MR Enterography
MR enterography is a dedicated MRI study of the small bowel after oral distension with a biphasic contrast agent. It shows the bowel wall, lumen, mesentery and extra-intestinal complications without ionising radiation.
The patient drinks 1 to 1.5 litres of an osmotically active, poorly absorbed solution (mannitol, sorbitol, polyethylene glycol or a methylcellulose preparation) over 45 to 60 minutes. This gives a dark lumen on T1 and a bright lumen on T2.
An antispasmodic (hyoscine butylbromide 20 mg or glucagon) is given just before and again during the study to reduce peristalsis.
The core sequences are coronal and axial T2-weighted single shot images, balanced steady state free precession images, diffusion weighted imaging, pre-contrast and dynamic post-contrast T1-weighted fat suppressed images (gadolinium 0.1 mmol each kg), and cine sequences of bowel motility. The study takes 30 to 45 minutes.
Crohn disease affects about 1 in 1000 people in high income countries, and the incidence is rising in many regions. The small bowel is involved in about two thirds of cases, and much of it lies beyond the reach of the colonoscope.
MR enterography has a sensitivity of 80 to 90% and a specificity of 85 to 95% for active small bowel disease, and it detects complications such as strictures, fistulas and abscesses.
It is the preferred cross-sectional test in young patients and for repeated follow up, because the cumulative radiation dose of repeated CT is avoided.
Indications.
- Known or suspected Crohn disease: diagnosis, extent, activity, complications and response to treatment.
- Unexplained chronic abdominal pain, diarrhoea or weight loss with a suspected small bowel cause.
- Suspected small bowel tumour (neuroendocrine tumour, adenocarcinoma, lymphoma, gastrointestinal stromal tumour), polyposis syndromes.
- Obscure gastrointestinal bleeding after negative endoscopy.
- Coeliac disease complications and suspected lymphoma.
- Suspected small bowel obstruction of low grade or intermittent type.
- Perianal and enteric fistulas, and post-operative recurrence.
- Intestinal tuberculosis and other infiltrative disease.
- Children and young adults who need repeated imaging.
Not for acute abdomen with perforation or complete obstruction, where CT is faster and the oral load is unsafe.
Contraindications and precautions.
- The usual MRI safety screen (pacemakers, clips, implants, metal fragments).
- High grade obstruction, perforation or toxic colitis: oral load is unsafe.
- Hyoscine is avoided in glaucoma, prostatic obstruction and tachyarrhythmia. Glucagon is avoided in phaeochromocytoma and insulinoma.
- Gadolinium: renal precautions apply, and a macrocyclic agent is used.
- Pregnancy: non-contrast MRI is acceptable if needed.
- Claustrophobia and inability to hold the breath for 15 to 20 seconds.
- Sorbitol and mannitol can cause diarrhoea and cramps. Warn the patient.
Preparation and technique.
The patient fasts for 4 to 6 hours. A bowel cleansing preparation is not needed.
The oral agent is drunk in divided portions over 45 to 60 minutes, with the last portion just before scanning. The patient lies prone or supine. Prone position spreads the loops and may reduce overlap, though supine is better tolerated.
Coronal and axial T2 images are taken first, then balanced steady state images and diffusion weighted images. The antispasmodic is given, and the T1 pre-contrast series is taken. Gadolinium is then given intravenously with dynamic images at about 30 and 70 seconds, followed by a delayed series.
Cine imaging for 10 to 20 seconds in the coronal plane shows peristalsis through stricture sites and tethering.
