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DOI: 10.1148/rg.243035120
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RadioGraphics 2004;24:689-702
© RSNA, 2004

EDUCATION EXHIBIT

Cross-sectional Imaging in Crohn Disease1

Akira Furukawa, MD, Takao Saotome, MD, Michio Yamasaki, MD, Kiyosumi Maeda, MD, Norihisa Nitta, MD, Masashi Takahashi, MD, Tomoyuki Tsujikawa, MD, Yoshihide Fujiyama, MD, Kiyoshi Murata, MD and Tsutomu Sakamoto, MD

1 From the Departments of Radiology (A.F., M.Y., K. Maeda, N.N., M.T., K. Murata) and Gastroenterology (T. Saotome, T.T., Y.F.), Shiga University of Medical Science, Seta Tsukinowa-cho, Otsu Shiga 520-2192, Japan; and the Department of Radiology, Koga Public Hospital, Koga, Japan (T. Sakamoto). Recipient of a Certificate of Merit award for an education exhibit at the 2002 RSNA scientific assembly. Received April 29, 2003; revision requested June 19 and received September 5; accepted September 5. All authors have no financial relationships to disclose. Address correspondence to A.F. (e-mail: akira@belle.shiga-med.ac.jp).


   Abstract
Top
 Abstract
LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
 References
 
The role of cross-sectional imaging in the diagnosis of Crohn disease has expanded with recent technologic advances in computed tomography (CT) and magnetic resonance (MR) imaging that allow rapid acquisition of high-resolution images of the intestines. To acquire images of diagnostic quality, administration of a fairly large amount of intraluminal contrast agent prior to examination and scanning with intravenous contrast material injection are necessary. Both CT and MR imaging are reported to have a sensitivity of over 95% for the detection of Crohn disease; however, they may not allow early diagnosis. Colonoscopy and conventional enteroclysis studies are indicated for patients with early-stage disease. At more advanced stages, CT and MR imaging can help identify and characterize pathologically altered bowel segments as well as extraluminal lesions (eg, fistulas, abscesses, fibrofatty proliferation, increased vascularity of the vasa recta, mesenteric lymphadenopathy). These modalities can also clearly depict inflammatory lesion activity and conditions that require elective gastrointestinal surgery, thereby aiding in treatment planning. In the clinical setting, CT is currently the imaging modality of choice at most institutions; however, it is expected that MR imaging will soon play a comparable role. CT or MR imaging should be included in a comprehensive evaluation of patients with Crohn disease, along with conventional imaging and clinical and laboratory tests.

© RSNA, 2004

Index Terms: Crohn disease, 70.262 • Enteritis, 74.24, 74.262 • Ileum, 742.262 • Intestines, CT, 74.1211 • Intestines, diseases, 74.24, 74.262 • Intestines, MR, 74.1214


   LEARNING OBJECTIVES FOR TEST 2
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
Introduction
 Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
 References
 
After reading this article and taking the test, the reader will be able to:


   Introduction
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
 References
 
Crohn disease is a chronic granulomatous inflammatory disease of the gastrointestinal tract with a tendency toward remission and relapse. Crohn disease can affect any part of the gastrointestinal tract from the mouth to the anus, often involving multiple discontinuous sites (Fig 1). The small intestine is involved in 80% of cases, most commonly at the terminal ileum. The colon is affected either with (50% of cases) or without (15%–20%) involvement of the small intestine (1). The cause of Crohn disease is not known; however, several factors are believed to be involved, including infections; intestinal mucosal immune system abnormalities; genetic, mesenteric, or vascular alterations; diet and ingested materials; and psychogenic factors (2). Crohn disease is common in northern Europe, North America, and Japan and is becoming more prevalent, with both genders equally affected and peak involvement in persons between 15 and 25 years of age (1,3). A familial tendency has been described with an increased risk of ulcerative colitis in relatives (2). The earliest change caused by the disease occurs in the submucosa and consists of lymphoid hyperplasia and lymphedema. Radiologic findings at this (early) stage include subtle elevations and aphthoid ulcers. As the disease progresses, it extends transmurally to the serosa (transmural stage) and beyond to the mesentery and adjacent organs (extramural stage). Aphthoid ulcers develop into linear ulcers and fissures to produce an ulceronodular or "cobblestone" appearance. The bowel wall is thickened by a combination of fibrosis and inflammatory infiltrates. Bowel obstruction, strictures, abscesses or phlegmon, fistulas, and sinus tracts are common complications of advanced disease. Although not common, toxic megacolon and neoplasms such as lymphoma and carcinoma may also occur (1,4,5).



