DOI: 10.1148/radiol.2403050818
(Radiology 2006;240:623-638.)
© RSNA, 2006
CT Imaging of Colitis1
Ruedi F. Thoeni, MD and
John P. Cello, MD
1 From the Departments of Radiology (R.F.T.) and Medicine (J.P.C.), University of California San Francisco, 505 Parnassus Ave, PO Box 0628, San Francisco, CA 94143-0628; and Departments of Radiology (R.F.T.) and Medicine (J.P.C.), Division of Gastroenterology, San Francisco General Hospital, San Francisco, Calif. Received May 13, 2005; revision requested July 12; revision received July 26; accepted September 6; final version accepted October 4; final review and update by R.F.T. March 3, 2006.
Address correspondence to R.F.T. (e-mail: Ruedi.Thoeni{at}radiology.ucsf.edu).
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ABSTRACT
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Computed tomography (CT) is widely used to assess patients with nonspecific abdominal pain or who are suspected of having colitis. The authors recommend multidetector CT with oral, rectal, and intravenous contrast material and thin sections, which can accurately demonstrate inflammatory changes in the colonic wall and help assess the extent of disease. In most cases, the final diagnosis of the type of colitis is based on clinical and laboratory data and colonoscopic and biopsy findings, but specific CT features help narrow the differential diagnosis. Ulcerative colitis is distinguished from granulomatous colitis (Crohn disease) in terms of location of involvement, extent and appearance of colonic wall thickening, and type of complications. Ulcerative colitis and Crohn disease (granulomatous colitis) are rarely associated with ascites, which is often seen in infectious, ischemic, and pseudomembranous colitis. Pseudomembranous colitis also demonstrates marked wall thickening and, occasionally, skip areas but is associated with broad-spectrum antibiotic treatment or chemotherapy. Neutropenic colitis is characterized by right-sided colonic and ileal involvement, whereas ischemic colitis is characterized by vascular distribution pattern and history. Diverticulitis is a focal asymmetric process with fascial thickening and inflamed diverticula. Dilatation of a thick-walled appendix with increased enhancement and adjacent stranding suggests appendicitis, but inflammatory changes may extend to the cecum and terminal ileum. Epiploic appendagitis is a focal rim-enhancing area next to the colon, usually without any substantial colonic wall thickening.
© RSNA, 2006
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INTRODUCTION
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Computed tomography (CT) is almost universally accepted as the primary screening modality for the evaluation of patients suspected of having colonic disease. CT has the advantages of wide availability and ease of performance. Key benefits of CT over alternative modalities are that it not only accurately demonstrates the bowel wall but also outlines the pericolonic soft tissues and adjacent structures. CT cannot demonstrate subtle superficial mucosal changes revealed on barium studies, but it is a highly sensitive method for the detection of intramural disease and extraluminal extension of colonic disease. CT can help assess inflammatory conditions, as well as facilitate comprehensive diagnosis and staging of abdominal neoplasms. In patients with symptoms of intestinal disease, CT can be used to help both diagnose other gastrointestinal disorders and differentiate them from abdominal diseases not involving the gastrointestinal system.
Patients with idiopathic inflammatory bowel disease (ulcerative colitis and Crohn disease), infectious colitis, pseudomembranous colitis, typhlitis or neutropenic colitis, ischemic colitis, diverticulitis, appendicitis, or epiploic appendagitis frequently present with abdominal pain. In many cases, the nature of the abdominal pain is nonspecific, and CT is the initial radiologic test ordered for its evaluation. CT can also demonstrate complications and assist in management of these conditions. With multidetector CT, scan times are very short, sections as thin as 0.625–1.25 mm can be obtained, and contrast material boluses are optimized. Multidetector CT can be used to obtain multiple reformations in different planes on the basis of isotropic voxels. The superb imaging quality of these reformations provides improved visualization of the extent of inflammatory changes in the colon and facilitates surgical planning. The added benefit of these multiplanar reformations is particularly evident for accurate assessment of the cecum and the terminal ileum. Furthermore, these reformations can often reduce to a minimum the number of images required for viewing by the clinician. Therefore multidetector CT has a pivotal role in today's diagnostic work-up of patients suspected of having colitis, but clinical information must be factored into the final diagnosis.
This article will review the CT technique for examining the colon, describe normal anatomic features of the colon, and contrast the clinical and CT appearances of inflammatory conditions of the colon. Typical imaging features that allow differentiation among the various types of colitis will be highlighted. The listed differential diagnoses are not meant to be all-inclusive; rather, they are the statistically more common diagnoses.
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TECHNIQUE
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Many patients who ultimately receive a diagnosis of some type of colitis initially present with nonspecific symptoms. Therefore, routine CT of the abdomen and pelvis usually is performed and, after review of the initial scans, may be augmented by delayed or decubitus views for improved visualization of the small or large bowel.
At our institution, patients routinely drink 900–1150 mL of a mixture of 2.5% diluted sodium amidotrizoate and meglumine amidotrizoate (Gastrografin; Bracco Diagnostics, Princeton, NJ) 30–45 minutes before the examination. Transit time through the small bowel can be increased with oral administration of 10 mg of metoclopramide (Reglan; AH Robbins, Richmond, Va) with the first dose of oral contrast material (1). Water or a special neutral contrast material (VoLumen; E-Z-Em, Westbury, NY) does not interfere with CT colonography or three-dimensional CT angiography, and the additives in VoLumen permit optimal small-bowel distention (so-called CT enterography).
At our institution, when CT of the abdomen and pelvis is performed, especially for possible colitis, we routinely use 500–1000 mL of water or a positive contrast agent (diatrizoate sodium meglumine, Hypaque; Amersham Health, Princeton, NJ) as a rectal contrast material. Rectal contrast material distends rectum and colon and helps prevent confusion between collapsed bowel wall and mural thickening due to inflammation. At some institutions, positive oral contrast material is given the night before the examination as an alternative approach. Rectal water helps better demonstrate the colonic wall (Fig 1) and its abnormalities and is our preference in all cases except when a colonic perforation or postoperative leak is suspected.
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Figure 1: Transverse CT image of normal rectosigmoid colon in a 45-year-old man. The wall of the rectosigmoid colon (arrows) is enhanced, and the colon is well distended with water.
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In addition, 150 mL of nonionic iodinated contrast material (iohexol, Omnipaque; Amersham Health) is administered intravenously at a rate of 3 mL/sec through a 20-gauge intravenous catheter, and multidetector CT is performed after an 80-second delay. In most cases, intravenous contrast material is critical for enabling detection of subtle bowel wall abnormalities that otherwise might not be visible; the portal venous phase usually is sufficient for demonstrating occlusion of mesenteric arteries and is excellent for depicting mesenteric veins. If the patient has known Crohn disease, it may be beneficial to scan the patient with a 45-second delay ("enteric" phase) rather than in the portal venous phase to optimize mural enhancement in small and large bowel.
