(Radiographics. 2000;20:399-418.)
© RSNA, 2000
CT Evaluation of the Colon: Inflammatory Disease1
Karen M. Horton, MD,
Frank M. Corl, MS and
Elliot K. Fishman, MD
1 From the Department of Radiology, Johns Hopkins Medical Institutions, 601 N Caroline St, Baltimore, MD 21287. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received February 26, 1999; revision requested March 29 and received May 14; accepted May 17. Address reprint requests to E.K.F. (e-mail: efishman@jhmi.edu).
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Abstract
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Computed tomography (CT) is valuable for detection and characterization of many inflammatory conditions of the colon. At CT, a dilated, thickened appendix is suggestive of appendicitis. A 1–4-cm, oval, fatty pericolic lesion with surrounding mesenteric inflammation is diagnostic of epiploic appendagitis. The key to distinguishing diverticulitis from other inflammatory conditions of the colon is the presence of diverticula in the involved segment. In typhlitis, CT demonstrates cecal distention and circumferential thickening of the cecal wall, which may have low attenuation secondary to edema. In radiation colitis, the clinical history is the key to suggesting the diagnosis because the CT findings can be nonspecific. The location of the involved segment and the extent and appearance of wall thickening may help distinguish Crohn disease and ulcerative colitis. In ischemic colitis, CT typically demonstrates circumferential, symmetric wall thickening with fold enlargement. CT findings of graft-versus-host disease include small bowel and colonic wall thickening, which may result in luminal narrowing and separation of bowel loops. In infectious colitis, the site and thickness of colon affected may suggest a specific organism. The amount of wall thickening in pseudomembranous colitis is typically greater than in any other inflammatory disease of the colon except Crohn disease.
Index Terms: Appendicitis, 751.291 • Appendix epiploica • Colitis, 75.26 • Colon, CT, 75.1211, 75.12115, 75.12117 • Colon, diverticula, 75.273 • Colon, ischemia, 75.266 • Colon, radiation effect, 75.269 • Crohn disease, 75.262 • Graft-versus-host disease, 75.269 • Typhlitis, 75.269
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Introduction
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The usefulness of computed tomography (CT) in patients with suspected colonic disease has been well documented. A unique feature of CT is its ability to accurately demonstrate the bowel wall as well as the pericolic soft tissues and adjacent structures. Therefore, abdominal CT is a highly sensitive method for the detection of intramural disease as well as extraluminal extension of colonic diseases.
CT is particularly valuable for detection and characterization of many inflammatory conditions of the colon, including appendicitis, epiploic appendagitis, diverticulitis, typhlitis, radiation colitis, inflammatory bowel disease, ischemic colitis, graft-versus-host disease, infectious colitis, and pseudomembranous colitis. This article reviews the technique of colon CT and describes the CT features of the normal colon and inflammatory conditions that affect the colon, with emphasis on distinctive imaging patterns that may help radiologists distinguish specific diseases.
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Technique of Colon CT
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Routine abdominal CT is usually performed after oral and intravenous administration of contrast material. At our institution, the patient routinely drinks approximately 1,000–1,250 mL of a 3% solution of oral diatrizoate sodium meglumine (Hypaque; Nycomed Amersham, Princeton, NJ) 60–90 minutes before the CT study. If specific colonic disease is suspected, it is important to adequately opacify the entire colon. Therefore, oral contrast material can be administered the night before the study as well as just prior to the study. This process ensures that the contrast material has reached the colon and is essential for optimal visualization. In urgent cases or in patients in whom limited rectosigmoid disease is suspected, positive contrast agents such as diatrizoate sodium meglumine can be administered via the rectum. Typically, 500–1,000 mL of contrast material is necessary to fill the colon. A topogram should be obtained to confirm adequate colonic opacification before the start of the CT study. Administration of air or water through a rectal tube to distend the colon has also been reported to be helpful (1). Unlike positive contrast agents, air and water do not interfere with virtual colonoscopy or three-dimensional CT angiography.
Although administration of intravenous contrast material is not essential for diagnosis of many colonic conditions, it is often helpful, especially if extracolonic extension of disease is also suspected. We routinely administer 100–120 mL of iohexol (Omnipaque 350; Nycomed Amersham) at a rate of 2–3 mL/sec. Scanning is begun 50 seconds after the start of contrast material injection, a delay that corresponds to the portal venous phase of liver imaging.
The abdomen should be routinely imaged from the diaphragm to the symphysis pubis. When spiral CT is used, 5-mm collimation is usually adequate, with a table speed of 8 mm/sec. These settings correspond to a pitch of 1.6. Reconstruction of the data is typically performed at 5-mm intervals. If three-dimensional imaging of the colon is desired, 3-mm collimation is optimal, with a table speed of 5 mm/sec (pitch of 1.7) and 2-mm reconstruction.
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Normal Colon
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The colon can usually be distinguished from the small intestine on the basis of appearance, caliber, and location. The teniae are three longitudinal bands approximately 8 mm wide that run the length of the colon and are located on the dorsomedial, dorsolateral, and anterior walls (Fig 1). The teniae merge where the appendix joins the cecum and at the rectosigmoid junction. Haustra are prominent sacculations formed in the spaces between the teniae. The prominence of the haustra depends on the contraction of the teniae. The appendices epiploicae are small packets of fat that run along the teniae and vary in size according to the nutritional status of the individual.
