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CT Enterography — Neutral Oral with Enteric-Phase Acquisition

Diaphragm to the pubic symphysis.

Typical, not policy

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.

Hyoscine butylbromide (Buscopan), or glucagon where unavailable

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, occasionally repeated. Where hyoscine is unavailable, glucagon roughly 0.25-1 mg intravenously is the usual substitute.
When
Immediately before the acquisition it is meant to cover — the effect is short-lived, so giving it early wastes it.
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 known Crohn disease: initial assessment, disease activity, and complications such as stricture, fistula or abscess.
  • Obscure gastrointestinal bleeding, where a multiphase variant may be used instead.
  • Suspected small bowel tumour.
  • Assessment of small bowel in a patient in whom MR enterography is unavailable or not tolerated.

Technique

  • Sources differ on the intravenous delay: the enteric phase is variously described at around 45-50 s and at around 60 s from the start of injection. What is not in dispute is that peak bowel wall enhancement is EARLIER than a standard portal venous phase, which is why this is a dedicated protocol rather than a routine abdominal study.
  • The `phase-portal-venous` slug is used here for that systemic venous window; the actual delay is protocol-specific and should be taken from the local card.
  • Distension is the study. An incompletely drunk oral preparation is the commonest reason a CT enterography answers nothing.
  • The final portion of the oral agent is often given immediately before the patient lies down, to distend the proximal small bowel that would otherwise have emptied.

Where it goes wrong

  • Positive oral contrast converts this into a routine abdominal CT and hides the mural hyperenhancement that defines active inflammation.
  • Under-distended loops look thick-walled and are the classic false positive; a collapsed loop cannot be called abnormal.
  • A standard portal venous delay flattens the mural hyperenhancement gradient.
  • Peristalsis blurs the wall; some protocols use an antispasmodic, though practice varies.
  • Repeated CT enterography in a young Crohn population accumulates dose — MR enterography is the usual alternative for serial assessment.

Clinical questions that reach this study

Acute gastrointestinal bleedingCrohn disease — small bowel assessment and monitoring

Contrast

Oral, neutralIodinated, intravenous

A large volume of neutral oral agent — consensus recommendations describe weight-based dosing of around 20 mL/kg to a maximum of about 1350 mL — drunk over roughly 45-60 minutes, plus typically 100-150 mL of non-ionic iodinated contrast at 3-5 mL/s.

  • Neutral means low attenuation (broadly under 20-30 HU), so that the enhancing bowel wall stands out against the lumen.
  • This is the whole reason the agent must NOT be positive. Mural hyperenhancement is the finding CT enterography exists to detect, and dense luminal contrast sitting against the mucosa hides exactly that. A positive-oral study booked as an enterography answers a different question and usually has to be repeated.
  • Typical agents are 0.1% low-density barium sulphate suspension, a 2-2.5% sorbitol or mannitol solution, or polyethylene glycol. Plain water is not adequate — it is absorbed too quickly to hold distension through the acquisition, which is the commonest reason a home-brewed protocol underperforms.
  • Split the volume: roughly a third at 60 minutes, a third at 40, a third at 20, then a final 200-250 mL immediately before the scan to distend the duodenum and proximal jejunum, which empty first.
  • The enteric phase is deliberate — around 45-50 s, earlier than a routine portal-venous abdomen — because mural enhancement peaks before parenchymal enhancement does. Scanning at 70 s flattens the mural-to-lumen difference the study depends on.

Acquisition

Breathing
Single breath-hold.
Reconstruction
Thin axial reconstructions with coronal reformats, which are the plane small bowel is followed in.
Preparation
Fasting for around four to six hours beforehand. A large volume of neutral oral agent (commonly a low-concentration barium sulphate or polyethylene glycol suspension) drunk over roughly 45-60 minutes, finishing shortly before the scan. Warn the patient the oral volume is large and often causes loose stool.

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.

  1. Neutral oral contrast (enterography preparation)Large-volume ingestion over roughly the 45–60 min before scanning (typically of the order of 1.3–2 L in divided doses). Confirm locally.

    A near-water-attenuation agent distends the lumen without hiding the wall. The transferable principle is that when the DISEASE IS IN THE WALL, the lumen must be made transparent rather than bright: a distended, low-attenuation lumen against enhancing mucosa is what makes mural hyperenhancement, wall thickening, stratification and comb-sign mesenteric hyperaemia measurable. Neutral agents are formulated to resist absorption so that distension survives to the terminal ileum. The cost is that they forfeit the one thing positive agents do best — an unenhanced-appearing lumen cannot demonstrate a leak — and that adequate distension depends on the patient completing an unpleasant volume on schedule, making this the phase most often defeated by preparation failure rather than by scanner technique.

  2. Enteric phase (peak small bowel mural enhancement)Published protocols genuinely disagree. The classical enteric phase is ~45–50 s after the start of injection (widely quoted simply as "50 s"), while other enterography protocols acquire at ~60–70 s, i.e. a conventional portal venous delay. Either way it is at or before a standard body venous delay, never after it. Confirm locally.

    Small bowel wall is thin, richly vascular and has a small interstitial space, so it fills and washes out faster than solid parenchyma and reaches peak mural enhancement EARLIER than the liver does. The transferable principle is that a protocol must be timed to the tissue that carries the diagnosis, and that mural hyperenhancement is a RELATIVE finding — an inflamed segment measured against a normal one — so the acquisition is placed where the gradient between them is widest. Every second of extra delay flattens that gradient as normal wall catches up. The phase is only readable at all against a distended, near-water-attenuation lumen, which is why it is inseparable from neutral oral preparation. The honest counterweight is that comparative work has not consistently shown better Crohn lesion detection at the enteric delay than at a portal venous one, so where a single acquisition must serve both bowel and liver a ~60–70 s compromise is defensible — the disagreement in the literature is real, not one camp being wrong.

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
  • Intravenous access adequate for the planned injection· radiographer at scan
  • Metformin and iodinated contrast· radiographer at scan
  • Child-sized technique and contrast dose· radiographer at scan
  • Pregnancy status before an ionising exposure· radiographer at scan
  • Kidney function and intravenous iodinated contrast· radiographer at scan

References