CT Colonography — Low Dose, Supine and Prone

Diaphragm to below the pubic symphysis in each position, so that the whole colon and rectum is covered including a redundant sigmoid.

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

Colorectal cancer screening where colonoscopy is declined, incomplete or contraindicated.
Completion imaging after an incomplete colonoscopy.
Symptomatic patients in whom colonoscopy carries excess risk, including the frail and those on anticoagulation.
Assessment of the proximal colon beyond an obstructing distal tumour.

Technique

Automated carbon dioxide insufflation is the recommended distension method, titrated to colonic pressure rather than to a fixed volume; carbon dioxide is absorbed faster than room air and causes less post-procedural discomfort.
Supine and prone acquisitions are both required. Where a patient cannot lie prone, a lateral decubitus acquisition substitutes.
Faecal tagging is treated as a mandatory part of the preparation in consensus guidance, not an optional adjunct.
Low-dose parameters are standard for screening; a tube current in the region of 50 mAs per position has been cited as a preferred setting except in larger patients.

Where it goes wrong

Inadequate distension of a segment — most often the sigmoid or transverse colon — makes that segment unassessable; recognising it during the study allows a repeat acquisition or a decubitus series while the patient is still on the table.
Untagged residual stool is indistinguishable from a polyp, which is why tagging is not optional.
Acquiring only one position removes the ability to show that a candidate lesion moves with gravity, which is how stool is excluded.
A single low-dose acquisition in a large patient is too noisy for confident 3D endoluminal review.
Suspected perforation or acute diverticulitis is a contraindication to insufflation, not a technique problem to work around.

Clinical questions that reach this study

Colorectal cancer — staging

Contrast

Oral, positive

Oral faecal tagging with dilute iodinated contrast or barium over the preparation period; no intravenous contrast in the screening protocol.

Carbon dioxide is used for colonic distension. It is an insufflated gas rather than a contrast agent and so is not represented as a contrast phase here.

Acquisition

Breathing: Breath-hold for each acquisition.
Reconstruction: Thin overlapping reconstructions — consensus guidance recommends collimation no wider than about 2.5 mm with roughly 20-30% overlap — for 2D and 3D endoluminal review.
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.

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.

Child-sized technique and contrast dose· radiographer at scan
Pregnancy status before an ionising exposure· radiographer at scan