- Pictorial Review
- Open Access
Small bowel MR enterography: problem solving in Crohn’s disease
© European Society of Radiology 2012
- Received: 10 December 2011
- Accepted: 20 February 2012
- Published: 17 March 2012
Magnetic resonance enterography (MRE) is fast becoming the first-line radiological investigation to evaluate the small bowel in patients with Crohn’s disease. It can demonstrate both mural and extramural complications. The lack of ionizing radiation, together with high-contrast resolution, multiplanar capability and cine-imaging make it an attractive imaging modality in such patients who need prolonged follow-up. A key question in the management of such patients is the assessment of disease activity. Clinical indices, endoscopic and histological findings have traditionally been used as surrogate markers but all have limitations. MRE can help address this question. The purpose of this pictorial review is to (1) detail the MRE protocol used at our institution; (2) describe the rationale for the MR sequences used and their limitations; (3) compare MRE with other small bowel imaging techniques; (4) discuss how MRE can help distinguish between inflammatory, stricturing and penetrating disease, and thus facilitate management of this difficult condition.
• MR enterography (MRE) is the preferred imaging investigation to assess Crohn’s disease. T2-weighted, post-contrast and diffusion-weighted imaging (DWI) can be used.
• MRE offers no radiation exposure, high-contrast resolution, multiplanar ability and cine imaging.
• MRE can help define disease activity, a key question in the management of Crohn’s disease.
• MRE can help distinguish between inflammatory, stricturing and penetrating disease.
• MRE can demonstrate both mural and extramural complications.
- Crohn’s disease
- Small bowel
Magnetic resonance enterography (MRE) is a radiological technique that has evolved in the last decade. It involves the use of magnetic resonance imaging (MRI) to assess the small bowel, following distension with an oral contrast agent. The advantages of this technique are that it involves no ionising radiation, is capable of multi-planar imaging, affords high-contrast resolution (with more detailed evaluation of bowel wall changes) and allows for cine-imaging. Its main indication at present is to evaluate small bowel involvement in patients with Crohn’s disease (CD).
CD is a chronic inflammatory bowel condition with onset usually in young adulthood. Twenty to thirty percent of patients are younger than 20 years old. The prevalence in the UK is estimated to be about 150 cases per 100,000 population . It can involve any part of the gastrointestinal tract. The small bowel, in particular the terminal ileum, is most commonly involved but perianal disease can occur in up to 40%. It is characterised histologically by the presence of non-caseating granulomas and trans-mural inflammation within the bowel, leading to erosions, ulceration and inflammatory stenosis. Enteric sinuses, fistulae, mesenteric phlegmons and abscess collections may complicate penetrating disease. With time, inflammation may become chronic and eventually lead to the development of fibrostenotic strictures.
The European Crohn’s and Colitis Organisation (ECCO) has recently published a revised consensus on the diagnosis and management of CD . Based on a systematic review of the literature, it recommends either computed tomography (CT) or MRE/MR enteroclysis as the current standards for assessing the small intestine due to their high diagnostic accuracy. It states that radiation exposure should be considered when selecting techniques. MRE can be used to help categorise the relative components of inflammatory, penetrating or stricturing disease in each individual patient. This advantage in combination with no radiation exposure makes MRE an ideal investigation for repeat bowel imaging in patients with CD.
A recent study looking into the diagnostic and therapeutic impact of MRE in CD has shown that MRE significantly increased the mean percentage confidence of clinicians for the presence or absence of small bowel disease and changed therapeutic strategy in 61% of patients .
The purpose of this pictorial review is to (1) detail the MRE protocol used at our institution; (2) describe the rationale for the MR sequences used and their limitations; (3) compare MRE with other small bowel imaging techniques; (4) discuss how MRE can assist the clinician in problem solving in CD.
