CT Urogram — Conventional Three Phase

Contrast

A single intravenous bolus, typically 100-150 mL at around 3 mL/s.

Acquisition

Breathing

Breath-hold for each acquisition.

Reconstruction

Thin axial reconstructions with coronal reformats and urographic maximum-intensity projections.

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.

1. 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.

2. Corticomedullary phaseTypically ~25–45 s after the start of injection; published renal-mass protocols quote delays across roughly 25–70 s, and the phase is defined by appearance — dense cortex against a still-unenhanced medulla — rather than by the clock. Confirm locally.

   Contrast has reached the renal cortex through the glomerular circulation but has not yet passed into the tubules, so the cortex is intensely enhanced while the medulla remains almost unenhanced and the corticomedullary junction is at its most striking. The transferable principle is that this is a VASCULAR phase of the kidney rather than a parenchymal one: it maps arterial supply and venous drainage, shows tumour thrombus in the renal vein and inferior vena cava, and exposes the degree of hypervascularity that separates clear cell from papillary and chromophobe tumours. Two weaknesses follow from the same physiology and are the reason it is never used alone. A lesion sitting in the low-attenuation medulla is camouflaged by it, so detection sensitivity is markedly lower here than in the nephrographic phase; and a small avidly enhancing cortical lesion can be indistinguishable from normal cortex, while a normal column of Bertin can be mistaken for one. It therefore characterises and maps — it does not detect.

3. 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.

4. 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