Because of the subtlety of many bowel wall changes, it is necessary to use thin-section, high-volume, rapid-bolus scanning techniques with state-of-the-art CT technology. For a 16–detector row CT scanner, scanning is recommended with a detector configuration of 16 sections at 0.625-mm section thickness and a table feed of 17.5 mm per rotation. For a 64–detector row CT scanner, 64 sections at 0.625 mm thickness can be obtained, and, for the same coverage, the total scan time is markedly reduced (to <5 seconds). The reconstruction section thickness and interval are both 5 mm, except for multiplanar reformations and three-dimensional CT colonography, for which a section thickness of 0.65–1.25 mm is desirable. Scanning should extend from the diaphragm to the symphysis pubis. Multiplanar reformations are helpful for demonstrating the extent of disease and provide detailed information on complications such as strictures, fistulae, and the exact location of small- or large-bowel obstruction. Delayed scans and/or decubitus views also can help clarify subtle bowel findings such as colonic leak or perforation, pneumatosis, fistulae, and sinus tracts.
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NORMAL COLON
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The normal transverse diameter of the colon varies greatly. The diameter of the cecum measures up to 9 cm, whereas that of the transverse colon is usually less than 6 cm. The diameter of the remainder of the colon is less than that of the transverse colon, but the caliber of the rectum can vary greatly among individuals.
The thickness of the colonic wall is best demonstrated if the colon is well distended and filled with air or water (Fig 1). In general, the normal colonic wall thickness should amount to no more than 3 mm (2). When the lumen of the bowel is distended, the normal bowel wall thickness is 1–2 mm; when the lumen is collapsed, the normal thickness can reach 3–4 mm.
The colon is located in the periphery of the abdomen and is well outlined by fat. Many variations of colonic position occur, depending on the length of the mesentery and the extent of incorporation into the retroperitoneal space. This is particularly true for the cecum. Redundancy of the colon can lead to anomalous location and, at times, to volvulus or obstruction. The sigmoid colon and cecum are most frequently affected by this complication. If the colon is located between the anterior abdominal wall and the liver or between the diaphragm and the liver, this anatomic variant is called the Chilaiditi sign. It usually is found incidentally and causes no symptoms. Occasionally the Chilaiditi syndrome develops and causes right upper quadrant pain, probably due to colonic distention, but symptoms from obstruction are rare (3,4). Alternatively, the colon may be located in spaces that normally are occupied by other structures. This can occur in patients with renal agenesis, a pelvic kidney, or after nephrectomy or other abdominal or pelvic surgery. The appendix is found in the retrocecal area in about 66% of patients and in the pelvis in 33%; on occasion, it can be located medial or even anterior to the cecum.
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IDIOPATHIC INFLAMMATORY BOWEL DISEASE
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Key Clinical Aspects
Both patients with Crohn disease and those with ulcerative colitis present with cramping abdominal pain, tenesmus (rectal urgency), and variable amounts of bright red rectal bleeding. Patients typically have frequent small-volume stools with a sense of incomplete evacuation. The age of onset for both Crohn disease and idiopathic ulcerative colitis is usually in patients in their early 20s, but late-age onset in some patients in their 60s or 70s may occasionally be confused clinically with ischemic colitis or colonic neoplasm.
Extraintestinal manifestations are common with Crohn disease and ulcerative colitis, particularly large-joint nondestructive arthritis, spondylitis, erythema nodosum, and pyoderma gangrenosum. In many instances, patients are treated for presumed hemorrhoidal disease for months before the clinical diagnosis of inflammatory bowel disease is suggested. Newer serologic tests are now available that have high sensitivity and specificity for inflammatory bowel disease. For ulcerative colitis, perinuclear antineutrophil cytoplasmic antibodies (pANCAs) are available; for Crohn disease, CD-specific anti–Saccharomyces cerivisiae (ASCA) and outer membrane porin to Escherichia coli (OmpC) antibodies are helpful in differentiating idiopathic inflammatory bowel disease from other bowel diseases.
Imaging Features
Many of the imaging features used to help diagnose inflammatory bowel disease on cross-sectional images have been derived from the appearance of the colon at barium enema examination (5). Barium studies and colonoscopy are better than CT for demonstrating superficial changes in the mucosa. CT usually does not depict early and superficial mucosal changes of inflammatory bowel disease. Often, the combination of these conventional morphologic features with CT attenuation patterns of the colonic wall and extracolonic manifestations enable a specific diagnosis, but clinical information and laboratory results must always be factored into the final diagnosis.
Despite considerable overlap between the CT features seen in Crohn disease and those seen in ulcerative colitis, some distinguishing signs can be detected (Table). Ulcerative colitis typically is left sided (Fig 2) or diffuse and only rarely is located on the right side alone. Ulcerative colitis may be associated with backwash ileitis that results in dilatation of the terminal ileum with a gaping ileocecal valve and, at times, a shaggy thin wall. Crohn disease demonstrates extensive involvement of the right colon and terminal ileum, which often is thick-walled and narrowed (string sign). In cases of marked stenosis of the ileocecal valve, the terminal ileum may show dilatation with thick walls (Fig 3). Diffuse colitis may occur in Crohn disease, but left-sided colonic involvement as the only manifestation of the disease is rare. In ulcerative colitis, wall thickening is symmetric and continuous; in granulomatous colitis, however, wall thickening is eccentric or asymmetric, with segmental involvement and skip areas. The asymmetric involvement in granulomatous colitis is typically seen along the mesenteric border, which can lead to pseudodiverticula along the antimesenteric border. These pseudodiverticula, or outpouchings, represent the uninvolved or less-involved areas of the bowel wall that are present opposite areas of scarring. In ulcerative colitis, the rectum always demonstrates inflammatory changes, unless the patient is being treated with enemas containing materials such as steroids or 5-aminosalicylic acid. In Crohn disease, however, the rectum may be spared. Rectal narrowing, thickening of the wall, and increased presacral space are typical in chronic ulcerative colitis and are well depicted at CT (Fig 4). These features result from the mural thickening, rectal narrowing, and proliferation of fat.
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Figure 2: Transverse CT image in a 34-year-old woman with ulcerative colitis. The wall of the sigmoid is thickened, and hyperemic mesenteric arteries (arrowheads) appear as bright dots next to the outer wall of the colon. This indicates active disease.
