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 an unenhanced CT of the kidneys, ureters, and bladder (the "CT KUB"). In acute flank pain it finds ureteral stones with about 97% sensitivity and 96% specificity.
- The job isn't just "spot the stone." It's where it is, how big it is, and whether the kidney upstream is backing up. Size drives fate: about 68% of ureteral stones 5 mm or smaller pass on their own, versus 47% of those over 5 mm and up to 10 mm.
- The classic gotcha: a phlebolith (a harmless calcified pelvic vein) masquerading as a ureteral stone. Don't get fooled.
- The danger combo is an obstructing stone plus infection: 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 an unenhanced CT covering the kidneys, ureters, and bladder, which radiologists call 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.
Stones don't form in the ureter, by the way. They grow quietly in the kidney, often for years, and only cause drama when one breaks loose and tries to leave. So the disease is really two stories: the renal stone sitting in a calyx (usually painless, often incidental) and the ureteral stone on the move (the emergency-department classic, which gets its own page at ureteral stones and complications).
Findings by modality
Radiograph (KUB)
The plain abdominal film is where stone imaging started, and it is still used to follow a known stone that you have already proven is visible on it. Calcium-containing stones are radiopaque, so they show as a small dense focus projected over the renal outline or along the expected course of the ureter. The trouble is everything else in the way: bowel gas, bone, the transverse processes of the lumbar spine, and the sacrum all hide stones, and pelvic phleboliths sit right where the distal ureter runs. Uric acid stones are the famous "radiolucent" stones, essentially invisible on a radiograph.
Ultrasound
Ultrasound is the radiation-free option, which makes it the first choice in pregnancy and in children. A stone is a bright echogenic focus with a clean posterior acoustic shadow behind it, because sound can't get through rock. On color Doppler, many stones also produce a twinkling artifact, a flickering mosaic of color behind the stone that can rescue a small stone hiding in an echogenic renal sinus. Ultrasound is much better at showing the consequence of a stone (a dilated collecting system) than the stone itself: the top of the ureter and the segment just behind the bladder are visible, but the long middle stretch is lost behind bowel gas. Watching for a ureteral jet in the bladder, the little puff of color when urine squirts in from each ureter, tells you that side is still draining at least partly.
CT (unenhanced)
This is the workhorse. Nearly every stone type is dense on CT, including the uric acid stones that vanish on the radiograph, because CT measures attenuation directly rather than through a stack of overlapping bowel. I read the study on thin sections in both the axial and coronal planes, and I trace each ureter from the renal pelvis to the bladder, slice by slice, every single time.
Beyond the stone itself, the unenhanced CT gives you the secondary signs that prove the stone is actually obstructing: a dilated ureter above it (hydroureter), a dilated collecting system (hydronephrosis), stranding of the fat around the ureter and kidney, and a swollen, slightly lower-attenuation kidney compared with the other side. The soft-tissue rim sign is a thin halo of ureteral wall wrapping the stone; a phlebolith has no such halo and often trails a comet tail of the collapsed vein it lives in.
The rare exceptions to "everything is bright on CT" are matrix stones and stones formed from certain protease-inhibitor drugs, which can be almost invisible. If the story is a stone but the CT shows only the secondary signs, keep those in mind rather than calling the study negative.
Dual-energy CT
Scanning at two energies lets the machine sort stones by chemistry, and dual-energy CT can separate a uric acid stone from a calcium-containing one. That matters because uric acid stones can sometimes be dissolved with medication rather than smashed or fished out.
The numbers
Size and location are the two facts that decide whether a stone passes or needs a urologist, so they are the two facts your report must nail. The passage figures below come from patients who were watched rather than treated, and they are the basis of the "small stones usually pass, big stones usually don't" rule.
| What | Threshold / value | Why it matters |
|---|---|---|
| Spontaneous passage, stone ≤5 mm | 68% (95% CI 46–85%) | The commonly used cutoff for "probably passes with time and analgesia" |
| Spontaneous passage, stone >5 to ≤10 mm | 47% | Roughly a coin flip; these patients need urology follow-up and often intervention |
| Passage by CT-measured size | 1 mm 87%; 2–4 mm 76%; 5–7 mm 60%; 7–9 mm 48%; >9 mm 25% | Passage falls steadily with every millimetre, so measure carefully |
| Passage by location | Proximal ureter 48%; mid ureter 60%; distal ureter 75%; ureterovesical junction 79% | The closer to the bladder, the shorter the trip and the better the odds |
| Unenhanced CT in acute flank pain | Sensitivity 97%, specificity 96%, accuracy 97% | The reason CT KUB replaced the intravenous urogram |
Notice that the size figures come from two different sources with two different ways of slicing the data, and they agree on the shape of the curve. Notice also what is not in the table: there is no single size above which a stone "cannot" pass. Size shifts the odds; it does not seal the verdict.
