CT Colonography — Contrast-Enhanced (Symptomatic or Frail)

Diaphragm to below the pubic symphysis; the intravenous-enhanced acquisition covers the abdomen and pelvis for staging.

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

Symptomatic patient with suspected colorectal cancer where a single study should both examine the colon and stage the abdomen.
Frail or reduced-preparation patients in whom intravenous enhancement compensates for poorer tagging.
Known colorectal cancer where the proximal colon must be cleared and the liver assessed at the same visit.

Technique

One acquisition (usually supine) is performed after intravenous contrast at a venous-phase delay of roughly 60-80 s from the start of injection; the other position is acquired unenhanced and low dose.
Reduced-preparation protocols lean more heavily on tagging and on intravenous enhancement, and are a recognised compromise in the frail.
The dose is higher than the screening protocol and this should be a deliberate choice tied to the symptomatic indication.

Where it goes wrong

Applying this protocol to asymptomatic screening adds contrast risk and dose for no screening benefit.
Enhancement does not rescue a poorly distended colon — distension remains the limiting factor.
A venous-phase acquisition through a gas-distended colon still needs the same 2D and 3D review discipline; the intravenous contrast is for the extracolonic staging question.

Clinical questions that reach this study

Colorectal cancer — staging

Contrast

Oral, positiveIodinated, intravenous

Faecal tagging as for the screening protocol — typically an iodinated tagging agent given in divided doses across the meals of the day before, or a dilute barium regimen of similar attenuation — plus a standard staging intravenous injection of roughly 100-150 mL of non-ionic iodinated contrast (300-370 mgI/mL) at around 3 mL/s with a saline chaser. Weight-based dosing of about 1.5 mL/kg is used in some centres.

The intravenous dose is the ordinary abdominopelvic staging injection rather than anything colonography-specific: it is there for the liver and the nodes, not for the colonic wall, and the endoluminal assessment still depends entirely on distension and tagging.
Tagging regimens differ substantially between consensus documents and between departments, and the reduced-preparation variants differ again. Confirm the local regimen — this is one of the numbers most likely to be wrong if copied from another centre.
Carbon dioxide insufflation is unchanged from the screening protocol and is not a contrast agent.

Acquisition

Breathing: Breath-hold for each acquisition.
Reconstruction: Thin overlapping reconstructions for endoluminal review plus a standard soft-tissue series.
Preparation: Cathartic bowel preparation with oral faecal tagging over the preceding day or two, per local regimen. Reduced-preparation regimens exist for frail patients and rely more heavily on tagging. Antispasmodic (for example hyoscine butylbromide) is often given immediately before insufflation where not contraindicated.

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.

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.
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