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 etiologies should be considered, as noted above. 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.

Figure 9-4 A 40-year-old male with viral gastroenteritis. Axial (A) and coronal (B) portal venous phase CT images reveal diffuse small bowel dilatation as well as mural thickening (arrows).

Crohn’s Disease, Small Bowel

Although Crohn’s disease can affect any portion of the gastrointestinal tract, the terminal ileum portion of the small bowel is the most commonly involved. Crohn’s disease may affect isolated segments of small bowel proximal to the terminal ileum. Usually, however, in cases of small bowel involvement, the colon is also affected.

Common gastrointestinal symptoms include abdominal pain, diarrhea, and weight loss. The crampy abdominal pain associated with Crohn’s disease may be temporarily relieved following defecation. The area of bowel involved may determine the nature of the diarrhea, with those affected by ileitis presenting with watery, large-volume stools.

Imaging Findings

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

The manifestations of Crohn’s disease of the small bowel typically demonstrate nonspecific CT signs of inflammation such as mural thickening. However, findings more specific are often identified. These 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 intra-abdominal fistulae and abscesses (Fig. 9-5).

Figure 9-5 A 24-year-old male with Crohn’s disease. Axial (A) and coronal (B) portal venous phase CT images demonstrate thickening of the terminal ileum (arrows) with prominence of the vasa recta along the mesenteric border (arrowheads), the so-called “comb” sign.

Small Bowel Diverticulitis

Meckel’s diverticulum, the most common congenital anomaly of the gastrointestinal tract, remains asymptomatic in the majority of patients. However, various complications can ensue, including inflammation, termed Meckel’s diverticulitis. Although less commonly seen than in the colon, diverticula formation may affect any portion of the small bowel from the duodenum to the ileum. Potential complications include bacterial overgrowth and malabsorptive states. Diverticulitis may also affect the small bowel.

Patients with small bowel diverticulitis may present with localized tenderness in the region affected; however, no specific signs and symptoms are pathognomonic for this diagnosis. Often, these patients are febrile and may present with a leukocytosis. Moreover, malabsorptive states may develop and patients may present with diarrhea and weight loss. Although this is a secondary finding unrelated to diverticulitis, it may add credence to a consideration of small bowel diverticulitis. Plain radiographs, not usually used in the diagnosis of small bowel diverticulitis, may show a focal ileus or “sentinel loop” sign in the area of inflammation.

Imaging Findings

Meckel’s diverticulitis may be diagnosed by CT. A Meckel’s 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). This might be complicated by frank perforation, possibly yielding an intra-abdominal abscess. In cases of diverticulitis unrelated to Meckel’s diverticulum, CT demonstrates focal luminal outpouchings with surrounding inflammatory changes. Again, gross perforation with abscess formation may also be seen.

Figure 9-6 A 38-year-old male with Meckel’s diverticulitis. Axial (A) and coronal (B) portal venous phase images reveal a tubular, blind-ending pouch (arrows) emanating from adjacent small bowel (arrowheads) with surrounding inflammatory changes, findings consistent with Meckel’s diverticulitis.

Appendicitis

Appendicitis is a common clinical concern in patients presenting to the emergency department with abdominal pain. In those patients presenting with the classic signs and symptoms, including right lower quadrant pain, leukocytosis, and fever, imaging may not be necessary in making the diagnosis of appendicitis. However, in those patients with atypical presentations or those requiring further characterization of the suspected diagnosis, imaging is often employed. Initially, the appendiceal lumen occludes secondary to a number of causes including fecaliths and lymphoid hyperplasia. Once occluded, intraluminal fluid continues to accumulate, distending the appendix and eventually increasing the intraluminal and intramural pressures to the point of vascular and lymphatic obstruction. Ineffective venous and lymphatic drainage allows bacterial invasion of the appendiceal wall and lumen. If untreated, perforation of the appendix may be a complication.

As noted, the classic description of patients presenting with appendicitis includes right lower quadrant pain, leukocytosis, and fever. Classically, these findings are preceded by anorexia and periumbilical pain. The migration of pain from the periumbilical region to the right lower quadrant is both sensitive and specific in the diagnosis of appendicitis. However, given the normal range of locations of the appendix as well as varying degrees of appendiceal inflammation, patients often present with atypical signs and symptoms.

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 the minority of cases, a calcified fecalith may be identified in the right lower quadrant. Also, the “sentinel loop” sign may be identified secondary 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 evaluating 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 may also 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, may also 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 pregnancy, 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 employed 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 those 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). There is a large variety in the diameter of the appendix in normal patients, with sizes ranging up to 1 cm. However, mean values range between 5 and 7 mm depending on whether or not 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.

Figure 9-7 A 46-year-old male with perforated appendicitis. Axial portal venous phase CT image (A) demonstrates an inflamed appendix with periappendiceal fat stranding (arrowhead) and phlegmon (white arrow), as well as an appendicolith at the appendiceal base (black arrow). Sagittal portal venous phase CT image (B) demonstrates extraluminal air (black arrow) consistent with perforation as well as the appendicolith (arrowhead).

Figure 9-8 A 20-year-old female with appendicitis. Axial (A) and sagittal (B) portal venous phase CT images demonstrate dilated appendix consistent with appendicitis (arrows). Though not necessary in this case given significant appendiceal dilatation, secondary signs of adjacent cecal wall thickening (arrowheads, B), the so-called “arrowhead” sign, may be useful in equivocal cases.

In pregnant patients, ultrasonography is typically employed initially, given risks of ionizing radiation. However, in those 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 employed to evaluate for suspected appendicitis in pregnant patients. 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.

Epiploic Appendagitis

Epiploic appendages are small fat-containing, serosal-covered outpouchings of the colon that project into the peritoneal cavity. Appendagitis represents torsion of these outpouchings with subsequent inflammation and thrombosis of the venous supply located centrally. Alternatively, spontaneous venous thrombosis has been described to increase the predilection of subsequent torsion.

Prior to the increasing utilization of CT, epiploic appendagitis was often misdiagnosed as acute appendicitis or diverticulitis given similar clinical presentations. Typically, symptoms include rapid onset of localized pain in the right or left flank, although more chronic torsion of the appendages may result in minimal or no symptoms. The pain is usually constant, and rebound tenderness is often elicited. Patients may present with low-grade fever and leukocytosis.

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 helpful when seen, the absence of this finding does not exclude the diagnosis.

Figure 9-9 A 42-year-old male with epiploic appendagitis. Axial (A) and coronal (B) portal venous phase CT images demonstrate a pericolonic, ovoid, fat-containing lesion (arrows) with a central high-attenuation focus (arrowheads) consistent with epiploic appendagitis.

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.

Omental Infarction

Omental infarction is often idiopathic in its etiology. It has been hypothesized that there may be a congenital anomalous, atypically tenuous blood supply to the omentum in some patients. However, a significant portion of cases are related to recent intra-abdominal surgery. Other associations include strangulation of the omentum such as in inguinal hernias that include portions of the omentum.

Omental infarction may present with acute abdominal pain, but, as in epiploic appendagitis, it typically represents a benign, self-limited disease process. Patients present with acute or subacute focal abdominal pain that may mimic the acute pain of appendicitis and diverticulitis. Again, patients may present with low-grade fever and leukocytosis. Occasionally, the area of infarction may be palpated as an intra-abdominal mass lesion. In some cases, especially those with more severe and prolonged symptoms, operative management is successfully employed.

Imaging Findings

Omental infarction is an alternative diagnosis that often presents with imaging features somewhat similar to 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 fat-containing lesion, as evidenced by a hyperechoic, noncompressible lesion in the region of the omentum.

Diverticulitis

Diverticulitis is a relatively common cause of abdominal pain and is predominantly seen to affect the colon. Diverticulitis represents the inflammation of small outpouchings, known as pulsion diverticula or pseudodiverticula of mucosa and submucosa through the regions of the underlying muscularis propria penetrated by the vasa recta. This is commonly secondary to raised intraluminal pressures common in Western, low-fiber diets and constipation; the sigmoid colon is postulated to be most commonly affected, as it has the smallest diameter leading to the greatest intraluminal pressures. Diverticulitis typically presents as localized, usually left-sided, abdominal pain. More severe disease, especially with the presence of complicating abscesses, may present with systemic symptoms such as fever and may develop a leukocytosis. These patients may also present with anorexia, nausea, and vomiting. The signs of peritonitis may develop with cases of more gross perforation.

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. 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 may also be encountered in patients with diverticulitis. Other complications include the formation of fistulae, most commonly colovesicular, as well as colovaginal, coloenteric, and colouterine. In those patients with a CT diagnosis of diverticulitis, follow-up colonoscopy is normally advised to rule out underlying malignancy masquerading as diverticulitis.

Figure 9-10 A 50-year-old male with diverticulitis. Axial (A) and coronal (B) portal venous phase CT images reveal thickening of the sigmoid colon with a central low-attenuation focus consistent with an intramural abscess (arrows).

Inflammatory Bowel Disease, Colon

Inflammatory causes of colitis include Crohn’s disease and ulcerative colitis, both of which may present with acute signs and symptoms. Crohn’s disease represents granulomatous, transmural inflammation of the bowel with skip lesions in which noncontiguous segments of bowel are affected. Skip lesions are not present in ulcerative colitis. Crohn’s disease most commonly demonstrates an ileocolic distribution, affecting both the colon and ileum, followed by disease isolated to the ileum, typically in its distal portion, or disease limited to the colon. Ulcerative colitis, on the other hand, involves the rectum with continuous involvement of more proximal bowel, and, unlike the transmural involvement of Crohn’s disease, is limited to the mucosa and submucosa with formation of crypt abscesses and mucosal ulceration.

Typical clinical presentations in Crohn’s disease and ulcerative colitis include abdominal pain, tenesmus, and diarrhea. A distinguishing feature may be the presentation of bloody diarrhea in patients with ulcerative colitis, a finding rarely seen in Crohn’s disease. Especially with more severe disease, systemic symptoms such as nausea and vomiting along with malaise and low-grade fever may develop. Typically, Crohn’s disease presents somewhat more insidiously and symptoms may have persisted for several years prior to diagnosis. Also, patients with Crohn’s disease often have perianal disease in the form of fistulae and sinus tracts. Whereas ulcerative colitis rarely involves the small bowel, Crohn’s disease more commonly causes weight loss secondary to malabsorption related to small bowel involvement.

Imaging Findings

In patients with inflammatory bowel disease and acute abdominal pain, intramural edema may be identified as thumb printing of the colonic wall. In patients with ulcerative colitis, the colon should be evaluated for signs of toxic megacolon as evidenced by a long segment of air-filled, significantly distended colon, often greater than 6 cm in diameter. Also, dilatation of the bowel proximal to areas of active inflammation or stricture may be identified. Secondary findings in patients with inflammatory bowel disease include extraintestinal manifestations, such as sacroiliitis, cholelithiasis, and renal stones, that may be identified on plain radiography.

