CT Mesenteric Angiogram — Biphasic
Diaphragm through the pelvis for the venous acquisition; the arterial acquisition covers the coeliac axis, superior and inferior mesenteric arteries and their branches.
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 acute mesenteric ischaemia of any mechanism — embolic, thrombotic, venous or non-occlusive.
- Suspected mesenteric venous thrombosis.
- Chronic mesenteric ischaemia before revascularisation.
- Suspected median arcuate ligament compression, where inspiratory and expiratory sagittal reformats help.
Technique
- Published biphasic protocols start the arterial acquisition around 25 s after the start of injection, or by bolus tracking, with a venous acquisition at roughly 60-70 s.
- Sources differ on the venous timing: some advocate around 90 s to ensure uniform opacification of the mesenteric veins, at the cost of slightly less crisp bowel wall enhancement.
- The unenhanced series helps identify hyperdense intravascular thrombus and pre-existing hyperdense bowel wall, and is included in several published protocols.
- The arterial phase is not just for the vessels: bowel wall enhancement in the arterial phase is part of the ischaemia assessment.
Where it goes wrong
- Positive oral contrast is the single most damaging error here: dense luminal contrast obscures bowel wall enhancement, which is the finding the study exists to detect, and streaks the mesenteric vessels.
- A single portal venous acquisition will miss an arterial embolus and cannot assess the arterial anatomy for revascularisation planning.
- Thick reconstructions hide small emboli in second- and third-order branches; thin sections reviewed on a workstation are part of the protocol.
- Delaying the study to wait for a creatinine result in a patient with clinical mesenteric ischaemia is itself a harm; the renal risk is downstream and manageable.
Clinical questions that reach this study
Contrast
Typically 100-150 mL of non-ionic iodinated contrast at around 4 mL/s with a saline chaser.
- No positive oral contrast. Water only, or nothing.
Acquisition
- Breathing
- Breath-hold for each acquisition.
- Reconstruction
- Thin axial source images with sagittal reformats along the coeliac and superior mesenteric artery origins, maximum-intensity projections and volume rendering.
- Preparation
- NO positive oral contrast. Dense luminal contrast obscures the bowel wall enhancement the study exists to assess. Water only, or nothing at all, depending on how unwell the patient is.
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.
- Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.
- CT angiography, bolus-trackedNo fixed delay. A monitoring ROI is placed in the target vessel (commonly the aorta) and acquisition triggers at a set attenuation rise — a ~100 HU threshold is widely used — followed by a short diagnostic delay of a few seconds for table movement and breath-hold instruction. Confirm locally.
This is not a separate physiological phase so much as a technique for hitting one reliably. The transferable principle is that a fixed delay assumes an average circulation, and the patients who most need vascular imaging — the shocked, the failing, the arrhythmic, the aneurysmal — are precisely those whose circulation time is furthest from average. Tracking the bolus in the target vessel replaces that assumption with a measurement, so peak arterial opacification is achieved in the individual patient. The trade-off is that the technique commits the scan to whatever the monitoring ROI sees: a badly placed ROI, a mistimed breath-hold or a threshold reached by a contralateral vein produces a systematically mistimed study rather than a slightly degraded one.
- 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