eMedicine Specialties > Radiology > Gastrointestinal

Crohn Disease: Imaging

Author: Yung-Hsin Chen, MD, Staff Physician, Department of Radiology, Nassau University Medical Center
Coauthor(s): Dahua Zhou, MD, Staff Physician, Department of Radiology, Nassau University Medical Center; David I Weltman, MD, Consulting Staff, S & D Medical, LLP; Director, Department of Radiology, Southside Hospital
Contributor Information and Disclosures

Updated: Jan 26, 2007

Radiography

Findings

The role of plain radiography is fairly limited. The 2 major purposes that it serves are (1) to assess the presence of intestinal obstruction and (2) to evaluate pneumoperitoneum prior to further radiological workup. Additional extraintestinal findings of sacroiliitis or oxalate kidney stones may be present. These further support the diagnosis of Crohn disease.

Double-contrast barium enema study is useful for diagnosing inflammatory bowel disease and for differentiating Crohn disease from ulcerative colitis, especially in the early phase of the disease. On double-contrast studies, early Crohn disease is characterized by discrete aphthoid ulcers, which are seen as punctate or slitlike collections of barium surrounded by radiolucent mounds of edema. The aphthoid ulcers are often separated by normal bowel and present as skip lesions.

On the contrary, ulcerative colitis extends proximally at various degrees from the rectum as a continuous area of disease that eventually leads to pancolitis. Early ulcerative colitis is characterized by a granular appearance on double-contrast examination as a result of edema and hyperemia of the mucosa. Thus, the 2 diseases can be differentiated on basis of radiographic findings.

As more severe Crohn disease develops, the small ulcers become enlarged and deeper, and they connect to one another, forming stellate, serpiginous, and linear ulcers. These ulcers are found most frequently in terminal ileum along the mesenteric border. These are pathognomonic of Crohn disease. On small-bowel series or enteroclysis, a mesenteric border ulcer appears as a long 1- to 2-mm barium collection that parallels a short, straight mesenteric border. A radiolucent collar usually parallels the linear barium collection at the margin of the ulcer. The antimesenteric border of the bowel is usually uninvolved and pulled into the ulcer collar, creating radiating folds.

As inflammation penetrates the submucosa and muscularis layers, deep knife like linear cleft form the basis of "cobblestoning" and fissure or fistula formation. They appear as a barium-filled reticular network of grooves that surround round or ovoid radiolucent island of mucosa. Eventually, transmural inflammation leads to decreased luminal diameter and limited distensibility. This leads to radiographic string sign that represents long areas of circumferential inflammation and fibrosis resulting in long segments of luminal narrowing.

Chronic inflammation in the lamina propria of the small intestine results in enlarged villi radiographically manifested as 0.5- to 2-mm, round or polygonal nodules. This fine mucosal nodularity occurs in the small intestine and should not be confused with the mucosal granularity seen in the colon of patients with ulcerative colitis.

Degree of Confidence

Aphthoid ulcers are detected on barium studies in 25-50% of patients with Crohn disease. These are identified in as many as 75% of surgical specimens with Crohn disease. Endoscopy is slightly superior to barium studies in the demonstration of isolated or a few aphthoid ulcers.

In a comparison of 23 patients with ulcerative colitis with 27 with Crohn disease, Laufer et al established the differentiating features the conditions using barium study. They found that ulcerative colitis involves granular mucosa, diffuse rectal involvement, and continuous inflammatory changes in the bowel with sparing of the terminal ileum. Crohn disease involves patchy rectal involvement with punched-out ulcers, ulcers on normal mucosa, and discontinuous bowel inflammation typically with involvement of the terminal ileum. These criteria are distinguishing in 95% of patients. The difficulty in differentiating the 2 illnesses reported occurs at later chronic stages, during which numerous remissions and exacerbations can result in discontinuous ulcerative colitis has become. Despite this difficulty, careful examination of the mucosal surface with barium study enables the distinction.

False Positives/Negatives

Mucosal nodularity or granularity in a small-bowel series is a nonspecific finding that can be seen in diseases that infiltrate or inflame the lamina propria, such as amyloidosis or radiation enteritis.

Small-bowel follow-though examination is limited by the speed of barium passage through the pylorus. If too slow, incomplete distention in the lumen of the bowel can cause short skip lesions, masses, or obstructing lesions in the small bowel to be missed.