Typically, in patients presenting acutely, a CT scan is acquired in the emergency department setting. Crohn’s disease demonstrates bowel wall thickening in the affected segments, which are most commonly right-sided; diffuse colitis may also be seen, although isolated left-sided involvement with Crohn’s disease is atypical (Fig. 9-11). Ulcerative colitis, unlike Crohn’s disease, demonstrates contiguous involvement of the bowel from the rectum proximally. Therefore, the CT findings are usually isolated to the left side. The bowel wall may demonstrate stratification, also known as the “water halo” sign, in which the bowel wall consists of two or three symmetrically thickened layers. In the case of three symmetrically thickened layers, also known as the “target” sign, the inner hypoattenuating layer is thought to represent edema within the submucosa. In patients with chronic inflammation, as in Crohn’s disease or ulcerative colitis, a pattern known as the “fat halo” sign may be seen in which there is three-layer stratification with the middle layer demonstrating fat attenuation (Fig. 9-12). The finding of a “fat halo” sign is more commonly associated with ulcerative colitis. Although these are nonspecific signs, taken in context with the remainder of the imaging findings as well as the clinical presentation, they may suggest a potential diagnosis of inflammatory bowel disease. Also, hyperenhancement of the bowel after contrast administration is useful in differentiating between chronically thickened loops of bowel secondary to fibrosis and those with active inflammation.

Figure 9-11 A 38-year-old male with Crohn’s disease. Coronal (A) and sagittal (B) portal venous phase images demonstrate significant cecal wall thickening (arrows) with oral contrast identified between the thickened haustra (arrowheads), the so-called “accordion” sign.

Figure 9-12 A 44-year-old male with chronic ulcerative colitis. Axial (A) and sagittal (B) portal venous phase CT images reveal fat attenuation within the wall of the sigmoid (arrows), the so-called “fat halo” sign.

Another classic finding in patients with Crohn’s disease is the so-called “creeping” fat, which represents fibrofatty proliferation of the mesenteric fat. Also, the vasa recta may be engorged in cases of acute inflammation, the so-called “comb” sign. Again, taken in isolation, this is not a specific finding for Crohn’s disease, as it can be seen in any hypervascular inflammation of the colon. As the rectum is typically involved in patients with ulcerative colitis, the chronic inflammation causes proliferation of the perirectal fat resulting in an increase in the presacral space.

Abscess and phlegmon formation are commonly encountered complications in patients with Crohn’s disease given its transmural nature. Abscesses can be successfully managed percutaneously to avoid unnecessary surgery. Another complication given the transmural extent of Crohn’s disease is the formation of sinus tracts and fistulae. Common fistulae include enteroenteric and enterocolonic as well as fistulae within the urinary system (Fig. 9-13). Other commonly encountered fistulae are perianal and enterocutaneous fistulae. Although less common, fistulas may also be encountered in patients with ulcerative colitis.

Figure 9-13 A 47-year-old male with Crohn’s disease. Coronal portal venous phase CT image reveals a colovesical fistula (arrow) with obstruction of the colon proximally (arrowhead) secondary to the inflammation related to the patient’s Crohn’s disease.

Unlike ulcerative colitis, Crohn’s disease affects the colon somewhat asymmetrically, as the mesenteric side may be more severely affected than the antimesenteric portion of the colon. This may result in the formation of sacculations, also known as pseudodiverticula.

Extracolonic manifestations of inflammatory bowel disease may be identified on abdominal CT and add specificity to a presumed diagnosis. These include sacroiliitis, which may be seen in up to 30% of patients with Crohn’s disease. These changes may result in eventual fusion around the sacroiliac joints in this patient population. Another manifestation of inflammatory bowel disease, which may be seen on abdominal CT, is primary sclerosing cholangitis. Although more commonly associated with ulcerative colitis, this may also be seen in association with Crohn’s disease. When primary sclerosing cholangitis is present, the typical CT findings include pruning, beading, or skip dilatations of the bile ducts. Additionally, the bile duct walls may be thickened on CT; expected normal bile duct wall thickness is approximately 1 to 2 mm. When incidentally identified, these extraintestinal manifestations of inflammatory bowel disease may increase confidence in the diagnosis in patients presenting with acute abdominal pain.

In patients with known inflammatory bowel disease presenting acutely with pain, MRI is being used more frequently given concerns of ionizing radiation associated with CT. Like CT, MRI may demonstrate focal bowel wall thickening associated with hyperenhancement after contrast administration as well as surrounding inflammatory changes. Associated findings described above, such as engorgement of the vasa recta, may also be identified on MRI, as may complications of inflammatory bowel disease including abscess or fistula formation. Perianal disease in patients with Crohn’s disease, in particular, is elegantly delineated with the soft tissue contrast afforded by MRI.

Infectious Colitis

Commonly encountered infectious etiologies of colitis include pseudomembranous colitis secondary to proliferation of Clostridium difficile related to antibiotic use, salmonella, shigella, yersinia, tuberculosis, amebiasis, schistosomiasis, E. coli, and cytomegalovirus, among others. The diagnosis of pseudomembranous colitis is often suspected as it is temporally related to antibiotic use. Another infectious cause of colitis is tuberculosis. Although the diagnostic dilemma may be somewhat simplified when patients display concurrent pulmonary tuberculosis, the abdominal manifestations may be seen in the absence of pulmonary infection. Neutropenic colitis or typhlitis typically affects the cecum and ascending colon, although the terminal ileum and appendix may also be affected.

The clinical presentation of infectious colitis is somewhat dependent on the underlying etiology, although patients generally present with diffuse, crampy abdominal pain and diarrhea, which may be profuse and bloody. Also, fever and leukocytosis may be associated. In some cases, the diarrhea may be persistent, prompting the presentation.

Imaging Findings

Plain radiographs are nonspecific, but mural edema may be identified within the colon. Again, as in cases of inflammatory bowel disease, one should consider and evaluate for the complication of toxic megacolon.

The CT imaging appearances of the infectious colitides are nonspecific, and there is considerable overlap among the various etiologies. The CT findings typically demonstrate pancolitis with marked colonic wall thickening, approaching the severity of Crohn’s disease (Fig. 9-14). The “accordion” sign may be seen as edematous haustral folds, separating the oral-contrast-filled lumen into narrow ridges, similar in appearance to an accordion. Although originally described as specific for patients with pseudomembranous colitis, this finding may be seen in any cause of severe colonic wall edema.

Figure 9-14 A 34-year-old male with fever and diarrhea secondary to Clostridium difficile colitis from recent antibiotic use. Axial (A) and coronal (B) portal venous phase CT images demonstrate multiple thickened loops of colon (arrows). Note the relative hypoattenuation of the thickened colon, well seen on the axial image, consistent with submucosal edema.

Tuberculous infection of the colon may have a somewhat more specific CT imaging appearance. In these patients, severe colonic wall thickening may be seen, commonly along with thickening of the terminal ileum, as the ileocecal area is the most frequently affected. Lymphadenopathy is classically described as low attenuation secondary to necrosis; calcification may also be seen. Like Crohn’s disease, tuberculosis is a well-known cause of intra-abdominal fistulae, as well as sinus tracts. Tuberculosis is also one of the causes of the cone-shaped cecum. Peritoneal thickening and ascites are common manifestations of intra-abdominal tuberculosis. Shallow mucosal ulcers may lead to significant gastrointestinal hemorrhage in patients with colonic involvement by tuberculosis.

The imaging characteristics of neutropenic colitis on CT are fairly nonspecific, demonstrating segmental bowel wall thickening, adjacent fat stranding, and fluid. The diagnosis of typhlitis should be entertained given these nonspecific imaging findings involving the cecum and/or descending colon in the immunocompromised patient population. Rapid diagnosis is critical in avoiding complications of frank bowel necrosis and perforation.

Foreign Bodies

Foreign bodies within the gastrointestinal tract are an uncommon cause of complications. However, in cases of bowel perforation without apparent alternative causes, foreign bodies should be a consideration. In more proximal portions of bowel, the foreign bodies are typically ingested. However, more distally in the rectum, the foreign bodies are also often inserted anally. Patients ingesting foreign bodies orally often have a mental disorder; patients in penitentiaries are also a frequent subset of this population. The ingested foreign bodies may include various household items, including metallic objects. The intentional ingestion of illicit narcotics may be seen in attempts to transport narcotics. Also, foodstuffs, including bones or the pits from fruits, may cause bowel injury. Various anally inserted foreign objects have been described including glass and metallic objects.

Patients with complications related to orally ingested foreign bodies may present with acute abdominal pain. If there is secondary bowel perforation, the signs and symptoms of peritonitis may ensue. Other complications include mechanical bowel obstruction with patients presenting with abdominal pain and distention, possibly with nausea and vomiting. Patients with anally inserted foreign bodies may present with pain, or may present to the emergency department unable to extract the intentionally placed foreign body.

Imaging Findings

Foreign bodies, depending on their composition, may be radio-opaque and readily identified on plain radiographs. However, smaller foreign bodies, as well as those of radiolucent composition, are not identified, although secondary findings of the related complications may be seen. These include evidence of free intraperitoneal air or possibly bowel obstruction. Patients with anally inserted foreign bodies may receive plain radiographs in order to localize the foreign body prior to definitive management. The intentionally ingested illicit narcotics, often wrapped in various protective covers such as balloons or condoms, may be radiographically apparent.

In patients with known foreign body ingestion, CT is infrequently employed for localization and evaluation of potential complications. Also, in patients with bowel perforation, and even in cases of obstruction, one may consider the possibility of a foreign body as the underlying etiology. The foreign bodies, depending on the composition, may also be subtle on CT (Fig. 9-15). However, when complications ensue, these may be helpful in localizing the foreign body. In cases of anally introduced foreign bodies, CT is particularly useful in identifying rectal injuries as evidenced by rectal wall thickening, surrounding fat stranding, and possibly free extra- or intraperitoneal air. CT is also, rarely, employed in the evaluation of illicit narcotic ingestion.

Figure 9-15 A 52-year-old female with perforated Meckel’s diverticulum secondary to fish bone ingestion. Axial (A) and coronal (B) portal venous phase CT images reveal a linear, hyperattenuating structure (arrows) consistent with a foreign body, which, in this case, happens to be located within a blind-ending tubular pouch (arrowheads). Operative repair revealed a fish bone perforating the patient’s Meckel’s diverticulum.

Vascular Diseases

Vascular diseases of the bowel include acute gastrointestinal bleeding and mesenteric ischemia.

Acute Gastrointestinal Bleeding

Acute gastrointestinal bleeding is often classified into upper and lower gastrointestinal categories based on the site of hemorrhage. Noninvasive imaging is uncommonly indicated in upper gastrointestinal hemorrhage, so only a discussion of lower tract bleeding follows. Acute lower gastrointestinal hemorrhage is a common cause of hospital admission with significant associated morbidity and mortality. The most common colonic etiologies include diverticulosis, ischemic and infectious colitis, colonic neoplasm, benign anorectal disease, and arteriovenous malformations (AVMs). Small bowel causes are a much less frequent etiology and are most commonly secondary to diverticulosis, AVM, and neoplasm.