The patient is fasted for 4-6 h prior to the study. In adults, oral contrast consists of at least 1-1.5 l of a 2.5% mannitol solution mixed with 0.2% carob bean gum. This solution acts as a hyperosmolar agent to draw fluid into the bowel. In paediatric patients, the volume of oral contrast is adjusted (between 300-1,000 ml) according to the weight of the patient. Mannitol is a biphasic agent that appears as low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. The patient drinks the oral contrast agent at regular intervals over a period of approximately 40 min prior to the study. The patient is imaged on a 1.5-T MRI scanner (Siemens, Erlangen, Germany), using a phased array surface coil, either in the supine or prone position (if no stoma is present). Coronal sequences are quicker to perform in the prone position, as there is a thinner volume of tissue to image. Although prone imaging has been shown to result in better small bowel distension, both positions are equivalent with respect to lesion detection and characterisation of changes in the bowel wall in CD .
Parameters for MRI protocol
(1) True FISP
(2) Cine True FISP
(4) T1 volumetric fat-saturated sequencea, b
Flip angle (degrees)
Field of view (mm)
Parallel imaging factor
Section thickness (mm)
Intersection gap (mm)
Number of sections per stack
Breath-hold time per stack
Number of stacks
2 or 3
2 or 3
2 or 3
1 or 2
The cine True FISP sequence involves repeatedly imaging the abdomen in the coronal plane sequentially from front to back over a period of 2 min. Twenty images are acquired in each coronal plane. Inflamed loops of bowel will demonstrate decreased peristalsis. An inflammatory small bowel stenosis should open up on cine imaging, compared with a fibrotic stricture where the stenosis will be constant and associated with more marked pre-stenotic dilatation.
Diffusion-weighted imaging (DWI) has recently been added to our MRE protocol with parameters as shown in Table 1. Three b values (b = 0, 100 and 800) are used with axial images through the upper and lower abdomen obtained. The purpose of this is to help identify actively inflamed loops of bowel by the presence of restricted diffusion within the affected bowel wall.
Other radiology techniques used to image the small bowel include barium follow-through, CT enterography, ultrasound and nuclear medicine techniques (such as positron emission tomography [PET]-CT and white cell scintigraphy).
With regards to the use of PET or PET-CT in the evaluation of CD, the sensitivity in the detection of active inflammation ranges between 73% and 90% when compared with clinical, endoscopic or biological markers of disease activity [5, 6]. Using PET alone, there is poor disease localisation, and specificity can sometimes be low , as other pathological or physiological processes may lead to increased bowel fluorodeoxyglucose (FDG) uptake. The main disadvantage of PET-CT is the use of ionising radiation; this is clearly not ideal in young patients who may require repeated imaging. It is also more time consuming to perform than CT or MRI alone. Hence for these reasons, PET-CT is not routinely used in the assessment of patients with CD.
A recent meta-analysis of 33 studies using ultrasound, MRE, scintigraphy and CT in the diagnosis of CD, showed no significant difference in the mean sensitivity . All these techniques showed a high sensitivity (> 84%) and specificity (90%) on a per patient basis . Hence the preference is based on availability and consideration of radiation exposure.
Enteroclysis is a technique whereby oral contrast is directly infused into the small bowel via a nasojejunal catheter at a high flow rate (e.g. 80 ml/min) in order to obtain optimal small bowel distension. Depending on the oral contrast agent used and preference of the radiologist, the small bowel can then be imaged using either fluoroscopy, CT or MRI. MR enteroclysis gives better mucosal detail than MRE  and is thus more likely to detect subtle aphthous or other types of mucosal ulceration. However, in the assessment of ileal CD, MRE has shown an equivalent accuracy and reproducibility to MR enteroclysis .
In addition to ileocolonoscopy, the gastroenterologist can use other techniques to evaluate the small bowel. Double balloon enteroscopy can examine the whole of the small bowel, either under conscious sedation or general anaesthesia. Video (or wireless) capsule endoscopy (VCE) involves the ingestion of a capsule that contains a camera and transmitter, which is propelled along the bowel by peristalsis. A video recorder carried on the patient records the signals and a computer later processes the data. A recent meta-analysis  looking at nine studies comparing VCE to other techniques in the diagnosis of CD (i.e. ileocolonoscopy, CT enterography/enteroclysis, push enteroscopy and MRI) showed VCE gave a greater diagnostic yield due to its ability to detect mucosal abnormalities. However VCE is not possible in stricturing disease, hence radiological imaging is usually required first to exclude this (unless a patency capsule is used).
Comparison of imaging modalities in the evaluation of Crohn’s disease
Good depiction of early bowel disease compared to CTE/MRE
Radiation burden; time consuming, operator and patient dependent leading to limited sensitivity; difficulty in assessing extramural complications
No radiation; may show terminal ileal disease well
Operator and patient dependent; comprehensive examination is not possible; time consuming
Fast (< 5 mins); greater spatial resolution than MRE; multiplanar reformats are possible; mural and extramural complications are seen
Radiation burden; early disease is not well seen; cine imaging is not possible
No radiation; high soft tissue contrast; multiplanar ability; shows mural and extramural complications; defines activity of disease; cine imaging is possible; can combine with perianal imaging
Longer scan time than CTE (20 mins); early disease is not well seen; suboptimal distention of proximal small bowel
Very good distension; can identify early ulceration, wall thickening, fistulae, sinus tracts
Radiation burden; invasive; time consuming
Nuclear medicine techniquesa
Similar diagnostic accuracy to CTE and MRE
Radiation burden; time consuming; poor localisation (unless PET-CT)
Evaluate small bowel mucosa; biopsy is possible
Requires sedation/anesthesia; extramural complications not assessed; risk of pancreatitis, bleeding, small bowel perforation
Evaluate small bowel mucosa
Cannot use in stricturing disease; battery exhaustion; poor localisation; extramural complications are not assessed
The multidisciplinary team meeting (consisting of the radiologist, gastroenterologist, histopathologist and surgeon) is an important forum where imaging, histopathology and clinical status of the patient can all be reviewed, in order to determine the most appropriate management. The following important questions can be addressed on MRE: (1) extent of small and large bowel involvement at first presentation; (2) distinction between active inflammatory and fibrotic stricturing disease; (3) recognition of penetrating disease with the development of extramural complications; (4) evaluation of response to medical therapy; (5) detection of recurrent disease following surgery. Each of these questions will be addressed in turn.
Extent of bowel involvement at first presentation
Distinction between active inflammatory and stricturing disease
A key question in the management of patients with CD is the assessment of disease activity. Active inflammation (characterised clinically by acute clinical symptoms and raised inflammatory markers) is usually treated medically (unless there are extra-mural complications), whilst fibrostenotic disease (characterised by obstructive symptoms) often requires surgery. In addition to dietary approaches, there are a number of drugs that can be used to induce and/or maintain remission in CD . In the last decade, anti-tumour necrosis factor alpha (TNF)-α antibodies (infliximab, adalimumab) have become an established and highly effective treatment for moderate to severe CD.
Endoscopy (with the use of an endoscopic severity score) with or without biopsy can be used to directly assess the degree of inflammation in the bowel. However, this is an invasive technique, which is restricted to the colon and distal 20-30 cm of ileum. There are several surrogate markers for disease activity that clinicians have traditionally used (including the Crohn’s Disease Activity Index [CDAI], C-reactive protein [CRP] and more recently faecal markers such as faecal calprotectin). All of these markers, however, have limitations. In a meta-analysis of seven studies (140 patients) looking at MRI to determine disease activity, MRI correctly graded disease activity in 91% of cases with frank disease . Recently, attempts have been made to derive an MR-based activity score (using MR features such as mural thickness and oedema) to assess disease activity, validated with ileocolonoscopy and histology as the standard of reference [14, 15].
Comparison of active inflammation and fibrostenotic disease on MRE
Features on MRE
Stratified mural enhancement
Increased mesenteric vascularity/adenopathy
Mesenteric phlegmon, abscess
Present in penetrating disease
Present in penetrating disease
Degree of mural thickening
It has been shown  that active inflammation (as defined by findings of inflamed mucosa or inflammatory infiltrates on endoscopy or histopathology respectively) has a greater degree of mural thickening on MRE (Fig. 4a, b) compared with chronic inflammation or fibrostenotic disease (mean maximal wall thickness 6.7 mm versus 3.3 mm respectively). A direct positive correlation has been demonstrated between the degree of acute inflammation on histopathology (defined by an acute inflammatory score) and the degree of mural thickening on MRE . This is not surprising as mural thickening is probably due to a combination of oedema and inflammatory infiltrates.