The transverse diameter of the colon varies greatly. The cecum generally has the greatest diameter, which is usually less than 9 cm in normal individuals. The transverse colon is usually less than 6 cm in diameter, and the descending colon and sigmoid colon are usually slightly smaller in caliber. The caliber of the rectum can vary significantly in normal individuals. In patients with mechanical obstruction or adynamic ileus, the diameter of the affected colonic segment can increase dramatically.
The wall of the colon is very thin and should measure less than 3 mm (2). In fact, it should be barely perceptible if the colon is well distended with contrast material or air. Gas, feces, and minimal fluid are normally present in the colon.
The colon normally frames the abdomen and is surrounded by homogeneous fat. Variations in colon position are not uncommon and are often clinically significant. For example, there is significant variation in cecal position depending on the length of its mesentery and the extent of retroperitonealization. Such variation can result in cecal volvulus. In addition, the colon can be redundant, resulting in drooping of the transverse colon into the lower abdomen or extension of the sigmoid colon out of the pelvis into the lower abdomen. Redundancy can also predispose to volvulus and obstruction. The colon can vary in location and in some instances may be interposed between the liver and anterior abdominal wall or between the liver and diaphragm (Fig 2). Chilaiditi syndrome is an anatomic variant in which there is interposition of the hepatic flexure and transverse colon between the liver and right hemidiaphragm. This variant is usually an incidental finding and is often transient; it rarely causes symptoms from obstruction or volvulus (3). In another common variant, the colon occupies an empty renal fossa secondary to nephrectomy, renal agenesis, or pelvic kidney (Fig 3).
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Figure 3. Colon in empty renal fossa in a 55-year-old man who underwent nephrectomy for renal cell carcinoma. Contrast material-enhanced spiral CT scan shows the colon (C) and spleen (S) located in the left renal fossa.
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Inflammatory Conditions
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Appendicitis
Acute appendicitis is a common cause of right lower quadrant pain. It occurs when the appendiceal lumen becomes occluded, resulting in an accumulation of fluid, appendiceal dilatation, inflammation, ischemia, and eventually perforation with possible abscess formation (Fig 4). Reported accuracy rates of spiral CT for diagnosis of appendicitis are as high as 98% (4,5). Compression-grade ultrasonography (US) is also a reliable technique for diagnosis of acute appendicitis, especially in children and thin adults. However, comparative studies of US and CT in adults show CT to be more accurate (6). We routinely use CT as the initial imaging modality in adults with suspected appendicitis. We perform US initially in children and pregnant female patients.
The normal appendix can often be reliably identified on routine CT scans of the abdomen (5). It appears as a small, thin-walled, tubular structure arising from the cecum between the ileocecal valve and the cecal tip (Fig 5). Filling of the normal appendix with oral contrast material is variable and is more common after retrograde filling of the colon (7). The appendix lies anterior and inferior to the cecum, although a retrocecal appendix occurs in up to 65% of patients (8). Air is a common finding in the normal appendix and can be seen in over 50% of normal individuals (9) (Fig 5). The appendix can vary in length, measuring up to 20 cm. The normal appendix is surrounded by homogeneous low-attenuation fat.
There is continued debate in the literature regarding the optimal technique for CT examination of patients with suspected appendicitis. Some investigators advocate a dedicated, focused appendiceal examination, which is more cost-effective than a full abdominal and pelvic CT study (10); however, a dedicated, focused appendiceal examination may limit the ability to establish alternative diagnoses. Also, there is controversy regarding the need for oral, rectal, or intravenous contrast material (4). CT protocols for appendiceal imaging vary from institution to institution and also depend on the experience of the radiologist. At our institution, oral and intravenous contrast material is routinely administered. Rectal contrast material is administered in urgent cases. Regardless of the contrast material used, thin (5-mm) collimation is performed. We routinely scan the entire abdomen and pelvis to allow diagnosis of conditions such as Crohn disease or ovarian pathologic conditions, which can mimic appendicitis.
At CT, an abnormal appendix appears dilated (>6 mm in diameter) and unopacified with a thickened wall that may homogeneously enhance after administration of intravenous contrast material (Figs 6, 7). The appendix may be filled with fluid or debris. Air is a common finding in both the normal and the inflamed appendix and does not necessarily indicate a patent appendix (9). An appendicolith is detected in up to 25%–40% of cases (11). The presence of an appendicolith along with pericecal inflammation or a mass is considered diagnostic for appendicitis (Fig 8).
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Figure 6. Appendicitis. Contrast-enhanced spiral CT scan shows a dilated, fluid-filled appendix (arrows). Minimal inflammatory changes are also present in the adjacent mesenteric fat.
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Figure 7. Appendicitis in a patient with acute-onset right lower quadrant pain. Spiral CT scan obtained with oral and intravenous contrast material shows enlargement of the appendix, which is filled with fluid (arrows). Adjacent inflammatory stranding is also present. Intravenous contrast material can help define the thickened walls of the appendix.