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Figure 3: Transverse CT image in a 39-year-old man with Crohn disease (granulomatous ileocolitis) shows thickening of the ileocecal tip (arrows) and marked thickening of the terminal ileum (arrowheads). The ileocecal valve is stenosed, as demonstrated by succus in the lumen and prestenotic dilatation.
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Figure 4: Transverse CT image in a 45-year-old man with chronic ulcerative colitis. Thickened rectal wall demonstrates the fat halo sign (arrowhead). Perirectal fat (arrows) is increased.
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Both types of idiopathic inflammatory bowel disease demonstrate bowel wall thickening, but the mean wall thickness in Crohn disease is usually greater (11 mm ± 5.1 [standard deviation]) than that in ulcerative colitis (7.8 mm ± 1.9) (6). The thickened bowel wall in inflammatory bowel disease often demonstrates stratification that consists of two or three thickened layers. Such patterns are best—and at times only—observed if intravenous contrast material is administered. Also, the use of rectal water can be of great help. The term halo sign (water halo and fat halo) is used to describe such an appearance on cross-sectional images and can refer to either the two- or the three-layered wall (7–9). Stratification with three layers also is referred to as the target sign.
The halo and target signs represent unequivocal evidence of bowel injury, often of an acute nature. These signs are nonspecific and can be seen in multiple colonic conditions, with the exception of malignancy. If the layering of the wall demonstrates the target sign, the layers consist of an outer ring of high attenuation (representing the muscularis propria), a middle ring of intermediate (Fig 5) or low attenuation (Fig 4), and an inner ring of high attenuation. The inner ring represents the inflamed mucosa. The middle ring is generally accepted as representing the submucosa. If it is of intermediate attenuation, it is edematous; if it is of low attenuation (in general, <–10 HU), it is infiltrated by fat (10).
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Figure 5: Transverse CT image in a 32-year-old woman with ulcerative colitis and bloody diarrhea demonstrates the double halo, or target, sign with inner (mucosa, arrow) and outer (muscularis propria, arrowhead) rings of high attenuation separated by a ring of low attenuation, which represents submucosa with edema. Little if any pericolonic stranding is seen.
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The water halo sign may be seen in both ulcerative and granulomatous colitis and represents acute disease. The fat halo sign represents chronic disease and is seen more often in ulcerative colitis than in Crohn disease (11). If the fat halo sign is observed in Crohn disease, it usually involves the small bowel; the large bowel is rarely involved, except for the right colon. Occasionally, the fat halo sign can be seen in patients who have undergone radiation or cytoreductive therapy. Intramural fat also may occur in the distal ileum and colon of patients without gastrointestinal disease, but the intramural fat layer is usually much thinner than that seen in inflammatory bowel disease (11). In Crohn disease, stratification of the bowel wall suggests active disease, whereas homogeneous enhancement suggests quiescence (12).
Small-bowel mural attenuation is highly correlated with disease activity, and active inflammation can be measured by quantifying mural enhancement on the basis of CT enterography findings (13). Once transmural fibrosis has developed, mural stratification no longer can be demonstrated, and the changes are not reversible because antiinflammatory drugs will not appreciably alter the thickness of the involved bowel segment.
In patients with idiopathic inflammatory bowel disease, CT is uniquely suited to help assess pericolonic involvement and complications. Pericolonic involvement is not as extensive in ulcerative colitis as in Crohn disease. In particular, ascites, which is often present in infectious types of colitis, is usually absent in inflammatory bowel disease. In ulcerative colitis, proliferation of perirectal fat is frequently present in chronic disease. This fat has slightly increased attenuation (10–20 HU) compared with normal mesenteric fat (–55 to –75 HU) and is traversed by nodular and streaky areas of soft-tissue attenuation. In Crohn disease, "creeping fat" or fibrofatty proliferation, represents an attempt by the body to contain the inflammatory process and results in separation of small-bowel loops (14). CT demonstrates an increase in fat in the mesentery that is of higher attenuation (20–60 HU) than subcutaneous fat because of edema and inflammatory cell infiltrates (Fig 6) (7). The sharp interface between the bowel wall and the mesentery is lost, and mesenteric lymph nodes measuring less than 1 cm (range, 3–8 mm) are usually present.
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Figure 6: Transverse CT image in a 24-year-old woman with Crohn ileocolitis shows thickening of terminal ileum and cecum (white arrows) with fibrofatty proliferation (arrowheads) in right lower quadrant. One enlarged lymph node (black arrow) is also depicted.
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Because the risks of carcinoma and lymphoma are increased in patients with Crohn disease, lymph nodes larger than 1 cm should be viewed with suspicion, and malignancy must be excluded (15). In inflammatory bowel disease, mesenteric vessels appear engorged (Fig 2), widely spaced, and, at times, tortuous as a result of hyperemia from the inflammatory process. The presence of engorged mesenteric vessels suggests active disease (16). The appearance of the mesenteric vessels in Crohn disease has been described as the comb sign, but this sign is not specific for active Crohn disease; it can be seen in any moderate to severe acute inflammatory condition of the small or large bowel. The comb sign may be used to differentiate active inflammatory bowel disease from lymphoma or metastatic carcinoma, which tend to be hypovascular (17).
Accurate identification of complications is important, because complications influence treatment and prognosis (18,19). Toxic megacolon can be seen in both types of inflammatory bowel disease and in infectious colitis, as well as in some other types of colitis. It is one of most feared complications. CT typically demonstrates marked colonic dilatation with intraluminal air and/or fluid and a distorted luminal colonic contour or an ahaustral pattern (Fig 7). The colonic wall often appears thinned, with an ill-defined and nodular inner margin, and some ascites may also be present. In severe ulcerative colitis, persistent colonic distention is predictive with regard to a subgroup of patients who will demonstrate a poor response to medical therapy and who are at higher risk for toxic megacolon and will require surgery (20). Even though CT can readily depict all these abnormalities, its findings cannot be used to predict clinical outcome unless complications are already evident, such as perforation, pneumatosis, or septic thrombosis (21). In patients with pancolitis or long-standing ulcerative colitis, the risk for colon cancer is markedly increased. Such a neoplasm can be visualized on CT images as an apple-core–like or plaquelike lesion.
In Crohn disease, a phlegmon may develop in the mesentery or omentum and is another common cause of separation of bowel loops. A phlegmon is an ill-defined inflammatory mass that either resolves completely with antibiotic treatment or progresses to an abscess. When a phlegmon is present, CT shows a soft-tissue mass with surrounding streaky areas of soft-tissue attenuation extending into the mesentery or omentum; the definition of surrounding organs is lost.
Abscesses are most frequently associated with small-bowel disease or ileocolitis and occur in 15%–20% of patients with Crohn disease (22,23). They usually have an enhancing "rind" with a low-attenuation center and may contain an air-fluid level, scattered internal gas bubbles, and internal septa. An abscess may extend into adjacent tissues or perforate and drain spontaneously into other bowel loops, muscles or organs (24,25). Abscesses usually are a result of a sinus tract, fistula, perforation, or surgical intervention in Crohn disease. At times, an abscess can be difficult to diagnose clinically because it may be masked by steroid therapy or misdiagnosed as an exacerbation of the underlying inflammatory bowel disease. CT demonstrates the exact location and extent of an abscess and, in the pelvis, can help determine whether a perianal abscess extends to or through the levator ani muscles (26). While barium studies can show indirect signs of an abscess on the basis of mass effect, spiculation, or demonstration of a fistula tract, unlike CT they cannot demonstrate the extent of the disease. CT permits appropriate planning of therapy, including the percutaneous drainage of these collections if larger than 3 cm in diameter (27).
Fistulas, which are a frequent complication of Crohn disease, can be well depicted at multidetector CT. Thin sections and coronal and/or oblique reformations are helpful in outlining the course of these fluid- and/or air-filled tracts that may have an enhancing rim. Enteroenteric, enterocolonic, enterovesical, enterocutaneous, perianal, and rectovaginal fistulae can be demonstrated readily (Fig 8). Positive contrast material can easily be detected in an enterocutaneous or perianal fistulous tract. In the case of a possible enterovesical or rectovaginal fistula, it is helpful to obtain the study with positive oral and rectal contrast material but no intravenous contrast material. In this fashion, positive contrast material in the bladder or vagina must have come from the intestinal tract. If necessary, the fistulous tract can be catheterized and its connection to the intestinal tract confirmed when unenhanced CT is performed.
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Figure 8a: CT images in a 37-year-old woman with Crohn disease (ileocolitis). (a) Transverse scan obtained following enteroclysis demonstrates ileoileal (arrow) and ileosigmoidal fistulae (arrowhead). The involved ileal and colonic wall is asymmetrically thickened. (b) Coronal reformation shows ileoileal fistula (arrow) quite well. Contrast enhancement in vagina (arrowheads) is due to an ileovaginal fistula. (Images courtesy of John Lappas, MD, University of Indiana.)
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Figure 8b: CT images in a 37-year-old woman with Crohn disease (ileocolitis). (a) Transverse scan obtained following enteroclysis demonstrates ileoileal (arrow) and ileosigmoidal fistulae (arrowhead). The involved ileal and colonic wall is asymmetrically thickened. (b) Coronal reformation shows ileoileal fistula (arrow) quite well. Contrast enhancement in vagina (arrowheads) is due to an ileovaginal fistula. (Images courtesy of John Lappas, MD, University of Indiana.)
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While fistulas are most frequently encountered in Crohn disease, actinomycosis, tuberculosis, histoplasmosis, and strongyloidiasis that involve the intestinal tract also can manifest with fistulae and must be considered in the differential diagnosis.
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INFECTIOUS COLITIS
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There is considerable overlap in the appearance of infectious types of colitis. They usually are confirmed clinically on the basis of stool analysis and/or colonoscopic imaging and biopsy results, but they may be detected incidentally with the aid of CT or because the symptoms are not clear cut. All these infectious types of colitis share the CT features of wall thickening, pericolonic stranding, and various degrees of ascites (28). Most cases of infectious colitis can manifest as pancolitis, some are more often limited to the right colon, including or excluding the ileum, such as colitis caused by salmonella, Yersinia, tuberculosis, and amebiasis. Other cases are predominantly left-sided, such as those caused by schistosomiasis, shigellosis, herpes, gonorrhea, syphilis, and lymphogranuloma venereum. Still others may be predominantly diffuse, such as those caused by cytomegalovirus and E coli. Since there is so much overlap in the CT appearance of these colitides, the diagnosis is often based on clinical history and the results of laboratory studies, but tuberculosis and amebiasis have sufficiently distinct features to warrant their individual discussion.
Tuberculosis
Key clinical aspects.—Colonic tuberculosis is usually acquired by ingesting contaminated milk products or, in a patient with pulmonary tuberculosis, by swallowing tracheobronchial secretions. The infection often involves the ileocecal valve area, although some patients have more distal segments of colon involved. The lesions tend to be transmural and intensely desmoplastic with a large amount of fibrous tissue. The inflammatory reaction may produce a masslike lesion called a tuberculoma, which has the appearance of a neoplasm. The lesions characteristically produce deep transverse ulceration of an irregular or "geographic" contour. Occasionally, enteroenteric fistulae or mixed tuberculous and bacterial abscesses are encountered. The diagnosis is established by culturing colonic biopsy specimens and acid-fast bacilli staining.
Imaging findings.—Abdominal tuberculosis is a diagnostic challenge, particularly when pulmonary tuberculosis is absent. It may mimic many other abdominal diseases, both clinically and radiologically. An early correct diagnosis, however, is important to ensure proper treatment and a favorable outcome. CT is the imaging modality of choice for the detection and assessment of abdominal tuberculosis. Barium studies remain superior for demonstrating superficial mucosal intestinal lesions.
Tuberculosis of the gastrointestinal tract mimics many of the features seen in Crohn disease. Thickening of the wall of the colon and terminal ileum may be more prominent than in Crohn disease (Fig 9). On CT images, lymph nodes are markedly enlarged and often are of low attenuation or are calcified. Fistulae and sinus tracts can be seen (Fig 10) but are less common than in Crohn disease. Segmental colitis, diffuse colitis, and short strictures that mimic carcinoma may be seen (29,30). A cone-shaped cecum caused by scarring, as well as hypertrophy of the ileocecal valve (Fleischner sign), can be seen on rare occasions (29).
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Figure 9: Transverse CT image in a 33-year-old man with tuberculous ileocolitis who had recently emigrated from Southeast Asia. The cecal wall (arrows) and terminal ileum (arrowheads) are markedly thickened.
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Figure 10: Transverse CT image in a 60-year-old man with fistula formation due to tuberculous colitis. Descending colon appears slightly thickened, and a fistulous tract (white arrows) is shown to extend from the colon to an abscess (black arrow) in the left psoas muscle and through the left paraspinal muscles into the subcutaneous tissue of the back, where another abscess (arrowheads) has formed.