There is no consensus regarding the initial imaging evaluation of patients presenting with acute lower gastrointestinal bleeding. Colonoscopy, technetium 99m (99mTc)–red blood cell scintigraphy, CT, and conventional mesenteric angiography are all successfully applied, depending on the given scenario. As colonoscopy may be limited in cases of severe, massive hemorrhage secondary to obscured visualization, alternate imaging for localization of the bleeding site is often required prior to possible operative intervention.

Patients with acute lower gastrointestinal hemorrhage report maroon stools or possibly bright red blood in the rectum, depending on the location of the hemorrhage. The signs and symptoms somewhat depend on the underlying etiology. Abdominal pain and diarrhea may be associated with infectious and inflammatory colitis. When severe, gastrointestinal hemorrhage may result in hemodynamic instability and shock.

Imaging Findings

Both ^99mTc-labeled erythrocytes and ^99mTc sulfur colloid are applied in the evaluation of acute gastrointestinal hemorrhage. However, ^99mTc-labeled erythrocytes offer the possibility for delayed imaging in cases of intermittent bleeding. Nuclear scintigraphy has been demonstrated as sensitive in the detection and accurate in the localization of the source of acute gastrointestinal hemorrhage. The diagnosis of gastrointestinal bleeding on nuclear scintigraphy depends on the visualization of an area of tracer localization that persists and is identified to move through the lumen of the bowel secondary to peristalsis. The transit within the bowel may be observed to be antegrade or retrograde; however, localization depends on the initial area of visualization.

Recently, CT has been shown to have a high diagnostic accuracy in both the detection and localization of massive gastrointestinal bleeding. Optimal results necessitate the distention of the bowel with a low-attenuation contrast agent, including water. Both unenhanced and arterial-phase intravenous contrast-enhanced images should be acquired. CT diagnosis relies on the visualization of an area of active arterial contrast extravasation. In some cases, CT may diagnose the underlying etiology of acute hemorrhage, such as in the case of colonic neoplasms.

Currently, given the noninvasive alternatives, the use of conventional mesenteric angiography as the initial imaging examination has decreased. However, catheter-directed therapy such as vasopressin and, more currently, super selective embolization techniques have been shown to be effective methods of treatment.

Mesenteric Ischemia

Mesenteric ischemia most commonly occurs secondary to long-standing atherosclerotic disease, with secondary acute or chronic thrombosis or embolization, as well as any cause of hypotension causing hypoperfusion on a background of narrowed mesenteric arteries. Other common etiologies occur in patients with embolization from proximal sources such as the heart secondary to mechanical heart valves or atrial fibrillation. Various other causes of mesenteric ischemia include vasculitides, bowel obstruction, radiation, corrosive ingestion, and drugs, including immunosuppressives and those agents involved in treating leukemia, lymphoma, and severe inflammation within the peritoneal cavity. Typically, depending on the etiology, segmental portions of the bowel are affected, although more diffuse involvement may also be seen.

The classic description of acute mesenteric ischemia is that of severe abdominal pain out of proportion to findings on physical examination. Typically, the pain is visceral and poorly localized. Nausea, vomiting, and diarrhea are associated. In severe cases, abdominal distention and peritonitis may ensue. In the minority of patients, gastrointestinal hemorrhage may occur.

Imaging Findings

The diagnosis of mesenteric ischemia may be suggested on plain radiographs by the presence of dilated loops of bowel demonstrating wall thickening, which may be seen as thumb printing, as well as more specific signs such as pneumatosis intestinalis, or air within the bowel wall, and portomesenteric venous gas. Alternatively, the area of bowel ischemia may cause a functional obstruction and the more proximal loops of bowel may become dilated.

PANCREATICOBILIARY DISEASE

Pancreaticobiliary disease is commonly related to inflammatory etiologies including acute and chronic pancreatitis and obstruction, often related to biliary stone disease. Usually the clinical diagnosis and management of uncomplicated pancreatitis do not rely on imaging diagnosis; however, the various complications of pancreatitis are typically identified and characterized on imaging. Also, biliary obstruction and the various complications of stone disease are a frequent cause of abdominal pain, and imaging is often employed in its diagnostic evaluation.

Acute Pancreatitis

Inflammation of the pancreas secondary to injury to the pancreatic parenchyma from release of pancreatic enzymes is a common cause of abdominal pain and may be associated with significant morbidity and mortality. In developed countries, the most common etiology for pancreatitis is alcohol abuse. Another frequent cause is biliary stone disease, which is obstruction of the pancreas by a stone lodging in the pancreatic duct, distal common bile duct, or ampulla of Vater. Other etiologies have been reported, including various medications such as steroids and NSAIDs, infectious etiologies including bacterial and viral pathogens, iatrogenic causes such as endoscopic retrograde cholangiopancreatography (ERCP) direct trauma, obstructing malignancies, hypertriglyceridemia, and ischemia, usually related to surgery. Congenital anomalies such as pancreas divisum have also been implicated in an increased incidence of pancreatitis. CT or MRI imaging of pancreas divisum includes the visualization of a dominant dorsal duct without evidence of communication between the dorsal and ventral ducts.

Patients with acute pancreatitis present with epigastric pain that is typically constant but may improve when the patient is supine. Patients commonly report anorexia, possibly with accompanying nausea and vomiting. Also, patients with acute pancreatitis may be febrile. Bowel sounds are often hypoactive secondary to ileus related to the local inflammation.

Imaging Findings

Plain radiographs may demonstrate a focal ileus secondary to the local inflammation related to the pancreas. Also, in patients with acute or chronic pancreatitis, the secondary signs of chronic pancreatitis as evidenced by parenchymal calcification may be identified.

Although imaging is not typically acquired in uncomplicated cases of acute pancreatitis, it does play a significant role in this disease process. Initial plain radiographs in patients with acute pancreatitis may demonstrate local ileus secondary to inflammation, the so-called “sentinel loop” sign. Other findings may include the visualization of calcifications within the pancreatic parenchyma in those patients presenting with acute on chronic pancreatitis (Fig. 9-16). The main role of ultrasonography in acute pancreatitis is in those patients with suspected gallstone pancreatitis; ultrasound may be used for confirmation of the presence of gallstones. Ultrasound may also be able to identify evidence of choledocholithiasis, although the examination of the distal aspect of the common duct is often limited on ultrasonography. Ultrasound may demonstrate the findings of acute pancreatitis as it affects the pancreatic parenchyma and surrounding peripancreatic tissues. Often, diffuse or focal enlargement of the pancreas is identified along with evidence of edema seen as abnormally hypoechoic parenchyma. Peripancreatic fluid may also be identified on ultrasonography. Although not required for the diagnosis of uncomplicated acute pancreatitis, when seen, typical CT findings include focal or diffuse enlargement of the gland as well as edema, identified as abnormal hypoattenuation of the parenchyma and stranding along with free fluid in the peripancreatic tissues (Fig. 9-17).

Figure 9-16 A 51-year-old male with chronic pancreatitis. Plain radiograph demonstrates multiple punctate foci of calcification overlying the expected location of the pancreas (arrow), a finding consistent with chronic pancreatitis.

Figure 9-17 A 34-year-old male with acute pancreatitis. Axial (A) and coronal (B) portal venous phase CT images reveal diffuse enlargement of the pancreas with peripancreatic fluid and stranding (arrowheads) consistent with uncomplicated pancreatitis.

In those patients suspected of having complicated acute pancreatitis, CT is usually the modality of choice. The use of CT, including the Balthazar CT severity index, has been shown to be valid in prognosticating acute pancreatitis based on the degree of CT findings of peripancreatic inflammation, phlegmon, and necrosis. Necrosis is evidenced by the absence of enhancement of the affected parenchyma when compared with adjacent areas of more normally enhancing pancreatic parenchyma. Therefore, when complicated pancreatitis is of clinical concern, CT scans are typically acquired, both with and without intravenous contrast, to evaluate for degrees of enhancement within the pancreatic parenchyma. The pancreatic necrosis may be seen to affect anywhere from small areas of the pancreatic parenchyma to virtually the entirety of the gland. If these patients receive follow-up CT examinations, the areas of necrosis typically do not regain enhancement and, when treated nonoperatively, may resorb, forming a focal cleft within the gland. If, on follow-up examinations, the initial area of pancreatic necrosis is noted to have increased in size, then these patients are more likely to undergo necrosectomy. A smaller subset of patients will go on to develop pancreatic necrosis after an initially unremarkable CT examination. Recent evidence suggests that these patients may be subject to a particularly high morbidity and mortality. Recently, novel techniques including perfusion CT have been described in the application to acute pancreatitis and may be clinically useful in assessing the status of pancreatic perfusion and possibly in predicting more severe ischemia and necrosis. Areas of pancreatic necrosis may become infected; these patients incur significant increases in morbidity and mortality. Although CT findings, such as gas bubbles within necrotic parenchyma, may be highly suggestive of infected pancreatic necrosis, patients typically undergo ultrasound or CT-guided fine needle aspiration for confirmation.

In severe cases of acute pancreatitis, there is often increased peripancreatic fluid secondary to inflammation. These findings have recently been shown to prognosticate both disease severity and 24-hour mortality, based on the findings of pleural effusions, ascites, or retroperitoneal fluid collections. Typically, pancreatic fluid collections are identified along the anterior or anterolateral aspect of the gland. Other common locations include the anterior pararenal space either bilaterally or asymmetrically depending on the precise location of pancreatic inflammation, as well as within the lesser sac. Less commonly, fluid collections are identified in the posterior pararenal space as well as within the peritoneum around the liver and spleen. These fluid collections also may dissect into the thorax, including the mediastinum.

Other complications that may be identified by CT in patients with acute pancreatitis include sequelae of injury to the splenic vasculature. These include splenic vein thrombosis, splenic infarction or subcapsular hemorrhage, and splenic artery pseudoaneurysm formation (Fig. 9-18). Pseudoaneurysms may communicate, rarely, with the pancreatic duct in a condition known as hemosuccus pancreaticus, in which bleeding occurs into the pancreatic duct and out of the ampulla of Vater to exit into the duodenum (Fig. 9-19). Treatment of splenic artery pseudoaneurysms may be successfully accomplished via transcatheter embolization, as well as by percutaneous techniques.

Figure 9-18 A 45-year-old female with acute pancreatitis. Axial (A) and coronal (B) portal venous phase CT images demonstrate complete thrombosis of the splenic, superior mesenteric, and portal veins (arrows).

Figure 9-19 A 27-year-old male with a history of pancreatitis and known gastroduodenal pseudoaneurysm status post attempted coil embolization with hemosuccus pancreaticus. Axial unenhanced (A) CT image demonstrates hyperattenuation within the main pancreatic duct (arrow) consistent with hemosuccus pancreaticus, confirmed on subsequent ERCP. Axial arterial phase CT image (B) demonstrates round, hyperattenuating focus (arrow) consistent with a gastroduodenal artery pseudoaneurysm secondary to the patient’s history of pancreatitis.