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Figure 8. Appendicitis in a patient with acute-onset right lower quadrant pain. Nonenhanced CT scan shows moderate inflammatory changes in the right lower quadrant. An appendicolith is identified (arrow).
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In some patients with suspected appendicitis, a distended appendix cannot always be confidently visualized; changes in the adjacent cecal tip such as focal cecal apical thickening, the arrowhead sign, or a cecal bar may be present and may help confirm the diagnosis. Focal cecal apical thickening occurs when appendiceal inflammation spreads contiguously to involve the cecal tip (12). The arrowhead sign is also caused by contiguous spread of inflammation from the appendix to the cecum, resulting in a triangular space between the thickened walls of the appendix (12). This sign is noted in up to 30% of cases when contrast material is administered rectally (13). A cecal bar occurs when a curved soft-tissue bar is interposed between the cecal lumen and an appendicolith. This finding has been reported in up to 10%–15% of cases after rectal administration of contrast material (5,12).
A hallmark of acute appendicitis is a varying degree of inflammatory thickening in the fat surrounding the diseased appendix. Stranding of the pericecal fat has a reported sensitivity of 100%, with an 80% specificity (5). Pericecal inflammatory changes without definite identification of an abnormal appendix are suspicious for acute appendicitis but not diagnostic, because many other conditions such as Crohn disease or cecal diverticulitis may demonstrate inflammatory changes. However, a dilated, thickened appendix is suggestive of appendicitis even without pericolic inflammation. Less specific signs such as paracolic gutter fluid or phlegmon may also be present (5).
Perforation is a potential complication of appendicitis and appears as small pockets of extraluminal air or rarely pneumoperitoneum. An appendiceal abscess appears as a pericecal fluid collection that may contain air or necrotic debris and is surrounded by inflammatory changes (Fig 9). Less common complications include hepatic abscess or small bowel obstruction.
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Figure 9. Periappendiceal abscess in an 80-year-old woman with right lower quadrant pain and fever. Contrast-enhanced CT scan shows an inflammatory mass with an air-fluid level in the right lower quadrant (arrow) and associated inflammatory changes in the pericecal fat. A periappendiceal abscess was discovered at surgery.
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Epiploic Appendagitis
Epiploic appendagitis is a rare inflammatory and ischemic condition that results from torsion or spontaneous venous thrombosis of one of the appendices epiploicae (14). The result is ischemia or infarction of the appendix epiploica and localized inflammation. Patients may present with sudden, severe, focal abdominal pain, which may mimic other conditions such as appendicitis. If diagnosed noninvasively, epiploic appendagitis can be managed conservatively (15). However, the diagnosis is typically not made until surgery. In these cases, the inflamed appendage is ligated and resected.
At CT, a 1–4-cm, oval, fatty pericolic lesion with surrounding mesenteric inflammation is considered to be diagnostic of epiploic appendagitis (16) (Fig 10). Adjacent cecal wall thickening and compression may occur. Rarely, a central high-attenuation "dot" can be identified within the inflamed appendage; this finding corresponds to the thrombosed vein (17).
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Figure 10. Epiploic appendagitis in a 16-year-old girl with severe left lower quadrant pain. Contrast-enhanced spiral CT scan shows a 2.5-cm-diameter mass (straight arrow) with fat attenuation and mesenteric stranding. The mass is adjacent to the descending colon (curved arrow) and was thought to represent inflammation of the appendix epiploica on the basis of the CT findings. The patient was treated conservatively, and the pain resolved within 24 hours.
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Diverticulitis
Diverticulosis is a common condition in Western society, affecting 5%–10% of the population over 45 years of age and approximately 80% of those over 85 years of age (18). Diverticula can occur anywhere throughout the colon but are most common in the sigmoid. They represent small outpouchings of the colonic mucosa and submucosa through the muscular layers of the wall. The outpouchings occur mainly where the vessels pierce the muscularis, between the mesenteric and antimesenteric teniae. This relationship of the diverticula to the penetrating blood vessels explains the propensity of diverticula to bleed. Diverticula vary in size but usually range from 2–3 mm up to 2 cm (Fig 11). Giant sigmoid diverticula have been reported (19). At CT, diverticulosis appears as small, air-filled outpouchings of the colonic wall, most abundant in the sigmoid colon (Fig 12). The wall of the involved colonic segment may appear thickened due to muscular hypertrophy.
Acute diverticulitis occurs when the neck of a diverticulum is occluded by stool, inflammation, or food particles, resulting in a microperforation of the diverticulum and surrounding pericolic inflammation. CT is well suited to evaluation of diverticular disease because it is able to demonstrate the wall of the colon as well as the surrounding pericolic fat. For imaging suspected diverticulitis and associated complications, careful attention to technique is necessary. We routinely use 5-mm collimation with an 8-mm/sec table speed (pitch of 1.6) and 5-mm reconstruction. Although oral contrast material can be helpful, diverticulitis typically affects the descending and sigmoid colon; therefore, scanning would need to be delayed 60–90 minutes after ingestion of oral contrast material. As an alternative, positive contrast material can be gently infused through a rectal tube to opacify the sigmoid and descending colon. We routinely administer intravenous contrast material (2 mL/sec) in patients with suspected diverticulitis because it aids in evaluation of complications such as diverticular abscess.