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  		Figure 1.  Mesenteric border ulceration and ileoileal fistula. Image from an air double-contrast enteroclysis study demonstrates typical straightening of the mesenteric border, a finding that indicates linear ulceration or ulcer scar. A relatively long segment of the bowel is affected at several sites, and multiple stenoses are also identified. A fistula (arrow) extends from the ileum to the adjacent ileal loop.

 
Endoscopy and barium studies are the principal tools for diagnosis and evaluation of Crohn disease; however, they are limited in their capacity to demonstrate the transmural or extramural extent of disease or extraintestinal complications. Although cross-sectional imaging may not help detect subtle mucosal lesions, it reveals pathologic changes of the intestinal mucosa and can help compensate for the limitations of conventional imaging. Computed tomography (CT) is currently the cross-sectional imaging modality of choice at most institutions; however, magnetic resonance (MR) imaging has also proved highly effective in this setting. The role of cross-sectional imaging in the diagnosis of Crohn disease has expanded with recent advances in CT and MR imaging technology, which allow rapid acquisition of high-resolution images of the intestines during a breath-hold examination. Both imaging modalities provide information that is crucial in the diagnosis of Crohn disease and in treatment planning (6–16). During CT or MR imaging, the intestines should be distended with a fairly large amount of intraluminal contrast material for better visualization of the anatomy and of morphologic changes caused by the disease. Contrast material is injected intravenously to demonstrate the presence of lesions and to help assess their inflammatory activity.

In this article, we review various imaging considerations in the diagnosis and evaluation of Crohn disease, including preparations, contrast agents, and scanning techniques. We also discuss and illustrate the characteristic imaging appearances of Crohn disease and the findings that indicate the presence of inflammatory lesion activity. In addition, we discuss the advantages and limitations and the clinical role of cross-sectional imaging in this setting.


   Imaging Considerations
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
 References
 
Imaging modalities used in the evaluation of Crohn disease include barium studies (barium follow-through, conventional enteroclysis studies, and barium enema studies), colonoscopy, ultrasonography, CT, and MR imaging. In the clinical setting, these modalities can help confirm the diagnosis; localize lesions and assess their severity, extent, and inflammatory activity; and identify the presence of extraintestinal complications and other entities that require surgical intervention. Advantages of cross-sectional imaging include its capacity to (a) demonstrate the transmural extent of inflammation, skip lesions beyond severe luminal stenoses, and intraperitoneal or extraintestinal complications, and (b) provide three-dimensional information and, with use of contrast material, vascular information. However, the limited spatial resolution of CT and MR imaging compared with enteroclysis studies results in lower rates of depiction of early disease manifestations and of fistulas and sinus tracts.

The detection and characterization of intestinal lesions on cross-sectional images require appropriate preparation and scanning techniques. The gastrointestinal tract should be empty and clean, with the lumen distended (17–20). Collapsed bowel loops may mimic a segment with wall thickening, an abscess, or enlarged lymph nodes, which can cause diagnostic error (5,11, 13,20,21). Patients with Crohn disease may be treated with totally intravenous nutrition; otherwise, they should fast for at least 3 hours prior to the examination (7). A fairly large amount of intraluminal contrast agent is administered prior to the examination to distend the bowel loops and allow correct assessment of pathologic changes in the intestinal wall. The contrast agent should allow imaging with (a) homogeneous luminal enhancement, (b) high contrast between the lumen and bowel wall, (c) minimal mucosal absorption, (d) absence of artifact formation, and (e) no significant adverse effects (1). Motion artifact from intestinal peristalsis is another potential problem. Antiperistaltic agents are commonly administered prior to scanning, particularly with MR imaging, to minimize bowel movement or contraction, and scanning time should be kept short. The use of intravenous contrast medium is indicated for better visualization of the bowel wall, extraintestinal structures, and lesions and for precise evaluation of the degree of inflammatory activity (6–16, 22–26).

