Routine CT Protocols
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Direct answer
A contrast CT protocol is defined above all by phase timing: late arterial phase around 30-40 seconds from injection, portal venous phase 60-70 seconds, and delayed (equilibrium or excretory) phases at 3-5 minutes and beyond. The liver mass protocol runs non-contrast, arterial and portal venous (with delayed imaging for washout and capsule); the renal mass protocol adds corticomedullary, nephrographic and excretory phases; CT pulmonary angiography fires at about 15-25 seconds or on bolus tracking in the pulmonary artery; CT urogram images the excretory phase at 5-10 minutes. Bolus tracking (a region of interest in the aorta triggering at roughly +100 HU) replaces fixed delays for vascular studies, and the technologist's timing accuracy decides whether the study answers the clinical question.
What you must remember
- Phase clock: late arterial 30-40 s, portal venous 60-70 s, equilibrium/nephrographic 90-120 s, delayed 3-5 min; excretory (urogram) 5-10 min.
- Bolus tracking: a region of interest in the aorta (or pulmonary artery for CTPA) triggers the scan at a threshold near +100 HU — the standard answer for vascular timing.
- Triple-phase liver: non-contrast, arterial, portal venous; hepatocellular carcinoma shows arterial hyperenhancement with portal/delayed washout — the pattern the protocol exists to capture.
- Renal mass protocol: corticomedullary (~35 s) for vascular anatomy, nephrographic (~100 s) — the most sensitive phase for small renal masses — and excretory for the collecting system.
- Pancreatic protocol: pancreatic phase 35-45 s plus portal venous, with water as oral contrast to distend the duodenum.
- CTPA: bolus tracking in the pulmonary artery, saline chaser, breath-hold; filling defects confirm or exclude embolism — late timing misses emboli.
- CT urogram: excretory-phase thin-section imaging of the whole urinary tract after contrast, for haematuria and urothelial tumours.
- Non-contrast phase first for stone disease (nearly all stones are visible on CT) and as the baseline for enhancement measurement; stroke protocols begin with non-contrast brain CT to exclude haemorrhage before thrombolysis.
- Oral contrast choices: water (neutral) for stomach and pancreatic studies; dilute iodinated positive contrast for bowel pathology; weight-based intravenous volume (commonly 1-1.5 mL/kg of 300-350 mgI/mL, 60-80 mL typical adult) with a 30-40 mL saline chaser.
Timing a triple-phase liver study
Walk a cirrhotic patient's protocol through the clock and every delay earns its place. The non-contrast phase maps baseline density — without it, later "enhancement" cannot be measured. The arterial phase at about 35 seconds — late arterial — is when hypervascular tumours glow: hepatocellular carcinomas steal arterial supply, and this phase alone catches them. The portal venous phase at 60-70 seconds is when normal liver parenchyma peaks via portal inflow; a tumour that blushed arterially now falls behind it — washout — a diagnostic signature impossible without both phases. Delayed imaging at 3-5 minutes captures capsule and the retention pattern of cholangiocarcinoma.
Now the failure modes the technologist polices. A patient who breath-holds differently between phases misregisters small lesions from phase to phase — coach the same breath-hold every time. And the contrast argument: enhancement measurement needs identical injection rate and volume, so "half the dose, same timing" silently corrupts the washout logic. The protocol is not a button labelled "triple phase liver" but a coordinated physiological experiment — tube, pump, patient and clock — and the radiographer runs three of the four.
Where students slip
Phase numbers are memorised as isolated facts and then shuffled: 35 seconds belongs to late arterial and pancreatic phases, 70 to portal venous, 100 to nephrographic, 5 minutes to excretory — build the ladder, do not collect digits. Second, the nephrographic phase is forgotten as the most sensitive for small renal masses; students name corticomedullary for its drama. Third, stroke sequencing: the non-contrast brain CT comes first because haemorrhage excludes thrombolysis; a protocol that leads with CTPA has ordered the study around the wrong question. And oral contrast chosen by habit — dense positive contrast outlining bowel — ruins a pancreatic protocol where water distension of the duodenum is the entire point.
Frequently asked questions
What are the standard contrast phases and their timings?
Late arterial about 30-40 seconds after injection, portal venous 60-70 seconds, equilibrium/nephrographic 90-120 seconds, and delayed or excretory phases from 3-5 minutes onward.
How does bolus tracking individualise timing?
A region of interest monitors enhancement in a vessel (aorta or pulmonary artery), triggering the scan when it reaches a set threshold (about +100 HU), correcting for each patient's circulation time.
Why does a liver mass protocol need arterial and portal venous phases?
Hepatocellular carcinoma enhances in the arterial phase and washes out relative to the portal-phase liver; capturing both patterns is the diagnostic basis of the study.
Which phase is most sensitive for small renal masses?
The nephrographic phase (about 100 seconds), when cortex and medulla enhance uniformly, exposing otherwise isodense small tumours.
How is CT pulmonary angiography timed?
By bolus tracking in the pulmonary artery or a fixed short delay of about 15-25 seconds with a rapid injection and saline chaser — late timing misses emboli as the bolus passes into systemic circulation.