Renal Stones (CT KUB)
- A renal stone is a tiny rock in your plumbing, and the pain it causes is wildly out of proportion to its size.
- The go-to test is a non-contrast CT of the kidneys, ureters, and bladder — the "CT KUB" — because almost every stone is bright white on CT.
- The job isn't just "spot the stone." It's to find where it's stuck, how big it is, and whether the kidney upstream is backing up (hydronephrosis).
- The classic gotcha: a phlebolith (a harmless calcified vein) can masquerade as a ureteric stone. Don't get fooled.
- Watch for the danger combo — an obstructing stone plus infection — which is a urological emergency, not a "drink water and wait" situation.
Few things in medicine produce a more memorable patient than a ureteric stone. They can't sit still, they can't find a comfortable position, and they will tell you — accurately — that it is among the worst pain a human can experience. Our job with imaging is to confirm the culprit, a mineral pebble roughly the size of a sesame seed, and figure out how much trouble it's causing on its way out.
Why a CT, and why no contrast
The kidneys make urine, which trickles down two narrow drainpipes (the ureters) into the bladder. A stone is just a crystal that grew too big to pass comfortably, and now it's wedged somewhere in that pipe like a cork in a straw.
The test of choice is a non-contrast CT covering the kidneys, ureters, and bladder — radiologists call it a CT KUB. Here's the clever part: we skip the IV contrast on purpose. Most stones are full of calcium, so they're already eye-searingly bright (high attenuation — they eat a lot of the X-ray beam). Squirt in contrast, which is also bright, and you'd be trying to spot a white pebble inside a white river. Leave the pipe dark and the stone lights up like a star in a night sky.
What I actually hunt for
Spotting the stone is step one, but the report needs the whole story. I work through a mental checklist:
| What I look for | Why it matters |
|---|---|
| The stone itself | Confirms the diagnosis; nearly all are bright white on CT. |
| Location | Stones love to lodge at three narrow points: where the ureter leaves the kidney, where it crosses the iliac vessels, and where it enters the bladder. |
| Size | The single best predictor of whether it'll pass on its own. Small ones usually pass; big ones often need help. |
| Hydronephrosis | A backed-up, swollen kidney upstream tells you the stone is genuinely obstructing flow. |
| Secondary signs | Stranding (haziness) of the fat around the ureter, and a swollen kidney, support an acute obstruction. |
That last point is worth dwelling on. A stone that's blocking the pipe makes urine pile up behind it, so the collecting system balloons out — hydronephrosis, which on CT looks like the kidney's central drainage spaces have been inflated like little water balloons. No backup usually means the stone isn't fully obstructing, or has already passed.
The classic traps
The great impostor is the phlebolith — a small, rounded, calcified pelvic vein that sits right where a ureteric stone would be and is also bright white. The tells: phleboliths are usually round with a lucent (dark) center, sit outside the ureter, and don't come with hydronephrosis or fat stranding. Follow the ureter carefully before you call a stone.
A second trap is the missed stone. Even uric-acid stones — the ones that are famously invisible on a plain X-ray — usually show up on CT, so the giveaway isn't the stone being lucent, it's failing to look. Track the ureter slice by slice along its whole length. Plain X-rays are far less reliable here — many stones hide behind bowel or bone, which is a big reason the CT KUB won out as the workhorse.
When it stops being routine
Most stones are a painful-but-survivable nuisance: manage the pain, push fluids, and wait for nature to do the plumbing.
The exception that earns a phone call: an obstructing stone in a patient with signs of infection — fever, a high white count, pus in the urine. That's an infected, blocked kidney, which can tip into sepsis fast and usually needs urgent drainage. If I see an obstructing stone and there's any hint of infection, that report goes from "FYI" to "call the urologist now."
It's also worth peeking at the kidney itself for pyelonephritis or signs of long-standing damage, since chronic obstruction quietly wears a kidney down over time.
The one thing to remember
A stone is a small rock in a narrow pipe. Find it, measure it, follow the pipe to make sure it isn't backing up the kidney — and never, ever confuse it with a harmless pelvic vein wearing a calcium costume. When in doubt: round with a dark center and no swelling behind it, leave it alone; bright, on the ureter, with a tense kidney upstream, that's your stone.