Radiography
Volume 17, Issue 1 , Pages 67-71, February 2011

Magnetic resonance imaging of the small bowel

Department of Radiology, St Mary’s Campus, Imperial College NHS Trust, London, UK

Received 2 April 2010; received in revised form 15 May 2010; accepted 5 June 2010. published online 26 July 2010.

Article Outline

Abstract 

Magnetic Resonance Imaging of the Small Bowel (MR Enterography, or MRE) is becoming increasingly popular as the first imaging modality for the diagnosis and follow-up of small bowel diseases. The inherent advantages of MRI, including excellent soft tissue contrast, multiplanar capability and lack of ionising radiation are well known. In addition, the use of luminal contrast agents in MRE has the added advantage of demonstrating the lumen and the wall directly, something not possible to achieve with conventional small bowel barium follow-through imaging.1

This review will highlight recent technical advances to this low cost, simple technique which is easily achievable in all hospitals. It will also review normal and abnormal radiological findings and highlight the value of this technique to both the clinician and patient alike in the investigation of small bowel disease.

Keywords: MRI, Small bowel

 

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1. Technique 

There are two MR requirements for a successful study. Very fast (sub-second) MRI sequences which freeze bowel movement and adequate bowel distension with a suitable luminal contrast medium.2

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2. Ultra-fast MRI sequences 

The pulse sequences are usually the combination of fast fluid sensitive sequences and fast post contrast gradient echo images.3

The ultrafast T2 sequences employed are either balanced gradient echo steady-state free precession (SSFP) or single shot fast spin echo (SSFSE). Each sequence has advantages and limitations. The SSFSE sequence has the benefit of lacking susceptibility or banding artefact, but the disadvantage of intraluminal flow voids, poorer spatial resolution and poor visualisation of the mesentery. It is therefore less widely used.4

Balanced gradient echo SSFP is known as FIESTA (fast imaging employing steady-state acquisition – General Electric, Milwaukee, USA), true FISP, (fast imaging with steady-state precession, Siemens, Berlin, Germany) or balanced FFE, (fast-field echo, Philips, Amsterdam, The Netherlands). It is very good at showing mural and extramural extension of disease, including lymphadenopathy and mesenteric stranding.4 The advantages of balanced gradient echo SSFP are lack of motion artefact, homogenous endoluminal opacifcation and high contrast between mural and intraluminal fluid.5, 6. The main limitation of the sequence is banding artefact (or off-resonance effect) which may result in the creation of black boundaries, particularly at air-soft tissue interfaces, obscuring subtle thickening of the bowel wall.4, 5 Fat suppression can be employed to reduce this artefact and improve the appreciation of mural and mesenteric T2 high signal inflammatory changes. Recently rapid repeated SSFP has been applied to produce cine images updated every 1.5s. This application shows great promise in enabling bowel peristalsis to be more comprehensively assessed and the presence of obstruction identified.

Intravenous contrast enhanced images are obtained by 2 or 3D gradient echo T1W sequences with fat saturation. Peak bowel wall enhancement usually occurs in the portal venous phase at 60–70s after intravenous injection. Post-gadolinium imaging is an essential adjunct since increased enhancement of the bowel wall is significant even in the presence of normal bowel wall thickness. Contrast enhancement is considered significant if greater than enhancement of renal cortex. In addition, post-gadolinium sequences are useful in the identification of fistulae.7

Combining ultrafast T2 and post-gadolinium GE fat saturated sequences ensures a comprehensive study, where the disadvantages of each sequence can be counteracted by the advantages of the other.5 Image quality is further improved with a spasmolytic agent such as Buscopan™ (Boeringer Ingelheim, Germany) or Glucagon being administered intravenously prior to the intravenous contrast in order to reduce motion artefact due to peristalsis.

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3. Bowel distension 

Adequate luminal distension is vital to evaluate mural pathology and differentiate between collapsed and abnormally thick bowel loops both on T2W imaging and post-gadolinium sequences.1, 4 Luminal contrast agents can be introduced via a naso-enteric tube (MR enteroclysis) or orally (MR enterography). MRI enteroclysis has been described as the ‘gold standard’ as it achieves superior bowel distension when compared to enterography,8 which results in better delineation of superficial mucosal changes.4 The placement of a naso-enteric tube requires fluoroscopic guidance and may be unpleasant for the patient. As a result most institutions do not carry out this type of procedure, particularly as similar distension can be achieved with oral contrast which is both simpler and often better tolerated by the patient.

Different contrast agents have been used to achieve an adequate distension of the entire small bowel. They may be positive (high signal), negative (low signal) or biphasic (low signal on one sequence, high signal on another).8 The usual volume is 1.5l, which is drunk fairly rapidly (over a period of 10min or so).