In the minority of patients with acute pancreatitis who develop pancreatic fluid collections, the fluid collections may go on to form pseudocysts. Typically, the acute fluid collections are resorbed; however, when encapsulated and well defined in shape, they are termed pseudocysts. Acute pancreatitis usually causes small ductal disruption secondary to necrosis of the parenchyma, which allows the fluid collections, and eventually pseudocysts, to form. Pseudocysts typically require at least 3 to 4 weeks to form after an episode of acute pancreatitis, usually up to 6 weeks. As they are formed from pancreatic secretions, these collections are normally amylase and lipase rich. Even when the prototypical encapsulated pseudocyst is formed, the smaller collections are highly likely to undergo complete involution with conservative management. Specifically, pseudocysts greater than 6 cm in diameter have been shown to have a significantly higher rate of operative management than smaller lesions. For the most part, pseudocysts are treated when they are symptomatic, are causing abdominal pain, are noted to enlarge on serial imaging, are noted to have become infected, or are causing hemorrhage or obstruction of adjacent bowel or bile ducts.

Chronic Pancreatitis

Chronic pancreatitis represents continued, irreversible inflammation leading to fibrosis and calcification. Eventually, patients may develop endocrine as well as exocrine insufficiency. The patient typically presents with chronic abdominal pain, and, unlike with acute pancreatitis, the pancreatic enzyme levels may be either normal or mildly elevated. The most common cause of chronic pancreatitis, like acute pancreatitis, is alcohol abuse. Other causes include hyperlipidemia, hypercalcemia, medications, cystic fibrosis, hereditary metabolic disorders, obstruction from congenital (pancreas divisum) or acquired (e.g., post-traumatic) causes, autoimmune pancreatitis, and idiopathic pancreatitis in up to 30% of cases. The role of gallstones in chronic pancreatitis remains controversial.

Patients with chronic pancreatitis describe intermittent bouts of epigastric abdominal pain that may radiate to the back. The pain may be induced by meals or occur independently of food. With ongoing chronic pancreatitis, exocrine insufficiency may result in diarrhea and weight loss. Also, endocrine insufficiency may develop.

Imaging Findings

As noted, plain radiographs may demonstrate the pancreatic parenchymal calcifications associated with chronic pancreatitis.

On CT, the findings of chronic pancreatitis include atrophy of the parenchyma, parenchymal calcifications, and dilatation of the pancreatic duct. Pseudocysts may also be visualized in patients with chronic pancreatitis.

The findings of pancreatic parenchymal calcification as echogenic, shadowing foci, as well as the secondary ductal dilatation seen in chronic pancreatitis, are well visualized by ultrasonography. Secretin-enhanced MR cholangiopancreatography provides a functional assessment of pancreatic exocrine function and has been shown to be useful in diagnosing early stages of chronic pancreatitis prior to the morphologic changes noted above.

Cholelithiasis/Choledocholithiasis

Gallstone disease is one of the most common gastrointestinal disorders and is estimated to have a prevalence of between 10% and 15% in the United States. Although the majority of patients with gallstone disease remain asymptomatic, approximately 20% will develop symptoms; this represents a significant disease burden in the adult population. The prevalence increases with increasing age and is more common in females. Additionally, increased rates of obesity in Western countries will likely further increase rates of gallstone disease. Most commonly in developed countries, gallstones are composed primarily of cholesterol, with calcium and pigments, including bilirubin and bilirubinate, composing the remaining minority of stones.

Patients with gallstone disease frequently describe symptoms of biliary colic. Generally, patients describe intermittent bouts of pain localized to the epigastrium or right upper quadrant, gradual in onset, and lasting up to several hours. Fatty meals may inconsistently be associated with bouts of biliary colic. Complications of gallstone disease including cholecystitis may infrequently be the initial presentations.

Imaging Findings

In the minority of patients, gallstones are identified on plain radiographs as calcified foci overlying the expected location of the gallbladder in the right upper quadrant.

Ultrasound is the method of choice for the noninvasive diagnosis of gallstones. Typically, gallstones are identified as freely mobile, echogenic foci demonstrating posterior acoustic shadowing. A small portion of gallstones may be affixed to the gallbladder wall and do not demonstrate mobility during the examination. The absence of posterior acoustic shadowing decreases the specificity of the ultrasound examination; however, discrete echogenic foci without shadowing most commonly represent gallstones. When the gallstones are seen within a contracted gallbladder, the wall echo shadow triad may be identified; the gallbladder mucosa is seen as a thin echogenic line anterior to a second echogenic line corresponding to the anterior wall of the gallstones with complete posterior acoustic shadowing of the more posterior aspect of the gallstones and gallbladder wall. This triad is helpful in differentiating the findings of gallstones from adjacent loops of air-filled bowel.

Although CT is not indicated in the primary detection of patients with suspected gallstone disease, gallstones may be incidentally identified on CT. Gallstones have various appearances on CT based on their composition; cholesterol stones have a range of attenuation values from 0 to several hundred Hounsfield units (HU), although the majority have attenuation values less than 100 HU. Pigmented stones have slightly higher attenuation values, although they also demonstrate a wide range of values. Recently, changing peak voltage settings during CT examination has been shown to significantly increase the detection rates of gallstones.

Cholecystitis

Cholecystitis represents the sequela of obstruction of the cystic duct, most commonly by a gallstone, followed by biliary stasis and secondary infection. In the minority of patients, acute cholecystitis occurs in the absence of gallstones, termed acalculous cholecystitis. These patients, who compose approximately 5% to 10% of all cases of acute cholecystitis, are generally critically ill with significant, prolonged bile stasis.

Patients with cholecystitis present with right upper quadrant pain secondary to peritoneal irritation from the inflamed gallbladder. Pain is typically described as severe and constant. Nausea and vomiting are associated, and the minority of patients present with fever. The Murphy’s sign, which is an inspiratory pause with palpation of the right upper quadrant, may be elicited.

Imaging Findings

In patients with suspected acute cholecystitis, ultrasonography is typically employed in the initial diagnostic evaluation. The ultrasound criteria for diagnosing acute cholecystitis include evidence of gallstones, gallbladder wall thickening, and distention, as well as elicitation of a sonographic Murphy’s sign. To elicit a sonographic Murphy’s sign, the patient is asked to breathe out with the ultrasound probe placed over the right upper quadrant. Upon the subsequent inspiration, if the patient halts the inhalation as the gallbladder passes beneath the ultrasound probe and is visualized, it is considered to be a positive sonographic Murphy’s sign (Fig. 9-20). The normal gallbladder wall is typically 2 to 3 mm in diameter; gallbladder wall thickness greater than 3 mm is considered abnormal in patients with suspected acute cholecystitis. A caveat in evaluating gallbladder wall thickness is that there are numerous causes of a thickened gallbladder wall. Gallbladder wall thickening can be seen in common systemic diseases such as cirrhosis, acute hepatitis, hypoproteinemia, renal failure, and congestive heart failure. Often, in patients with gallbladder wall thickening secondary to acute cholecystitis, areas of lucency corresponding to subserosal edema may be seen within the wall. Finally, in patients with acute cholecystitis, the gallbladder is typically significantly distended secondary to obstruction. A measurement of greater than 10 cm in the long axis of the gallbladder or 5 cm in the transverse or anterior-posterior axis is normally used to signify abnormal distention. Complications include gangrenous cholecystitis, which may demonstrate a striated pattern of gallbladder wall thickening with alternating hyperechoic and hypoechoic layers. Increasingly specific signs in more severe cases include the presence of sloughed mucosa seen within the gallbladder lumen as demonstrated by thin hyperechoic bands. In some cases of severe cholecystitis, hemorrhage may be present within the lumen, termed hemorrhagic cholecystitis. In these cases, ultrasound commonly demonstrates the findings of gangrene, in addition to nonshadowing intraluminal echoes, which may entirely fill the lumen of the gallbladder. Finally, emphysematous cholecystitis represents a rare but life-threatening complication, often associated with diabetes, in which foci of air are identified within the wall and/or lumen of the gallbladder. On ultrasound, echogenic foci or bands are seen with posterior ring-down artifact, or “dirty shadowing.” Often, the presence of the gas obscures actual visualization of the gallbladder wall. These findings are specific for this life-threatening complication and should be recognized and treated urgently.

Figure 9-20 A 43-year-old male with acute cholecystitis. Ultrasound images (A and B) demonstrate echogenic, shadowing gallstones (arrows) with gallbladder wall thickening (arrowheads) in patient in whom a sonographic Murphy’s sign was elicited. Axial (C) and sagittal (D) portal venous phase CT images reveal a gallstone (arrows) as well as a thickened gallbladder wall (white arrowheads). Although not acquired in the arterial phase of contrast, hepatic hyperenhancement consistent with secondary inflammation is nevertheless seen (black arrowheads).

CT is often used for patients with nonspecific clinical presentations of abdominal pain or equivocal findings on the initial ultrasound examination. The findings of acute cholecystitis, including its complications, should be recognized on CT. As on ultrasound, CT demonstrates gallbladder wall thickening, pericholecystic fluid, and possibly pericholecystic fat stranding. In most cases, the obstructing gallstone may be identified, although this is less likely on CT than on ultrasound, as discussed above. In those patients who receive intravenous contrast, especially when imaged during an arterial phase of enhancement, there may be evidence of secondary inflammation of the liver around the gallbladder fossa (see Fig. 9-20). This is seen as relative hyperenhancement of this portion of the liver parenchyma. CT may also demonstrate the complications of acute cholecystitis. In those patients with a gangrenous cholecystitis, the most specific CT imaging findings are shown to be direct evidence of the sloughed membranes within the lumen, irregularity of the gallbladder wall, and perforation including pericholecystic abscess. Hemorrhage is also clearly identified on CT, as evidenced by increased attenuation of the intraluminal bile. Finally, emphysematous cholecystitis is also demonstrated on CT by direct evidence of air within the gallbladder wall and/or lumen.

Hepatobiliary scintigraphy is also a common modality in assessing patients with acute cholecystitis. The examination depends on a nuclear medicine technologist, who may not be available at all hours, as well as on the length of the examination itself, so it is not typically employed as a first-line modality. The technologist administers a radionuclide in the form of a 99mTc-labeled iminodiacetic acid derivative, which is taken up by hepatocytes and excreted into the bile ducts. This technique offers a functional assessment of the patency of the cystic duct. Normally, the radionuclide is seen within the bile ducts and gallbladder lumen before being excreted into the duodenum and proximal small bowel. Obstruction of the cystic duct eliminates visualization of the gallbladder lumen. This technique offers very high diagnostic accuracy in the assessment of acute cholecystitis. Increased specificity and the diagnosis of more severe or gangrenous cases of acute cholecystitis are achieved by the visualization of a “rim” sign of relatively increased tracer localization to the hepatic parenchyma surrounding the gallbladder fossa.