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Particularly in developing countries, if CT demonstrates peritoneal thickening, ascites, abdominal lymphadenopathy, and thickened intestinal walls, a diagnosis of abdominal tuberculosis should be considered (31). Endoscopic and sometimes laparoscopic specimens are needed for a definitive diagnosis, which is based on the presence of caseating granulomas or positive cultures for acid-fast bacillus. Correct identification of gastrointestinal tuberculosis is important because administration of steroids on the basis of a mistaken diagnosis of idiopathic inflammatory bowel disease can have disastrous consequences.
Differential diagnosis.—The most important differential diagnosis of tuberculous colitis is Crohn disease. The best diagnostic feature for distinguishing between tuberculosis and Crohn disease is the fact that in tuberculous colitis, separation of bowel loops is usually due to adenopathy, whereas in Crohn disease this change is usually caused by fibrofatty proliferation. In addition, in tuberculous colitis the nodes are larger and mural stratification is absent (32). Occasionally, a short thick-walled stricture seen on CT images can be mistaken for carcinoma, and colonoscopic biopsy is needed for proof.
Amebiasis
Key clinical aspects.—Entamoeba histolytica is a protozoon that is endemic in various parts of the tropical and subtropical world, including some areas of the United States. It may live in the large bowel in its cyst form without harming the host (commensalism) or, for as yet poorly understood reasons, invade tissue as a trophozoite, where it produces invasive amebiasis of the colon. Most patients with acute amebiasis have colonic diarrhea with bleeding (small volume of bloody stools). Occasionally, brisk colonic bleeding may occur. The initial lesions (the cecum is usually involved first, but the entire colon may be involved) are small punched-out ulcerations (2–5 mm in diameter) having a sharp cookie-cut appearance. The base of the ulcerations is usually clean of debris, and the adjacent mucosa is normal. In rare instances, the inflammation may be chronic and an ameboma may form with a masslike appearance. The diagnosis can be established on the basis of stool culture results, serologic analysis, sigmoidoscopy, or a combination of the three.
Imaging findings.—Radiologically, amebiasis manifests as acute fulminant colitis with ulcerations and skip lesions (33,34). Diffuse colitis can occur, but the right colon (Fig 11a) and rectum tend to be most severely affected. With disease progression, the cecum may become cone-shaped. Even after treatment, residual deformity or strictures may remain. The terminal ileum is usually spared (Fig 11b), although on occasion the last few centimeters may be involved. Focal involvement of the appendix is rare, but amebiasis may mimic appendicitis clinically. Fulminant amebic colitis may progress to toxic megacolon and perforation. Amebomas, consisting of focal areas of pronounced granulation tissue formation, develop in 10% of cases (35) and are most common in the right colon. These amebomas can be confused with a neoplasm (36,37). Liver abscess is the most common complication of amebiasis and is well depicted on CT images.
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Figure 11a: Transverse CT images in a 31-year-old man with amebiasis who had a history of diarrhea and recent travel to North Africa. (a) Cecal wall is thickened (arrow), but terminal ileum (arrowheads) is not involved. (b) Hepatic flexure (arrows) also demonstrates marked thickening, but descending colon (arrowhead) appears normal.
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Figure 11b: Transverse CT images in a 31-year-old man with amebiasis who had a history of diarrhea and recent travel to North Africa. (a) Cecal wall is thickened (arrow), but terminal ileum (arrowheads) is not involved. (b) Hepatic flexure (arrows) also demonstrates marked thickening, but descending colon (arrowhead) appears normal.
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Differential diagnosis.—All types of colitis that produce ulcerative rectocolitis (coloproctitis) or cecitis need to be considered. The coned-shaped cecum, without involvement of the terminal ileum, is unique to amebiasis. Typhlitis or neutropenic colitis can mimic amebiasis, but the patient's history and laboratory test results can allow a clear distinction.
Pseudomembranous Colitis
Key clinical aspects.—Clostridium difficile is a ubiquitous Gram-positive organism associated with the use of broad-spectrum antibiotics. The organism itself does not invade the colonic mucosa, and the entire spectrum of injury is due to exotoxins A and B. Many individuals harbor the organism as a true commensal bacterium until the use of antibiotics permits overgrowth of the resistant C difficile. Virtually all antibiotics other than vancomycin have been associated with pseudomembranous colitis. Chemotherapeutic agents also can cause the condition. The patient with pseudomembranous colitis presents with diffuse watery diarrhea and abdominal cramps. The rectosigmoid colon is almost invariably involved, with 3–8-mm in diameter, creamy, white, elevated plaques or nodules. Histopathologic examination typically shows volcano-like eruptions of fibrin and leukocytes from mucosal crypts. While most patients have a superficial mucosal disease, on occasion the disease can progress to toxic megacolon with transmural injury. The definitive diagnosis is made with the aid of colonoscopy and stool cultures positive for C difficile toxin.
Imaging findings.—The radiologic diagnosis of pseudomembranous colitis is important because the lack of aggressive treatment may lead to increased morbidity. Mild cases may demonstrate changes on CT images that are barely detectable, but severe cases show a markedly thickened colonic wall with "thumbprinting," low attenuation from mucosal and submucosal edema, irregular mucosal contour with polypoid protrusions, pericolonic stranding, and ascites (Fig 12) (38–40). The appearance of the colon may resemble that of an accordion (41); this appearance is caused by trapping of positive contrast material between thickened haustral folds, but this sign is not specific (Fig 13). Owing to transmural inflammation, the colonic diameter is often enlarged. After administration of intravenous contrast material, the target sign may be seen with enhanced mucosa and serosa. The average wall thickness is 14.7 mm (range, 3–32 mm) (38).
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Figure 12: Transverse CT image in a 56-year-old man with pseudomembranous colitis who was undergoing antibiotic treatment for endocarditis. In the sigmoid colon, a shaggy thickened bowel wall with alternating areas of necrosis (arrows) and plaques is visible.
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Figure 13: Transverse CT image in a 25-year-old man with pseudomembranous colitis who was undergoing antibiotic treatment for sepsis. Extensive wall thickening throughout the colon is evident. The accordion sign (arrows) is seen in the transverse colon, and ascites (arrowheads) is also noted.
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Complications of untreated pseudomembranous colitis include toxic megacolon and intestinal perforation. Today, toxic megacolon is more often caused by pseudomembranous colitis than by ulcerative colitis. CT has a positive predictive value of 88% for the diagnosis of C difficile colitis and can help initiate (42) and monitor the response to medical treatment with vancomycin and metronidazole (43). Pseudomembranous colitis may manifest as pancolitis and be right sided, left sided traveling from the rectum, or even segmental (38,40).