In general, 18-20% of findings are false-negative on barium study, as compared with endoscopic detection. However, barium enema has a 95% accuracy rate in distinguishing Crohn disease from ulcerative colitis.

Computed Tomography

Findings

The role of CT in the evaluation of Crohn disease is well accepted. The ability of CT to depict bowel involvement and extraluminal pathology (eg, abscess, obstruction, fistula) makes it an essential imaging tool for patient care. The earliest CT finding of Crohn disease is bowel wall thickening, which usually involves the distal small bowel and colon, although any segment of GI tract can be affected. Typically, the luminal thickening is 5-15 mm.

Ulcerations in the mucosa can be detected on thin-section CT, although small-bowel series or enteroclysis is more sensitive to the early mucosal changes of Crohn disease. In addition, mesenteric stranding, increase in mesenteric fat, local adenopathy, fistula, and abscess are readily and commonly identified on CT scans.

Edema or mild inflammation of the mesenteric fat results in fat of increased attenuation, the so-called hazy fat on CT. Greater inflammation or fibrosis of fat results on CT in attenuating linear bands of soft tissue coursing through the mesentery. On CT, an ill-defined inflamed mass of mixed attenuation may represent a phlegmon or early abscess formation. Enlarged lymph nodes are usually seen in proximity to the bowel wall along the mesenteric course of the vascular bundle.

On CT scans, abscesses appear as well-defined, round or oval masses of fluid attenuation, and they are often multilocular. Pockets or bubbles of gas usually result from fistulous communication with bowel or, less likely, from infection by gas-producing organisms.

One limitation of CT has been in the area of delineating active versus inactive disease. The presence of mesenteric stranding does not reliably signify active disease because residual mesenteric thickening can remain during remission.

The introduction of multidetector-row CT scanners with thinner collimation and faster intravenous injections of contrast material have allowed more detailed evaluation of the bowel. The enhancement of the bowel wall after intravenous contrast enhancement is correlated with the enlargement of the feeding vessel and hyperemia during active disease. In an article by Del Campo et al, patients with active disease had a bowel wall attenuation of 95 HU, as compared with 65 HU in patients with disease in remission. The ability to measure bowel wall enhancement may prove valuable in treating patients with Crohn disease.

Degree of Confidence

CT should be the first radiologic procedure performed in patients with acute symptoms and suspected or known Crohn disease. The ability to directly demonstrate the bowel wall, adjacent abdominal organs, mesentery and retroperitoneum makes CT superior to barium studies in diagnosing the complications of Crohn disease. CT directly demonstrates bowel wall thickening, mesenteric edema and lymphadenopathy as well as phlegmon and abscess.

Although barium is more sensitive in demonstrating the presence of fissures and fistulas, CT is superior in demonstrating the sequelae of these tracks (eg, air in the urinary bladder in enterovesical fistula).

False Positives/Negatives

The sensitivity of CT for Crohn disease is estimated to be 71%, with lower detection of early mucosal disease as compared with barium studies. A recent study by Philpotts et al has shown that the CT findings of Crohn disease considerably overlap with those of infectious, radiation, ulcerative, and ischemic colitides.

However, certain distinguishing features including differences in wall thickness and attenuation; the distribution of colonic wall involvement; the presence or absence of abscesses, fistulas, small-bowel disease, and mesenteric fibrofatty proliferation have all been cited in delineating Crohn disease from other forms of enterocolitis. In using the mentioned features, CT can attain positive predictive value above 90% and a diagnostic accuracy as high as 93%.

There is considerable overlap between CT findings of ulcerative colitis and Crohn disease. Despite this fact, certain defining features of each disease have been characterized. Ulcerative colitis is predominantly a mucosal disease. However, with progression of illness, there is hypertrophy of the muscularis often by 40 fold, increase submucosa fatty deposition, and thickening of the lamina propria from round cell infiltration which all leads to bowel wall thickening. On average, thickening of the luminal wall is 7.8 mm in ulcerative colitis, which is less than the amount of wall thickening typically seen with Crohn disease.

Submucosal fat as mentioned is a prominent finding in chronic ulcerative colitis and is one of the defining features of the mural stratification seen in ulcerative colitis. In comparison, Crohn disease has transmural involvement that over time leads to replacement of submucosal fat with fibrosis and loss of mural stratification. In distinction to ulcerative colitis, Crohn disease also has several extraluminal CT findings including mesenteric fibrofatty proliferation and abscess.

