CT Abdomen and Pelvis — Rectal Contrast (Anastomotic Leak)

Diaphragm to the perineum, so that a low pelvic leak and any presacral collection are included.

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 colorectal or ileorectal anastomotic leak after surgery.
Suspected colovesical or colovaginal fistula.
Assessment of a pelvic collection for communication with the bowel lumen.
Assessment of a defunctioned distal segment before stoma reversal.

Technique

Instillation is gentle and gravity-fed; the endpoint is patient tolerance or reflux beyond the anastomosis, not a fixed volume.
The enteric interval here is minutes, not hours, and that is the practical advantage of the rectal route over the oral one. Gravity instillation of 500-1000 mL takes roughly 5-10 minutes on the table, and the diagnostic acquisition follows immediately — there is no transit period to wait out, which is why a leak study can be done acutely without the 60-90 minute oral preparation.
Where a stoma or a defunctioned distal limb is being studied, instillation is through a soft catheter (a Foley with the balloon left deflated or only just seated) and may need a further few minutes for contrast to reach beyond the anastomosis. That endpoint is confirmed, not assumed.
The rectal contrast is the diagnostic agent — the intravenous phase is for the surrounding inflammatory change and bowel perfusion.
The intravenous component runs on its own clock: a portal venous acquisition at roughly 60-80 s from the start of injection, given after the rectal instillation is complete so that the two are not competing for the same few minutes.
A scout or limited pelvic acquisition after instillation can confirm adequate filling before the full study.
Volumes, dilutions and instillation times vary between departments and none of them are settled numbers. Confirm locally.

Where it goes wrong

Insufficient instillation gives a false-negative: the contrast never reaches the anastomosis under study.
Over-vigorous instillation under pressure risks converting a contained leak into a free one.
Barium instead of a water-soluble agent is contraindicated where leak is suspected.
A study cropped above the perineum misses a low anterior resection anastomosis.

Clinical questions that reach this study

Suspected intra-abdominal abscess or collection Suspected appendicitis Suspected malrotation with midgut volvulus Major trauma — whole-body (pan-scan) versus selective CT Suspected acute diverticulitis Suspected bowel obstruction Suspected gastrointestinal perforation Suspected acute mesenteric ischaemia Acute pancreatitis — severity and complication assessment Suspected acute cholecystitis Obstructive jaundice and suspected biliary obstruction Suspected cholangiocarcinoma Colorectal cancer — staging Rectal cancer — local staging and restaging Crohn disease — small bowel assessment and monitoring Suspected perianal fistula or perianal sepsis Acute colitis Suspected deep vein thrombosis Acute pelvic pain in the reproductive age group (including suspected PID and tubo-ovarian abscess)Adnexal mass — characterisation and risk stratification Staging or completion CT in a known solid tumour Response assessment on systemic therapy Suspected or newly diagnosed lymphoma Metastatic disease with no identified primary

Contrast

RectalIodinated, intravenous

Typically 3-5% dilute water-soluble iodinated contrast — of the order of 500-1000 mL — instilled per rectum or through a stoma by gravity from a bag held about a metre above the table, to patient tolerance rather than to a fixed volume, plus the standard intravenous injection where not contraindicated.

Barium is avoided entirely when a leak is suspected: extraluminal barium causes a granulomatous peritonitis that water-soluble contrast does not.
Gravity, never a pressurised system, and stop at the point of discomfort. The study is looking for a leak; creating one is the failure mode.
Rectal contrast is NOT routine for diverticulitis or for undifferentiated abdominal pain. It is for a specific question about luminal communication, and using it reflexively adds preparation time and patient discomfort for no diagnostic gain.

Acquisition

Breathing: Single breath-hold.
Reconstruction: Thin axial reconstructions with sagittal reformats through the pelvis, which is the plane a low anastomosis is best assessed in.
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.

Rectal contrastInstilled through a rectal catheter immediately before the acquisition, under gravity, to patient tolerance. Confirm locally.
Retrograde filling opacifies the distal lumen directly, without waiting for transit. The transferable principle is that a distal question is best answered from the distal end: an antegrade oral agent arrives late, diluted and unreliably, whereas retrograde instillation guarantees opacification and, just as importantly, guarantees a pressure head across the segment of interest. That pressure is the diagnostic mechanism — extraluminal contrast becomes positive evidence of a leak or fistula rather than an inference from adjacent gas or fluid. Water-soluble agents are used rather than barium precisely because a leak is what is being looked for, and spilled barium causes chemical peritonitis.
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