MR Enterography — small bowel
Diaphragm to pubic symphysis, covering the whole small bowel, the colon and the perianal region where relevant.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
Premedication
Given to make the acquisition work, not to treat the patient. Doses are typical — confirm against your local protocol and prescribe within your own governance.
Bowel paralysis. Peristalsis blurs the bowel wall over the acquisition, and mural detail is the finding in every one of these studies — an unparalysed study is not a slightly worse study, it is often a non-diagnostic one.
- Dose and route
- Typically 20 mg intravenously immediately before acquisition. Where hyoscine is unavailable, glucagon roughly 0.25-1 mg intravenously (or about 1 mg intramuscularly) is the usual substitute. Glucagon is NOT a benign substitute: it is contraindicated in phaeochromocytoma, where it provokes catecholamine release and hypertensive crisis, and in insulinoma or glucagonoma, where it causes rebound hypoglycaemia. It also raises blood glucose transiently, which matters in diabetes, and commonly causes nausea and vomiting.
- When
- Immediately before the acquisition it is meant to cover — the effect lasts only a few minutes, so giving it early wastes it. MR enterography is where this bites hardest: the examination runs 20-30 minutes, far longer than the drug, so many centres split the dose, giving part before the high-resolution T2 and coronal sequences and part again before the post-contrast series. A single dose at the start of an MR enterography leaves the post-contrast acquisition — the one showing mural hyperenhancement — unprotected.
- Do not give if
- If glucagon is used instead of hyoscine, it carries its OWN absolute contraindications rather than inheriting a clean slate: phaeochromocytoma, where it provokes catecholamine release and hypertensive crisis, and insulinoma or glucagonoma, where it causes rebound hypoglycaemia. It also raises blood glucose transiently, which matters in diabetes, and commonly causes nausea and vomiting.
- MHRA Drug Safety Update (February 2017), issued after nine reported deaths mostly from cardiac arrest: in patients with cardiac disease, monitor the patient and ensure resuscitation equipment AND staff trained to use it are readily available before giving it. This is an availability requirement, not a caution to note and move past.
- Contraindicated in untreated angle-closure glaucoma, myasthenia gravis, megacolon, and significant tachyarrhythmia.
- Caution in prostatic enlargement with urinary retention, and in significant cardiac disease.
- Warn the patient about transient blurred vision, and that they must not drive until it resolves — this is the practical consequence people forget to mention.
- If unsuitable
- Proceed without it and accept some motion, or substitute glucagon where the contraindication is to hyoscine specifically — but not before excluding phaeochromocytoma and insulinoma, in which glucagon is itself contraindicated. Neither is a reason to cancel the study.
When to use it
- Suspected or newly diagnosed Crohn disease, to establish small bowel extent, inflammation and complications.
- Monitoring known Crohn disease for activity and response to therapy, repeatedly and without radiation.
- Suspected stricture, penetrating disease, fistula or abscess.
- Suspected small bowel tumour or polyposis surveillance.
- Assessment of the small bowel in coeliac disease with alarm features.
Technique
- Coronal and axial balanced steady-state free precession for anatomy and rapid survey, with and without fat suppression.
- Coronal and axial single-shot fast spin-echo T2, plus fat-suppressed T2 for mural oedema.
- Cine balanced steady-state acquisition through the small bowel for motility, acquired before the antiperistaltic agent or as a separate assessment.
- Diffusion-weighted imaging with low and high b values, which is increasingly used and in some protocols substitutes for intravenous contrast.
- Post-gadolinium fat-suppressed 3D T1 in coronal and axial planes; the useful enhanced acquisition for bowel wall is earlier than a conventional portal venous phase, at roughly 40-70 seconds, and practice differs.
Where it goes wrong
- Under-distension from incomplete oral intake — the commonest cause of a non-diagnostic study, and a collapsed loop both hides and mimics disease.
- Scanning too long after ingestion, by which time the oral agent has passed into the colon and the proximal small bowel has collapsed.
- Omitting the antiperistaltic agent, leaving motion artefact across the high-resolution sequences.
- Post-contrast timing too late, so mural hyperenhancement has faded.
- Coverage stopping above the perineum in a patient whose question includes perianal disease.
Clinical questions that reach this study
Contrast
Oral neutral or biphasic agent, typically around 1-1.5 litres of a mannitol or polyethylene glycol solution drunk over roughly 45-60 minutes. Intravenous macrocyclic gadolinium at a typical single dose of about 0.1 mmol/kg. An antiperistaltic agent (hyoscine butylbromide or glucagon) is given at the scanner.
- Reported oral volumes and agents vary substantially between centres; the common ground is a large-volume neutral or biphasic agent and a staged ingestion schedule.
Acquisition
- Breathing
- Breath-hold for the structural sequences; the cine motility acquisition is a repeated single-slice breath-hold or free-breathing series.
- Preparation
- Fast for approximately 4-6 hours, then drink the oral preparation over roughly 45-60 minutes before scanning; this is a long appointment and needs to be booked as one. An antiperistaltic agent is given intravenously or intramuscularly at the scanner; screen for glaucoma, cardiac disease and prostatism per the local policy for hyoscine butylbromide or glucagon. Patients who cannot tolerate the oral volume will produce an under-distended, non-diagnostic study — flag vomiting or obstruction at vetting.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.
- Portal venous phaseTypically ~60–90 s after the start of injection. Confirm locally.
The portal vein and hepatic veins are opacified and the liver is at maximum parenchymal enhancement; bowel wall, mesentery, spleen and peritoneum are all well enhanced. The transferable principle is that HYPOVASCULAR lesions are most conspicuous when the BACKGROUND peaks, so this is the single most productive general-purpose abdominal phase and the correct default when the question is "what is wrong in this abdomen?". Its corollary is the classic error: a hypervascular lesion that was obvious 30 s earlier can become isodense and invisible here, so a normal portal venous study never excludes hypervascular disease.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Prior contrast reaction and elective premedication· nurse pre scan
- Kidney function and gadolinium-based contrast· radiographer at scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- MR safety screening for implants and foreign bodies· radiographer at scan
- Gadolinium in known or possible pregnancy· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan