Magnetic resonance enterography is a
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...
technique used to evaluate
bowel
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
wall features of both upper and lower
gastro-intestinal tract, although it is usually used for small bowel evaluation.
It is a less invasive technique with the advantages of no
ionizing radiation
Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
exposure, multiplanarity and high contrast resolution for soft tissue.
The term MR enterography and MR enteroclysis are similar, but the first is referred to a MR exam with orally administered enteric contrast media, and the second to a more invasive technique in which enteric contrast media is administered through the
fluoroscopy
Fluoroscopy (), informally referred to as "fluoro", is an imaging technique that uses X-rays to obtain real-time moving images of the interior of an object. In its primary application of medical imaging, a fluoroscope () allows a surgeon to see t ...
-guided positioned
nasojejunal tube
A feeding tube is a medical device used to provide nutrition to people who cannot obtain nutrition by mouth, are unable to swallow safely, or need nutritional supplementation. The state of being fed by a feeding tube is called gavage, enteral fee ...
.
The need for imaging assessment of small bowel diseases comes from the limits of traditional endoscopy in evaluating ileum loops, as other modern techniques such as
capsule endoscopy
Capsule endoscopy is a medical procedure used to record internal images of the gastrointestinal tract for use in Medical diagnosis, disease diagnosis. Newer developments are also able to take biopsies and release medication at specific locati ...
are not routinely performed as it is seldom available in most centers. Over the past several years assessment of small bowel diseases was performed by Barium follow through, or upper and lower gastrointestinal series, that provided plan film of bowel loop lumen, thanks to the swallowing or instillation of radiopaque agents mixed with water or other neutral contrast media. Gastrointestinal series allow to depict lumen caliber, gross mucosal alterations and wide fistulous tract, but were poorly diagnostic for submucosal or extraluminal features. CT scan instead provides cross sectional and multiplanar images of intraluminal and extra-mucosal, extra-luminal or even extra-enteric features, but costing higher radiation dose.
The spread of MR technique has revolutionized the diagnostic imaging of the small bowel loop, restricting CT scan to particular situations, such as emergency or MR contraindications like patients with pacemaker implant, recently implanted vascular/bilious stent or other ferromagnetic prosthesis/devices. It is a safe, multi-planar imaging modality with high soft tissue contrast resolution that does not expose to ionizing radiation, so it is feasible for young patients or when several follow up are required.
Preparation
Cathartic preparation should be performed in order to clean residual stool from bowel loops from to allow a better visualization of mucosal features and an easier luminal distention as well. This type of preparation usually implies a fiber restricted diet and intake of water solution with laxative effect few days before the exam, and abstaining from food intake starting from six hours prior to the study.
Use of enteric contrast media is recommended, aiming to distend small bowel loops, and it is administered orally at regular intervals approximately 40 minutes before the study.
The type of endo-luminal contrast media varies among negative contrast media, consisting of superparamagnetic agents that evoke low signal both in T1 and T2 weighted images, positive contrast media, represented by paramagnetic agents, that produce high signal on both sequences, or biphasic contrast media, that gives high signal intensity in T2 and low intensity in T1.
This latter, that consists of water,
methyl cellulose
Methyl cellulose (or methylcellulose) is a compound derived from cellulose. It is sold under a variety of trade names and is used as a thickener and emulsifier in various food and cosmetic products, and also as a bulk-forming laxative. Like cell ...
or
polyethylene glycol
Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular wei ...
, is the most used, because of the wide availability, low cost, good patient compliance, and good taste. Water enema may be administered as well in order to distend bowel loop (MR-colonography).
Intravenous contrast media increases diagnostic capability of enterography MRI. Although it is better tolerated than iodinated contrast media used for CT-scan, the use of gadolinium-based
contrast agent
A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
should always be preceded by kidney function assessment, in order to reduce the risk of
nephrogenic systemic fibrosis
Nephrogenic systemic fibrosis is a rare syndrome that involves fibrosis of the skin, joints, eyes, and internal organs. NSF is caused by exposure to gadolinium in gadolinium-based MRI contrast agents (GBCAs) in patients with impaired kidney funct ...
, and prophylactic protocol in case of previous allergic reactions.
Antispasmodic
An antispasmodic (synonym: spasmolytic) is a pharmaceutical drug or other agent that suppresses muscle spasms.
Smooth muscle spasm
One type of antispasmodics is used for smooth muscle relaxation, especially in tubular organs of the gastrointestina ...
agents may be used to reduce the motion artifacts due to
peristalsis
Peristalsis ( , ) is a type of intestinal motility, characterized by symmetry in biology#Radial symmetry, radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an wikt:anterograde, anterograde dir ...
.
Protocol
High field MR scanners and the use of multi-channel phased array surface coil are suggested in order to obtain adequately diagnostic images.
The subject drinks 1.5 litres of oral contrast (3% mannitol)
over 30 to 45 minutes before the scan. After that
venous access is obtained and Buscopan (
hyoscine butylbromide
Hyoscine butylbromide, also known as scopolamine butylbromide and sold under the brandname Buscopan among others, is an anticholinergic medication used to treat abdominal pain, esophageal spasms, bladder spasms, biliary colic, and renal colic. ...
) is given to reduce the gastrointestinal tract movement, thus reducing motion artifact on MRI scan.
The patient is placed in prone position, thus provides better separation of bowel loops and reduces breathing movement-artifacts. Although MR enterography protocols may vary among different hospitals/institutions, the main sequences are the following:
# Axial and coronal balanced
steady-state free precession imaging
Steady-state free precession (SSFP) imaging is a magnetic resonance imaging (MRI) sequence which uses steady states of magnetizations. In general, SSFP MRI sequences are based on a (low flip angle) gradient echo MRI sequence with a short repetitio ...
(SSFP, commercial name FISP)
# Axial and coronal
single-shot-fast spin echo (commercial name HASTE) with
fat saturation
# Axial and coronal 3D
spoiled gradient echo (commercial name VIBE) before and after
gadolinium contrast
MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI).
The most commonly used compounds for contrast enhancement are gadolinium-based contrast agents (GBCAs). Suc ...
administration
# Axial
Diffusion Weighted Imaging (DWI) sequences,
using at least 2 B-value
# Cine loop technique using SSFP sequences
Indications
The most common indication of MR enterography is diagnosis and follow up of inflammatory and
neoplastic
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
small bowel disease.
Risks and contraindications
Risks and contraindications are the same of any MR exam.
References
* Fidler JL, Guimaraes L, Einstein DM. MR Imaging of the Small Bowel. RadioGraphics 2009; 29:1811–1825
* Ilangovan R, Burling D, George A, Gupta A, Marshall M, and Taylor SA. CT enterography: review of technique and practical tips. Br J Radiol. 2012 Jul; 85(1015): 876–886
* Lo Re G, Midiri M, et Al. Crohn's disease. Radiological features and clinical-surgical correlations; Cap.12:107-113; Cap.14:128-133
{{Medicine, state=collapsed
Magnetic resonance imaging