Choledocholithiasis

Common bile duct stones typically occur secondary to passage from the gallbladder and represent a cause of increased morbidity and mortality in gallstone disease. Less commonly, the stones may form within the common duct itself. The classic presentation includes colicky right upper quadrant pain, which may be associated with jaundice secondary to biliary obstruction. Alternatively, obstructing common bile duct stones may be complicated by infection, and the classic Charcot triad of fever, right upper quadrant pain, and jaundice may be the initial presentation. Common bile duct stones are also a significant cause of pancreatitis.

As in patients with cholelithiasis, biliary colic is a common presenting symptom in patients with choledocholithiasis. Less frequently, complications of choledocholithiasis, including biliary obstruction with possible cholangitis, may be the initial presenting clinical scenario.

Imaging Findings

In patients with a clinical presentation suggestive of choledocholithiasis, ultrasound may be initially employed to confirm the presence of gallstones as well as to evaluate for the secondary signs of common duct stones such as the presence of biliary dilatation. The common duct stone itself may be directly visualized by ultrasound as an echogenic, shadowing, intraluminal filling defect: however, in the majority of cases, the stones are not directly visualized.

In the acute setting, especially patients presenting with nonspecific signs and symptoms, CT may be one of the initial imaging modalities used. Similar to gallstones, common duct stones display a wide range of attenuation values from isoattenuating to bile to heavily calcified. Therefore, there is a limit to the diagnostic accuracy of routine CT techniques in detecting choledocholithiasis. Optimal protocols for the detection of common bile duct stones are usually considered to include imaging in the absence of oral or intravenous contrast so as to limit interference with the visualization of the hyperattenuating common duct stones. Recently, MDCT has demonstrated moderate sensitivity and specificity in this diagnosis, using routine CT protocols that include intravenous and oral contrast in patients with nonspecific abdominal pain. CT findings of choledocholithiasis include the direct visualization of the common duct stone, often surrounded by lower attenuating bile, the so-called “target” sign. Other common imaging findings include the presence of a hyperattenuating stone seen eccentrically located within the bile duct surrounded by a crescent of lower attenuating bile, termed the “crescent” sign.

In patients with moderate or high clinical suspicion of common duct stones, MR cholangiopancreatography (MRCP) and/or ERCP may be used initially, depending on the degree of clinical confidence. On MRCP, choledocholithiasis is diagnosed by the direct visualization of the common duct stones as low-signal, intraluminal filling defects. Biliary calculi are readily identified as low-signal filling defects surrounded by hyperintense bile using MRCP techniques. Differential considerations for intraluminal filling defects on MRCP include pneumobilia, blood clots, vascular pulsation artifact, and susceptibility artifact from adjacent cholecystectomy clips, among others. Pneumobilia is differentiated by its nondependent location within the bile ducts.

Hepatolithiasis

The presence of intrahepatic bile duct stones proximal to the confluence of the left and right intrahepatic bile ducts is termed hepatolithiasis. This entity is particularly prevalent in East Asia and is thought to be related to bile stasis and recurrent bacterial infections. Typically, the stones are pigment stones with a significant cholesterol component as well. Parasite infection by Ascaris lumbricoides, Clonorchis sinensis, or Opisthorchis viverrini has been implicated as the initial insult followed by recurrent pyogenic seeding of the susceptible bile ducts. Other initial insults include malnutrition. The cycle of recurrent bacterial infections is termed recurrent pyogenic cholangitis.

Patients with recurrent pyogenic cholangitis may present with the signs and symptoms of acute cholangitis: right upper quadrant pain, fever, and jaundice. Also, patients may present with complications related to recurrent infection of the bile ducts, including bacteremia or sepsis, focal hepatic abscess, or, eventually, cholangiocarcinoma.

Imaging Findings

Interestingly, ultrasound has demonstrated difficulty in detecting the biliary stones associated with recurrent pyogenic cholangitis secondary to the soft, “mudlike” consistency of the stones. In some cases, ultrasound may directly visualize parasites within the biliary ducts.

The typical CT findings of hepatolithiasis include direct visualization of the intrahepatic stones, as well as intra- and extrahepatic biliary dilatation and stricture formation (Fig. 9-21). Hepatic parenchymal atrophy may also be identified. During the acute stages, recurrent pyogenic infections demonstrate the typical findings of inflammation on CT, including bile duct wall enhancement and adjacent hepatic parenchymal hyperenhancement. Complications of the acute disease include intrahepatic abscess and biloma formation. The dreaded long-term complication of recurrent pyogenic cholangitis is the development of cholangiocarcinoma. This typically develops in atrophic, stone-containing segments of the hepatic parenchyma and is usually associated with narrowing of the adjacent portal vein. As on ultrasound, the presence of intrabiliary parasites may be directly identified on CT.

Figure 9-21 A 42-year-old male with a history of chronic recurrent cholangitis. Axial portal venous phase CT image (A) demonstrates the presence of hepatolithiasis (arrowhead) as well as intraductal biliary dilatation (arrow) secondary to stricture related to recurrent infection. Coronal T2 MRI image (B) confirms the presence of an intrahepatic bile duct stone (arrow).

DISEASES OF THE LIVER

Hepatitis is a relatively common clinical presentation, the diagnosis and management of which typically does not require imaging. However, unsuspected cases may undergo imaging and the radiologist should be aware of the typical imaging characteristics and associated findings of the various hepatitides. Focal hepatic infections, on the other hand, often require imaging both in the diagnosis as well as in the characterization of the underlying etiology. In addition, image-guided therapies are often used in the management of focal hepatic infections.

Focal Hepatic Infections

Pyogenic hepatic abscesses account for the majority of hepatic abscess cases in the United States. Pyogenic hepatic abscess formation is most commonly associated with cryptogenic, post-traumatic, and biliary etiologies. Portal venous seeding secondary to infections drained by the portal venous vasculature, such as appendicitis or diverticulitis, accounts for approximately 10% of cases.

Although bacterial infections are the most common cause of hepatic abscess in the United States, worldwide, Entamoeba histolytica is a cause of significant morbidity and mortality. Amebic liver abscess is a common extraintestinal manifestation of E. histolytica, an infection with prevalence of approximately 10% worldwide. This infection is rare in the United States; the affected patients are typically from countries where the infection is more common, including Africa, Central and South America, and India, as well as East Asian countries, or they have recently traveled to these areas. Classically, on gross pathology, these collections are filled with blood products, forming a brown, thick consistency described as “anchovy paste.”

Finally, Echinococcus granulosus is a common parasitic infection that may result in focal hepatic infection. Like amebiasis, infection of the liver occurs through the invasion of the mucosa of the bowel followed by the portal venous system. Rare in the United States, echinococcal disease is common in the Mediterranean region, as well as in areas where sheep raising is more common. The composition of the hydatid cyst results in its characteristic imaging features. At histopathology, it is composed of three layers: the outer pericyst and the inner endocyst separated by a thin membrane termed the ectocyst. The outer pericyst is actually composed of compressed liver parenchyma and fibrosis.

Patients with pyogenic hepatic abscesses typically present with right upper quadrant pain, fever, and malaise. Depending on the size and number of abscesses, the presentation may be acute or more chronic and indolent with weight loss or fever of unknown origin. When irritation of the right hemidiaphragm is associated with the hepatic abscess, patients may complain of referred pain to the right shoulder. Patients with amebic liver abscesses also typically complain of right upper quadrant pain, and the majority of patients present with fever. The presentation of patients with a hydatid cyst involving the liver is variable and may be due to complaints of a mass in the right upper quadrant or upper abdominal discomfort. Cyst rupture may result in a more acute presentation of pain with peritonitis.

Imaging Findings

Ultrasonography may be employed in the diagnosis of pyogenic liver abscess. The typical imaging findings are of a relatively hypoechoic, heterogeneous collection surrounded by normal-appearing liver parenchyma. Often, internal echoes are identified secondary to debris and pus. However, early in the development of these infections, they may be of a more phlegmonous nature and appear ill-defined and hyperechoic relative to the hepatic parenchyma. As they mature, they take on the more classic cystic-appearing features. These collections may also contain air secondary to the presence of gas-forming organisms, and the typical hyperechoic foci demonstrating ring-down artifact are seen on ultrasound. Amebic liver abscesses demonstrate findings similar to findings of pyogenic infections on ultrasonography; hypoechoic, intraparenchymal collections often demonstrate homogeneous, low-level echoes internally. As it is on CT, the typical imaging appearance of echinococcal disease of the liver may be identified on ultrasonography; uni- or multilocular collections with possible identification of smaller, daughter cysts are common.

On CT, the abscesses are relatively hypoattenuating with the surrounding liver parenchyma (Fig. 9-22). Often, surrounding, smaller collections are identified in the vicinity of the largest abscess cavity. Hyperenhancement of the surrounding liver parenchyma may be identified, possibly related to local portal venous obstruction secondary to the acute inflammation. As on ultrasound, there may be direct evidence of air within the collection. Complications of liver abscesses, including venous thrombosis, are seen in up to half of patients. The hepatic venous system is affected nearly as often as the portal veins. Contrast-enhanced CT demonstrates the nonenhancing hypoattenuating tubular structures of the thrombosed veins.

Figure 9-22 A 70-year-old male with choledocholithiasis and hepatic abscess. Axial (A) and coronal (B) portal venous phase CT images demonstrate large bile duct stones (arrows) with a focal intrahepatic fluid collection (arrowheads) consistent with the patient’s known pyogenic abscess.

CT findings of amebic liver abscesses are similar to pyogenic liver abscesses in that a well-defined, hypoattenuating, peripherally enhancing collection is typically identified. Serologic testing, in the appropriate patient population with typical CT findings, often solidifies the diagnosis. Extrahepatic extension of the infection is relatively common, a possible discriminating imaging finding when considering this diagnosis. Extension into the pleural space, chest wall, pericardium, bile ducts, bowel, and surrounding viscera has been described.

Both ultrasound and CT may diagnose the findings of echinococcal disease within the liver. Typically, these cystic lesions are identified as uni- or multilobular collections. Often, small daughter cysts are seen within the larger lobules, a finding highly specific for echinococcal disease. Another fairly specific sign for echinococcal disease is the so-called “water lily” sign, in which the endocyst ruptures and is seen floating dependently within the outer layers of the cyst. Complications, when affecting the liver, include rupture into the peritoneal cavity or into the biliary tracts.

In all three of the aforementioned focal hepatic infections, radiology may play a role in management. In those patients presenting with pyogenic abscesses, ultrasound or CT-guided percutaneous needle aspiration or catheter placement combined with antibiotic therapy results in a definitive treatment for most patients. In patients with amebic liver abscess, medical management with metronidazole is successful in the majority of patients. However, those with more severe disease burden may benefit from percutaneous drainage using ultrasound or CT guidance. Finally, in patients with echinococcal liver disease, surgery remains the cornerstone of treatment. However, percutaneous therapy has been shown to be successful, often in conjunction with scolicidal agents. Typically, patients are treated with albendazole prior to intervention to avoid the risk of anaphylaxis by causing the cyst material to become nonantigenic.