Negative agents have the advantage of improving contrast between lumen and bowel wall on T2W MRI in the presence of bowel wall inflammation. They can also improve visualisation of abscesses, which might otherwise be mistaken for fluid-filled bowel loops.8 Negative agents include oral super paramagnetic iron oxide. Positive and biphasic agents include a combination of water and a non-reabsorbed osmotic agent such as Mannitol, an insoluble fibre such as Locust Bean Gum or a combination of the two.9 Water alone is inadequate as it is rapidly reabsorbed.

Mannitol is a readily available sugar alcohol which cannot be absorbed and therefore acts as an osmotic agent, retaining water in the gut lumen. It has the advantage of having a pleasant mildly sweet taste, but the disadvantage of causing moderate to severe diarrhoea about 1h post-administration. Locust bean gum, an insoluble fibre derived from carob seeds holds water in the gut and is tasteless. This is more expensive than the Mannitol and less acceptable for oral ingestion. However, it does not cause diarrhoea. Both Mannitol and locust bean gum distend the small bowel satisfactorily to a diameter of around 2cm. The choice of contrast agents will therefore depend on local availability, costs and preferences. Mannitol, despite its osmotic effects, is a practical choice given its availability from hospital pharmacies and acceptability to the patients. It can be easily made up using existing IV solutions which are available in all hospitals.

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4. How we do it – and what is the patients’ experience? 

At our institution we ask the patients to fast for 4h prior to the study. The patient then drinks 500ml of 5% Mannitol mixed with 700ml tap water. The patient drinks 1l initially and then a further 200ml just before getting on the table and lying prone to improve bowel loop separation.4 MR imaging starts at 30min after drinking with an initial SSFP sequence to identify whether oral contrast has reached the terminal ileum. The entire study usually takes 30–35min. Patients are warned that they are likely to experience moderate to severe diarrhoea within an hour due to the laxative effect of the Mannitol so that they can plan their journey home accordingly.

In our practice we perform coronal and axial sequences using a phased array body coil and a large field of view (40cm) to include the abdomen and pelvis. Initially coronal and axial SSFP T2W imaging is performed. Following intravenous spasmolytic administration and gadolinium injection, coronal and axial post-gadolinium fat saturated volumetric gradient echo imaging is performed. This includes three phases in the coronal plane at 15, 45 & 90s post-injection. The full details are as follows:

∗All use torso coil full field of view. The LOC=40 FOV (field of view) with a slice thickness of 6mm.

∗FIESTA CORONAL T2 (with breath holds, which cover the whole abdomen).FOV 40, Slice thickness=4mm, flip angle=70 and TE=minimal full.

∗FIESTA AXIAL T2 (with breath hold).FOV=38; slice thickness=5mm; All other parameters are the same as for FIESTA CORONAL T2.

∗LAVA CORONAL (after Buscopan injection, post-gadolinium, (0.1mmol/kg) at 15, 45 and 90s with breath hold).FOV=38; slice thickness=6 and location for slab=32.

∗LAVA AXIAL (to include terminal ileum).The same as for LAVA CORONAL, other than: FOV=34; slice thickness=6mm, and location for slab=40.

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5. Imaging findings 

5.1. Normal appearance 

On T2 WI, luminal Mannitol solution returns high signal intensity and is clearly demarcated from the low signal bowel wall. The normal wall thickness of the distended bowel is 1–2mm (Fig. 1). Following contrast enhancement, T1W fat saturated gradient echo MRI demonstrates homogenous wall enhancement (at a thickness of 1–2mm if distended and 3–4mm if collapsed) with a low signal intensity lumen (Fig. 2).

5.2. Crohn’s disease 

MRE is most commonly requested in individuals with suspected or known Crohn’s disease (CD). It is used to assess disease activity and guide medical or surgical treatment. Some debate exists as to whether mild to moderate mucosal disease can be reliably detected with MRI given the current limits of spatial resolution.1 Some studies have described superficial mucosal erosions and ulcers, but the consensus is that capsule endoscopy has greater sensitivity in mild to moderate disease.5, 10 MRE certainly has a high sensitivity for detecting transmural disease.7, 11, 12 Features of active CD on MRE are: bowel wall thickening (>4mm), bowel wall enhancement (greater than the renal cortex) and layered enhancement. Early inflammation leads to submucosal oedema and mucosal hyperenhancement, both of which contribute to either a stratified or target appearance, depending on the orientation of the bowel loop. When inflammation extends beyond the bowel wall into the mesentery, mesenteric hypervascularity is seen as the ‘comb’ sign of engorged vasa recta on both T2 and post-gadolinium imaging. Perienteric inflammation is seen well on T2W imaging and reactive adenopathy can be identified on post-gadolinium imaging as enhancing normal or enlarged lymph nodes, indicating active inflammation (Fig. 3, Fig. 4).

