As initial studies reported that magnetic resonance imaging (MRI) was useful for the evaluation of the small intestine, this modality has become increasingly important in the diagnosis, assessment and exclusion of small bowel disease. The use of MRI for the assessment of inflammatory bowel disease is increasing, at the expense of the current primary imaging modality and computed tomography (CT) enterography. MRI has many advantages over CT, including a lack of radiation exposure, lower prevalence of adverse events, availability of dynamic information, higher resolution and better soft-tissue contrast. New MRI techniques, including diffusion-weighted imaging, spectroscopy, motility study, positron-emission tomography-MRI and molecular imaging, are currently under investigation to improve the diagnosis, follow-up and management of the disease.
Keywords: Magnetic resonance imaging MRI, bowel disease, tomography
| Diagnosis|| |
The diagnosis of Crohn's disease (CD) is a clinical one and can be quite difficult, given that the presenting symptoms can be insidious and non-specific. The diagnosis of CD is made on the basis of symptoms and endoscopic and radiological findings.
| Radiological Techniques|| |
Barium small bowel follow-through studies and enteroclysis
The advantage of a small bowel barium study is that it achieves good mucosal detail, and the distension achieved with enteroclysis is reported to improve visualisation of fistulae, sites of small bowel obstruction and mural or intraluminal filling defects such as small bowel neoplasms. Barium studies have a limited role in the diagnosis of acute small-bowel obstruction or ileus and in the assessment of extraluminal disease, and patients are often referred for additional computed tomography (CT) assessment help characterise small bowel lesions or stage small bowel tumours.
In CD, findings of barium studies include stenosis of the ileocaecal valve, luminal narrowing and ulceration of the terminal ileum, irregular thickening and distortion of the valvulae conniventes, mesenteric and mural thickening causing bowel loop separation and loop adhesions resulting in mass effect.
Severe CD produces a 'cobblestone' appearance, with deep transverse and longitudinal ulcerations bordered by areas of oedema creating a chequered mucosal relief pattern. Chronic CD leads to circumferential thickening of the bowel wall and can progress to fibrotic strictures, in which irreversible deposition of the extracellular matrix causes impaired peristalsis, fixed luminal narrowing and bowel obstruction [Figure 1].
|Figure 1: Conventional enteroclysis. (a) Mucosal ulcers (arrows); (b) typical cobblestone-like nodular filling defects and ulceration and (c) stenotic loop (arrows). Reproduced from Gatta et al. This is an open access article distributed under the terms of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0.), which permits others to distribute, remix, adapt and build up on this work, for commercial use, provided the original work is properly cited|
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In patients with inflammatory bowel disease (IBD), ultrasonographic findings are non-specific but can be used to guide further studies and to evaluate the effects of treatment. When peroral techniques are used to distend the bowel, the sensitivity and specificity of ultrasonography in the detection of IBD range from 78% to 90% and from 83% to 95%, respectively.
However, ultrasonography is an operator-dependent technique and its use is limited in patients with a large abdomen [Figure 2].
|Figure 2: (a) Transverse, (b) longitudinal and (c) Doppler ultrasonographs of the terminal ileum. The terminal ileum is thickened, measuring 5.5 mm in single-wall thickness, and shows the target sign on the transverse scan. Doppler examination reveals hyperaemia. Reproduced from Radiopaedia.org. Case courtesy of Dr Robert Jones, Radiopaedia.org. From the case Crohn's disease: ultrasound findings. This is an open access article distributed by the terms of the creative commons attribution (CC BY 3.0) license, which permits others to distribute, remix, adapt and build up on this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/3.0|
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CT has proven to have a very high sensitivity (81%–94%) and specificity (96%) for determining the level and cause of high-grade small-bowel obstruction and is now the investigative method of choice for this indication.
