CT Urogram — Split Bolus

Upper poles of the kidneys to the pubic symphysis, including the whole bladder.

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

Visible (macroscopic) haematuria work-up in an adult, for upper tract urothelial assessment.
Follow-up surveillance of known upper tract urothelial carcinoma.
Assessment of a filling defect or obstruction identified on another study.
Suspected congenital or acquired collecting system abnormality where the whole urothelium must be seen.

Technique

The point of the split bolus is that the nephrographic and excretory phases coincide in one acquisition, reducing the number of exposures.
Published delays for the combined acquisition after the second injection are commonly around 90-120 s, with total imaging delays from the first injection reported anywhere from about 8 to 14 minutes; an 8-minute total delay with the larger volume in the second bolus performed best in one optimisation study.
Hydration, and in some protocols a low dose of intravenous furosemide, improves ureteric distension and dilutes the excreted contrast so that small filling defects are not obscured.
Prone positioning or a repeat limited acquisition is used where a ureteric segment fails to opacify.

Where it goes wrong

Dense unopacified-to-opacified layering in the bladder can hide a bladder tumour; a supplementary prone or delayed acquisition is often needed.
Incomplete ureteric opacification is common and a non-opacified segment cannot be reported as normal.
Very dense excreted contrast causes streak and can obscure small urothelial lesions — dilution by hydration is part of the protocol, not an afterthought.
Using a routine portal venous CT abdomen and pelvis to answer a haematuria question misses upper tract urothelial disease.
The unenhanced series must not be dropped: without it, an excreted-contrast-filled system cannot be distinguished from a stone.

Clinical questions that reach this study

Staging or completion CT in a known solid tumour Indeterminate renal mass or suspected renal cell carcinoma Bladder cancer — pretreatment staging Visible haematuria — suspected urothelial cancer

Contrast

Iodinated, intravenous

A total of roughly 100-150 mL of non-ionic iodinated contrast (300-370 mgI/mL) at around 2-3 mL/s with a saline chaser — weight-based dosing of about 1.5-2 mL/kg is used in many centres — divided into two injections rather than given as one. A representative split is about 30-50 mL first, then the remaining 70-100 mL after an interval of roughly 8-10 minutes, so that a single acquisition contains both nephrographic and excretory enhancement.

Reported contrast splits vary — 30/70, 50/50 and 70/30 have all been studied, with a larger second portion favoured in at least one optimisation study.
The two numbers that must travel together are the inter-injection interval and the delay after the second injection: the first portion has to have reached the collecting system by the time the second is producing a nephrogram, and changing one without the other collapses the protocol back into a single-phase study.
Injection rate is deliberately lower than an angiographic protocol. Nothing here depends on a tight bolus, and a slower injection is better tolerated through the smaller cannulae common in an outpatient haematuria population.

Acquisition

Breathing: Breath-hold for each acquisition.
Reconstruction: Thin axial reconstructions with coronal reformats and coronal maximum-intensity projections through the collecting systems and ureters.
Preparation: Oral or intravenous hydration per local pathway to promote ureteric distension and dilute the excreted contrast. Some centres use low-dose intravenous furosemide or prone positioning to improve ureteric opacification. No positive oral contrast.

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.
Nephrographic phaseTypically ~85–120 s after the start of injection. Confirm locally.
Contrast has passed through the glomeruli into the tubules, so cortex and medulla enhance uniformly and the corticomedullary distinction resolves. The transferable principle is that a HOMOGENEOUS background is what makes a small lesion detectable: during the earlier corticomedullary phase the unenhanced medulla is itself relatively low attenuation and hides medullary lesions, whereas a uniform nephrogram makes any focal attenuation difference stand out. This is therefore the detection phase for renal parenchymal masses, and the phase in which a striated or delayed nephrogram becomes an interpretable sign of obstruction or pyelonephritis.
Excretory (pyelographic / urographic) phaseTypically ~5–15 min after injection; many CT urography protocols use a ~8–15 min delay, sometimes split-bolus or multi-acquisition. Confirm locally.
Contrast has been excreted into and has mixed with urine, opacifying the collecting systems, ureters and bladder. The transferable principle is that this phase images a LUMEN by filling it, so it answers questions about the urothelial surface — filling defects, strictures, calyceal anatomy, leaks and fistulae — and answers almost nothing about parenchyma. Its two structural weaknesses follow from the same physiology: incomplete or asymmetric opacification renders segments uninterpretable rather than normal, and dense intraluminal contrast masks the very calculi that an unenhanced series would have shown.

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