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  • Fig. 3 

    (a) (Coronal T2 FIESTA) showing skip lesions which are concentric, symmetrical or asymmetrical (pseudo sacculation). (b) showing inflammatory mesenteric stranding within the distal ileum (yellow arrow). (c) (Post-Gadolinium) showing wall thickening (yellow arrow), stratified enhancement, ‘Comb sign’ –mesenteric hyperaemia (red arrow) and lymph nodes (green arrow). (d) (Axial T2 FIESTA) showing concentric, asymmetrical bowel wall thickening.

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  • Fig. 4 

    (a) (Coronal FIESTA) showing large featureless dilated small bowel in the proximal jejunum which narrows inferiorly to a strictured segment. There is fat hypertrophy of the adjacent mesentery, which is typical of Crohn’s. (b) (post-Gad) showing contrast enhancement of the inflammatory stricture.

In established CD, an important question is whether active inflammation is present or not. Identifying the cause of wall thickening requires comparison of T1 and T2W sequences since active inflammation will produce high signal intensity within the bowel wall, whereas fibrostenotic disease due to long standing fibrosis will be low signal.13 Other signs which help differentiate between acute inflammation and fibrosis include deposition of submucosal fat, which should be of similar appearance to mesenteric fat. So-called ‘creeping fat’ within the mesentery can also be identified in long standing IBD.

Detection of extramural disease is critical and studies show that MRE is superior to contrast follow-through studies for this. Sinuses and fistulae, common complications of CD, are fluid-filled tracts which show peripheral enhancement and can be identified on T2W and post-gadolinium MRI. The tract may be predominantly inflammatory, in which case there will be uniform contrast enhancement with less central fluid content. Fistulae occur in one third of patients with CD14 and can be divided into internal (IF) and external (EF). The EFs manifest in the perianal region (54%) and drain to the cutaneous surface. In contrast, IFs are less common and are more difficult to diagnose and treat. They may form between different parts of the small and large bowel and may involve other abdominal organs and structures such as the bladder, vagina, or abdominal wall, without draining to the cutaneous surface. They represent an impending risk for small bowel obstruction and abscess formation. IFs may be simple or complex in there course. Complex IFs take on a stellate appearance, with multiple tracts radiating to multiple in-pulled bowel loops.4 Abscesses contain fluid, with or without associated air and an enhancing wall. Bowel obstruction can also be identified and multiphasic cine acquisition can be useful to ascertain whether obstruction is due to active inflammation or fibrostenotic disease.

5.3. Small bowel tumours and small bowel obstruction 

Small bowel tumours are rare and difficult to diagnose, hence there is little work on the sensitivity of MRI in detecting such small bowel masses. However it may be at least equivalent to capsule endoscopy in patients with small bowel polyposis.13 MRE has the advantage of superb soft tissue characterization, exploiting the presence of a biphasic oral contrast agent. These features are lacking in CT enterography. Fig. 5 demonstrates a patient with a jejunal adenocarcinoma diagnosed on MRE.

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  • Fig. 5 

    (a) (Coronal FIESTA) and (b) (axial FIESTA) showing circumferential thickening of the proximal Jejunum due to adenocarcinoma. (c) showing intense contrast enhancement of the tumour.

In acute small bowel obstruction, abdominal CT shows a high sensitivity in identifying the cause and is widely available. As such it is the first line investigation in clinical practice. MRE would however be useful in this clinical scenario, having the advantage of no ionising radiation and thus a greater number of times that imaging can be performed. MRE may be of use in low grade small bowel obstruction, especially at demonstrating subtle transition points of obstruction due to adhesions and particularly in the presence of soft tissue bands (Fig. 6).

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6. Conclusion 

MRE benefits from the inherent advantages of MRI: no ionising radiation, multiplanar imaging, exquisite soft tissue contrast and the ability to use different sequences to maximise information from the study. Its volumetric capabilities as well as the ultrafast cine sequences are further areas for future development. Sub-second imaging and palatable, easily available oral contrast agents have rendered this technique popular with both patients and clinicians alike. The simplicity of MR enterography when compared to enteroclysis means that is can be easily used in any hospital. It has become the first line investigation of suspected and confirmed IBD and has great potential in the investigation of other suspected small bowel pathologies.

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References 

  1. Masselli Gabriele, Casciani Emanuele, Polettini Elisabetta, Gualdi Gianfranco. Comparison of MR enteroclysis with MR enterography and conventional enteroclysis in patients with Crohn’s disease. Eur Radiol. 2008;18:438–447
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PII: S1078-8174(10)00078-7

doi:10.1016/j.radi.2010.06.001

Radiography
Volume 17, Issue 1 , Pages 67-71, February 2011

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