Imaging in renal colic: non-contrast CT or ultrasound

What it is

In suspected renal colic, imaging answers three questions: is there a stone, where is it and how big, and is the kidney obstructed or infected. Low-dose non-contrast CT of the abdomen and pelvis (often called CT KUB) is the reference test: it shows almost every stone whatever its composition, measures it, and also finds other causes of flank pain. Ultrasound uses no radiation and is the first test in pregnancy and children, and a reasonable first test in many young adults: it shows hydronephrosis and stones in the kidney or at the junction with the bladder, but often misses stones in the middle of the ureter. A plain abdominal X-ray adds little to diagnosis but helps follow a stone already known to be radiopaque.

What it looks like

• On non-contrast CT a stone is a very bright (calcium-dense) dot in the line of the ureter; nearly all stones are dense on CT, including uric acid stones that do not show on X-ray.
• Secondary signs of obstruction: hydronephrosis and hydroureter down to the stone, stranding of the fat around the kidney, and a slightly swollen kidney.
• Soft-tissue rim sign: a thin ring of swollen ureteric wall around the bright dot means a ureteric stone.
• Comet-tail sign: a bright dot with a soft-tissue tail is usually a phlebolith (a calcified pelvic vein), not a stone; phleboliths also often have a lucent centre.
• Stones lodge where the ureter narrows: the pelviureteric junction, where it crosses the iliac vessels at the pelvic brim, and the vesicoureteric junction (the narrowest).
• On ultrasound a stone is a bright echo with a dark acoustic shadow behind it; colour Doppler may show a twinkling artefact, and the ureteric jet into the bladder may be absent on the blocked side.

Why it looks that way

• Calcium makes a stone attenuate X-rays strongly, so it is bright on CT and, if large enough, on plain film.
• A stone that blocks urine flow raises pressure above it, so the collecting system and ureter dilate and fluid leaks into the fat around the kidney.

Differential and pitfalls

• Most stones of about 5 mm or less pass on their own; larger stones and stones high in the ureter are less likely to pass.
• An obstructed kidney with fever or signs of sepsis is a urological emergency: the infected urine must be drained (stent or nephrostomy).
• Mild hydronephrosis on ultrasound with no visible stone is common in renal colic: the stone may have passed or be hidden in the mid ureter.
• In an older patient with a first episode of «renal colic», think of a leaking abdominal aortic aneurysm; CT shows it.
• Other mimics on CT: appendicitis, diverticulitis, ovarian torsion, ectopic pregnancy, pyelonephritis; if CT without contrast cannot answer, contrast may be added.

Next step

• Typical colic in a stable adult: low-dose non-contrast CT; in pregnancy or a child, ultrasound first.
• Recurrent stone former with a known radiopaque stone: ultrasound plus plain X-ray can follow it and spare radiation.
• Fever with an obstructed kidney, a single kidney, both sides blocked or rising creatinine: call urology at once for drainage.
• Pain out of proportion, an older patient or no stone on CT: ask the radiologist about other causes and consider CT with contrast.

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Acute Renal Colic (StatPearls (NCBI Bookshelf), 2024); Ureterolithiasis (StatPearls (NCBI Bookshelf), 2024); Urolithiasis (StatPearls (NCBI Bookshelf), 2023).

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