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Everything you need to know about Computed Tomography (CT) & CT Scanning

Colon: Non Neoplastic Disease: Crohn's Disease: CT Scanning in Clinical Decision Making


Karen M. Horton ,MD and Elliot K. Fishman, MD




Despite the availability and widespread use of endoscopy today, radiologic imaging studies continue to play a valuable role in the diagnosis and management of patients with Crohn’s disease. Contrast studies such as the upper gastrointestinal series, the small bowel series and the barium enema are still important diagnostic tools to evaluate patients with inflammatory bowel disease and allow excellent visualization of mucosal disease. These examinations, however, give only limited information about extraluminal extension of disease or extraintestinal manifestations.

Recent advancements and improvements in spiral CT technology have made CT a valuable and necessary adjunct to traditional contrast examinations in patients with Crohn’s disease. New spiral CT scanners allow faster data acquisition combined with more rapid contrast infusion and narrow collimation. The average CT examination of the abdomen and pelvis can be performed in less than 60 seconds! This speed and narrow collimation results in excellent evaluation of the entire gastrointestinal tract. Another distinct advantage of CT over contrast studies, is its ability to accurate image the image the wall of the gastrointestinal tract as well as extraluminal extension of disease and adjacent organs. This chapter will review the role of CT in the diagnosis and management of patients with Crohn’s disease.


CT Technique

Optimal CT imaging of the gastrointestinal tract requires attention to technique, adequate oral contrast, as well as intravenous contrast. Insufficient oral contrast can result in collapsed loops of normal bowel which can simulate disease or can result in misdiagnosis of diseased loops as normal undistended loops. We use the following protocol in patients with known or suspected inflammatory bowel disease.

Approximately 750-1000 cc of a 3% oral iodinated contrast solution is administered 60-90 minutes before the scan and an additional 250 cc of oral contrast immediately before the start of the scan. This should provide adequate contrast opacification of the stomach and small bowel. If esophageal pathology is suspected, Esoph-o-Cat paste (EZM Co, Westbury NY) can also be administered immediate prior to the scan to coat the esophagus. If colonic involvement is suspected, the colon should be adequately opacified. In outpatients, oral contrast can be administered the night before the study as well as prior to the scan to ensure that contrast opacifies the colon as well as the small bowel. Alternatively, in urgent cases, or in patients in whom limited colonic disease is suspected, a 3-4% iodinated contrast solution can be administered gently through a rectal tube immediately prior to the scan.

The administration of intravenous contrast is necessary for complete evaluation of patients with inflammatory bowel disease, especially if extraintestinal disease or complications such as abscess are suspected. We routinely administer 100-120 cc of Omnipaque 350 (Nycomed, Princeton, NJ) at a rate of 2-3 cc/sec. injected through a peripheral vein using a mechanical injector pump. Imaging should begin 45-50 seconds after initiation of contrast injection.

The abdomen and pelvis should be routinely imaged from the level of the diaphragm through the perineum. It is important that CT scanning extend through the perineum. In a series of patients by Yousem et al, perirectal-perianal abnormalities are demonstrated on CT in up to 37% of Crohn's patients. Using spiral CT, 5 mm collimation can be performed with a table speed of 8 mm/sec., with reconstruction every 5 mm. If necessary additional scan can be obtain through specific areas of concern utilizing thinner collimation (3mm). Also, multiplanar reconstructions or 3D imaging can be helpful problem solving tools in difficult. In a series of patients with Crohn’s disease by Raptopoulos et al, multiplanar reconstruction improved observer confidence and was found to be complementary and often superior to conventional barium studies.


Primary Disease

The normal wall thickness of a loop of distended small bowel or colon is less than 3 mm. When it is distended and opacified with contrast agent, the normal bowel is not definable on CT. The esophageal wall should measure less than 3 mm when distended. The normal gastric wall can measure up to 5-7 mm.

The earliest perceptible change on CT in patients Crohn’s disease is wall thickening, usually involving the distal small bowel and colon . Since contrast studies such as the small bowel series or enteroclysis are the most sensitive radiologic techniques for detecting the earliest mucosal changes of inflammatory bowel disease, CT is usually reserved for the patients with a known diagnosis in whom the extent of disease or the presence of complications is suspected. However, CT may be the first modality to suggest the disease in patients presenting with nonspecific complaints.