Bowel wall signal intensity on T2-weighted images
Degree of mural enhancement
Active inflammation results in an increase in bowel wall enhancement compared with adjacent uninvolved bowel [19–23]. Less avid mural enhancement is seen in fibrostenotic disease. Some studies have used dynamic contrast enhanced MRI [20–23] to obtain either semi-quantitative or quantitative measurements of enhancement. Results suggest that inflamed bowel segments have steeper initial enhancement slopes, and faster volume transfer coefficients (Ktrans) [21, 23]. A linear relationship between the degree of bowel wall enhancement and degree of inflammatory activity on histopathology has, however, not been clearly established .
Pattern of mural enhancement
Active inflammation may show either increased mucosal enhancement or a stratified pattern (Fig. 6b, 7b) of mural enhancement [17, 18]. The latter is due to the presence of submucosal oedema, where there is avid enhancement of the mucosa and the muscularis propria/serosa but relatively reduced enhancement of the submucosa. Chronic inflammation and fibrostenotic disease may show either a heterogeneous, homogeneous (Fig. 9b)  or stratified pattern of enhancement . It is postulated  that the stratified pattern of enhancement seen in fibrostenotic disease may be due to the presence of active disease on the background of mural fibrotic change or the presence of mural fibrosis itself (which theoretically would then show delayed enhancement).
Dilatation can occur upstream to either an inflammatory stenosis or fibrostenotic stricture (Fig. 9a, b). This is significant when the small bowel diameter is > 3 cm. As mentioned earlier, cine imaging is helpful in distinguishing between active inflammatory and fibrotic strictures.
‘Comb’ sign and mesenteric adenopathy
Recognition of penetrating disease and development of extramural complications
Sinus and fistula formation
Mesenteric phlegmon and abscess collections
Evaluation of response to medical therapy
Detection of recurrent disease following surgery
As the terminal ileum is the commonest site of disease in CD, the majority of patients eventually requiring surgery typically have an ileocolic resection. Stricturoplasties may also be performed in short (< 3 cm) fibrotic strictures causing obstructive symptoms. Patients are routinely re-evaluated by ileocolonoscopy at 6-12 months post-resection. Post-surgical recurrence, defined as the appearance of new lesions on endoscopy, radiology or pathology, occurs in 80% of patients at the site of the neoterminal ileum/anastomosis (Fig. 8) within the 1st year following resection [29, 30]. Rutgeerts et al.  have shown that patients with absent or very mild lesions on endoscopy following surgery have a very good prognosis, whereas patients with more severe lesions on endoscopy develop early clinical recurrence. Limitations of ileocolonoscopy are that intubation of the neoterminal ileum may not be possible due to stenosis, more proximal small bowel disease will not be assessed and extramural complications will not be demonstrated. Recently both MR enteroclysis  and MR enterography  have been suggested as useful alternatives, showing good concordance with ileocolonoscopy in the detection of recurrence and in the differentiation of high and low risk patients.
Imaging has an important role in the management of CD. Although various imaging modalities are available, MRE is increasingly becoming the first line investigation for such patients. It can be useful at both initial diagnosis and follow-up. It is the preferred investigation due to its lack of ionising radiation and its ability to depict both mural and extramural complications. MRE can be performed prior to VCE in excluding strictures where VCE can then be used to detect subtle mucosal disease. In combination with blood and faecal biomarkers and endoscopy, MRE can assist the treating clinician in distinguishing between inflammatory stenoses amenable to medical therapy and fibrostenotic disease requiring surgery. The value of DWI remains to be defined, but it may well have an adjunctive role in the assessment of disease activity and response evaluation.
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