Differential diagnosis.—Pseudomembranous colitis produces one of the most severe degrees of wall thickening among all types of colitis with the exception of Crohn disease. The accordion sign can also be seen in patients with ischemia, cirrhosis, and infectious types of colitis caused by cytomegalovirus, cryptosporidiosis, and salmonellosis (41). Pseudomembranous colitis also demonstrates skip areas that can further confuse the diagnosis, but the wall usually appears shaggier and more irregular than it does in Crohn disease. The presence of ascites in up to 35% of cases of pseudomembranous colitis is another feature that distinguishes it from Crohn disease (in which ascites is very rare) (44). Cytomegalovirus and ischemic colitis also may be associated with ascites and marked wall thickening, but the wall is usually not quite as thick as it is in pseudomembranous colitis. For definitive diagnosis, the clinical history and laboratory results must be considered.
Neutropenic Colitis
Key clinical aspects.—Neutropenic enterocolitis, also known as typhlitis, is encountered in patients with leukemia or other immunosuppressive conditions (eg, acquired immunodeficiency syndrome) and after transplantation or chemotherapy for malignancy. The mucosal changes are usually confined to the cecum and ascending colon, with characteristic diffuse hyperemia, edema, and superficial ulceration. Colonoscopic findings of neutropenic enterocolitis are difficult to distinguish from those of idiopathic ulcerative colitis or even infectious colitis, except that the changes are confined to the right colon in patients with neutropenic colitis while the rectum and sigmoid are usually involved in ulcerative colitis and infectious colitis. To prevent transmural necrosis and perforation, prompt diagnosis and supportive therapy with intensive broad-spectrum (including fungal) antibiotics and supplemental nutrition are necessary (45,46). Surgical resection is often needed for patients who develop these complications.
Imaging findings.—CT is the modality of choice in patients suspected of having typhlitis because there is a high risk of perforation during colonoscopy or barium enema. On conventional radiographs, ileocecal dilatation may be seen; on CT images, marked thickening of the cecum and ascending colon is present, and the terminal ileum also frequently demonstrates wall thickening (Fig 14) (47,48). The wall of the colon and small bowel is circumferentially thickened, and there is pericolonic stranding and fluid (Fig 15). Pneumatosis may also be identified. Sepsis, abscess formation, intramural perforation, intestinal necrosis, hemorrhage, or any combination of the above may occur in severe cases (49). CT can also be used to monitor the success of treatment by showing a decrease in the thickness of the colon wall or by demonstrating complications such as pneumatosis when there is bowel wall necrosis or pneumoperitoneum when there is a silent perforation.
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Figure 15: Transverse CT image in a 45-year-old male transplantation patient with neutropenic colitis shows marked wall thickening in ascending colon (arrow) associated with pericolonic stranding and ascites (arrowheads).
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Differential diagnosis.—Distinction of typhlitis from Crohn disease, appendicitis, or other infectious or ischemic changes that involve the terminal ileum, cecum, and ascending colon is difficult on the basis of CT appearance alone, but CT results can often suggest the correct diagnosis (46,50) if the clinical history is considered.
Ischemic Colitis
Key clinical aspects.—With the increase in the number of people in the general population over 70 years of age, ischemic colitis is now encountered more frequently. The pathophysiologic characteristic of ischemic colitis almost invariably is a sudden loss of blood flow due to myocardial infarction, arrhythmia, embolus, thrombosis, shock, or trauma. While the loss of blood flow sets the stage for ischemia, the colonic mucosal changes are due primarily to the sudden restoration of blood flow through vasculature, with "leaky capillaries" brought about by superoxides. The clinical findings can be catastrophic but in most instances are minimal, with mild abdominal pain and hemodynamically trivial rectal bleeding. The event associated with the loss of blood flow may be clinically silent. Lack of sufficient collateral flow and thrombotic and embolic events produce dramatic mucosal changes in "watershed" distributions such as the distal transverse colon and distal descending colon. In a low-flow states brought about by arrhythmia, the usual pattern of distribution is, classically, the sigmoid colon and descending colon, the distal transverse colon, or the ascending colon. The definitive diagnosis is based on colonoscopic findings. The classic colonoscopic features include focal submucosal hemorrhages, red-purple blebs, superficial ulcerations, necrotic ulcerations, and dusky purple mucosa. So dramatic are these findings that identification of them just proximal to a normal rectum is pathognomonic for ischemic colitis.
Imaging findings.—The extent and severity of the ischemic changes and their CT appearance vary with the underlying cause and may involve the celiac axis and the superior and/or inferior mesenteric artery. Therefore, involvement of the colon follows a vascular distribution pattern that can be readily recognized on CT images. Causes of critical reduction of blood flow to the bowel range from occlusions of mesenteric arteries or veins to complicated bowel obstruction and overdistention. The ischemic changes may be segmental or more diffuse. Watershed areas in the sigmoid colon near the rectosigmoid junction (Fig 16) and in the transverse colon near the splenic flexure (Fig 17) are particularly vulnerable to ischemia caused by hypovolemia.
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Figure 16: Transverse CT image in a 71-year-old man with ischemic colitis due to arrhythmia shows ascites and marked thickening of sigmoid colon associated with multiple large nodular defects, which are the CT analog of thumbprinting (arrows) on radiographs. Note that rectal wall (arrowheads) is normal.
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Figure 17: Transverse CT image in a 22-year-old woman with ischemic colitis after blunt abdominal trauma to right flank demonstrates marked thickening of hepatic flexure and right colon, with abrupt transition (arrows) between abnormal and normal wall in the transverse colon.
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In elderly patients, left-sided ischemic changes are due typically to hypoperfusion, whereas right-sided ischemic changes predominate in younger patients and are usually related to hemorrhagic shock due to penetrating or blunt trauma (51). Ischemic changes in the rectum are rare because of the excellent collateral blood supply. However, in elderly patients with atherosclerosis and precipitating factors such as a major illness or hemodynamic disturbance, the diagnosis of ischemic proctosigmoiditis should be considered when wall thickening confined to the rectum and sigmoid colon is seen in association with perirectal fat stranding (52,53). Ischemic changes in the colon may also be seen in cocaine users, including crack users, and may manifest in the right colon or in the rectosigmoid colon (54).
Radiation-induced changes often appear similar to ischemic changes but are localized to the radiation port, which is commonly in the pelvis, where rectal wall thickening with edema can be seen in the acute stage and increased intramural and perirectal fat can be seen in the chronic stage.
CT has emerged as the initial imaging method for assessing intestinal ischemia. In nontransmural ischemic colitis, bowel wall thickening, thumbprinting, and pericolonic stranding with or without ascites can be seen on CT images (55). In these cases, CT usually demonstrates the double halo or target sign. After reperfusion of the ischemic bowel wall, the sign may be produced by edema in the submucosa and appear as low attenuation or by hemorrhage and appear as high attenuation. If hemorrhage into the bowel wall occurs in patients with intestinal ischemia, the hematoma may produce localized wall thickening or a mass. Such findings may also be encountered in patients being treated with anticoagulation therapy or in patients with hemophilia where long segments of bowel may be involved. Colonic wall attenuation changes on CT images cannot be used to help diagnose or predict infarction (56). Bowel wall thickening is more pronounced if the bowel ischemia is caused by occlusion of the mesenteric vein than by occlusion of the mesenteric artery alone (57).
If there is total vascular occlusion without reperfusion (infarction), the colonic wall remains thin and unenhancing, associated with dilatation of the lumen (Fig 18). In these cases, CT may demonstrate a thrombus in the corresponding mesenteric vessel. If water is used as oral and rectal contrast material, a CT angiogram and multiplanar reformations can be obtained. High-quality thin-section multidetector CT images obtained in the portal venous phase are often sufficient to demonstrate occlusion of the arteries; at the same time, portal venous phase images are better than arterial phase images for assessment of mesenteric veins and colonic wall changes (57).
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Figure 18a: Ischemic colitis in an 81-year-old woman with myocardial infarction. (a) Anteroposterior CT scout view shows air (arrows) in wall of right colon and small- and large-bowel dilatation. (b) Transverse CT image demonstrates air (arrowheads) in wall of right colon, with lack of wall enhancement and pericolonic stranding indicative of infarction.
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Figure 18b: Ischemic colitis in an 81-year-old woman with myocardial infarction. (a) Anteroposterior CT scout view shows air (arrows) in wall of right colon and small- and large-bowel dilatation. (b) Transverse CT image demonstrates air (arrowheads) in wall of right colon, with lack of wall enhancement and pericolonic stranding indicative of infarction.
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If the ischemia is transmural, strictures may form. Occasionally, a toxic megacolon develops. Pneumatosis and/or venous gas are ominous signs when associated with bowel wall thickening and are due to bowel infarction (Fig 18). Pneumatosis coli or pneumatosis intestinalis can be diagnosed by demonstrating air bubbles in the colonic or intestinal wall. The gas bubbles are arranged in a linear fashion and are best visualized with the window settings for bone or lung.
Differential diagnosis.—For accurate diagnosis of ischemic colitis, the patient's clinical history is critical and the vascular distribution pattern is very helpful. Depending on involvement of the colon, infectious or idiopathic pancolitis and right- or left-sided colitis need to be considered, but absence of involvement of the rectum favors ischemia when coupled with an appropriate history, except in elderly patients with atherosclerosis and identifiable risk factors. Pneumatosis can be a benign condition associated with chronic obstructive pulmonary disease, steroid treatment, or scleroderma, but it may also be associated with severe colitis or graft-versus-host disease. Pneumatosis coli or intestinalis can also be seen in patients with infectious colitis or after radiation therapy or chemotherapy or in patients with acquired immunodeficiency syndrome (58).
Diverticulitis
Key clinical aspects.—Diverticulitis is a common condition that particularly affects the elderly population but may be seen in younger patients, occasionally even in those younger than 40 years of age. Acquired colonic diverticula usually form at the site of nutrient arteries, the vasa recta, that extend into the submucosa, because these locations are the weakest areas of the colonic wall. They therefore occur on the mesenteric side and may be related to increased intramural pressure. The prevalence of diverticula increases dramatically in the descending and sigmoid colon, but diverticula may form anywhere in the colon. These structures are actually pseudodiverticula because they do not contain all the layers of the true bowel wall.
In pathophysiologic terms, diverticulitis occurs as a consequence of microperforation at the tip of a diverticulum, usually a left-sided colonic diverticulum. Diverticulitis is characterized clinically by abdominal pain, cramping, low-grade fever, and altered stool habits, including both diarrhea and constipation. Diverticulitis is rarely associated with gross bleeding or iron-deficiency anemia. Today, the diagnosis of diverticulitis usually is based on CT findings.
Imaging findings.—On CT images, diverticula appear as small outpouchings of the colonic wall that usually are filled with air. They are most abundant in the sigmoid colon but may occur anywhere except the rectum. In many cases of colonic diverticulosis, muscular hypertrophy causes the involved segment to appear thickened. Stool, food particles, or inflammation may occlude the diverticulum and cause microperforation of the diverticulum and the pericolonic inflammation that results in acute diverticulitis. CT is superior to barium enema in depicting early and subtle changes of diverticulitis, as well as complications. On CT images, diverticulitis appears as focal thickening of the colonic wall associated with pericolonic stranding (59). In sigmoid diverticulitis, fascial thickening close to the pelvic side wall is a reliable sign, even for early diverticulitis (Fig 19). Visibly engorged mesenteric vessels due to hyperemia supply the inflamed segment. In 30% of cases of diverticulitis, an inflamed diverticulum can be visualized (Fig 20) (59).
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Figure 19: Transverse CT image in a 65-year-old woman with early diverticulitis who had left lower quadrant pain and leukocytosis reveals wall thickening and multiple small diverticula (arrows) in a long segment of the sigmoid colon. Fascial thickening (arrowheads) along left pelvic side wall indicates mild diverticulitis.
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Figure 20: Transverse CT image in a 57-year-old woman with diverticulitis of sigmoid colon shows wall thickening (large arrows) in sigmoid colon and scattered diverticula. An inflamed diverticulum (arrowheads) and mesenteric fluid (small arrow) also are seen.
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CT is particularly suited to depict complications of diverticulitis, such as abscess formation, fistula, and perforation. An abscess appears as a fluid-filled collection next to the colon that may contain air bubbles or an air-fluid level and/or debris of various degrees of soft-tissue attenuation. Inflammatory stranding of the pericolonic fat always surrounds this fluid collection. CT can be used for guidance of percutaneous drainage of an abscess large enough to be amenable to such a procedure and can thus help prevent emergency surgery.
A colovesical fistula is suspected in a patient with air in the bladder and a thickened bladder wall next to the inflamed colonic segment (Fig 21). If such a colovesical fistula is present, it may be helpful to administer positive rectal contrast material without intravenous contrast material to obtain evidence that the high-attenuation material originated from the colon. However, the scanning time with multidetector CT is so short that intravenous contrast material excreted by the kidney usually has not yet reached the bladder where it might create confusion.
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Figure 21: Transverse thin-section multidetector CT image in a 70-year-old man with diverticulitis and colovesical fistula shows eccentric wall thickening (arrows), pericolonic stranding, and air (arrowhead) in the bladder.