The typical bowel thickness of a segment involved with Crohn's disease is 5 to 15 mm. This bowel thickening is usually symmetric and diffuse and on thin-section CT ulcerations in the mucosal surface can sometimes be detected . Eccentric thickening or "skip" areas can be present. Although bowel wall thickening is a nonspecific finding which can occur in a variety of inflammatory and neoplastic diseases, the appearance and degree of wall thickening on CT can sometimes aid in diagnosis. For instance, in a study of colonic wall thickening on CT, Philpotts et al, noted the mean colon wall thickness in Crohn’s colitis was 13 mm and the appearance was homogeneous. This was significantly greater than in ulcerative colitis, where the mean wall thickness was 7.8 mm and the appearance heterogeneous. In addition to wall thickening, CT may demonstrate a low-density zone or halo within a thickened bowel loop, either small bowel or colon. Usually, the low-density zone is of soft tissue density, representing submucosal edema, or can be of fat density, due to submucosal fat deposition . This "halo" was originally thought to be specific for inflammatory bowel disease but can also be seen in radiation enteritis, intestinal ischemia, and typhlitis.

The second area commonly involved by Crohn's disease is the mesentery. The normal mesentery is homogeneous and has an attenuation similar to that of fat, measuring in the range of -90 and -130 Hounsfield units. The interface between the bowel and mesentery is usually sharply defined. The earliest inflammatory changes of Crohn's disease are detected by inflammatory stranding in the mesentery adjacent to diseased bowel loops . With more advanced disease, mesenteric inflammation can increase with or without the formation of abscess or phlegmon . Fistulae from bowel may extend into the mesentery and appear as linear tracts.

In patients with long-standing Crohn's disease, there may be increased fat around affected loops of large or small bowel. This increased fat can simulate a mass or abscess on small bowel series. The increased mesenteric or pericolonic fat seen on CT scan corresponds to the "creeping" mesenteric fat noted at surgery and pathologically . Increased mesenteric fat is specific for Crohn’s disease. However, increased perirectal fat can be seen in patients with ulcerative colitis as well.

Mesenteric adenopathy can be present in patients with Crohn’s disease. Usually the mesenteric nodes are small, ranging between 3 to 8 mm. Inflammatory nodes typically measure less than 1 cm in short axis. If mesenteric or retroperitoneal nodes measure greater than 1 cm in short axis, the possibility of superimposed malignancy (adenocarcinoma or lymphoma) should be considered, as there is an increased incidence of malignancy in patients with Crohn's disease.



Although CT may be the first modality to suggest the diagnosis of Crohn’s disease in patients presenting with nonspecific abdominal pain, the major clinical application of CT is in the definition of the extent of disease and in the detection of complications such as abscess and fistulae. At our institution, patients with known Crohn's disease are referred for CT when they experience a change in clinical course, such as increasing abdominal pain, diarrhea, or distention. CT is valuable in evaluation of these patients and can affect patient management in 28% of patients with symptomatic Crohn's disease. CT may reveal previously unexpected findings which subsequently lead to a change in medical or surgical management.


CT is valuable in patients with suspected small bowel or colonic obstruction and can frequently determine the cause of obstruction and whether there is evidence of strangulation or ischemia. In patients with Crohn's disease, small bowel obstruction can result from stricture, inflammatory masses, adhesions following surgical resections or rarely intussusception . CT is also helpful in distinguishing obstruction from ileus. A definite advantage of CT over barium studies is its ability to detect additional diseased segments distal to the site of obstruction.


Although contrast studies can define the presence of enteric fistulae, the extent and involvement of adjacent organs or structures can be difficult to define on standard radiographs. Additionally, a fistulous tract may be edematous and therefore may not fill with contrast. In this cases, the fistula may go undetected on conventional contrast studies, but can still be visualized on CT. Finally, the fistulous tracts may be in an area such as the perianal, perirectal or gluteal regions, which are technically difficult to evaluate on barium studies CT, on the other hand, has none of these limitations. Regardless of the location of the fistulous tract, CT examination can define its presence as well as define, by direct visualization, its true extent. Extension into muscle, viscera, spine, or bladder can all be easily detected on CT using a properly designed examination with careful attention to scanning techniques. On CT, one potential limitation is that although a fistulous tract may be detected, it is sometimes impossible to be certain whether the tracts are patent, unless contrast material actually opacifies the tract itself. In patients with enterocutaneous fistulae contrast can be injected through the skin opening before the CT scan to opacify the tract and to evaluate its course.