Computed Tomography
Various types of intraluminal contrast media are used to provide positive (Figs 2, 3) or negative(Fig 4) contrast between the bowel lumen and surrounding structures (5,9–12,17,21). Positive contrast agent with high attenuation at CT aids in differentiating bowel loops from enlarged lymph nodes or an extramural fluid collection such as an abscess. The presence of small bowel obstruction or fistulas is also well appreciated. However, with the use of positive intraluminal contrast material, mural enhancement after intravenous injection may obscure subtle Crohn lesions. The use of negative intraluminal contrast agents with low attenuation facilitates depiction of the wall of normal and diseased bowel segments, particularly after intravenous contrast material administration (8). The following technique for intraluminal contrast material administration is common to both CT and MR imaging. Between 1,500 and 2,000 mL (or more) of contrast material is administered orally 45–90 minutes prior to the examination (9,10). To provide adequate and uniform distention of the bowel loops, patients are asked to steadily ingest the contrast material over a 20–60-minute period (7,10). The contrast material may be administered through a nasojejunal catheter at a rate of 100–250 mL/min with the help of a roller pump (11–13). CT or MR imaging performed with this technique is called CT or MR enteroclysis (6,12,13,16,27,28). An increased rate of infusion or dual-phase intubation with an initial flow rate of 80–120 mL/min followed by a rate of 200 mL/min is recommended (6,13,29) to achieve reflex intestinal atony and thereby minimize motion artifact. Use of a nasojejunal catheter allows better luminal distention but causes patient discomfort. If necessary, 300– 1,000 mL of contrast agent can be administered transrectally (22). CT scans are obtained from the dome of the liver to the level of the perineum to cover the entire course of the intestine. Imaging with the patient in the prone position is recommended to disperse the small bowel loops (10). With a single–detector row helical CT scanner, a collimation of 5–8 mm and a pitch of 1–2 are often used (9–12,27). With multi–detector row CT scanners, thinner collimation (0.5–2.5 mm) is possible. Sections with a 5–7-mm thickness are usually obtained; however, thinner sections, overlapping reconstructed images, or multiplanar reformatted images can also be obtained. Intravenous administration of contrast material is essential, and 100–150 mL of iodinated contrast material is injected at a rate of 2.5–4 mL/sec with a delay time of 40–70 seconds.



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  		Figure 2a.  Use of positive intraluminal contrast medium. Contrast material-enhanced CT scans of the abdomen obtained at the level of the renal hilum (a) and lower pelvis (b) after oral administration of 1,500 mL of 2% barium suspension demonstrate uniform enhancement of the small bowel and colon.

 


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  		Figure 2b.  Use of positive intraluminal contrast medium. Contrast material-enhanced CT scans of the abdomen obtained at the level of the renal hilum (a) and lower pelvis (b) after oral administration of 1,500 mL of 2% barium suspension demonstrate uniform enhancement of the small bowel and colon.

 


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  		Figure 3a.  Use of positive intraluminal contrast medium. CT scans of the pelvis (b obtained caudad to a) clearly show wall thickening of the ileal loops and an ileoileal fistula (arrow in b).

 


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  		Figure 3b.  Use of positive intraluminal contrast medium. CT scans of the pelvis (b obtained caudad to a) clearly show wall thickening of the ileal loops and an ileoileal fistula (arrow in b).

 


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  		Figure 4.  Use of negative intraluminal contrast medium. Coronal contrast-enhanced reformatted CT scan of the abdomen obtained after oral administration of 1,500 mL of nonabsorbable liquid (Niflec; Ajinomoto Pharma, Tokyo, Japan) with a nasoenteric tube demonstrates uniform distention of the intestinal loops. Thickness of the bowel wall and degree of enhancement can clearly be assessed.

 
MR Imaging
Various kinds of intraluminal contrast agents have been proposed for MR imaging (6,20,28,30–37) and are classified as positive, negative, or biphasic. Positive agents produce high intraluminal signal, and negative agents produce little or no intraluminal signal, regardless of the applied pulse sequence. Biphasic contrast agents may produce either high or low signal depending on the pulse sequence used, usually demonstrating low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images. Negative or biphasic contrast agents seem to be more suitable for assessing the small bowel (6,28). An antiperistaltic agent is injected to minimize potential artifact caused by bowel movement or contraction. Imaging with the patient prone is recommended for separating bowel loops and decreasing the scanning volume. This position is also safe for patients should they vomit. To increase the signal-to-noise ratio, use of abdominal phased-array radiofrequency coils is preferred. Coronal images are usually obtained with a 4–7-mm section thickness, a 128–256 x 256 matrix, and a field of view of 350 x 350 mm or more to cover all the intestinal loops (6–8,13–16,20,28). Thicker sections (eg, 70–180 mm) are obtained at MR fluoroscopy to monitor the infusion process (6,13). Acquisition of additional axial, sagittal, or multiplanar images may be necessary for precise evaluation. A comprehensive imaging protocol should include both T1- and T2-weighted imaging to detect and characterize each lesion. T1-weighted imaging with intravenous contrast material is essential for assessing inflammatory lesion activity. True fast imaging with a steady precession (FISP), half-Fourier acquired single-shot fast spin-echo, T2-weighted turbo spin-echo, and gadolinium-enhanced fat-suppressed spoiled gradient-echo sequences are preferable for MR enteroclysis, and a combination of these sequences is recommended (6,16,28). However, the availability of these pulse sequences for MR imaging currently depends on the manufacturer and grade of the imaging equipment. The various pulse sequences complement one another in demonstrating the anatomy as well as intestinal and extraintestinal lesions and their inflammatory activity. Additional axial T1- and T2-weighted images may be obtained to evaluate the extramural extent of a lesion and the presence of complications. The advantages and disadvantages of each pulse sequence are discussed in the following paragraphs (6,28).

