Crohn disease — small bowel assessment and monitoring

ACR AC Crohn Disease (2020); ECCO-ESGAR diagnostic guideline

A lifelong disease of young people, which makes cumulative radiation the dominant vetting consideration and MR enterography the default. The recurring protocol failure is not the modality but the preparation: without luminal distension the study answers nothing.

Suspected Crohn disease, or known disease being assessed for activity, extent, stricturing or penetrating complications.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MR Enterography — small bowel

MRI Small Bowel (MR Enterography)

What we'd amend, and why

The distinction that matters clinically is active inflammation, which responds to drugs, versus fibrotic stricture, which does not — and MRI is the only modality that separates them non-invasively. Mural oedema on fat-suppressed T2, restricted diffusion and avid layered enhancement indicate active disease; a thickened segment with low T2 signal and delayed homogeneous enhancement indicates fibrosis. Cine acquisitions add whether an apparent stricture is fixed or merely a transient contraction. A large-volume neutral oral agent is what makes any of this visible: an undistended loop is indistinguishable from a thickened one.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-gadolinium-renal — Kidney function and gadolinium-based contrast; checked by Radiographer at the scanner
rulerule-pregnancy-gadolinium — Gadolinium in known or possible pregnancy; checked by Radiographer at the scanner
rulerule-fasting-prep — Patient preparation: fasting, enteric contrast and lactation advice; checked by Scheduling team
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(3)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.
Patient preparation: fasting, enteric contrast and lactation advice
Confirm the correct preparation was issued and followed. Three separate things are commonly conflated and should not be: (1) routine intravenous iodinated or gadolinium contrast requires NO fast — the ACR Manual states fasting is not required before routine intravascular contrast administration; (2) oral or rectal contrast protocols have their own timing which is a protocol requirement, not a fast; (3) planned sedation or anaesthesia does require fasting, on anaesthetic rather than contrast grounds.
Scheduling teamBefore the scan
Flags back if: The patient has not taken the oral or rectal preparation on which the protocol depends, or is incorrectly fasted or unfasted for a planned sedation or anaesthetic.
The oral preparation is the examination. Roughly an hour of staged drinking of a large-volume neutral agent is what distends the bowel, and a patient who arrives having drunk only part of it produces a study that cannot be reported for activity. Booking should tell the patient this explicitly, and a poorly distended study is a repeat rather than a report.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Children and adolescents

Role	Study & protocol	Why this answers the question
First line	Paediatric abdominal ultrasound Paediatric abdominal ultrasound	A slim abdominal wall and thin body habitus make the terminal ileum accessible to high-frequency transducers, so bowel wall thickness, mural stratification and hyperaemia on Doppler can be assessed directly and repeatedly at no radiation cost and with no sedation. It is the natural first look in a child and can be repeated as often as the disease demands.
Second line	MRI Small Bowel (MR Enterography) MR Enterography — small bowel	MR enterography maps the whole small bowel, including segments ultrasound cannot reach, and does so without radiation in a patient who will need this examination many times over a lifetime. Tolerance of the oral preparation, not the scan, is usually the limiting factor in younger children.
Problem solving	CT Enterography CT Enterography — Neutral Oral with Enteric-Phase Acquisition	Reserved for the acutely unwell child in whom an abscess or perforation must be excluded quickly and MRI cannot be delivered, using paediatric dose parameters.