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Colovaginal fistula may occur in women as a complication of diverticulitis. It usually develops in women who have undergone hysterectomy. In these cases, positive rectal contrast material is seen in the vagina. If the patient complains of a foul-smelling vaginal discharge but the fistula is not demonstrated on CT images, postvoiding repeat scans of the lower pelvis may demonstrate such a fistula.
Pneumoperitoneum is not a common complication in diverticulitis, but contained microperforation may be visualized as small extraluminal pockets of air or extravasation of contrast material.
Differential diagnosis.—An important feature in the distinction of diverticulitis from other inflammatory conditions of the large bowel and from ischemia is the presence of diverticula. If diverticular disease is present in a patient with known Crohn disease, complications associated with diverticulitis are more frequent, such as abscess formation or intramural fistulae. If diverticulitis involves the right colon, it may mimic appendicitis or other inflammatory conditions that predominantly involve the right colon. If a colovesical fistula is present in a patient with diverticulitis, the fistula tends to form in the left (often posterior) wall of the bladder, since it typically arises from the sigmoid colon. In contrast, the fistula in a patient with Crohn disease usually originates from the terminal ileum and is located on the right anterior wall of the bladder.
The most important differential consideration in the diagnosis is the distinction between diverticulitis and colon cancer. Features that indicate diverticulitis are long-segment involvement (>10 cm), pericolonic stranding, engorged mesenteric vessels, and fluid in the mesentery (60). Features that indicate colon cancer are a focal concentric mass with overhanging shoulders and pericolonic nodes (61). However, because the distinction between a benign and a malignant process cannot always be made on CT images, it is important to recommend colonoscopy as a follow-up examination in all such patients to prevent missing a colon cancer with microperforation.
Appendicitis
Key clinical aspects.—A patient with acute appendicitis usually presents clinically with right lower quadrant pain, obstipation, low-grade fever, and paraumbilical pain, but the variability in clinical presentation is extraordinary. Patients may present with diarrhea, unexplained fever, signs and symptoms suggestive of small-bowel obstruction, or even sepsis. The patient's white blood cell count is usually elevated, and there may be a "shift" to more immature white cells in the peripheral blood. Older patients and/or those with diabetes, neuropathy, altered mental status, or inability to communicate frequently present without any abdominal pain. Given the size of the patient population with appendicitis, it is important to consider the possibility of appendicitis until or unless a normal appendix is visualized. CT or ultrasonography (US) helps establish the diagnosis in a patient with an elevated white blood cell count.
Imaging findings.—We routinely use CT as the modality of choice for diagnosing possible appendicitis, except in children and women of child-bearing age. In these latter patients, graded compression US is preferred as the initial test; only if the results of this test are inconclusive is CT performed by using a low-amperage technique (62,63). Reported accuracy rates for helical CT are 96%–100% (64,65).
There is continued debate about the best CT technique for aid in diagnosing appendicitis. There are controversies regarding (a) the use of a focused examination (CT of the pelvis only, usually starting below the lower pole of the right kidney) versus an unfocused examination (CT of the abdomen and pelvis) and (b) the use of oral contrast material alone versus rectal contrast material alone versus both oral and rectal contrast material. At our institution, we use oral, rectal, and intravenous contrast material and scan the entire abdomen and pelvis. We may use 2.5-mm-thick reconstructions and decubitus or delayed-scan series in the pelvis for improved visualization of the appendix and the surrounding small and large bowel. Scanning the entire abdomen and pelvis permits us to exclude conditions that could mimic appendicitis, such as Crohn disease or other infectious types of colitis that have a predilection for the right colon. The limited evaluation achieved with the focused CT protocol of the pelvis for possible appendicitis is the major reason that this approach is only rarely taken today. Intravenous contrast material is very helpful in the differential diagnosis of appendicitis, which is of particular importance in female patients.
The diameter of the normal appendix has been reported to measure 6 mm or smaller when filled with fluid and 8 mm or smaller when filled with air (66), but others have found that the collapsed appendix can measure up to 6 mm and the fluid- or air-filled appendix, up to 10 mm (67). The diagnosis of appendicitis cannot be made by relying on measurements of appendiceal diameter alone. On CT images, the diameter of an inflamed appendix is enlarged, with a thickened wall that demonstrates increased enhancement. These findings are most reliable for the diagnosis of appendicitis. The inflamed appendix may contain fluid, debris, and/or air. The most important secondary sign of appendicitis is inflammatory stranding in the fat surrounding the appendix, with a reported sensitivity of 100% and specificity of 80% (67).
Absence of periappendiceal inflammation does not exclude appendicitis. Pericecal inflammation in the absence of a visualized appendix is suggestive of appendicitis. This is not diagnostic, however, because other conditions in the right lower quadrant can manifest in a similar fashion, including Crohn disease, typhlitis, and right-sided diverticulitis.
One or more appendicoliths can be detected in 25%–40% of adult patients and in an even higher percentage of children, but the diagnosis of appendicitis is definitive only if other signs of appendicitis are present (68,69). Other ancillary signs that are helpful, particularly if the appendix is not well visualized, are thickening of the cecal tip, the so-called arrowhead sign, and the cecal bar sign (Fig 22). Cecal thickening is present when inflammation from the appendix spreads to the cecum, and the arrowhead sign is seen when contrast material in the cecal tip produces an arrowhead-shaped collection near the occluded appendiceal orifice (70). The cecal bar sign occurs when a curved strip of cecal wall thickening is seen between the cecal lumen and an appendicolith. Less useful signs are paracolic gutter fluid, extraluminal air, and phlegmon (67). Inflammation may also contiguously involve the terminal ileum (Fig 23).
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Figure 22: Transverse CT image in a 21-year-old man with right lower quadrant pain due to appendicitis shows an appendicolith (arrowhead) separated from cecal lumen by cecal wall thickening (cecal bar sign) (black arrow). Dilatation of inflamed appendix with air and debris (white arrow) is demonstrated.
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Figure 23: Transverse CT image in a 34-year-old man with acute right lower quadrant pain caused by appendicitis with perforation. Appendiceal lumen (short black arrow) is enlarged and shows wall enhancement. Mesenteric stranding (white arrow), free intraperitoneal air (arrowhead), and thickened terminal ileum (long black arrow) due to contiguous inflammation are seen.
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Appendicitis may be complicated by appendiceal perforation, periappendiceal abscess, small-bowel obstruction, and, rarely, liver abscess secondary to septic pyelophlebitis. In rare cases, perforation can appea
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ABSTRACT
INTRODUCTION