True FISP Sequence.— The true FISP sequence provides motion-free, high-resolution images similar to T2-weighted images of the intestine, mesentery, and vasculature in 1.5 seconds. However, this sequence is prone to susceptibility artifacts from intraluminal air and from "black boundary" artifact due to the chemical shift phenomenon, which may obscure subtle bowel wall thickening (Fig 5). The black boundary artifact can be eliminated with use of fat suppression.



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  		Figure 5.  Imaging with a true FISP sequence. Coronal true FISP MR image clearly depicts fluid-filled bowel loops. The dark line surrounding the bowel represents black boundary artifact, which may hinder the precise assessment of bowel wall thickening.

 
Half-Fourier Acquired Single-Shot Fast Spin-Echo Sequence.— The half-Fourier acquired single-shot fast spin-echo sequence provides heavily T2-weighted images in less than 1 second. This sequence does not suffer from susceptibility or chemical shift artifacts but is prone to motion artifact such as intraluminal flow void. Mesenteric structures such as mesenteric vessels and lymph nodes are not clearly demonstrated due to k-space filtering effects (Fig 6b). Use of fat saturation allows acquisition of thicker sections to demonstrate all the bowel loops on a single image (Fig 6a). This sequence can be used for MR fluoroscopy.



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  		Figure 6a.  Imaging with a single-shot fast spin-echo sequence. (a) Coronal fat-suppressed single-shot fast spin-echo MR image shows motion artifact from intraluminal flow in the intestine (arrows). Because only fluid can produce bright signal intensity with this sequence, a thicker section can be obtained to demonstrate the course of the entire intestine on a single image. This sequence can be used for MR fluoroscopy. (b) Coronal non-fat-suppressed single-shot fast spin-echo MR image shows excellent contrast between the bowel wall and surrounding structures. Mesenteric structures are not clearly visible with this pulse sequence; however, bowel folds at the jejunum are well appreciated with both pulse sequences.

 


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  		Figure 6b.  Imaging with a single-shot fast spin-echo sequence. (a) Coronal fat-suppressed single-shot fast spin-echo MR image shows motion artifact from intraluminal flow in the intestine (arrows). Because only fluid can produce bright signal intensity with this sequence, a thicker section can be obtained to demonstrate the course of the entire intestine on a single image. This sequence can be used for MR fluoroscopy. (b) Coronal non-fat-suppressed single-shot fast spin-echo MR image shows excellent contrast between the bowel wall and surrounding structures. Mesenteric structures are not clearly visible with this pulse sequence; however, bowel folds at the jejunum are well appreciated with both pulse sequences.

 
T2-weighted Turbo Spin-Echo Sequence.— The T2-weighted turbo spin-echo sequence is a multisection sequence that provides T2-weighted images with excellent soft-tissue contrast. This sequence is insensitive to susceptibility and black boundary artifacts but is prone to motion artifacts. Images covering all the bowel loops can be obtained within 20 seconds.

Gadolinium-enhanced Fat-suppressed Spoiled Gradient-Echo Sequence.— The gadolinium-enhanced fat-suppressed spoiled gradient-echo sequence provides T1-weighted images with excellent visualization of the enhancing bowel wall, which contrasts well with the low-signal-intensity mesenteric fat and negative intraluminal contrast material (Fig 7). Morphologic features and degree of enhancement both aid in assessing disease activity. Images covering the bowel loops in their entirety can be obtained within 30 seconds. Scanning is performed after a bolus intravenous injection of 0.1–0.2 mmol/kg of gadopentetate dimeglumine with a delay time of 40–80 seconds. Either a two- or three-dimensional data acquisition technique is used.



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  		Figure 7a.  Imaging with a gadolinium-enhanced spoiled gradient-echo sequence. (a) Non-fat-suppressed single-shot fast spin-echo MR image shows a Crohn lesion with prominent wall thickening at the distal ileum (arrows). (b, c) On unenhanced (b) and contrast-enhanced (c) spoiled gradient-echo MR images, the involved ileal segment shows intense wall enhancement compared with normal bowel segments, a finding that indicates an inflammatory active lesion.