The perirectal and perianal regions are reportedly involved in up to 60 percent of patients with colonic involvement of Crohn's disease. The perirectal region is difficult to evaluate with any imaging modality other than CT. In a recent review we found that 81.5 percent of patients referred to us had perirectal and/or perianal disease. CT can provide very clear definition of the boundaries of the perirectal zone, allowing for the detection of even the smallest fistulous tract or perirectal abscess . Although most of these tracts extended only into the perirectal-perianal fat planes, others extended into adjacent viscera (e.g., prostate, vagina), muscle (obturator internus, gluteus maximus), or bone (hip joint). The full definition of the extent of fistulas is important in planning medical or surgical management.


The second major complication in the patient with Crohn's disease is the development of an abscess. The location of the abscess in patients with Crohn's disease can involve any of the major viscera, such as the liver and spleen, or be deep in the retroperitoneal , or pelvic cavities. Abscess can also invole adjacent mucles or bone . CT is ideally suited for evaluating all these regions in a single examination.

One of the more common areas for an abscess to develop is between loops of bowel, i.e., an interloop abscess. Interloop abscess is very difficult to diagnose on conventional contrast studies such as the small bowel series. CT with contrast opacification of bowel loops can help detect even small abscesses and define their relationship to bowel. A CT can also be used for planning the patient's therapy, such as planning surgical vs percutaneous drainage under CT guidance. Van Sonnenberg and colleagues have shown that CT can be particularly helpful in patients who are not surgical candidates or in delaying surgery until the patient is in a better medical and nutritional state.


Over the years, numerous studies have attempted to determine the relationship between Crohn's disease and the development of malignancies. Recent literature supports that there is an increased risk of small bowel adenocarcinoma, colorectal cancer and possibly cholangiocarcinomas in patients with Crohn's disease. Although screening patients with Crohn's disease is controversial at this time, it is clear that CT should play an important role in cancer detection and staging . Virtual colonoscopy, which is a 3D CT of the colon, is currently under investigation as a quick non invasive tool for screening patients at increased risk for colon cancer.


Extraintestinal Complications of Crohn's Disease

Extraintestinal complications in patients with Crohn’s disease can also be evaluated with CT. The most common areas of extraintestinal involvement include the liver and biliary tree, the urinary tract, and the musculoskeletal system.

Hepatobiliary Tract

The most common extraintestinal manifestation of Crohn's disease is fatty infiltration of the liver which can be due either to the patient's poor nutritional state or steroids used to treat the bowel disease. Fatty liver appears as diffuse low attenuation on CT . Pericholangitis or sclerosing cholangitis is rare but occurs with increased frequency in the patient with Crohn's disease. Primary bile duct carcinomas also occur with increased frequency but are very unusual. The presence of gallstones is common, particularly in patients following distal bowel resection or when there is extensive disease in the distal ileum. Although some gallstones can be visualized on CT, ultrasound is much more sensitive for evaluation of the gallbladder and for the detection of calculi.

Urinary Tract

The urinary tract can be involved in Crohn's disease. Renal calculi are not uncommon and are often of the oxalate type due to involvement of the terminal ileum and resultant problems of malabsorption. Thin collimation (3mm) noncontrast CT can be performed to detect renal calculi and obstructing ureteral stones in symptomatic patients. This quick, noncontrast scan often obviates the need for intravenous pyelography. At our institution, non contrast CT is currently the first modality performed in patients with suspected ureteral obstruction.