How good is the test
For unenhanced helical CT in patients presenting with acute flank pain, the classic figures are a sensitivity of 97% and a specificity of 96%, with an overall accuracy of 97%. That is about as good as a diagnostic test gets, and it is why the intravenous urogram, with its contrast injection and its series of delayed films, quietly disappeared from stone work.
Ultrasound and the plain radiograph are less sensitive, particularly for small stones and for anything in the mid ureter, but no robust pooled accuracy figure exists for either, so I will leave it at "worse than CT, and worse still for the middle of the ureter" rather than quote a number.
What could it be instead
The stone's mimics are almost all other bright dots in the abdomen and pelvis, and the whole game is proving that your dot sits inside the ureter with a backed-up system above it.
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Phlebolith | Round calcification in the pelvis, exactly where the distal ureter runs | Lucent center, comet tail of vein trailing off it, sits outside the traced ureter, no hydroureter above it |
| Calcified iliac artery plaque | Dense focus near the pelvic brim where the ureter crosses the vessels | Curvilinear, follows the vessel wall in successive slices, no rim sign |
| Calcified lymph node or appendicolith | Bright, roughly stone-sized, right lower quadrant | Sits in a node or in the appendix on coronal images, ureter traced separately and clear |
| Passed stone with residual edema | Hydroureter, stranding, and no stone anywhere | Trace the whole ureter to the bladder and look in the bladder itself; the stone may already be sitting there |
| Pyelonephritis without a stone | Swollen kidney, perinephric stranding, flank pain and fever | No calculus, no hydronephrosis, and a striated or wedge-shaped nephrogram if contrast is given (see pyelonephritis) |
| Non-urinary flank pain (appendicitis, diverticulitis, a ruptured aneurysm) | Same emergency-department chief complaint | The unenhanced CT shows the real culprit; this is why we scan the whole abdomen and pelvis, not just the kidneys |
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 usually have a lucent (dark) center and a comet tail, they sit outside the ureter, and they 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 show up on CT, so the giveaway isn't the stone being lucent, it's failing to look. Track the ureter along its whole length; a stone at the ureterovesical junction on the last few pelvic slices is the one that gets missed at the end of a long shift.
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."
Two other things I check on every stone CT. First, the other kidney: if it is absent, shrunken, or full of its own stones, an obstructing stone on this side is suddenly obstructing the patient's entire urine output. Second, the stone burden back in the kidneys, because a large branching staghorn calculus filling the collecting system is a different management conversation from a single tiny pebble in a lower-pole calyx.
Reporting
The urologist wants five things, and if you give them in the first two lines they will love you.
- Is there a stone, and where exactly (renal calyx, renal pelvis, ureteropelvic junction, proximal, mid or distal ureter, ureterovesical junction, bladder).
- Size in millimetres, measured on the plane that shows it largest; this is the number that steers the passage odds above.
- Obstruction: hydroureter and hydronephrosis (and how much), perinephric and periureteric stranding, renal swelling.
- Complications: perinephric fluid suggesting a ruptured fornix, gas in the collecting system, or a dilated system full of debris in a febrile patient.
- The rest of the kidneys and the other side: total stone burden, any staghorn, the health of the contralateral kidney, and any non-urinary cause of the pain.
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.
References
- Smith RC, Verga M, McCarthy S, Rosenfield AT. Diagnosis of acute flank pain: value of unenhanced helical CT. AJR Am J Roentgenol 1996;166(1):97–101. Used for: the 97% sensitivity, 96% specificity, and 97% accuracy of unenhanced helical CT for ureteral stone in acute flank pain, in Key Points, "The numbers", and "How good is the test".
- Preminger GM, Tiselius HG, Assimos DG, et al. 2007 Guideline for the management of ureteral calculi. J Urol 2007;178(6):2418–2434. Used for: the spontaneous passage estimates of 68% (95% CI 46–85%) for stones ≤5 mm and 47% for stones >5 to ≤10 mm, in Key Points, the Measurement chips, and "The numbers".
- Coll DM, Varanelli MJ, Smith RC. Relationship of spontaneous passage of ureteral calculi to stone size and location as revealed by unenhanced helical CT. AJR Am J Roentgenol 2002;178(1):101–103. Used for: the passage rates by CT-measured size (1 mm 87%; 2–4 mm 76%; 5–7 mm 60%; 7–9 mm 48%; >9 mm 25%) and by location (proximal 48%, mid 60%, distal 75%, ureterovesical junction 79%) in "The numbers".
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