 


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  		Figure 7b.  Imaging with a gadolinium-enhanced spoiled gradient-echo sequence. (a) Non-fat-suppressed single-shot fast spin-echo MR image shows a Crohn lesion with prominent wall thickening at the distal ileum (arrows). (b, c) On unenhanced (b) and contrast-enhanced (c) spoiled gradient-echo MR images, the involved ileal segment shows intense wall enhancement compared with normal bowel segments, a finding that indicates an inflammatory active lesion.

 


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  		Figure 7c.  Imaging with a gadolinium-enhanced spoiled gradient-echo sequence. (a) Non-fat-suppressed single-shot fast spin-echo MR image shows a Crohn lesion with prominent wall thickening at the distal ileum (arrows). (b, c) On unenhanced (b) and contrast-enhanced (c) spoiled gradient-echo MR images, the involved ileal segment shows intense wall enhancement compared with normal bowel segments, a finding that indicates an inflammatory active lesion.

 

   CT and MR Imaging Findings in Crohn Disease
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
 CT and MR Imaging...
Diagnosis of Crohn Disease
 Conclusions
 References
 
In patients with proved or suspected Crohn disease, cross-sectional images should be analyzed specifically for the presence and character of a pathologically altered bowel segment (wall thickness, pattern of attenuation, degree of enhancement, length of involvement) (Figs 8–10), stenosis and prestenotic dilatation (Fig 11), skip lesions, fistulas, abscess, fibrofatty proliferation, increased vascularity of the vasa recta (comb sign), mesenteric adenopathy, and other extraintestinal disease involvement (5,12). The normal thickness of the wall of the small intestine and colon is 1–2 mm and 3 mm, respectively, when the lumen is distended (6,18,19,38). Any portion of the bowel wall that exceeds 4–5 mm is considered abnormal (7,12). Bowel wall thickening, usually ranging from 1–2 cm, is the most consistent feature of Crohn disease on cross-sectional images (5,11). The number of lesions and extent of involvement are accurately depicted. The bowel wall of the involved segment may have a homogeneous or stratified appearance (alternating layers of higher or lower attenuation or signal intensity) at CT or MR imaging. Mural stratification ("target" or "double halo" appearance) is often seen in active lesions, particularly after the intravenous administration of contrast medium. An inflamed bowel wall demonstrates marked enhancement after intravenous contrast material injection, and the intensity of enhancement correlates with the degree of inflammatory lesion activity (5). The lumen of the normal small intestine is less than 2.5 cm in diameter. Luminal narrowing and associated prestenotic dilatation are easily recognized. Deformity of bowel loops such as pseudodiverticulum formation caused by asymmetric involvement by longitudinal ulcers and ulcer scars is well demonstrated on both axial and coronal images. However, early-stage lesions such as enlarged lymph follicles, slight distortion of the bowel folds, and tiny aphtae are not consistently visible at either CT or MR imaging due to inadequate spatial resolution (5,28). Fibrofatty proliferation of the mesentery is commonly seen adjacent to involved bowel segment in Crohn disease (Fig 12). Both CT and MR imaging demonstrate fibrofatty proliferation, which has slightly increased CT attenuation and slightly decreased MR imaging signal intensity compared with normal fat separating the bowel loops. Abscess and phlegmon can occur in the small bowel mesentery (Fig 13), abdominal wall (Fig 14), or psoas muscle (Fig 15) or around the anus (Fig 16). Abscesses and phlegmon are well demonstrated at CT and fat-saturated T2-weighted MR imaging and can be distinguished reliably, which aids in management planning. Fistulas (Figs 17, 18) and sinus tracts (Fig 19) are also depicted; however, the reported sensitivity of MR imaging for depicting sinus tracts is 50%–75% when a conventional enteroclysis study is used as a reference (6,16). Mesenteric lymphadenopathy ranging from 3 to 8 mm in size is depicted at both CT and MR imaging with a true FISP or T2-weighted turbo spin-echo sequence (6,16,28). If these sequences are not available, axial T1- or T2-weighted spin-echo imaging should be added. When lymph nodes are larger than 10 mm, lymphoma and carcinoma must be excluded.



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  		Figure 8a.  Active Crohn lesions at the distal ileum with contiguous involvement from the ascending colon to the sigmoid colon. (a) Coronal reformatted CT scan from 1-mm-thick axial source images clearly shows bowel wall thickening at the distal ileum (arrowheads) and colon (arrows). The thickened wall has a stratified appearance. Increased mesenteric vascularity ("comb sign") is noted around the involved segment. These findings are suggestive of active lesions from Crohn disease. (b) Gadolinium-enhanced spoiled gradient-echo MR image shows markedly increased enhancement of the involved bowel wall segment and an increased number of mesenteric vessels around the segment (arrows).