Magnetic Resonance Imaging

Findings

Traditionally, MRI has had a well-defined role in evaluation of anorectal complications of Crohn disease. With a regular fast spin-echo technique, the pathologic entities of a fistula, a sinus tract, and an abscess can be detected in the static anorectal region by using MRI.

Sinus tracts and fistulas often appear hyperintense on T1-weighted images and hyperintense on T2-weighted images because of their fluid content. With fat suppression, the fluid signal is further intensified and easily seen as being hyperintense on T2-weighted images. An abscess often appears as an isolated collection of high-signal-intensity areas on the T2-weighted image, especially in ischioanal fossa. Defining whether abscess, fistula, or sinus tract is above or below the levator ani muscle is important for drainage, because any part of the abscess above the levator ani muscle will not drain adequately in the inferior direction, and vice versa.

Recently, the development of faster pulse sequences (eg, single-shot fast spin-echo and gradient-echo sequences) and higher-gradient systems has made T1- and T2-weighted breath-hold imaging possible. This breath-hold imaging has been a major breakthrough in overcoming physiologic motion artifacts in abdominal imaging. It has made routine abdominal MRI feasible. Because of a decrease in cumulative radiation exposure and because of the possibility of attaining high-quality coronal images correlating with barium studies, MRI is currently being investigated for monitoring disease activity in Crohn disease.

The parameters of active disease from multiple investigations have included wall thickening, fibrofatty proliferation, and bowel wall enhancement with gadolinium-based contrast agents. During active inflammation, gadolinium enhancement of the bowel wall can be seen on T2-weighted images, and it is easily differentiated from normal adjacent bowel. The enhancement pattern described by Koh et al is "layered" and specific to Crohn disease.

Wall thickening has been variable in active disease, as described in many reports. The general consensus is that concentric bowel wall thickening greater than 4 mm is suggestive of active disease. In study by Maccioni et al, active disease is characterized by a thickened bowel wall with gadolinium enhancement, but inactive disease is not. Fibrofatty proliferation is hyperintense on T2-weighted images and related to regional mesenteritis or edema and dilatation of local vessels in both active and nonactive disease.

Fat-suppressed T2-weighted images can also be used for differentiation because they fat-suppressed show high signal intensity in active disease and low or absent signal intensity in nonactive disease. Madsen et al evaluated the use of T2-weighted gadolinium-enhanced imaging in assessing wall thickening in response to treatment.

Currently, gadolinium-enhanced spoiled gradient-echo MRI is the most useful study for depicting mural changes during active disease. Dilute barium sulfate and water can be used to distend the bowel and act as a negative contrast agent on fat-suppressed T2-weighted spoiled gradient-echo images.

Another possible imaging sequence in use has been the single-shot fast spin echo sequence, in which T2-weighted images are acquired by using half-Fourier transformation and a long echo train. Each image section is acquired independently in less than 1 second, and the method eliminates physiologic motion from bowel and the need for breath holds. Compared with gradient-echo techniques, this method is less sensitive to motion artifact. Both water and dilute 2% barium sulfate serve as positive intraluminal contrast agents in single-shot fast spin-echo sequences. Low et al and Marcos and Semelka have found gradient-echo imaging to be more sensitive than other methods in determining the severity of Crohn disease, and they favor the use of gadolinium-enhanced gradient-echo MRI.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with troublemoving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Degree of Confidence

Gadolinium-enhanced spoiled gradient-echo MRI has reported sensitivity of 85-89%, a specificity of 96-94%, and an accuracy of 94-91% for active disease as compared with single-shot fast spin-echo MRI, which has a sensitivity of 51-52%, a specificity of 98-96%, and an accuracy of 83-84% (Low, 2002).

False Positives/Negatives

In general, most investigators currently favored the use of gadolinium-enhanced gradient-echo imaging for monitoring Crohn disease activity with MRI. In the gradient-echo sequence, active disease is best correlated with mural thickening, as shown with gadolinium enhancement.

False-positive findings most often occur when gadolinium enhancement is present without mural thickening. False-negative findings most often occur when distension of the bowel is suboptimal.

Ultrasonography

Findings

Ultrasonography can be an alternative to CT in the evaluation of the intraluminal and extraluminal manifestations of Crohn disease. The normal GI wall appears as 5 concentric, alternating echogenic and hypoechoic layers; this appearance is known as the gut signature. The GI wall has an average thickness of less than 5 mm.

