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CT Abdomen and Pelvis — Positive Oral Contrast

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.

When to use it

  • Suspected bowel leak or enterocutaneous fistula, where luminal contrast outside the lumen is the diagnosis.
  • Distinguishing an interloop abscess from unopacified bowel in a complex post-operative abdomen.
  • Assessment of an abdominal wall hernia or complex post-surgical anatomy where bowel course is unclear.
  • A very thin patient with little mesenteric fat, where bowel loops are otherwise hard to separate.

Technique

  • Water-soluble agent rather than barium wherever perforation or leak is suspected.
  • The preparation time is the protocol: scanning before the contrast has reached the segment of interest gives all of the delay and none of the benefit.
  • The interval is set by the segment in question, and this is the number that should be agreed at vetting rather than assumed at the scanner. Roughly 15-30 minutes opacifies the stomach and duodenum; roughly 45-60 minutes reaches the mid small bowel; roughly 60-90 minutes is what the terminal ileum and caecum need, and 90-120 minutes is not unreasonable in a patient with slow transit, obstruction or recent surgery. Opacifying the whole colon takes several hours and is not achievable within a same-day acute study.
  • A common split against that interval is roughly half the volume at the start, a quarter at about the halfway point, and the last 200-300 mL immediately before the patient lies down — the final portion exists to fill the stomach and duodenum, which will have emptied by the time the distal bowel is ready.
  • The intravenous component is timed independently and unchanged: a portal venous acquisition at roughly 60-80 s from the start of injection. The oral interval and the intravenous delay are two separate clocks and confusing them is how a study ends up with neither done properly.
  • This protocol should be a deliberate answer to a specific question, not a default applied to every abdominal request.
  • Volumes and intervals vary more between departments than almost any other part of a CT protocol, and the numbers above are typical rather than settled. Confirm locally.

Where it goes wrong

  • Positive oral contrast blocks any subsequent angiographic, urographic or enterographic use of the same visit — the decision is one-way for that appointment.
  • Dense luminal contrast causes streak artefact that degrades assessment of the pancreas and mesenteric vessels.
  • It obscures the bowel wall enhancement needed to assess ischaemia and inflammatory bowel disease activity.
  • Scanning before adequate transit produces an unopacified distal small bowel and a study that answers nothing.

Clinical questions that reach this study

Suspected intra-abdominal abscess or collectionSuspected appendicitisSuspected malrotation with midgut volvulusMajor trauma — whole-body (pan-scan) versus selective CTSuspected acute diverticulitisSuspected bowel obstructionSuspected gastrointestinal perforationSuspected acute mesenteric ischaemiaAcute pancreatitis — severity and complication assessmentSuspected acute cholecystitisObstructive jaundice and suspected biliary obstructionSuspected cholangiocarcinomaColorectal cancer — stagingRectal cancer — local staging and restagingCrohn disease — small bowel assessment and monitoringSuspected perianal fistula or perianal sepsisAcute colitisSuspected deep vein thrombosisAcute pelvic pain in the reproductive age group (including suspected PID and tubo-ovarian abscess)Adnexal mass — characterisation and risk stratificationStaging or completion CT in a known solid tumourResponse assessment on systemic therapySuspected or newly diagnosed lymphomaMetastatic disease with no identified primary

Contrast

Oral, positiveIodinated, intravenous

Typically a dilute water-soluble iodinated agent at roughly 2-3% (about 20-30 mL of concentrate made up to a litre), or a dilute barium suspension of similar attenuation, given as approximately 1000-1350 mL split across the preparation period, plus the standard intravenous injection. Dilution matters: undiluted contrast is dense enough to produce streak artefact and to mask mucosal enhancement.

  • A typical split is around 600-800 mL from about 60-90 minutes before, then a further 200-300 mL immediately before scanning to fill the stomach and duodenum, which empty first. Volumes and intervals vary widely between departments — confirm locally.
  • Roughly 60-90 minutes is what it takes to opacify the distal small bowel, and that delay IS the cost of this protocol. In an acute presentation it is not a neutral choice.
  • Withhold in the patient with a reduced conscious level or an unprotected airway, and in anyone likely to go to theatre imminently — aspiration of contrast is the harm, and an anaesthetist will not thank you for a stomach full of it.

Acquisition

Breathing
Single breath-hold.
Reconstruction
Thin axial reconstructions with multiplanar reformats.
Preparation
Fasting for a few hours is common practice where IV contrast is planned, though it is not a safety requirement for iodinated contrast. Oral contrast, if any, is a protocol decision and should not be given by default.

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. Positive oral contrastIngested in divided volumes beginning roughly 60 min before scanning (commonly split doses at about 60, 45 and 15 min), so that the bolus front reaches the distal small bowel. Confirm locally.

    A dense intraluminal agent labels bowel as bowel. The transferable principle is that the hardest structure to exclude on abdominal CT is unopacified bowel masquerading as something else — a collection, a mass, a leak, an abscess — and that the problem is worst exactly where mesenteric fat planes are thinnest, in the cachectic and the paediatric patient. Positive agents also demonstrate luminal continuity, so extraluminal contrast becomes direct evidence of perforation, leak or fistula. The trade-offs are symmetrical and important: the same density obscures mucosal enhancement, defeats bowel-wall assessment, degrades CT angiographic and three-dimensional reconstructions, and creates streak artefact, which is why several high-volume indications deliberately use a neutral agent instead.

  2. 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
  • 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