 


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  		Figure 8b.  Active Crohn lesions at the distal ileum with contiguous involvement from the ascending colon to the sigmoid colon. (a) Coronal reformatted CT scan from 1-mm-thick axial source images clearly shows bowel wall thickening at the distal ileum (arrowheads) and colon (arrows). The thickened wall has a stratified appearance. Increased mesenteric vascularity ("comb sign") is noted around the involved segment. These findings are suggestive of active lesions from Crohn disease. (b) Gadolinium-enhanced spoiled gradient-echo MR image shows markedly increased enhancement of the involved bowel wall segment and an increased number of mesenteric vessels around the segment (arrows).

 


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  		Figure 9a.  Active Crohn lesions at the distal ileum. (a) Image from an air double-contrast enteroclysis study demonstrates deformity of the distal ileum associated with linear (arrows) and aphthoid (arrowheads) ulcers. (b) Fat-suppressed single-shot fast spin-echo MR image (70-mm section thickness) demonstrates similar deformity at the distal ileum (black arrowhead) as well as the entire course of the intestine (white arrowheads). (c) Coronal non-fat-suppressed single-shot fast spin-echo MR image (5-mm section thickness) clearly demonstrates bowel wall thickening at the involved segment (arrowheads). (d) On a gadolinium-enhanced spoiled gradient-echo MR image, the bowel wall demonstrates intense enhancement.

 


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  		Figure 9b.  Active Crohn lesions at the distal ileum. (a) Image from an air double-contrast enteroclysis study demonstrates deformity of the distal ileum associated with linear (arrows) and aphthoid (arrowheads) ulcers. (b) Fat-suppressed single-shot fast spin-echo MR image (70-mm section thickness) demonstrates similar deformity at the distal ileum (black arrowhead) as well as the entire course of the intestine (white arrowheads). (c) Coronal non-fat-suppressed single-shot fast spin-echo MR image (5-mm section thickness) clearly demonstrates bowel wall thickening at the involved segment (arrowheads). (d) On a gadolinium-enhanced spoiled gradient-echo MR image, the bowel wall demonstrates intense enhancement.

 


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  		Figure 9c.  Active Crohn lesions at the distal ileum. (a) Image from an air double-contrast enteroclysis study demonstrates deformity of the distal ileum associated with linear (arrows) and aphthoid (arrowheads) ulcers. (b) Fat-suppressed single-shot fast spin-echo MR image (70-mm section thickness) demonstrates similar deformity at the distal ileum (black arrowhead) as well as the entire course of the intestine (white arrowheads). (c) Coronal non-fat-suppressed single-shot fast spin-echo MR image (5-mm section thickness) clearly demonstrates bowel wall thickening at the involved segment (arrowheads). (d) On a gadolinium-enhanced spoiled gradient-echo MR image, the bowel wall demonstrates intense enhancement.

 


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  		Figure 9d.  Active Crohn lesions at the distal ileum. (a) Image from an air double-contrast enteroclysis study demonstrates deformity of the distal ileum associated with linear (arrows) and aphthoid (arrowheads) ulcers. (b) Fat-suppressed single-shot fast spin-echo MR image (70-mm section thickness) demonstrates similar deformity at the distal ileum (black arrowhead) as well as the entire course of the intestine (white arrowheads). (c) Coronal non-fat-suppressed single-shot fast spin-echo MR image (5-mm section thickness) clearly demonstrates bowel wall thickening at the involved segment (arrowheads). (d) On a gadolinium-enhanced spoiled gradient-echo MR image, the bowel wall demonstrates intense enhancement.

 


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  		Figure 10a.  Inactive Crohn lesion at the terminal ileum. (a) Image from a barium study demonstrates typical straightening of the mesenteric border at the terminal ileum. (b) Non-fat-suppressed single-shot fast spin-echo MR image clearly shows asymmetric bowel deformity at the terminal ileum. No bowel wall thickening is seen at the involved segment (cf Fig 9c).

 


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  		Figure 10b.  Inactive Crohn lesion at the terminal ileum. (a) Image from a barium study demonstrates typical straightening of the mesenteric border at the terminal ileum. (b) Non-fat-suppressed single-shot fast spin-echo MR image clearly shows asymmetric bowel deformity at the terminal ileum. No bowel wall thickening is seen at the involved segment (cf Fig 9c).