Febrile or septic — suspected abscess or perforation

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Sepsis changes what is being asked. The activity question — inflammatory against fibrotic, and how much drug the disease needs — is a question about the next few weeks, and enterography answers it beautifully after an hour of staged drinking that a septic patient will not manage and should not be asked to. The question in front of this patient is whether there is a drainable collection, a free perforation or a phlegmon, and a standard portal-venous abdomen answers it now, with no preparation, in a few minutes on the table. It is the same reasoning that puts an unprepared study ahead of a prepared one anywhere else in the abdomen: the preparation buys mural detail, and mural detail is not what is being asked for. There is a second reason not to defer, which is that corticosteroids and biologic therapy blunt the signs — a Crohn patient on a biologic with an intra-abdominal abscess can have a soft abdomen and an unremarkable examination, so the clinical threshold for scanning is lower here than the physical signs suggest.
Second line	MRI Small Bowel (MR Enterography) MR Enterography — small bowel	Enterography is not cancelled by this arm, it is deferred by it. Once the sepsis is controlled and the collection drained, the disease still has to be mapped — extent, activity, stricturing, and whether a fistula tracks from a segment that will need resecting rather than escalating drugs — and MRI is the study that does that without adding to a radiation burden this patient will accumulate over decades. In a young adult with a first penetrating presentation, arranging the enterography for the recovery phase at the same time as the CT is the sequence that avoids a second CT later.
Problem solving	CT Enterography CT Enterography — Neutral Oral with Enteric-Phase Acquisition	Where the patient is well enough to take the oral preparation and both questions are genuinely live at once — a septic complication and the extent of disease before an operation that is already being planned — enteric-phase CT enterography answers both in one acquisition and saves a second study.

Perianal sepsis is a different field of view and a different study; it has its own card and a general abdominal CT will not answer it.
The arm is entered on fever alone, which is deliberately broad, and the dose decision still has to be made honestly. A young adult with a flare and a low-grade temperature who is otherwise well is not automatically a CT: where a bedside ultrasound can see the collection in a slim abdomen, or an MR enterography can be delivered the same day, that is the better answer. CT is for the patient in whom the answer is needed now, and "now" is a clinical judgement rather than a property of the thermometer.
Count the previous scans. This is a disease of young people who accumulate imaging over decades, so each CT should be able to name the question it is answering that the last one did not.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Small Bowel (MR Enterography) MR Enterography — small bowel usually appropriate	The distinction that matters clinically is active inflammation, which responds to drugs, versus fibrotic stricture, which does not — and MRI is the only modality that separates them non-invasively. Mural oedema on fat-suppressed T2, restricted diffusion and avid layered enhancement indicate active disease; a thickened segment with low T2 signal and delayed homogeneous enhancement indicates fibrosis. Cine acquisitions add whether an apparent stricture is fixed or merely a transient contraction. A large-volume neutral oral agent is what makes any of this visible: an undistended loop is indistinguishable from a thickened one.
Reasonable alternative	CT Enterography CT Enterography — Neutral Oral with Enteric-Phase Acquisition	For a first assessment in an unwell adult, or where MRI capacity or tolerance is the barrier, CT enterography answers the same anatomical questions faster. It uses the same neutral oral distension and an earlier enteric-phase acquisition than a routine abdomen, because peak bowel wall enhancement occurs before the conventional portal venous window.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	The acutely septic or peritonitic patient needs the abscess or perforation question answered now, and a standard portal-venous abdomen does that without the hour of oral preparation an enterographic protocol requires. It is a complication study, not an activity study.

Pitfalls

Accepting a standard CT abdomen and pelvis as an assessment of disease activity. Without neutral luminal distension and enteric-phase timing, mural thickening and enhancement cannot be assessed.
Allowing positive oral contrast, which conceals the mural enhancement that is the entire measurement.
Sending a young adult repeatedly to CT for what is a lifelong monitoring question. Cumulative dose in this population is the dominant vetting consideration, and the modality decision made at each flare is what determines it — MR enterography and ultrasound carry none of it, and a CT should be able to state the question it is answering that the previous one did not.
Reading a collapsed loop as thickened bowel, the commonest false positive on an underprepared study.
Treating a stricture as fibrotic without the T2 and enhancement features, which changes the patient from a drug pathway to a surgical one.

Priors — what to pull first

Compare with the previous enterography using the same measurement points; disease activity in Crohn disease is judged as change, not as an absolute appearance.
Check for previous resections, because a neoterminal ileum at an anastomosis is the commonest site of recurrence and needs specific attention.

What makes a good request

Say what is being asked. Activity, a stricture, a fistula and an abscess are four different questions with different urgency, and the answer to the last one is often a same-day CT.
This population accumulates imaging over decades, so a repeated CT strategy carries a dose burden that a repeated MRI strategy does not.

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Paediatric abdominal ultrasound MR Enterography — small bowel CT Enterography — Neutral Oral with Enteric-Phase Acquisition CT Abdomen and Pelvis — Portal Venous Phase

Confirm locally

MR Enterography — small bowel: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.