Hepatitis

There are a myriad of underlying etiologies for acute hepatitis, including toxic causes such as alcohol and various medications, infectious causes, and autoimmune causes, among others. Typically, imaging is not utilized in the evaluation and diagnosis of hepatitis, although the findings of hepatitis should be recognized if imaging is acquired for another indication or if the diagnosis of hepatitis is unsuspected.

Patients with diffuse inflammation of the liver or acute hepatitis may present with a wide range of clinical signs and symptoms from mild forms to fulminant hepatic failure. Often, abdominal discomfort, anorexia, nausea, and vomiting as well as jaundice may be associated.

Imaging Findings

Two distinct ultrasound patterns have been described in patients with acute hepatitis: the bright liver and the “starry sky” appearance. The bright liver refers to increased parenchymal echogenicity and attenuation of sound as it passes through the abnormal liver parenchyma. This pattern is nonspecific and can be seen in patients with acute alcoholic hepatitis, cirrhosis, chronic hepatitis, and diffuse fatty infiltration of the liver. The second pattern, called “starry sky,” refers to diffuse decrease in echogenicity of the liver secondary to edema of the hepatocytes, causing the walls of the portal venules to be seen as echogenic areas on the dark background of the edematous hepatic parenchyma. This is also a nonspecific finding and has been described in other pathologies involving the liver including Burkitt’s lymphoma and in fasting patients. Another finding that may be identified on ultrasound in patients with acute hepatitis is diffuse gallbladder wall thickening. Viral hepatitides are associated with gallbladder wall thickening, and fairly significant periportal lymphadenopathy may be seen.

Typical CT findings of acute hepatitis are nonspecific and may include findings of periportal edema, also known as a periportal “collar,” hepatomegaly, and ascites. As on ultrasound, gallbladder wall thickening and lymphadenopathy may be identified in patients with viral hepatitides.

DISEASES OF THE GENITOURINARY TRACT

Genitourinary disease, especially nephrolithiasis and its complications, represents a significant cause of abdominal pain and commonly requires imaging in its diagnosis. Infectious genitourinary diseases, such as pyelonephritis, less commonly require imaging diagnosis; however, the complications of the various genitourinary infections may necessitate an imaging evaluation. Finally, acute vascular insults of the kidney, including infarcts and renal vein thrombosis, are commonly diagnosed using cross-sectional imaging.

Nephrolithiasis

Renal calculi are usually caused by crystallization of the supersaturated stone-forming materials from urine. Calcium is the most common stone-forming material with uric acid constituting the second largest component. Numerous other less common components are described, including xanthine, cystine, struvite, and precipitation of medications such as protease inhibitors in the HIV population, among others. Alternatively, renal pathology may initiate the formation of crystals within the renal tubules that are extruded into the renal collecting system to undergo further growth. Ureteral calculi are most commonly secondary to renal calculi passing distally into the ureters.

Nephrolithiasis and ureterolithiasis present with pain in the region of the flanks, which may radiate into the groin, especially with distal progression of the stones into the ureter. Costovertebral tenderness is commonly elicited in patients with nephrolithiasis. Also, hematuria is often associated.

Imaging Findings

Nephrolithiasis may be identified as focal calcific densities projecting over the renal shadows. The expected course of the ureters should be analyzed for evidence of ureteral calculi. Also, bladder calculi may be identified on plain radiographs. In patients with a known history of renal calculi who have undergone lithotripsy, plain radiographs may be used to evaluate for residual renal or ureteral calculi. When multiple ureteral calculi are identified following lithotripsy, it is termed Steinstrasse. The translation of this German term is “stone street.”

Ultrasound is often employed in patients presenting with acute renal failure and may demonstrate findings of nephrolithiasis, possibly with signs of obstruction. Renal calculi are seen as echogenic foci, typically demonstrating posterior acoustic shadowing. Also, signs of hydronephrosis and hydroureter may be identified on ultrasound in patients with acute obstruction. Proximal and distal ureteral stones may be clearly identified on ultrasonography; however, ultrasound is limited in the evaluation of the entirety of the ureter, and, therefore, sensitivity is significantly less than with CT in detecting ureteral calculi.

Renal stone CT protocols are usually acquired without the use of oral or intravenous contrast, which may obscure the underlying stones. CT has a high diagnostic accuracy in the detection of renal and ureteral calculi and may be used to differentiate among stones of various chemical compositions. Recently, ultra-low-dose CT with radiation dose equivalent to a KUB has been shown to be diagnostically sufficient in evaluating renal and ureteral calculi. Renal stones are most commonly composed of calcium in the form of calcium oxalate, calcium phosphate, and calcium urate. Other common stones are struvite, uric acid, and cystine stones. These most common forms of renal stones are all readily identified by routine CT techniques (Fig. 9-23). However, in patients with HIV with a clinical suspicion of nephrolithiasis, protease-inhibitor induced stones, because they are not routinely identified on CT, should be a consideration.

Figure 9-23 A 59-year-old female with obstructing ureteral calculus. Axial unenhanced CT image (A) demonstrates mild diffuse enlargement of the right kidney with hydronephrosis (arrow) and mild perinephric stranding (arrowhead). Coronal unenhanced CT image (B) demonstrates hydronephrosis (arrowhead) and the ureteral stone (arrow).

Secondary signs of acute ureteral obstruction include enlargement of the kidney, which often demonstrates diffusely decreased attenuation secondary to edema, perinephric stranding, and dilatation of the ureter and collecting system. Three areas of narrowing in the course of the ureter are common areas for the identification of obstructing ureteral stones. These include the ureteropelvic junction (UPJ), the pelvic brim as the ureter crosses into the pelvis, and the ureterovesical junction. Ureteral stones, when identified in the pelvis, may demonstrate a soft tissue “rim” sign surrounding the calculus, distinguishing a ureteral calculus from adjacent pelvic vein phleboliths.

Pyelonephritis

Pyelonephritis is typically caused by E. coli or Enterococcus faecalis in the outpatient community population. In patients who fail therapy, or in whom symptoms progress, ultrasound or CT may be used to evaluate the various complications of pyelonephritis.

Patients with pyelonephritis may describe a preceding lower urinary tract infection including dysuria, frequency, and lower abdominal pain. Subsequently, patients often develop complaints of flank pain, which may be either uni- or bilateral. Fever and leukocytosis may be associated.

Imaging Findings

Uncomplicated pyelonephritis may demonstrate an enlarged, swollen kidney with areas of increased or decreased renal parenchymal echogenicity. More so in children, power and color Doppler techniques have been used to diagnose pyelonephritis based on focal areas of hypovascularity.

On CT, simple pyelonephritis may demonstrate a striated nephrogram as well as ill-defined hypoattenuating areas within the parenchyma. This is caused by the decreased excretory function of the renal tubules secondary to obstruction by inflammatory debris, vasospasm, and surrounding parenchymal edema. Delayed enhancement secondary to vasospasm may be identified on the affected side. Other findings associated with pyelonephritis on CT may be thickening and hyperenhancement of the urothelium. Also, perinephric stranding may be identified.

Renal Abscess

Renal abscess formation is a potential complication of pyelonephritis, the suspicion of which typically entails the use of imaging for characterization. The imaging characteristics of the abscess, when identified, are used for planning management.

Patients with a renal abscess commonly present with fever, chills, and abdominal pain. Associated urinary tract symptoms may also be reported. Leukocytosis is typically associated.

Imaging Findings

The distinction between a focal renal abscess and acute focal bacterial nephritis has important implications for subsequent management. Renal abscesses usually demonstrate ultrasound findings of a complex, focal, hypoechoic region within the parenchyma, often with internal echotexture but with evidence of increased through-transmission (Fig. 9-24). Focal bacterial nephritis, it has been reported, lacks the increased through-transmission of a renal abscess.

Figure 9-24 A 37-year-old female with renal abscess. Ultrasound image (A) reveals a focal hypoechoic area within the renal parenchyma with scattered internal echoes (arrow). Axial (B) and coronal (C) portal venous phase CT images reveal a corresponding focal hypoattenuating area within the parenchyma of the right kidney, findings consistent with a renal abscess.

CT demonstrates focal, low-attenuation areas that may extend into the perirenal soft tissues. These areas are more readily identified with intravenous contrast, thereby increasing the attenuation differences between normally enhancing renal parenchyma and the nonenhancing abscess collection. Often, secondary CT signs of inflammation accompany a renal abscess, including perinephric fat stranding.

In patients with a renal abscess, percutaneous drainage via ultrasound or CT guidance may be used to successfully manage the patients, an approach that has been shown to have outcomes similar to those of surgical management. However, smaller abscess collections may be treated successfully using intravenous antibiotics.

Emphysematous Pyelonephritis

Emphysematous pyelonephritis represents a severe renal parenchymal infection, almost exclusively encountered in diabetic patients. The most commonly cultured organism in patients with emphysematous renal tract disease (ERTD) is E. coli. As this entity results in significant increases in morbidity and mortality, emphysematous pyelonephritis should be considered in all diabetic patients with signs and symptoms of pyelonephritis.

Patients with emphysematous pyelonephritis often present with fever and flank pain. In severe cases, crepitus may be appreciated. Also, given the severity of the infection, a significant portion of patients may present with frank septic shock.

Imaging Findings

CT is primarily used as the initial imaging modality for evaluating complicated pyelonephritis, including emphysematous infections. CT clearly demonstrates gas within the renal parenchyma that may extend into the perirenal soft tissues. A classification system based on imaging has been shown to have prognostic implications. Two types of emphysematous pyelonephritis are described: type I without, and type II with associated renal or perirenal fluid collections. Those with associated fluid collections have a more favorable prognosis, hypothesized to be secondary to a more favorable immune response as evidenced by the presence of fluid collections. The diagnosis of emphysematous pyelonephritis is a medical emergency and typically requires emergent nephrectomy, although successful management has been described using percutaneous drainage or medical therapy.

Xanthogranulomatous Pyelonephritis

Xanthogranulomatous pyelonephritis (XGP) represents a chronic inflammatory condition of the renal parenchyma. XGP has been associated with urinary tract infections secondary to E. coli and Proteus mirabilis and is nearly exclusively seen in patients with underlying urinary tract obstruction, commonly secondary to stone disease. Histopathologically, lipid-laden “foamy” macrophages accompany both acute and chronic inflammatory cells. Most commonly, the kidney is affected diffusely, although focal involvement of a portion of the parenchyma may also be identified. CT and ultrasound demonstrate the findings of XGP, although, usually, definitive preoperative diagnosis is difficult as renal malignancy remains on the differential in most cases.

Patients with XGP present with signs and symptoms of a chronic illness: anorexia, weight loss, fevers, and flank pain. As XGP may create a fistula with adjacent soft tissues, patients may initially present with cutaneous fistulas.

Imaging Findings

Few ultrasound imaging characteristics are available to distinguish XGP from a renal tumor. This is because the most common ultrasound finding is that of a circumscribed, solid renal mass, which may be hyper-, iso-, or hypoechoic with respect to the renal cortex. Hydronephrosis and renal calculi are commonly identified on ultrasound.

CT demonstrates renal enlargement with fluid-filled cavities replacing the renal parenchyma, often with extension into the perinephric tissues, findings suggestive of XGP. Again, hydronephrosis and renal calculi may be identified in patients with XGP (Fig. 9-25). CT evidence of fistula formation with surrounding tissues including bowel or skin lends credence to a suspected diagnosis of XGP.

Figure 9-25 A 50-year-old female with xanthogranulomatous pyelonephritis (XGP). Axial (A) and coronal (B) portal venous phase CT images demonstrate significant enlargement of the left kidney with large renal calculi (black arrows) and large, low attenuation infiltration of the renal parenchyma (white arrows) characteristic of XGP.

Pyonephrosis

In patients with fever, chills, and flank pain, as well as obstruction of the urinary tract established on imaging, pyonephrosis should be highly suspected. Pyonephrosis represents infection of an obstructed urinary tract and is a medical emergency. Because this scenario of infection within the obstructed collecting system represents “pus under pressure,” these patients can rapidly deteriorate and develop septic shock and possible irreversible deterioration of renal function.

Imaging Findings

Typically, ultrasound is indicated to demonstrate obstruction of the affected urinary system as evidenced by hydronephrosis and possibly hydroureter. Also, fluid–fluid levels and coarse or possibly low-level echoes are described in pyonephrosis.

On CT, the findings of renal obstruction as evidenced by hydronephrosis are identified. The obstructing lesion may also be characterized on CT. CT findings of inflammation may be identified, including inflammatory fat stranding around the collecting system and ureter as well as hyperenhancement of the urothelium. Thickening of the urothelium of the renal pelvis may also be identified.

Prompt placement of a percutaneous nephrostomy tube is indicated in these patients with pyonephrosis, who often eventually require subsequent definitive management to treat the underlying obstruction.

Emphysematous Cystitis/Pyelitis

Emphysematous cystitis represents a rare, fulminant infection of the bladder, again associated with underlying diabetes. Repeated urinary tract infections, neurogenic bladder, and bladder outlet obstruction have also been described in association with emphysematous cystitis. Commonly isolated organisms include E. coli and Enterobacter aerogens. Common causes of air within the bladder should be considered and excluded, such as recent instrumentation like Foley catheter placement or cystoscopy. Less common alternative causes of air within the bladder include fistulas with adjacent loops of bowel. Clinical history is often helpful in excluding these considerations. Unlike emphysematous pyelonephritis, emphysematous cystitis is usually successfully treated conservatively with antibiotics.

Emphysematous pyelitis refers to gas within the renal collecting system. Typically, the gas is secondary to emphysematous infections of the bladder and kidney, although primary infection of the urothelium is a less common cause. Again, other causes of air within the bladder and collecting system should be excluded as detailed above. Emphysematous pyelitis, as it is most commonly secondary to infection of the bladder or kidney, is normally managed by treating the underlying abnormality.

Patients with emphysematous cystitis often have fairly subtle clinical presentations, and therefore a high clinical suspicion is necessary in the appropriate patient population. Complaints include dysuria, fever, and hematuria. Although not sensitive, a report of pneumaturia is highly specific to this diagnosis.

Imaging Findings

Plain radiographs may demonstrate mottled or curvilinear lucencies overlying the expected location of the bladder wall. Also, an air–fluid level may be identified in patients with intraluminal air; this level is noted to change position with the patient. The intraluminal air may also outline the irregular mucosal surface, distorted by submucosal blebs, termed the beaded necklace appearance.

In patients with emphysematous cystitis, CT is highly sensitive for the finding of air localized within the bladder lumen or wall. It is important to exclude air within the renal parenchyma in these cases.

Renal Infarct

Nontraumatic renal infarcts are typically secondary to embolic phenomena, which may be secondary to atrial fibrillation, left ventricular thrombus in patients with prior myocardial infarction, and atherosclerotic disease in the aorta. Other considerations in cases of renal infarction include infective endocarditis and vasculitides, overwhelming shock and hypoperfusion, sickle cell disease, and iatrogenic injury.

Patients with renal infarction usually present with nonspecific signs and symptoms. Acute onset of low back pain in patients with risk factors of thromboembolic disease has been described as suggestive of renal infarction. Patients may present with fever and leukocytosis as well as hematuria and proteinuria.

Imaging Findings

The majority of patients with renal infarction demonstrate peripheral, wedge-shaped areas of decreased attenuation on contrast-enhanced CT. Smaller subsets of patients have multifocal areas of low attenuation or global infarction. Sickle cell disease may demonstrate multiple “slitlike” areas of infarction, possibly secondary to thrombosis of small venous channels. In cases of global renal infarction, the cortical “rim” sign is seen as a hyperattenuating rim around the cortex as the cortical tissue remains perfused by proximal branches of the renal artery and collateral vessels. As the majority of renal infarcts are secondary to embolic sources, the remainder of the abdomen, including the spleen and bowel, should be scrutinized for further complications of embolic phenomena.

Renal Vein Thrombosis

The most common etiology of renal vein thrombosis is nephrotic syndrome, which leads to a hypercoagulable state with systemic effects and with a predilection for renal vein thrombosis. Another relatively frequent cause of renal vein thrombosis is secondary to extension of renal cell carcinoma. Less likely causes are in patients with renal transplants or hypercoagulability secondary to causes other than nephrotic syndrome. Although usually asymptomatic, renal vein thrombosis may present with acute flank pain and hematuria, similar to nephrolithiasis.

Imaging Findings

Ultrasound can demonstrate enlargement of the affected kidney and potentially may demonstrate direct evidence of the thrombus within the renal vein, as evidenced by an echogenic intraluminal filling defect. Unlike in the renal transplant population, intrarenal arterial Doppler studies in native kidneys have shown inadequate sensitivity and specificity for diagnosing renal vein thrombosis.

CT clearly demonstrates thrombus within the renal vein and may aid in characterizing the underlying etiology, especially in cases of renal cell carcinoma. Other CT findings in renal vein thrombosis include an enlarged kidney with prolonged corticomedullary phase and delayed nephrogenic and excretory phases. Also, significant prolongation of parenchymal enhancement may be seen on follow-up examinations up to many hours later. Because complications include the potential for pulmonary embolism, extension of the thrombus into the inferior vena cava should be evaluated using ultrasound or CT.

DISEASES OF THE SPLEEN

The overwhelming majority of splenic abnormality identified on imaging is benign and represents an incidental finding. However, several splenic abnormalities may present with acute abdominal symptoms, including focal infections of the spleen. Also, vascular insults such as splenic infarcts and torsion, related to “wandering” spleen, may present acutely and require imaging in their diagnosis.

Splenic Abscess

Patients with splenic abscesses commonly have an underlying predisposition to infection, such as diabetes mellitus, leukemia, intravenous drug abuse, or HIV positivity. Other associations include endocarditis and recent splenic infarction from various causes. A variety of bacteria are commonly isolated in these cases, including E. coli, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus, Mycobacterium organisms such as tuberculosis, and fungal organisms. Unilocular abscesses are typically secondary to pyogenic abscesses.

Patients with splenic abscesses typically present with fever and abdominal pain, often localized to the left upper quadrant. Diaphragmatic irritation can cause referred pain in the shoulder, called the Kehr sign. Patients also report pleuritic chest pain.

Imaging Findings

When seen on ultrasound, splenic abscesses appear as focal, hypoechoic areas, often with internal echoes or debris. Rarely, gas may be identified as focal echogenic areas with posterior “dirty shadowing.”

Splenic abscesses are for the most part low attenuation on CT. They may demonstrate peripheral rim enhancement and possibly contain gas. Mycobacterial and fungal abscesses typically demonstrate several or many small, hypoattenuating ovoid lesions throughout the splenic parenchyma. Echinococcus is another relatively common cause of splenic abscess. As in other areas of involvement, echinococcal disease of the spleen demonstrates the typical multiloculated cystic appearance, possibly with more specific visualization of daughter cysts.

In cases of pyogenic abscess, management may include splenectomy; however, percutaneous ultrasound or CT-guided drainage has been demonstrated as a viable alternative in selected patients.

Splenic Infarction

Splenic infarction is a cause of acute abdominal pain and fever in patients who commonly have underlying hematologic or thromboembolic disorders. However, a small subset of patients remains asymptomatic, and splenic infarction is often incidentally identified on imaging studies obtained for other reasons. Hematologic disorders include both benign diseases such as sickle cell disease and malignant lymphoproliferative disorders including lymphoma and leukemia. Thromboembolic etiologies typically include atherosclerotic disease or emboli originating from the heart, as in endocarditis or atrial fibrillation, or are secondary to prosthetic mitral valves. Splenic vein thrombosis secondary to a variety of causes, including pancreatitis, may result in splenic infarction. As detailed below, a dramatic cause of splenic infarction is torsion of a “wandering” spleen.

Splenic infarction is often incidentally identified on imaging for unrelated clinical indications. However, especially when larger, splenic infarction can cause abdominal pain, typically in the left upper quadrant. As well, these patients may be febrile.

Imaging Findings

The findings of splenic infarction can be seen on both ultrasound and CT as peripheral, hypoechoic in the case of ultrasound, and hypoattenuating wedge-shaped defects in the case of CT.

Less classic appearances of splenic infarction include multiple focal, heterogeneous areas of infarction as well as massive areas involving significant portions of the splenic parenchyma. In contrast to unenhanced CT, intravenous-enhanced CT has been shown to improve detection of splenic infarcts significantly. Complications in cases of splenic infarction are secondary infection and abscess formation, as well as pseudocyst formation, hemorrhage, and even rupture. In cases of chronic splenic infarcts, as seen in patients with sickle cell disease, the spleen appears atrophic and diffusely calcified, a finding that may be apparent on plain radiographs as focal calcification in the left upper quadrant.

One consideration in patients with sickle cell disease is the possibility of splenic sequestration, which might have a somewhat similar imaging appearance to splenic infarction in some cases. Although this is typically a clinical diagnosis based on hematocrit decrease and splenic enlargement, imaging may provide a diagnosis in unsuspected cases. In splenic sequestration, CT findings of multiple, peripheral, nonenhancing areas or diffuse low attenuation involving the majority of the splenic parenchyma are identified.

Splenic Torsion

Splenic torsion results secondary to twisting of the spleen around its vascular pedicle and is associated with the rare condition of “wandering” spleen. The term is used when the spleen is not identified in its expected location but seen elsewhere in the abdomen or pelvis. The underlying cause for the ectopic location is hyperlaxity or abnormal development of the supportive ligaments of the spleen. More commonly seen in children, wandering spleen has been identified in adults, most commonly women. Patient presentation in cases of splenic torsion varies from acute to chronic, intermittent pain. An even less common cause of acute abdominal pain that demonstrates similar imaging appearances, although on a somewhat smaller scale, is the acute torsion of an accessory spleen, or splenule.

Patients with splenic torsion typically present with acute abdominal pain. As these cases are associated with wandering spleen, patients may report a more prolonged history of intermittent abdominal pain. Less commonly, splenic torsion may be asymptomatic with clinical presentation prompted by a palpable abdominal mass caused by the torsed spleen.

Imaging Findings

In cases of torsion related to wandering spleen, a hypo- or avascular mass is identified in the peritoneal cavity on ultrasound. In cases of torsion of a splenule, a smaller hypovascular mass is identified adjacent to the normal-appearing spleen.

CT also demonstrates a hypo- or nonenhancing mass in cases of complete infarction, usually surrounded by inflammatory changes. CT may be used to directly identify the twisted vascular pedicle, described as having a whorled appearance. In cases of torsion of a splenule, a hypovascular soft tissue mass is identified on CT in the left upper quadrant adjacent to a normal spleen.

EMERGENT IMAGING FINDINGS

Several imaging findings are considered emergent findings because they suggest underlying pathology requiring urgent intervention. These include pneumoperitoneum, which, although possibly suggested clinically, is typically an imaging diagnosis. The findings of pneumoperitoneum demand urgent attention, even though there are various underlying causes, some of which are benign, as discussed below. Additionally, intra-abdominal hemorrhage also warrants urgent attention both in acute clinical management and in imaging evaluation to diagnose the underlying cause. Acute urinary and biliary obstruction both have various underlying causes, and although the uncomplicated obstruction does not usually represent an emergent, life-threatening situation, various complications of the obstructions may indeed represent true emergencies as detailed below.

Pneumoperitoneum

Pneumoperitoneum refers to free air within the peritoneal cavity. Common etiologies include perforation of a hollow viscus and iatrogenic causes such as recent surgery and orogenital insufflations, which are the most common cause overall, and extension from other compartments of the body, including pneumomediastinum. In cases of pneumoperitoneum secondary to surgery, one should expect appropriate resolution during the postoperative period. The majority of cases are noted to resolve within 2 days based on radiography; however, many patients have minute areas of free air for several days and even up to several weeks on CT scans postoperatively.

Because the signs and symptoms of pneumoperitoneum are usually related to the underlying etiology, there are a myriad of clinical presentations. When there is perforation of a hollow viscus, peritonitis may ensue; this might be preceded by focal abdominal pain, based on the underlying etiology. Benign causes of pneumoperitoneum, such as those secondary to orogenital insufflations, may be asymptomatic and only incidentally identified on imaging.

Imaging Findings

Plain radiographs are often initially employed to detect free intraperitoneal air. Typically, a supine radiograph, in addition to an upright view of the chest, is acquired to evaluate for free intraperitoneal air (Fig. 9-26). Left lateral decubitus views of the abdomen and lateral chest radiographs are further options. In cases of upright radiographs or left lateral decubitus views, acquisition with the central ray of the x-ray beam at the highest level of the peritoneal cavity has been shown to increase sensitivity. Various signs of pneumoperitoneum on abdominal radiographs have been described: they include the Rigler’s sign, in which air is seen on both sides of the bowel wall; the falciform ligament sign, in which air outlines the falciform ligament; the “football” sign, in which air outlines the confines of the peritoneal cavity; the “inverted V” sign, in which air outlines the medial umbilical folds; and the right upper quadrant air sign, in which a focal, typically triangular, collection of gas is seen in the right upper quadrant. At least one of these signs has been reported in slightly more than half of patients with pneumoperitoneum on supine abdominal radiographs. The “cupola” sign of air outlining the median subphrenic space has also been described in the minority of patients with pneumoperitoneum on supine radiographs.

Figure 9-26 A 76-year-old female with perforated duodenal ulcer and free intraperitoneal air. Plain upright chest radiograph (A) demonstrates lucency (arrow) beneath the diaphragm on the left, adjacent to the air-filled stomach, consistent with free intraperitoneal air. Axial CT (B) using lung windows clearly demonstrates free intraperitoneal air (arrow) adjacent to the stomach, as on the radiograph.

CT is the most sensitive modality for detecting small volumes of pneumoperitoneum. In cases of hollow viscus perforation, it has been shown to be highly accurate in the localization of the site of perforation, especially when thin collimation images and multiplanar reformations are viewed. The findings of focal bowel wall thickening, localized air bubbles, and direct visualization of a rent within the wall of the bowel have been shown to be accurate predictors of the site of perforation.

Spontaneous Hemoperitoneum

Spontaneous hemoperitoneum, in the absence of trauma, may be seen secondary to a variety of causes including splenic, hepatic, vascular, coagulopathic, and gynecologic. Typically a patient presents with acute symptoms including abdominal pain possibly associated with hypotension. Spontaneous splenic hemorrhage may be secondary to underlying lymphoproliferative disease, infection, or rupture of a splenic artery pseudoaneurysm commonly caused by pancreatitis, among a myriad of other underlying etiologies. Ruptured splenic artery aneurysm, which represents the most common splanchnic vessel aneurysm territory, is also a potential cause of spontaneous splenic hemorrhage.

Spontaneous hemorrhage originating from the liver is usually associated with hepatic tumors, both benign and malignant. As in any splanchnic vascular territory, rupture of hepatic arterial aneurysms or pseudoaneurysms may cause spontaneous hemorrhage. Benign hepatic tumors predisposing to spontaneous hemorrhage include adenomas, while hepatocellular carcinoma and, less likely, metastases account for the malignant etiologies (Fig. 9-27). Spontaneous hemorrhage secondary to other benign hepatic lesions, including focal nodular hyperplasia and cavernous hemangioma, has been described but is much less frequently observed.

Figure 9-27 A 57-year-old female with ruptured hepatocellular carcinoma. Ultrasound image (A) demonstrates free intraperitoneal fluid (arrow) with evidence of dependently layering debris (arrowhead) consistent with hemoperitoneum. Axial (B) and coronal (C) portal venous phase images demonstrate the patient’s hepatoma (black arrows) with a central hyperattenuating focus (arrowhead) seen on the axial image consistent with active hemorrhage. Perihepatic hemorrhage is also seen on the CT images (white arrows).

Another significant cause of spontaneous intra-abdominal hemorrhage, affecting both intraperitoneal and retroperitoneal compartments, is coagulopathy, including medical anticoagulation. Hemorrhage secondary to coagulopathy most commonly involves the rectus sheath, iliopsoas, and retroperitoneal compartments. Less commonly, the hemorrhage is within the peritoneum, and, rarely, it involves a solid visceral organ such as the spleen or liver. Patients often present with acute abdominal pain and, as the majority of cases involve anticoagulant or antiplatelet administration, are generally known to be coagulopathic.

Patients with spontaneous retroperitoneal hemorrhage often present with acute abdominal pain. When the hemorrhage is extensive, patients may present with hypotension and even frank shock.

Imaging Findings

In these cases, ultrasound can identify hemoperitoneum as free fluid with homogeneous internal echoes. Additionally, a clot may appear as a focal heterogeneous collection, which might seem to be masslike but demonstrates no internal vascularity.

CT is often used as the initial imaging modality in cases of suspected hemoperitoneum or to evaluate for the underlying etiology when hemoperitoneum is identified on ultrasound. On CT, a focal region of high-attenuation clotted blood, known as the “sentinel clot” sign, is useful in localizing the underlying etiology of hemorrhage.

On CT, in cases of spontaneous splenic hemorrhage, the underlying etiology may be clearly visualized as a focal or infiltrative mass lesion in cases of malignancy, as diffuse enlargement with or without focal hypoattenuating areas in the case of infection, or as a pseudoaneurysm. In these cases, the CT protocol should include intravenous contrast to allow characterization of intraparenchymal abnormalities and clear visualization of the splenic artery. CT affords potential characterization of the etiology of spontaneous hepatic hemorrhage; however, the hemorrhage may obscure the underlying cause in some cases. In this subset of patients, short-term follow-up CT or MRI is typically utilized to evaluate the etiology once the hemorrhage has organized or resolved.

In coagulopathic patients, CT demonstrates the hematoma in the various intra-abdominal compartments, and, in the majority of cases, a hematocrit effect, or the separation of the cellular components from the plasma, is visualized. This is a sensitive and specific finding suggesting underlying coagulopathy.

Spontaneous Retroperitoneal Hemorrhage

In addition to rupture of the various visceral organ related aneurysms and pseudoaneurysms, a significant cause of spontaneous retroperitoneal hemorrhage is rupture of an abdominal aortic aneurysm (AAA). Although less commonly encountered than spontaneous hemorrhage from underlying hepatic and splenic pathology, spontaneous renal and pancreatic hemorrhage can be severe and life-threatening. The most common causes of spontaneous renal hemorrhage include benign and malignant lesions, commonly renal angiomyolipoma and renal cell carcinoma, respectively. Other causes of renal hemorrhage include vasculitides such as polyarteritis nodosa (PAN) and Wegener’s granulomatosis.

Imaging Findings

Ultrasound may identify the retroperitoneal hemorrhage and possibly characterize the underlying etiology, as in the case of the renal tumors mentioned above; however, CT is often employed.

In cases of ruptured AAA, CT is often the first-line imaging modality (Fig. 9-28). CT protocols may include the use of intravenous contrast to evaluate for active extravasation, although unenhanced CT may readily identify cases of AAA rupture. In cases of rupture, CT demonstrates hematoma within the retroperitoneum, contiguous with the aorta. Often, when large, the hemorrhage is identified within multiple retroperitoneal compartments. CT findings that increase the specificity of the diagnosis of ruptured AAA include the “crescent” sign, which refers to hyperattenuating clefts within mural thrombus or the aneurysmal wall itself; a “draped” aorta, which refers to displacement of the aorta onto the spine with lateral “draping” over a vertebral body; discontinuity of the aortic wall; and frank active contrast extravasation. Ruptured AAA represents a surgical emergency that warrants rapid intervention. In cases of renal tumors, specific findings of angiomyolipoma may be identified, such as areas of macroscopic focal fat. However, these can become somewhat obscured by hemorrhage. Also, in cases of renal malignancy, the underlying mass may become obscured and follow-up imaging by CT and MRI may be necessary for identification and characterization of the mass lesion. The spatial resolution and multiplanar capabilities of the current generation of CT scanners offer the ability to accurately diagnose vascular abnormalities secondary to vasculitis, such as the multiple small aneurysms characteristic of PAN. The common causes of spontaneous hemorrhage specific to the pancreas include hemorrhage related to pseudocysts that cause pseudoaneurysm formation in the surrounding arteries secondary to exposure to the various pancreatic enzymes. Other causes include pancreatic malignancies, a rare etiology.