 


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  		Figure 11a.  High-grade small bowel obstruction at the distal ileum caused by Crohn disease. Contrast-enhanced axial (a, b) and coronal reformatted (c) CT scans demonstrate luminal narrowing at the distal ileum in a relatively long bowel segment (straight arrows) associated with prominent dilated proximal loops (curved arrow in c). The wall of the involved segment has a stratified appearance associated with an increased number of adjacent mesenteric vessels (comb sign) (arrowheads in a and c).

 


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  		Figure 11b.  High-grade small bowel obstruction at the distal ileum caused by Crohn disease. Contrast-enhanced axial (a, b) and coronal reformatted (c) CT scans demonstrate luminal narrowing at the distal ileum in a relatively long bowel segment (straight arrows) associated with prominent dilated proximal loops (curved arrow in c). The wall of the involved segment has a stratified appearance associated with an increased number of adjacent mesenteric vessels (comb sign) (arrowheads in a and c).

 


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  		Figure 11c.  High-grade small bowel obstruction at the distal ileum caused by Crohn disease. Contrast-enhanced axial (a, b) and coronal reformatted (c) CT scans demonstrate luminal narrowing at the distal ileum in a relatively long bowel segment (straight arrows) associated with prominent dilated proximal loops (curved arrow in c). The wall of the involved segment has a stratified appearance associated with an increased number of adjacent mesenteric vessels (comb sign) (arrowheads in a and c).

 


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  		Figure 12.  Fibrofatty proliferation. Contrast-enhanced CT scan of the lower abdomen shows a proliferation of fat tissue around the ascending colon. The tissue has a heterogeneous appearance with increased attenuation. The wall of the ascending colon is thickened and demonstrates intense enhancement (arrow).

 


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  		Figure 13.  Abscess in the small bowel mesentery. Contrast-enhanced CT scan of the pelvis shows a loculated fluid collection in the mesentery surrounded by a thin wall.

 


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  		Figure 14.  Abscess in the abdominal wall. Contrast-enhanced CT scan shows the right abdominal rectus muscle and subcutaneous fat tissue with increased enhancement. Air bubbles are seen within the abdominal wall.

 


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  		Figure 15.  Iliopsoas muscle abscess. Contrast-enhanced CT scan of the pelvis shows an air-containing abscess in the right iliopsoas muscle (arrow).

 


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  		Figure 16.  Perianal abscess. Contrast-enhanced CT scan obtained at the bottom of the pelvis demonstrates a fluid-containing abscess around the anus.

 


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  		Figure 17.  Duodenocolic fistula. Single-shot fast spin-echo MR image demonstrates a fistula (arrow) between the duodenum and the ascending colon.

 


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  		Figure 18a.  Enterocutaneous fistula. Conventional fistulogram (a) and fat-suppressed single-shot fast spin-echo MR fistulogram (b) clearly demonstrate an enterocutaneous fistula (arrows).

 


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  		Figure 18b.  Enterocutaneous fistula. Conventional fistulogram (a) and fat-suppressed single-shot fast spin-echo MR fistulogram (b) clearly demonstrate an enterocutaneous fistula (arrows).

 


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  		Figure 19a.  Sinus tracts. (a) Image from a barium enema study shows sinus tracts at the descending colon (arrowheads). (b) Coronal single-shot fast spin-echo MR image demonstrates the descending colon with wall thickening and containing a tract (arrowhead).

 


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  		Figure 19b.  Sinus tracts. (a) Image from a barium enema study shows sinus tracts at the descending colon (arrowheads). (b) Coronal single-shot fast spin-echo MR image demonstrates the descending colon with wall thickening and containing a tract (arrowhead).

 
The inflammatory activity of Crohn lesions is well appreciated at both CT and MR imaging (Figs 7–9, 11, 20–22) (22–26). Findings include thickened bowel wall with marked contrast material enhancement, mural stratification, pericolic or perienteric hypervascularity (comb sign), hyperintensity of the bowel wall on T2-weighted images, lymph node enlargement, and extramural complications such as phlegmon and abscess.



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  		Figure 20.  Mural stratification. Contrast-enhanced CT scan of the pelvis shows Crohn disease involvement of the distal ileum. The thickened bowel wall has low attenuation owing to fluid in the lumen and is surrounded by alternating layers of higher or lower attenuation in a concentric pattern.

 


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  		Figure 21.  Comb sign. Contrast-enhanced CT scan of the lower pelvis shows a diseased segment of the distal ileum (arrowheads) with prominently dilated adjacent mesenteric vessels.

 


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  		Figure 22.  Prominent wall thickening. Contrast-enhanced CT scan demonstrates prominent, strongly enhanced wall thickening with a stratified appearance at the ascending colon. An inflammatory lesion extends beyond the wall to the adjacent region.