At CT, diverticulitis appears as segmental wall thickening and hyperemia with inflammatory changes in the pericolic fat (Fig 13). The key to distinguishing diverticulitis from other inflammatory conditions that affect the colon (eg, inflammatory bowel disease, ischemia) is the presence of diverticula in the involved segment. Also, diverticulitis typically occurs in the descending or sigmoid colon. However, diverticulitis can occur anywhere in the colon where diverticula are present. When diverticulitis involves the right colon, it may mimic other conditions such as appendicitis. In these cases, a careful search for diverticula in the right colon (on images from the present study or earlier studies [CT or barium enema]) will help suggest the diagnosis.
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Figure 13. Diverticulitis. CT scan obtained with oral and intravenous contrast material shows wall thickening in the sigmoid colon (arrows) with adjacent inflammatory changes in the pericolic fat.
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CT also allows detection of other complications of diverticulitis such as diverticular abscess, colovesical fistula, and perforation and is more sensitive than contrast enema examination (20,21). An abscess can be seen in up to 30% of cases and appears as a fluid collection with surrounding inflammatory changes (22) (Fig 14). The center of the collection may contain air or air-fluid levels or demonstrate low attenuation, which represents necrotic debris. When an abscess is detected, CT can provide guidance for percutaneous drainage, which can eliminate the need for emergent surgery, thus permitting elective single-stage resection after proper patient preparation (23).
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Figure 14. Diverticular abscess. CT scan obtained with oral and intravenous contrast material shows moderate wall thickening in the sigmoid colon (S) with significant adjacent inflammatory changes and stranding of the pericolic fat. A 3-cm-diameter fluid collection (*) is seen between the sigmoid colon and bladder (B), a finding compatible with a diverticular abscess.
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A colovesical fistula is suspected when air is seen in the bladder and there is thickening of the bladder wall adjacent to a diseased segment of bowel (usually the sigmoid) (20) (Fig 15). If a colovesical fistula is suspected, it is helpful to administer rectal contrast material without intravenous contrast material. Then, if positive contrast material is present within the bladder, it must have originated from the colon and the diagnosis of a colovesical fistula is confirmed. Crohn disease is another inflammatory condition that can result in an enterovesical fistula. However, in patients with Crohn disease, the fistula classically forms between the diseased terminal ileum and the bladder and will therefore be located on the right anterior surface of the bladder. Because diverticulitis typically involves the sigmoid colon, fistulas tend to occur in the left posterior portion of the bladder. In addition to demonstrating the presence and location of a fistula, CT can be helpful in planning subsequent surgery.
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Figure 15. Colovesical fistula. CT scan obtained with oral and intravenous contrast material shows moderate wall thickening in the sigmoid colon (S) with adjacent inflammatory changes and stranding of the pericolic fat. Focal wall thickening is seen in the left posterior part of the bladder adjacent to the inflamed sigmoid (arrow). A moderate amount of air is also present in the bladder, a finding compatible with a colovesical fistula. Small collections of retained barium are identified within diverticula.
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Focal contained perforations can also be a complication of diverticulitis and appear as small extraluminal pockets of air or extravasation of oral contrast material. Pneumoperitoneum is not a common finding in patients with diverticulitis and microperforations.
One potential pitfall of diagnosis of diverticulitis with CT is overlapping imaging findings in diverticulitis and colon cancer (Fig 16). In a study by Padidar et al (24) of 69 patients with sigmoid diverticulitis and 29 patients with sigmoid colon cancer, the presence of fluid in the root of the sigmoid mesentery and engorgement of adjacent sigmoid mesenteric vasculature favored the diagnosis of diverticulitis. Conversely, the presence of pericolic lymph nodes suggests the diagnosis of colon cancer rather than diverticulitis (25). However, in some cases it may not be possible to distinguish diverticulitis from colon cancer with CT alone, and histologic samples will be required.
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Figure 16a. Diverticulitis. CT scans obtained with oral contrast material show focal thickening of the sigmoid colon (straight arrow) with adjacent inflammatory changes in the pericolic fat. This appearance can be difficult to distinguish from that of colorectal cancer. However, the presence of mesenteric fluid (curved arrow in a) favors diverticulitis.
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Figure 16b. Diverticulitis. CT scans obtained with oral contrast material show focal thickening of the sigmoid colon (straight arrow) with adjacent inflammatory changes in the pericolic fat. This appearance can be difficult to distinguish from that of colorectal cancer. However, the presence of mesenteric fluid (curved arrow in a) favors diverticulitis.
