Acute colitis

ACR AC Acute Nonlocalized Abdominal Pain; toxic megacolon CT literature

Imaging rarely names the cause of a colitis, but it does two things that change management on the day: it shows the distribution, which narrows the differential, and it finds the complications that make this an urgent surgical problem.

Diarrhoea, often bloody, with abdominal pain and systemic upset, either as a first presentation or as a flare of known inflammatory bowel disease.

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

Immunosuppressed

Changes the differential rather than the modality.

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

CT Abdomen and Pelvis — Portal Venous Phase

CT Abdomen and Pelvis

What we'd amend, and why

Intravenous contrast turns a non-specific thick colon into an interpretable finding. Submucosal oedema between an enhancing mucosa and an enhancing serosa produces the stratified target appearance of active inflammation, whereas a thickened segment that fails to enhance at all indicates ischaemic non-viability, and those two look identical without contrast. The portal venous phase also demonstrates the complications that change the plan within the hour: pneumatosis, portal venous gas, perforation and pericolic collections.

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

How was this decided?

pathwayadult — Adults
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
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(4)nothing for you to do

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

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
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.

Immunosuppressed or neutropenic

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	The study is the same and the reasons for it are not. Neutropenic enterocolitis is a right-sided, caecal disease of the patient a week or two out from cytotoxic chemotherapy, and the finding that makes the diagnosis is circumferential caecal and ascending colonic wall thickening with pericolic stranding — which needs a cross-sectional study, because a supine radiograph shows it only once the wall has become thick enough to be a mass or the bowel has perforated. The portal-venous injection carries the same weight it does in any colitis, separating an inflamed but perfused wall from a non-enhancing dead one, and it adds pneumatosis, portal venous gas and perforation, which are the findings that turn a medical problem into a surgical one. What is different is the threshold. Neutropenia and corticosteroids blunt the physical signs, so peritonism arrives late or not at all and the abdomen can be soft in a patient with a perforated caecum; typhlitis, cytomegalovirus colitis, graft-versus-host disease and Clostridioides difficile all present the same way and are separated largely by distribution. And there is no endoscopic fallback here — biopsy in a profoundly neutropenic, often thrombocytopenic patient carries a perforation and bleeding risk that imaging does not, which makes the scan the primary investigation rather than a preliminary one.

The plain-film monitoring shortcut used in acute severe ulcerative colitis does not transfer to this patient: caecal wall thickening is the finding and a radiograph does not show it.
Wall thickness is graded and reported as a number here, because the threshold at which it is described influences whether the patient is managed medically or referred for surgery, and because serial comparison is how the response is judged.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Intravenous contrast turns a non-specific thick colon into an interpretable finding. Submucosal oedema between an enhancing mucosa and an enhancing serosa produces the stratified target appearance of active inflammation, whereas a thickened segment that fails to enhance at all indicates ischaemic non-viability, and those two look identical without contrast. The portal venous phase also demonstrates the complications that change the plan within the hour: pneumatosis, portal venous gas, perforation and pericolic collections.
Reasonable alternative	Abdominal radiograph Abdominal radiograph — supine	For the inpatient already diagnosed with acute severe colitis, the daily monitoring question is transverse colonic diameter and mucosal islands, and a supine abdominal film answers it at a fraction of the dose. The number the surgeons act on is a transverse colonic diameter beyond roughly 5.5 to 6 cm — or a caecum beyond about 9 cm — in a patient with systemic toxicity, which is toxic megacolon and a surgical conversation the same day, so the measurement belongs in the report rather than an impression of dilatation. It is monitoring, not diagnosis: it cannot show mural enhancement and it can miss a perforation in a dilated colon, so new pain, a falling diameter with a deteriorating patient, or free gas on the film all convert the question back to CT.

Pitfalls

Accepting an unenhanced study when the real question is ischaemic versus inflammatory, which is an enhancement question.
Permitting positive oral contrast, which obscures the mucosal surface.
Repeating CT daily for a monitoring question that a plain film answers at a fraction of the dose.
Reporting a thickened colon without the distribution. Segment and territory carry most of the diagnostic information here.
Treating an undistended, collapsed sigmoid as colitis — a common false positive when the colon is empty.
Describing colonic dilatation without measuring it. Toxic megacolon is a number in a toxic patient — a transverse colon beyond roughly 5.5 to 6 cm — and an impression of "dilated bowel" does not start the surgical conversation that measurement does.
Sending a young patient with a flare of known inflammatory bowel disease for CT when the question is severity rather than a complication. This population accumulates scans over decades, and the complication question — collection, perforation, ischaemia — is what justifies the dose.

Priors — what to pull first

Check for a known diagnosis of inflammatory bowel disease and the extent documented at previous endoscopy, which reframes the study as a flare assessment.
Compare colonic calibre with any earlier film or scan; dilatation is judged as change as much as by an absolute measurement.

What makes a good request

Distribution is the most useful discriminator available to imaging: a segment limited to a watershed territory suggests ischaemia, right-sided disease suggests infection, and continuous disease extending proximally from the rectum suggests ulcerative colitis.
In known acute severe colitis under medical treatment, the daily question is colonic diameter and free gas, which does not need repeated cross-sectional imaging.

How these studies are acquired

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

CT Abdomen and Pelvis — Portal Venous Phase Abdominal radiograph — supine

Confirm locally

CT Abdomen and Pelvis — Portal Venous Phase: 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.