 
Investigation of the diagnostic accuracy of cross-sectional imaging for Crohn disease has been limited; however, the overall sensitivity and specificity of CT on a per patient basis are reported to be 94%–100% and 95%, respectively (9,12). Sensitivity increases to 98% in the diagnosis of transmural or extramural Crohn disease but is only 70% for early-stage disease. Acquiring multiplanar images in addition to axial images significantly improves observer confidence in image interpretation even if it does not reveal additional abnormalities (10). The overall sensitivity of MR imaging on a per patient and per segment basis is reported to be 96%–100% and over 85%, respectively (6–8). Gourtsoyiannis et al (6) reported a 100% sensitivity on a per lesion basis when superficial lesions were excluded.


   Diagnosis of Crohn Disease
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
Conclusions
 References
 
The diagnosis of Crohn disease should include assessment of (a) the presence, severity, and extent of disease, (b) inflammatory lesion activity, and (c) the presence of extraintestinal complications to aid in treatment planning, which largely depends on imaging findings, particularly those at cross-sectional imaging. Patients with active inflammatory disease are commonly treated with a combination of nutrition management and a variety of medical therapies. Azathioprine and 6-mercaptopurine should be used in patients with fistulas. Surgery is required in fistulating disease that is not responsive to medical therapy, in fibrostenosing disease, and in patients with massive intestinal hemorrhage, perforations, or abscesses (39). Colonoscopy, barium studies, and CT are the most commonly used imaging modalities. Early manifestations of Crohn disease such as enlarged lymphoid follicles, erosions, and aphtoid ulcers (5,9) are well appreciated at colonoscopy and barium studies. In transmural and extramural stages of Crohn disease, pathologic changes in the intestine (1,5,12) are consistently depicted and inflammatory activity can be assessed with colonoscopy, barium studies, and CT. CT is preferred for evaluation of the transmural extent of disease (degree of wall thickening), skip lesions beyond severe luminal stenoses, and intraperitoneal or extraintestinal complications. MR imaging is an emerging technique in this field and is expected to play a role similar to that of CT. The clinical efficacy of MR imaging has been investigated, and favorable results have been reported as described in this article. High soft-tissue contrast, static and dynamic imaging capabilities, and the absence of ionizing radiation exposure represent advantages of MR imaging over CT (39). On the other hand, MR imaging is more time consuming, less readily available, and more expensive (39). Advantages of CT over MR imaging include greater availability, shorter examination times, flexibility in choosing imaging thickness and planes after data acquisition with multi–detector row CT, and higher spatial resolution. Precise indications for MR imaging in the diagnosis of Crohn disease and its use as a complement to CT or other imaging procedures need further investigation. Clinical management decisions might be influenced by the presence of unsuspected additional lesions that were seen only at CT or MR imaging, as reported by Fishman et al (40) and Turetschek et al (12), with management changes occurring in 28% and 62% of cases, respectively. Cross-sectional imaging should be included or even performed as a primary examination in the clinical evaluation of Crohn disease, along with conventional imaging and clinical and laboratory tests. Cross-sectional imaging should be used to evaluate for the presence of entities that indicate elective gastrointestinal surgery (eg, marked prestenotic dilatation [severe stenosis], skip lesions, fistulas, perforations, abscesses).


   Conclusions
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
References
 
Appropriate treatment planning in a patient with Crohn disease requires correct assessment of the severity, extent, and inflammatory activity of lesions and of the presence of extraintestinal complications. CT and MR imaging with intraluminal and intravenous contrast material are limited in the depiction of subtle mucosal lesions but provide excellent visualization of most intestinal lesions and demonstrate their mural and extramural extent. Complications such as phlegmon, abscesses, sinus tracts, and fistulas are also demonstrated. Disease activity and entities that indicate elective gastrointestinal surgical intervention are well appreciated, which aids in selecting appropriate treatment options. In the clinical setting, CT has been the cross-sectional imaging modality of choice at most institutions. Although the precise clinical role of MR imaging in the diagnosis of Crohn disease has yet to be determined, this modality has yielded excellent results in this setting and is expected to play a role equal to that of CT. Cross-sectional imaging, whether CT or MR imaging, should be included in a comprehensive clinical evaluation of Crohn disease, along with conventional imaging and clinical and laboratory tests.


   Footnotes
 
Abbreviation: FISP = fast imaging with a steady precession


   References
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Imaging Considerations
 CT and MR Imaging...
 Diagnosis of Crohn Disease
 Conclusions
 References
 

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