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Typhlitis
Typhlitis, also known as neutropenic enterocolitis, occurs in neutropenic patients undergoing treatment for a malignancy, most frequently patients with acute leukemia who are receiving chemotherapy (26). However, typhlitis has also been reported in patients with aplastic anemia, lymphoma, or acquired immunodeficiency syndrome and after kidney transplantation (27). Patients present with fever, watery or bloody diarrhea, and abdominal pain, which may be localized to the right lower quadrant. Typhlitis is characterized by edema and inflammation of the cecum, the ascending colon, and sometimes the terminal ileum (Fig 17). The inflammation can be so severe that transmural necrosis, perforation, and death can result. The mechanism of the condition is not known, but it is probably due to a combination of ischemia, infection (especially with cytomegalovirus), mucosal hemorrhage, and perhaps neoplastic infiltration (27). Treatment consists of bowel rest, total parenteral nutrition, antibiotics, and aggressive fluid and electrolyte replacement. Resolution usually corresponds with adequate return of functioning neutrophils. Surgery is indicated in patients with uncontrollable gastrointestinal bleeding, obstruction, abscess, transmural necrosis, free intramural perforation, or uncontrolled sepsis. At surgery, all necrotic bowel tissue must be resected (28,29).
CT is the study of choice for diagnosis of typhlitis due to the risk of bowel perforation with colonoscopy or contrast enema examination. CT demonstrates cecal distention and circumferential thickening of the cecal wall (Fig 18), which may have low attenuation secondary to the edema (30). Inflammatory stranding of the adjacent mesenteric fat is a common finding. Detection of complications such as pneumatosis, pneumoperitoneum, and pericolic fluid collections is important because they indicate a need for urgent surgical management (29,30). CT is also helpful in assessing response to treatment and identifying patients in need of surgical resection (intramural air) (27).
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Figure 18. Typhlitis in a patient with acute myelogenous leukemia. Nonenhanced CT scan shows inflammation and marked thickening of the cecum (arrowheads), findings compatible with typhlitis. Minimal pericecal inflammation is present in the adjacent mesenteric fat. The descending colon (D) appears normal.
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Owing to the involvement of the cecum, differentiation of typhlitis from appendicitis or Crohn disease can be difficult on the basis of CT findings alone. However, the clinical presentation and history are usually distinctive.
Radiation Colitis
Radiation therapy can result in injury to the colon. Over one-half of patients receiving more than 3,000 cGy of radiation therapy to the pelvis will experience acute proctitis, which manifests as pain, diarrhea, tenesmus, and rectal bleeding (31). The proctitis is typically treated symptomatically, is self-limited, and does not require imaging.
Acute radiation injury to the small intestine and colon occurs during or within a few weeks of radiation exposure. Patients present with self-limited diarrhea. This form of acute radiation injury is usually recognized clinically and treated conservatively. Imaging is typically not necessary for diagnosis. If CT is performed during the acute phase of radiation injury, nonspecific wall thickening and inflammatory stranding will be demonstrated in the affected region, which is typically the rectosigmoid in patients who have undergone pelvic irradiation for prostate or cervical cancer. The CT appearance is nonspecific, but the clinical history will help suggest the cause of the colitis.
Chronic radiation injury of the colon and rectum can lead to a variety of complications, with most patients presenting 6–24 months after completion of radiation therapy (32). Such injury is the result of radiation-induced endarteritis. The sigmoid colon and rectum are the most commonly affected because radiation therapy is often given for pelvic disease (33). CT findings include nonspecific wall thickening (Fig 19), typically in the rectum (Fig 20); increased pelvic fat and thickening of the perirectal fibrous tissue are also seen (34). Strictures and fistulas are possible complications. The clinical history is the key to suggesting the diagnosis because the CT findings can be nonspecific.
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Figure 20. Radiation proctitis in a 38-year-old woman who underwent radiation therapy for cervical cancer. CT scan of the pelvis shows marked thickening of the cervix and vagina (curved arrow) with necrosis (open arrow), findings compatible with known locally invasive cervical cancer. Marked thickening of the rectum due to radiation colitis is also present (solid straight arrows).
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Inflammatory Bowel Disease
Although classic barium studies remain the principal tool for diagnosis and evaluation of suspected inflammatory bowel disease, CT can sometimes aid in differentiating Crohn disease and ulcerative colitis when results of barium studies are equivocal. In addition, although barium studies allow exquisite mucosal detail, they provide little information about extraluminal disease, which is often important for accurate diagnosis. CT has the advantage of allowing visualization of the bowel wall as well as adjacent structures and therefore plays an important role in detection of complications of inflammatory bowel disease.
There may be considerable overlap between the CT findings in Crohn disease and in ulcerative colitis. However, there are often certain features that may help distinguish the two. Extensive involvement of the right colon and small intestine is more common in Crohn disease (Fig 21), although involvement of the left colon does occur. In contrast, ulcerative colitis is typically left sided (Fig 22) or diffuse and only rarely involves the right colon exclusively (33). At CT, the most frequent finding in both Crohn disease and ulcerative colitis is wall thickening. The mean wall thickness in Crohn disease (11–13 mm) is usually greater than in ulcerative colitis (7.8 mm) (33,35). Wall thickening in ulcerative colitis may be diffuse and symmetric, whereas wall thickening in Crohn disease may be eccentric and segmental with skip regions. The asymmetry of the disease involvement, which typically occurs along the mesenteric border of the intestine, can result in the formation of pseudodiverticula along the antimesenteric border. Pseudodiverticula are small outpouchings of the colonic wall that occur opposite regions of fibrosis and scarring.
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Figure 21. Crohn disease. CT scan obtained with oral contrast material shows moderate thickening of the terminal ileum (curved arrow) and cecum (straight arrow) with adjacent inflammatory changes in the pericolic fat.
