaging Findings

Plain radiographs demonstrate dilated bowel loops, often diffusely when the cause is systemic. When it is a result of focal inflammation such as pancreatitis, a more localized ileus may be identified and may be useful in localizing intraabdominal disease.

CT has been shown to have a high diagnostic accuracy in distinguishing paralytic ileus from mechanical causes. The CT findings typical of a mechanical cause of obstruction such as a distinct transition point will not be identified in cases of adynamic ileus, which demonstrate mildly dilated loops of bowel diffusely.

Imaging Findings

Plain radiographs are often nonspecific but may demonstrate mildly dilated loops of small bowel, similar to adynamic ileus. In more severe cases, the degree of small bowel dilatation may approach the appearance of a small bowel obstruction.

CT imaging, when acquired, is also often nonspecific and may demonstrate mild to moderate small bowel dilatation and mural thickening ( Fig. 9-4 ). Often, the small bowel is diffusely affected; however, certain infections may present in more specific locations such as the proximal small bowel in cases of giardiasis. In the immunocompromised host, a myriad of other infectious causes should be considered, as previously noted. Typically, nonspecific small bowel wall thickening and mucosal irregularity are identified in cases of cytomegalovirus and cryptosporidium. In patients affected by Mycobacterium avium intracellulare, hepatic and splenic enlargement, jejunal wall thickening, and enlarged soft tissue attenuation or, less commonly but more characteristically, low-attenuation lymphadenopathy are described.

Imaging Findings

In patients presenting with acute abdominal symptoms related to Crohn disease of the small bowel, plain radiographs may demonstrate dilatation of proximal bowel loops because of a functional mechanical obstruction resulting from enteritis. Mural thickening of the affected small bowel loops also may be visualized on plain radiographs.

The manifestations of Crohn disease of the small bowel typically demonstrate nonspecific CT signs of inflammation such as mural thickening. However, more specific findings are often identified. These findings include isolation to the terminal ileum, hypervascularity and prominence of the vasa recta, the “comb” sign, increased mesenteric fat surrounding loops of small bowel, and “creeping” fat, as well as intraabdominal fistulae and abscesses ( Fig. 9-5 ).

Imaging Findings

Meckel diverticulitis may be diagnosed by CT. A Meckel diverticulum is evident as a blind-ending pouch of variable length containing fluid, air, or particulate debris, often located near the midline but seen anywhere from the right lower quadrant to the mid abdomen. When the diverticulum is inflamed, mural thickening and surrounding inflammatory stranding may be identified ( Fig. 9-6 ) and might be complicated by frank perforation, possibly yielding an intraabdominal abscess. In cases of diverticulitis unrelated to Meckel diverticulum, CT demonstrates focal luminal outpouchings with surrounding inflammatory changes. Again, gross perforation with abscess formation also may be seen.

Imaging Findings

Abdominal radiographs have been shown to have little clinical utility in patients with suspected appendicitis; therefore, cross-sectional imaging may be the initial diagnostic examination of choice. In a minority of cases, a calcified fecalith may be identified in the right lower quadrant. Also, the “sentinel loop” sign may be identified in relation to appendiceal inflammation.

In the younger population, and especially in females, ultrasonography may be used as an initial imaging evaluation given radiation concerns involved with CT. Ultrasound has been shown to have a high diagnostic accuracy in the evaluation for suspected appendicitis. Typical findings include the visualization of a blind-ending tubular structure measuring greater than 6 mm in diameter during graded compression. When present, an appendicolith also may be identified as an echogenic, shadowing focus within the lumen of the appendix. Secondary signs of active inflammation, including free intraperitoneal fluid, may be seen. The complications of appendicitis, which include rupture and abscess formation, also may be detected using ultrasound. Technical limitations of ultrasound include patients with an obese body habitus; moreover, given the wide variety of locations of the normal appendix, those located more posteriorly within the peritoneal cavity pose increased difficulty for evaluation.

Pregnant women often pose a diagnostic challenge when presenting with suspected appendicitis. Typically, ultrasound examination is completed to rule out other causes of abdominal pain in this patient population, such as ectopic pregnancy or ovarian torsion. However, as the gravid uterus enlarges, the appendix becomes displaced from its expected location in the right lower quadrant, and it may be difficult to visualize. Unfortunately, the normal appendix is highly unlikely to be visualized by ultrasonography in pregnant women, and further imaging is often requested to definitively exclude appendicitis. However, when the appendix is visualized on ultrasound, the findings of acute appendicitis are similar to those in nonpregnant patients and include the visualization of a blind-ending tubular structure measuring greater than 6 mm in diameter using a graded compression technique.

When the ultrasound results are equivocal or the appendix is not visualized, CT is often used in patients with suspected appendicitis. In older or significantly obese patients, CT may be the initial imaging examination. CT has been shown to have a very high diagnostic accuracy in the diagnosis of appendicitis. In patients with appendicitis, the appendix appears enlarged, often with surrounding inflammatory changes, including the free intraperitoneal fluid. When present, appendicoliths are readily identified on CT ( Fig. 9-7 ). The diameter of the appendix varies widely in typical patients, with sizes ranging up to 1 cm. However, mean values range between 5 and 7 mm depending on whether the appendix is distended with air. Therefore, in a patient with an appendix measuring slightly greater than the standard cutoff value of 6 mm, secondary signs of inflammation should be sought, such as hyperenhancement, periappendiceal fat stranding or fluid, fascial thickening, or edema at the origin of the appendix as evidenced by thickening of the adjacent cecum, the so-called “arrowhead” sign ( Fig. 9-8 ). Filling of the appendix by orally or rectally introduced positive contrast material is a useful means of excluding obstruction of the appendix and, therefore, acute appendicitis. When the appendix is not visualized, this finding, in the absence of right lower quadrant inflammation, carries a high negative predictive value of appendicitis.

In pregnant patients, ultrasonography is typically used initially, given risks of ionizing radiation. However, in patients with an inconclusive ultrasound examination, both CT and, with increasing frequency, magnetic resonance imaging (MRI) are often used. The findings of acute appendicitis on CT in pregnant patients are similar to those in the nonpregnant population.

Given concerns regarding radiation dose to the fetus, MRI is frequently used to evaluate pregnant patients for suspected appendicitis. MRI offers high diagnostic accuracy and is an excellent modality for excluding appendicitis. The appendix is considered normal when it is less than or equal to 6 mm in diameter or is filled with air or contrast material. As on CT, secondary findings such as periappendiceal inflammation are used to increase specificity when the appendix is at the upper limits of normal size. As the gravid uterus enlarges, the cecum and therefore the appendix may be in atypical locations, displaced superiorly by the uterus. Therefore, it is helpful to identify the landmarks of the terminal ileum and cecum in attempting to localize the appendix on MRI in pregnant patients.

Imaging Findings

Given the increasingly routine use of CT imaging in patients with abdominal pain, the imaging manifestations of epiploic appendagitis have been well described. The characteristic imaging findings include an ovoid lesion containing fat, abutting the colon, with surrounding inflammatory stranding ( Fig. 9-9 ). A central high-attenuation focus has been described and is thought to be related to a thrombosed vein. Although this finding is helpful when it is seen, the absence of this finding does not exclude the diagnosis.

Although epiploic appendagitis is typically a CT diagnosis, findings have been well described using both ultrasonography and MRI. The diagnosis using these modalities also includes the demonstration of the ovoid fatty lesion with surrounding inflammatory changes.

Imaging Findings

Omental infarction is an alternative diagnosis that often presents with imaging features somewhat similar to those of epiploic appendagitis. CT reveals an ill-defined focal area of fat with evidence of inflammation and heterogeneous attenuation that may appear somewhat masslike. Follow-up CT imaging often demonstrates a more well-defined area of fat attenuation with heterogeneous attenuation throughout.

Like epiploic appendagitis, omental infarction can be readily diagnosed with ultrasound. Ultrasonography may identify the findings of a lesion that contains fat, as evidenced by a hyperechoic, noncompressible lesion in the region of the omentum.

Imaging Findings

Although ultrasound has been shown to be accurate in the diagnosis of acute diverticulitis, this disease is more commonly diagnosed using CT. CT scans typically involve the use of oral and intravenous contrast material. Left-sided diverticulitis predominates with the typical CT imaging findings, including colonic wall thickening and pericolonic fat stranding seen in close proximity to a diverticulum. Typically, diverticulosis is noted throughout a longer segment of adjacent colon, although only a single diverticulum is necessary to cause acute diverticulitis. Often, several small foci of extraluminal gas may be identified in the region of inflammation.

Multiple complications are associated with acute diverticulitis, including intramural sinus tracts or abscess, as well as extracolonic phlegmon or abscess formation ( Fig. 9-10 ). Frank perforation with gross free intraperitoneal air also may be encountered in patients with diverticulitis. Other complications include the formation of fistulae that are most commonly colovesicular but may be colovaginal, coloenteric, and colouterine as well. In patients with a CT diagnosis of diverticulitis, follow-up colonoscopy is normally advised to rule out underlying malignancy masquerading as diverticulitis.