Kidney stones

What it means

Urine carries dissolved minerals, and when those minerals become too concentrated they can crystallize and clump into a hard stone inside the kidney. Kidney stones (medically, nephrolithiasis or renal calculi) range from tiny grains to stones more than a centimeter across. Trouble starts when a stone moves out of the kidney into the ureter, the narrow tube draining to the bladder, where it can lodge and block the flow.

Why it appears on a CT or MRI report

A non-contrast CT is the most sensitive test for stones and can pinpoint their number, size, and exact location, as well as any backup of urine behind a blockage (hydronephrosis). Reports usually give the stone's size in millimeters and where it sits — within the kidney, at the junction where the ureter leaves the kidney, or lower down near the bladder. Size and position strongly influence whether a stone is likely to pass on its own. Stones are sometimes also seen incidentally on scans done for other reasons.

What it usually means

Many small stones (roughly under 5 mm) pass on their own with fluids, time, and pain control. Larger stones, or stones causing a persistent blockage, are more likely to need a procedure — such as shock-wave treatment to break them up or a scope passed to remove them. A blocked, infected kidney is the scenario doctors most want to avoid, because it can become serious quickly. Beyond the immediate episode, stones tend to recur, so prevention through hydration and dietary changes matters for anyone who has had one.

When to follow up

Seek urgent care for severe one-sided back or flank pain, blood in the urine, or pain with nausea and vomiting — the classic picture of a stone on the move. Get emergency care if you also have fever, chills, or feel very unwell, since a blocked stone with infection is a medical emergency. Milder or incidentally found stones warrant a non-urgent conversation about prevention and monitoring.

A plain-language way to picture it

Think of hard water leaving scale inside a pipe. Most of the time the water flows past the grit without trouble. But if a chip of scale breaks loose and wedges in a narrow bend, it dams the pipe, pressure builds behind it, and the whole system aches until the blockage clears. That narrow bend is the ureter, and the wedged chip is the stone.