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Figure 22. Ulcerative colitis in a 27-year-old man. Contrast-enhanced CT scan shows minimal diffuse thickening of the sigmoid colon with minimal inflammatory stranding. The remainder of the colon was normal (not shown).
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The halo sign, a low-attenuation ring in the bowel wall due to deposition of submucosal fat, is seen more commonly in ulcerative colitis than in Crohn disease (Figs 23–25). In Crohn disease, the bowel wall tends to enhance homogeneously, although edema within the wall may result in low attenuation (33). Proliferation of mesenteric fat is seen almost exclusively in Crohn disease (Fig 24), whereas proliferation of perirectal fat is nonspecific and can be present in Crohn disease, ulcerative colitis, pseudomembranous colitis, or radiation colitis (Fig 25) (33). Mesenteric lymphadenopathy suggests Crohn disease rather than ulcerative colitis, although this finding is certainly not specific for inflammatory bowel disease.
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Figure 24. Crohn disease. CT scan obtained with oral contrast material shows low-attenuation submucosal fat in the ascending colon (arrow) and fibrofatty proliferation of adjacent mesenteric fat (*).
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Figure 25. Ulcerative colitis. CT scan of a patient with long-standing ulcerative colitis shows a submucosal halo of fat within the rectum (arrow). There is also perirectal fibrofatty proliferation (*).
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Complications of inflammatory bowel disease can be imaged with CT; in the case of Crohn disease, CT has been shown to affect disease management in 28% of cases (35). Abscesses are detected almost exclusively in Crohn disease and not in ulcerative colitis (33,36). An abscess can be confined to the bowel wall and pericolic fat or involve adjacent structures such as the bladder, psoas muscle, and pelvic sidewall (Fig 26). CT can also be used for guidance when draining intraabdominal abscesses, although we usually prefer US guidance when possible. US allows real-time imaging, is faster than CT guidance, and avoids unnecessary exposure to ionizing radiation.
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Figure 26. Crohn disease. CT scan obtained with oral and intravenous contrast material shows an abscess involving the iliacus muscle (black arrow) with a fistula to the anterior abdominal wall (white arrow).
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Fistulas can also be reliably detected with CT. Enterovesical, enterocutaneous, perianal, and rectovaginal fistulas have all been detected with CT. If an enterovesical fistula is suspected, it is often helpful to perform CT with oral or rectal contrast material but no intravenous contrast material. If positive contrast material is detected in the bladder, it must have originated from the intestine, thus confirming the presence of an enterovesical fistula. If intravenous contrast material is administered, positive contrast material can reach the bladder via the ureters or intestine. Other CT findings of enterovesical fistula include air in the bladder and focal bladder wall thickening adjacent to a diseased bowel loop. Enterocutaneous, perianal, and rectovaginal fistulas may be diagnosed by detecting oral or rectal contrast material within the actual fistulous tract. As an alternative, for greater sensitivity, positive contrast material can be injected into the fistula and its connection to the intestine can be determined. In our experience, this technique is usually more successful when performed with real-time fluoroscopy rather than CT, since the technique usually requires probing and repositioning of the catheter during injection. However, in difficult cases, the fistulous tract can be injected under fluoroscopic guidance and nonenhanced CT can then be performed, if necessary.
Ischemic Colitis
Ischemic colitis is a common cause of abdominal pain in the elderly. Most patients are over 70 years of age, and many have a history of heart disease (37). Ischemic colitis results when blood flow to the colon is compromised, usually as a result of hypoperfusion and vasospasm of the splanchnic arteries. Clinical situations associated with nonocclusive ischemic colitis include hemorrhagic or septic shock, heart failure with low cardiac output, and the use of pressor drugs such as digitalis (38). Colonic ischemia can also result from occlusion of the mesenteric vasculature by a thrombus, embolus, or invasive tumor. Both arterial and venous occlusion can result in colonic ischemia.
The extent and severity of the ischemia vary with its cause (hypoperfusion vs trauma vs thrombus) and the vessels involved (celiac artery, superior or inferior mesenteric artery). Therefore, ischemic colitis can be diffuse (Fig 27) or segmental (Fig 28). Watershed areas of the colon (the splenic flexure and rectosigmoid) are particularly susceptible to ischemia due to hypovolemia. These regions represent areas of relatively poor perfusion at the border of major vascular territories. Although left-sided involvement is typical in elderly patients with hypoperfusion, right-sided colonic ischemia and necrosis has been reported in young patients as a complication of hemorrhagic shock after blunt or penetrating trauma (38). In a study of 14 young adult patients by Ludwig et al (38), right-sided colonic ischemia and necrosis was observed after nonocclusive ischemia resulting from hypovolemic shock. The explanation for the right-sided predominance is not clear but may be related to inconsistent development of the marginal artery of the right colon or poor collateral blood supply to the right colon (39,40).
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Figure 27. Diffuse ischemic colitis. CT scan obtained with oral and intravenous contrast material shows diffuse, low-attenuation thickening of the colonic wall (arrows). This is an example of the halo sign.