In the case of active Crohn disease, the wall thickness can range from 5 mm to 2 cm with either partial or total loss of layering, which reflects transmural edema, inflammation or fibrosis. With severe inflammation, the wall appears diffusely hypoechoic with a central hyper echoic line that corresponds to the narrowed lumen. Peristalsis is reduced or absent, and the diseased segment is noncompressible and rigid with a loss of haustra.

Ultrasonography can depict ballooning of the less involved segments, which is seen as focal sacculation or outpouching. These findings reflect the skip lesions found in Crohn disease. The accuracy of ultrasonography is further improved with the use of color Doppler imaging. The use of Doppler imaging is helpful in the detection of hyperemia of an inflamed bowel wall and adjacent fat during active disease.

With transmural inflammation, edema and fibrosis of the adjacent mesentery occurs, leading to fingerlike projections of mesenteric fat that creeps over the serosal surface of the bowel. This creeping fat eventually envelops the diseased bowel segment. On sonograms, this appears as a uniform hyperechoic mass, which is classically seen at the cephalic margin of the terminal ileum. With long-standing disease, this becomes more heterogeneous or even hypoechoic.

In active Crohn disease, reactive mesenteric nodes are enlarged and may coalesce to form a conglomerate mass. On sonograms, enlarged nodes can be seen as oval hypoechoic masses in the mesentery. With confluence, they become lobulated mass of various sizes.

Many complications of Crohn disease can also be seen in their ultrasonographic forms. Phlegmon appears as hypoechoic mass with irregular borders and no identifiable wall or fluid. Abscess appears as a fluid collection with thickened wall containing air or echogenic debris. Obstruction appears as dilated hyperperistaltic fluid filled segments. Perforation appears as bright echoes with distal acoustic shadows outside the boundaries of bowel loops.

Fistula, on the other hand, appears as a hypoechoic tract. If gas is present in the fistulous tract, it contains hyperechoic foci with acoustic shadowing. Palpation of diseased loops during sonography enables tract identification. In addition sonography should be able to identify gas bubbles in abnormal locations, eg, air in bladder or vaginal vault, retroperitoneum, subcutaneous tissue and urachal remnant.

Degree of Confidence

The detection of bowel wall thickening varies widely. Detection rates range from 22% to 89%. The large variation presumably reflects differences in technique, operator experience, and ultrasound equipment. Determination of the extent of the disease is not always possible, and correlation between wall thickening and the clinical activity of disease is poor.

False Positives/Negatives

The loss of gut signature and bowel wall thickening is a nonspecific finding. It is found in infectious, ischemic, neoplastic, and radiation-induced conditions. In addition, the detection of bowel wall changes in Crohn disease varies significantly because of operator dependence.

The literature states that the differentiation between hypoechoic foci from creeping fat versus from phlegmon or edema may be difficult or nearly impossible. Proponents of CT have also stated that the specificity of color Doppler imaging is still unknown. In general, the confidence level of the radiologist in interpreting the results is operator dependent, and it is often lower than that of CT. For these reasons, ultrasonography has not been the favored modality for imaging Crohn disease.

Nuclear Imaging

Findings

Leukocytes labeled with either technetium-99m-HMPAO or indium-111 can be used to assess for active bowel inflammation in inflammatory bowel disease. Compared to the¹¹¹ In label, the^99m Tc HMPAO label has better imaging characteristics and can be imaged much sooner after injection. However, imaging must typically be done within an hour after injection of^99m Tc HMPAO labeled leukocytes as there is normal excretion into the bowel after this time, unlike¹¹¹ -labeled leukocytes, which have no normal bowel excretion.

Degree of Confidence

Molnar et al found that^99m Tc HMPAO leukocyte scan in active Crohn disease had a sensitivity of 76.1% and specificity of 91.0% compared to CT sensitivity of 71.8% and specificity of 83.5%. While leukocyte scan may be better in the detection of segmental inflammatory activity, CT is superior for the detection of complications.

False Positives/Negatives

False-positive bowel activity can be seen with gastrointestinal bleeding, swallowed leukocytes (eg, from uptake related to sinusitis or nasogastric tubes), or activity related to indwelling enteric tubes. In addition, leukocyte uptake is not specific for Crohn disease and will be seen in most infectious or inflammatory bowel processes. As mentioned above, there is often normal bowel excretion of^99m 99m HMPAO leukocytes if imaged after the first hour of the injection.

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