The major drawback of CT is patients' exposure to ionising radiation, which has received much attention in recent years given that the IBD population (CD in particular) is likely to require multiple imaging studies over the course of the disease [Figure 3].
|Figure 3: Selected, (a) axial and (b) coronal computed tomography images with a contrast between oral and intravenous therapy. The terminal ileum and sigmoid and descending colon show mural thickening and contrast enhancement of the mucosa. Prominent perienteric and pericolic vasculature are highlighted by fibrofatty proliferation of the mesentery. Vascular dilatation and tortuosity of the vasa recta give the comb sign. Reproduced from Radiopaedia.org. From the case hidden diagnosis case. This is an open access article distributed by the terms of the creative commons attribution (CC BY 3.0) license, which permits others to distribute, remix, adapt and build up on this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/3.0|
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Positron emission tomography/computed tomography: An emergent role in evaluating patients with inflammatory bowel disease
The use of fluorodeoxyglucose (FDG) in positron-emission tomography (PET)/CT is the only modality available that allows both functional and morphological visualisation of the whole gastrointestinal tract, as well as detection of extraintestinal areas of inflammation.
The advantages of PET/CT with FDG include:
- Improved spatial localisation compared to PET with FDG without CT
- Reduced FDG uptake in fibrous strictures (indicating failure of medical therapy), compared to non-fibrous areas
- Improved performance for detecting colon inflammation compared to CT and magnetic resonance (MR) enterography [Figure 4].
|Figure 4: From left to right: examples of positron-emission tomography, computed tomography and positron-emission tomography/computed tomography, and the corresponding endoscopic appearance. (a) Deep ulcers with cobblestones in the left colon, appearing as a thickened segment with a prominent increase in F-FDG uptake on positron-emission tomography/computed tomography and (b) no endoscopic lesion in the caecum, contrasting with the thickening of the bowel wall and increased uptake of FDG on positron-emission tomography/computed tomography. This research was originally published in JNM. Louis E, Ancion G, Colard A, Spote V, Belaiche J, Hustinx R. Noninvasive sssessment of Crohn's disease intestinal lesions with F-FDG positron-emission tomography/computed tomography. J Nucl Med. 2007;48:1053–9. © Society of Nuclear Medicine and Molecular Imaging, Inc|
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Magnetic resonance imaging
The excellent soft-tissue contrast, direct multiplanar imaging capabilities, new ultrafast breath-holding pulse sequences, lack of ionizing radiation and availability of a variety of oral contrast agents make magnetic resonance imaging (MRI) well suited to a critical role in the imaging of small bowel disorders.,,
With the increasing awareness of radiation exposure, there has been an increased global interest in implementing techniques that either reduce or eliminate radiation exposure, particularly for patients with CD, who are likely to require multiple imaging studies over the course of their disease.
Two major techniques are used to achieve bowel distension using MRI as follows:
- MR enterography with oral contrast administration has been used as the primary MRI modality in CD, with high sensitivity, specificity and interobserver agreement 
- MR enteroclysis, with an infusion of the contrast through a nasojejunal tube, provides superior small bowel distension. The optimal distension of small bowel loops is crucial to evaluate bowel wall pathologies correctly because collapsed bowel loops can hide lesions or mimic disease by suggesting a pathologically thickened bowel wall in collapsed segments, and the visualisation of small polypoid masses that do not produce obstruction is difficult.,
MR enteroclysis delineates superficial changes better than MR enterography, and this aspect has to influence the revealing and localising of the disease in patients with only superficial manifestations.
Magnetic resonance imaging sequences
Several different pulse sequences are available for imaging the small bowel. The main diagnostic sequences can be divided into the T2-weighted sequences that consist of the half-Fourier acquisition single-shot turbo spin-echo imaging (HASTE) techniques (single-shot fast spin echo [SSFSE], HASTE and single-shot turbo spin echo) and the balanced gradient echo (fast imaging employing steady-state acquisition, true fast imaging with steady-state precession, balanced fast field echo and balanced steady-state free precession) sequences.
Contrast-enhanced T1-weighted sequences are obtained using a gradient echo technique with fat saturation. The most commonly used sequence in small bowel imaging is fast low-angle shot using both two-dimensional (2D) and three-dimensional (3D) acquisitions. These are routinely used to identify increased enhancement in an inflamed bowel wall [Figure 5].