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Radiologic assessment of potential ischemia traditionally consisted of plain radiography of the abdomen, barium studies, and angiography. However, with continued technologic advancement, CT is being used with increasing frequency in evaluation of patients with suspected colonic ischemia. In patients with colonic ischemia, CT typically demonstrates circumferential, symmetric wall thickening with fold enlargement (Fig 27). A double halo or target sign may also be present. The wall may demonstrate low attenuation due to edema or high attenuation, which indicates intramural hemorrhage. Inflammatory changes in the pericolic fat may also be present (41). In cases involving occlusive ischemia, CT can often demonstrate thrombus within the splanchnic vessels (Fig 29) or invasion of vessels by tumors such as pancreatic cancer.
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Figure 29a. Ischemic colitis in a 65-year-old man with abdominal pain after surgery for pancreatic cancer. Contrast-enhanced spiral CT scans show marked edema and thickening of the small bowel (SB) and colon (C) to the level of the splenic flexure, findings compatible with ischemia due to thrombus in the superior mesenteric vein (arrow in a). There is also moderate atherosclerosis of the aorta and ascites.
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Figure 29b. Ischemic colitis in a 65-year-old man with abdominal pain after surgery for pancreatic cancer. Contrast-enhanced spiral CT scans show marked edema and thickening of the small bowel (SB) and colon (C) to the level of the splenic flexure, findings compatible with ischemia due to thrombus in the superior mesenteric vein (arrow in a). There is also moderate atherosclerosis of the aorta and ascites.
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Pneumatosis with or without air in the mesenteric vessels or portal vein is an ominous finding in patients with colonic ischemia and suggests necrosis (Figs 30, 31). CT is more sensitive than plain radiography in detection of pneumatosis and sometimes allows identification of the cause (42). However, CT detection of pneumatosis coli is not pathognomonic for colonic ischemia and has been reported in benign conditions, including as a late manifestation in patients with acquired immunodeficiency syndrome (43). Thus, CT findings can only suggest the diagnosis of ischemic colitis in the appropriate clinical setting.
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Figure 31. Ischemic colitis in a critically ill patient. CT scan obtained with oral and intravenous contrast material shows pneumatosis involving the ascending colon and transverse colon (arrows), findings compatible with ischemia and infarction.
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Most cases of transient colonic ischemia due to hypovolemia are treated conservatively and result in full resolution. In such cases, the colon gradually returns to normal. However, some patients go on to develop scarring and stricture in the involved segment. In patients with severe ischemia and pneumatosis, urgent surgical resection is necessary to prevent progression to infarction and death.
Graft-versus-Host Disease
Graft-versus-host disease is a complication of allogeneic bone marrow transplantation that occurs when the donor lymphocytes in the graft mount an immunologic attack against the host. The skin, liver, and gastrointestinal tract (predominantly the ileum and colon) are the primary organs affected.
Findings at CT include small bowel and colonic wall thickening, which may result in luminal narrowing and separation of bowel loops (Fig 32). Prolonged coating of both the small intestine and colon with contrast material has been reported in patients with severe mucosal disease. In these cases, the barium can actually become incorporated into the bowel wall as the mucosal ulcers heal (44) (Figs 33, 34). This intramural dissection of barium is not pathognomonic for graft-versus-host disease because it has been described in other conditions that cause severe mucosal ulceration, such as ischemic colitis (45).
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Figure 32a. Graft-versus-host disease in a 35-year-old man after bone marrow transplantation. Contrast-enhanced spiral CT scans show diffuse thickening and edema of the small intestine (a) and colon (b). There is also mesenteric stranding and edema, which are compatible with inflammation.
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Figure 32b. Graft-versus-host disease in a 35-year-old man after bone marrow transplantation. Contrast-enhanced spiral CT scans show diffuse thickening and edema of the small intestine (a) and colon (b). There is also mesenteric stranding and edema, which are compatible with inflammation.
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Other CT findings include a halo of decreased attenuation within the wall, as well as inflammatory changes in the mesentery (46). Relative to processes like Crohn disease or radiation enteritis, the length of small bowel involvement in graft-versus-host disease is more extensive. Once again, the clinical history is most helpful in making the specific diagnosis.
Infectious Colitis
There are many causes of infectious colitis. Bacterial causes include Shigella, Salmonella, Yersinia, Campylobacter, Staphylococcus, and Chlamydia trachomatis. Fungal infections such as histoplasmosis, mucormycosis, and actinomycosis can involve the colon. Viral causes of colitis include herpesvirus, cytomegalovirus, and rotavirus. Amebiasis and tuberculosis can also cause a colitis, which can resemble inflammatory bowel disease. In general, the infectious colitides are typically diagnosed clinically and do not require CT for detection or differential diagnosis. However, they may be identified at CT incidentally or in cases in which the clinical symptoms are not straightforward.
At CT, patients with infectious colitis from any cause typically have wall thickening (Fig 35), which usually demonstrates homogeneous enhancement. Low attenuation representing edema may be detected within the wall. Ascites or inflammation of the pericolic fat may also be present (33). Multiple air-fluid levels may be present in the colon due to increased fluid and fluid feces. There is considerable overlap of the appearances of infectious colitis at CT, and laboratory studies are necessary for definitive diagnosis.