Since Crohn's disease often involves multiple loops of bowel with subsequent fibrosis and adhesions, the ureters can be involved, resulting in obstruction. The right ureter is typically involved more frequently than the left, since Crohn’s disease often occurs in the right lower quadrant. Finally, the bladder can be involved with thickening due to adjacent inflammation or in more severe cases to the development of enterovesical fistula. A carefully performed CT examination should be able to detect the presence, extent and etiology of the fistula. In addition to detecting the fistula, by defining its location and the affected zone of the bladder and bowel, surgical planning is facilitated. Of note, if an enterovesical fistula is suspected, intravenous contrast should not be administered. Then, if contrast is detected in the bladder, it must be oral contrast which entered the bladder though a enterovesical fistula.

In a series of 275 patients with Crohn's disease, 22 patients had bladder abnormalities, ten of which were enterovesical fistulas. In eight of the ten cases the fistulas were from small bowel to bladder, and in two cases they were from sigmoid colon to bladder. In the ten patients in whom other imaging studies were done (cystoscopy, intravenous pyelography, barium enema) all produced false-negative results. In our experience, fistulas between the small bowel and bladder in Crohn's disease usually occur anteriorly on the right side of the bladder. This is in contradistinction to the situation in patients with diverticulitis, in whom the involvement is usually posterior or posterior on the left side. This is one of the findings that helps us determine the exact cause of an enterovesical fistula. Crohn's disease is the second most common cause of enterovesical fistulas, the most common being diverticulitis.

Musculoskeletal System

The musculoskeletal system is involved in up to 60 percent of patients with Crohn's disease. The most common finding is sacroiliitis commonly resembling the HLA B-27 antigen positive type. Sacroiliitis in Crohn's disease tends to be bilateral with erosions and sclerosis on both sides of the joint space. In more severe cases, bone ankylosis may be seen. These patients may present clinically with back pain and findings suggestive of exacerbation of the primary disease process in the bowel. CT scans using bone settings (window width 1776H, window center 176H) can detect the bone involvement. CT is more sensitive than plain films in detecting the presence of sacroiliitis.

One of the skeletal complications of Crohn's disease not related to the primary process but to therapy is the development of avascular necrosis of the femoral heads. Although standard x-ray films can detect avascular necrosis, CT has been shown to be more sensitive than standard radiology in its early detection. We have seen several patients with pelvic pain suggesting either pelvic abscess or perirectal disease who have had unsuspected avascular necrosis. The use of CT in these cases obviously has an impact on subsequent patient management.

Finally, we have seen several cases of sacral osteomyelitis secondary to enteric fistulas and subsequent abscess. This should always be considered in patients with inflammatory bowel disease and back pain.


Future Directions

A definite limitation of the radiologic evaluation of Crohn’s disease has been determining the activity of disease. Although, initial reports of contrast-enhanced CT scans suggested that enhancement could be used to determine disease activity, this has not proved reliable in any large series of patients to date. However, multidetector- array CT, the latest advancement in CT technology may be useful in this regard. These new scanners combine multiple rows of detectors and faster gantry speeds, thus allowing examinations to be performed in a few seconds. This speed combined with faster IV injection rates (3-5cc/sec) should allow the acquisition of functional data. For instance enhancement of a segment of small bowel can be evaluated over time. From this data, perfusion rates can be calculated, which may help determine disease activity.

Another advancement in CT scanning of the gastrointestinal tract relates to oral contrast. Traditionally high density oral agents (dilute barium or iodinated agents) are used to opacify the gastrointestinal tract. However, these high density agents limit visualization of the adjacent enhancing bowel bowel. Investigations are underway assessing the use of alternative agents such as water as a potentially useful oral contrast agents, as they allow better visualization of the enhancing wall (Figure 11). More research is necessary to determine the optimal oral contrast agent.

Three dimensional volume rendering of CT data is becoming more available, and may come to play a role in the evaluation of patients with Crohn’s disease. The ability to view the gastrointestinal tract in 3 dimensions with CT eliminates the problem of superimposed loops.



The key advantage of CT scanning is its ability to detect a wide variety of potential complications in patients with Crohn's disease on a single examination with high sensitivity and specificity. It is therefore our policy to evaluate the patient with Crohn's disease who has a changing clinical pattern with a CT scan. CT scans can be done on an outpatient or inpatient basis. The study can be performed quickly, is not invasive, and requires minimal patient compliance. It is a valuable adjunct to endoscopy and contrast studies in the management of patients with inflammatory bowel disease.

© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.