|Figure 5: Magnetic resonance enteroclysis in a 21-year-old male with active Crohn's disease. (a) Coronal true-FISP and (b) half-Fourier acquisition single-shot turbo spin-echo imaging images show mucosal irregularity (arrows) as thin lines of high signal intensity, longitudinally or transversely (fissure ulcers) orientated within the thickened wall in the terminal ileum consistent with diffuse ulcerations in Crohn's ileitis; (c) an axial true-FISP sequence detects wall thickening of terminal ileum as well as the caecal wall (arrows); (d) an axial fat-suppressed T2-weighted half-Fourier acquisition single-shot turbo spin-echo imaging sequence MRI shows a high-signal-intensity bowel wall (arrows) and fluid surrounding the distal ileum (small arrow); and (e) coronal and (f) axial contrast T1-weighted gradient echo sequences with fat-saturated images showing marked contrast enhancement, with avid enhancement of the mucosa of the terminal ileum and caecal walls. Note the high-signal-intensity linear structure caused by increased vascularity (small arrows in [e]) close to the mesenteric border of the involved small bowel segment, the so-called comb sign. These MR findings are indicative of active Crohn's disease. Reproduced from Masselli. This is an open access article distributed under the terms of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0.), which permits others to distribute, remix, adapt and build up on this work, for commercial use, provided the original work is properly cited|
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T2-weighted sequences are generated by rapid acquisition and relaxation enhancement with ultrafast acquisition time. They are known as half-acquisition SSFSE or SSFSE sequences, depending on the manufacturer. They are heavily T2-weighted sequences, complementary to gadolinium-enhanced gradient echo sequences, and produce a high contrast between the lumen and the bowel wall. As these sequences are highly resistant to magnetic susceptibility or chemical shift artefacts, the wall thickness may be evaluated accurately. Moreover, the sinus tracts and fistulas are well visualised. These sequences are sensitive to intraluminal motion and there may be intraluminal low-intensity signal artefacts.
Balanced steady-state free precession sequences
Balanced steady-state free precession sequences are characterised by two unique features as follows: (1) a very high signal-to-noise ratio and (2) a T2/T1-weighted image contrast.
The recent development of faster pulse sequences provides an opportunity to create a film of cine images. Cine imaging confers the ability to observe the motion of intestinal segments over a relatively short period and in real time. It provides high temporal, spatial and contrast resolution for monitoring bowel contractions.
After intravenous administration of gadolinium-containing contrast material (0.1–0.2 mmol/kg), dynamic coronal 3D T1-weighted gradient echo sequences with fat suppression are obtained at time intervals of 45–55, 70 and 180 s. These intervals are institutionally specific. Delayed axial and coronal post-contrast 2D or 3D T1-weighted sequences with fat suppression are acquired following dynamic imaging. Although rapid transit to the right colon is seen in some patients, most patients require a delay of at least 40–60 min from contrast material ingestion to imaging.
Diffusion-weighted imaging (DWI) has long been used in several parts of the body such as the brain. Although the application of DWI to assess the bowel is a relatively new trend, DWI may yield comparable performances for detecting and assessing ileal inflammation in CD. The high signal intensity in DWI and the restricted diffusion of the bowel wall have also has been linked to the detection of acute inflammation [Figure 6].
|Figure 6: Ileocaecal Crohn's disease in a 17-year-old girl. (a) The inflamed caecum (arrows) is thickened and hyperintense compared to the psoas muscle in the half-Fourier acquisition single-shot turbo spin-echo imaging image. (b) The post-gadolinium axial volumetric interpolated breath-hold examination image shows layered enhancement (from the inside moving outwards: enhancing mucosa, non-enhancing hypointense submucosal oedema and enhancing muscular layer and serosa). (c) The mucosa is more restricted than the other layers on the axial diffusion-weighted image. All of these findings are suggestive of active inflammation in the caecum. Reproduced from Chavhan et al. This is an open access article distributed under the terms of the creative commons attribution-Non-commercial-ShareAlike 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited|
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| Conclusion|| |
In recent years, several radiological techniques have been developed for the study of the small bowel. Each technique is characterised by its profile of advantages and disadvantages.,
A successful approach for the radiologist depends on the local availability of services and clinical expertise. Consideration should always be given to new investigative methods with the utility benefit of reduced radiation exposure, single-study techniques or those with increased diagnostic sensitivity.
In recent years, MR enterography has become a part of standard diagnostic modality in CD. Novel MRI techniques such as DWI, motility studies, PET-MRI and molecular imaging might further contribute to the diagnosis and management of this chronic inflammatory disease.
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Conflicts of interest
There are no conflicts of interest.
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] [Full text]
Mohamed Walaaeldin Elfaal,
Department of Radiology, Thumbay Clinic, Ajman
United Arab Emirates
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]