CT KUB — Low Dose Unenhanced
Upper poles of the kidneys to below the pubic symphysis, so the vesicoureteric junctions and the whole bladder are included.
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
- Acute flank pain with suspected ureteric colic.
- Known stone disease with new or persistent pain, or before intervention.
- Assessment of stone burden, position and density before lithotripsy or ureteroscopy.
- Follow-up of a stone under conservative management.
Technique
- A low-dose technique is the standard of care, not a variant: published effective doses of roughly 3-4 mSv for low-dose acquisition compare with about 10-12 mSv for a conventional unenhanced abdomen and pelvis, with a sensitivity of around 97% and specificity around 95% for stone disease.
- Iterative or deep-learning reconstruction is what makes the dose reduction tolerable in terms of noise.
- Dose should be scaled up in the larger patient rather than held at the standard-size benchmark.
- Intravenous contrast is not merely unnecessary but actively harmful to the question: opacified collecting systems conceal stones.
Where it goes wrong
- Adding intravenous contrast to a stone protocol hides the stone.
- Cropping the coverage above the symphysis misses distal ureteric and vesicoureteric junction stones, which are the commonest site of obstruction.
- Phleboliths are routinely mistaken for distal ureteric stones on axial images alone; coronal tracking of the ureter resolves most of them.
- Very low dose in a large patient produces a noisy study in which a small stone is undetectable — the low-dose benchmark is defined for a standard-sized patient.
- Indinavir and some matrix stones are poorly attenuating and can be invisible.
Clinical questions that reach this study
Contrast
Acquisition
- Breathing
- Single breath-hold.
- Reconstruction
- Thin axial reconstructions with coronal and sagittal reformats; coronal images are how the ureteric course is followed and how a stone is distinguished from a phlebolith.
- Preparation
- No oral or intravenous contrast. A comfortably full bladder helps assess vesicoureteric junction stones but should not delay an acutely painful patient.
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.
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.
- Child-sized technique and contrast dose· radiographer at scan
- Pregnancy status before an ionising exposure· radiographer at scan