MR Enterography: a technical review
Magnetic resonance enterography (MRE) is an imaging technique that has evolved in the last 15-20 years. It involves the use of magnetic resonance imaging (MRI) to assess the small bowel. The advantages of this technique are that MRE uses no ionising radiation, is capable of direct (not reconstructed) multi-planar imaging, offers high-contrast resolution and can provide cine-imaging.
Indications for MRE
The most common indication is to evaluate patients with Crohn disease (CD)
Other indications include:
small bowel malignancies
Routine medication should not be stopped.
Patients should not eat or drink any solid food for 4-6 hours, in an attempt to reduce motion artifacts arising from bowel peristalsis.
Only non-sparkling water is permissible.
Technique & Patient Positioning
Have patients empty their bladder before imaging. A full bladder may cause discomfort to the patient, resulting in motion artifacts and blurry images.
Patients should be placed in the prone position. Prone position results in better small bowel distention and reduced motion artifacts.
There is no single preferred oral contrast agent for MRE. Recommended agents include mannitol (with or without locust bean gum), PEG, sorbitol and lactulose.
Oral contrast agents used for MRE can be categorized as positive contrast agents, negative contrast agents, and biphasic agents.
The most commonly used MRE contrast agents are biphasic, exhibiting high signal intensity on T2-w imaging (allowing for detection of wall thickening, endoluminal abnormalities, and transmural ulcers) and low signal intensity on T1-w imaging (enhancing detection of mucosal enhancement and hypervascular endoluminal masses).
Most patients manage to ingest contrast orally without too much difficulty.
Compliance can be optimized by giving the patient input into choice of flavoring and keeping drinks chilled.
The optimal volume of oral contrast is 1-1,5 L.
Mannitol solution as follows: 7.5 mg locust bean gum and 250ml Mannitol 20% in 1250ml of water.
Mannitol uptake should take place as follows: 750ml per 20-30 min.
The total ingestion time of oral contrast should be 45-60 minutes.
Patients are instructed to ingest the oral contrast at a steady pace to obtain uniform distention throughout the small bowel.
When scanning patients with a stoma, the stoma should be plugged before oral contrast ingestion.
Antispasmodic agent should be administered during the MRE.
The timing of antispasmodic agent administration should consider the susceptibility of the applied MRI sequences to motion artifacts (fig. 1).
Our recommendation is to administer the antispasmodic agent right before the dynamic contrast-enhanced MR imaging.
First line antispasmodic agent is intravenous (IV) hyoscine butylbromide (trade name: Buscopan). The recommended dose is 20 mg.
Second line antispasmodic agent is IV glucagon. The recommended dose is 1 mg.
Fig. 1 illustrates the effect of timing on antispasmodic agent administration. Early administration of the antispasmodic agent leads to motion artifacts and poor image quality (left image), while correct administration (right before the dynamic imaging) results in high-quality dynamic contrast-enhanced MR imaging (right image).
MR Sequences and protocol
Coronal and Axial T2-w single-shot fast spin echo (SS-FSE/HASTE) without fat suppression.
Coronal and Axial FIESTA/True-FISP without fat suppression.
Coronal and/or Axial T2-w single-shot fast spin echo (SS-FSE/HASTE) with fat suppression.
Non-enhanced Axial T1-w GRE with fat suppression.
Non-enhanced Coronal T1-w GRE with fat suppression followed by dynamic contrast-enhanced Coronal T1-w with fat suppression.
Contrast-enhanced Axial T1-w with fat suppression (delayed phase).
MRE dynamic acquisitions are divided as follows: arterial (30 sec), enteric (45 sec), venous (70 sec) and delayed (≥ 90 sec) phases.
In patients with known or suspected inflammatory bowel disease, contrast-enhanced sequences should be obtained during the enteric (45 sec) and/or venous phase (70 sec).
In patients with suspected chronic gastrointestinal (GI) bleeding contrast-enhanced sequences should be obtained during the arterial (30 sec), enteric (45 sec) and/or portal venous phase (70 sec) phase.
Diffusion-weighted imaging (DWI) and cine FIESTA/True-FISP are optional sequences. However, we strongly suggest their routinely usage because these sequences provide very useful biological and functional information.
DWI can better identify the inflamed bowel loops in active phase. Any number of b values between 0 and 1000 s/mm2 provide adequate imaging. This correlates with actively inflamed bowel and lymph nodes, with the appearance of bowel fibrosis on DWI not yet defined.
Cine FIESTA is a fast sequence to evaluate bowel peristalsis and is particularly helpful in cases of obstruction or to confirm presence of strictures.
The MR protocol is summarized in the table below.
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