Find the stone, then prove it is obstructing, then say the two things the urologist acts on: size and position.
Orient first
- Non-contrast CT is the reference test. Almost every urinary calculus is dense on CT, including those invisible on radiographs — the exception is the pure indinavir stone.
- The two numbers that decide management are the STONE SIZE and its POSITION. Small distal stones usually pass; large proximal ones usually do not.
- Obstruction is proved by the SECONDARY SIGNS, not by the stone alone: hydronephrosis, hydroureter to the level of the stone, perinephric and periureteric stranding, and an enlarged kidney.
- A phlebolith is the great mimic. It is round, may have a central lucency, and has a "comet tail" of adjacent vessel; a stone is usually angular with a soft-tissue rim sign.
Acquire the study
- Non-contrast CT of the kidneys, ureters and bladder, thin slices, from the top of the kidneys to below the bladder base.
- Review on CORONALS to follow the ureter along its length — this is far easier than tracking it on axials.
- Use a bone or wide window to separate a calculus from adjacent contrast or vessel.
- ULTRASOUND is the study in pregnancy and in children: look for hydronephrosis, ureteric jets and the twinkling artefact behind a stone.
The manoeuvre
- Assess both kidneys for hydronephrosis and grade it, and compare sides.
- Look for perinephric fat stranding and a bulky kidney on the symptomatic side — supportive evidence of acute obstruction.
- Follow the ureter from the renal pelvis to the bladder on coronals, on both sides.
- When you find a dense focus, prove it is IN the ureter: look for the soft-tissue RIM SIGN of the ureteric wall around it, and for hydroureter above and a normal-calibre ureter below.
- Measure the stone in the axial plane and, ideally, its maximum dimension on the plane that shows it best. State the size in millimetres.
- State the POSITION precisely: proximal, mid or distal ureter, vesicoureteric junction, or intrarenal with the calyx named.
- Count all stones and note any in the contralateral kidney.
- Look for complications: pyonephrosis (needs urgent decompression), forniceal rupture with urinary extravasation, an obstructed solitary kidney.
- Look for the alternative diagnosis — appendicitis, diverticulitis, an aneurysm, ovarian pathology — because a fifth of these scans find something else.
What confirms it
- A dense calculus within the ureter WITH hydronephrosis or hydroureter above it and periureteric stranding.
- The rim sign helps separate a ureteric stone from a phlebolith.
What licenses you to exclude it
- A properly performed non-contrast CT with no calculus and no secondary signs excludes an obstructing stone.
- Consider a recently passed stone — the secondary signs outlast the calculus, so hydronephrosis with no stone and a normal distal ureter is a recognised picture worth stating.
- ⚠️ On ULTRASOUND, absence of hydronephrosis does not exclude obstruction, particularly early or when the patient is dehydrated.
The classic misread
- Calling a phlebolith a stone — look for the comet tail and the rim sign.
- Not stating stone size and position, which are the only two facts the referrer needs.
- Missing pyonephrosis, which is the emergency in this pathway.
- Failing to report the alternative diagnosis in a stone-negative study.