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Figure 35. Infectious colitis from Escherichia coli in a 52-year-old man with abdominal pain and severe bloody diarrhea. CT scan obtained with oral and intravenous contrast material shows moderate thickening of the colon (arrows) and inflammatory changes in the mesenteric fat. E coli was cultured from stool.
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The portion of colon affected may suggest a specific organism. For instance, most cases of infectious colitis are limited to the right colon (Shigella, Salmonella), although diffuse involvement also occurs (cytomegalovirus, E coli) (27). In contrast, gonorrhea, herpesvirus, and C trachomatis (lymphogranuloma venereum) typically involve the rectosigmoid. In schistosomiasis, involvement is usually confined to the descending and sigmoid colon because the adult worms have a tendency to enter the inferior mesenteric vein.
Pseudomembranous Colitis
Pseudomembranous colitis results from toxins produced by an overgrowth of the organism Clostridium difficile and results in a profuse watery diarrhea with abdominal pain and fever (47). Although first described as a complication of antibiotic therapy, pseudomembranous colitis has also been associated with hypotensive episodes, chemotherapeutic agents, and abdominal surgery.
At histologic analysis, the condition is characterized by pseudomembranes (Fig 36), which represent an exudate of necrotic cells from the denuded mucosa. The diagnosis is typically made with stool assay for the C difficile toxin, but the clinical presentation is often nonspecific. Radiologists should be familiar with the CT findings because the diagnosis may not be suspected clinically; if not treated aggressively, pseudomembranous colitis can result in significant morbidity and mortality (48,49). Treatment with metronidazole and vancomycin is usually effective. Some inpatients with a fulminant form of pseudomembranous colitis may not respond to medical therapy and therefore will require surgical intervention such as colectomy (47,50). However, a recent article by Kawamoto et al (51) suggests that CT findings alone do not allow reliable prediction of which patients with pseudomembranous colitis will require surgical intervention. In addition to suggesting the diagnosis of pseudomembranous colitis, the radiologist can evaluate the extent and severity of the disease and detect potential complications.
The most common (but nonspecific) CT finding in pseudomembranous colitis is thickening of the colonic wall, which may be circumferential or eccentric (Fig 37). In one series, the colonic wall thickness ranged from 3 to 32 mm (mean, 14.7 mm) (52). In another series, the mean colonic wall thickness was 10.7 mm (33). In general, the amount of bowel wall thickening in pseudomembranous colitis is greater than in any other inflammatory or infectious disease of the colon except Crohn disease; this is a helpful differential point (53). At CT, the wall thickening in pseudomembranous colitis is often more irregular and shaggy than in Crohn disease, in which it is usually symmetric and homogeneous (52). In patients with pseudomembranous colitis, the bowel wall may have low attenuation due to edema or may enhance significantly after intravenous administration of contrast material secondary to hyperemia (Fig 38).
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Figure 37. Pseudomembranous colitis. Spiral CT scan obtained with oral and intravenous contrast material shows marked wall thickening throughout the colon (thickness, 15 mm) and pericolic inflammation. The thickening in the transverse colon is asymmetric.
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Figure 38. Pseudomembranous colitis. Spiral CT scan obtained with oral and intravenous contrast material shows marked wall thickening throughout the colon. The wall is of low attenuation, which is compatible with edema or inflammation, and there is significant enhancement of the mucosa due to hyperemia. There is also moderate pericolic inflammation and ascites.
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In addition to wall thickening, the colon is often dilated, probably due to the transmural inflammation. Mild pericolic stranding may also be present. Although pericolic stranding is a nonspecific finding, occurring in many other inflammatory and infectious colonic diseases, the pericolic stranding in pseudomembranous colitis is often disproportionately mild relative to the marked colonic wall thickening, since the condition predominantly affects the mucosa and submucosa (54).
The target sign, originally described in ulcerative colitis and Crohn disease, has also been reported in pseudomembranous colitis. When haustral folds are significantly thickened, they can appear as broad transverse bands that may trap oral contrast material. This appearance is known as the accordion sign (52) (Fig 39). The accordion sign is very suggestive of pseudomembranous colitis but typically occurs only in severe cases and is therefore not a sensitive indicator.
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Figure 39. Accordion sign in a patient with pseudomembranous colitis. Spiral CT scan obtained with oral contrast material shows marked wall thickening throughout the colon. The colonic wall is so thick that only minimal contrast material can be seen sandwiched between the thick walls, creating the appearance of an accordion. There is also pericolic inflammation and ascites.
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In its classic form, pseudomembranous colitis is a pancolitis. However, in some cases, it may begin in the rectum and progress retrogradely to involve the left colon. It can also be limited to the right side of the colon with sparing of the left colon in up to 30%–40% of cases (53). Involvement of isolated segments of the colon and rectum has also been reported (52,53) (Fig 40). In addition, pseudomembranous enteritis has been reported in patients with ileostomies or defunctionalized loops of small intestine (55,56). Ascites has been reported in up to 35% of patients with pseudomembranous colitis, both as a direct complication of the infection and due to coexisting conditions such as portal hypertension (53,57). Thus, ascites may be a helpful differentiating point between pseudomembranous colitis and Crohn disease. However, ascites is not uncommon in oth
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Abstract
Introduction
