Pyelonephritis & Renal Infection
- Pyelonephritis is a kidney infection — usually a urine infection that climbed upstream from the bladder.
- It's mostly a clinical diagnosis (fever, flank pain, nasty urine). Imaging is for the sick, the not-improving, and the "is there a complication?" crowd.
- On contrast CT, the classic look is a wedge of kidney that lights up less and later — patchy "striated" stripes pointing toward the center.
- The findings you must not miss are the complications: an obstructing stone, a frank abscess, and the surgical emergency that is gas in the kidney (emphysematous pyelonephritis).
Most kidney infections never need a scanner. Someone shows up with a fever, a sore flank, and urine that could be classified as a biohazard, and the diagnosis is made before radiology even wakes up. So why do we image at all? Because every so often the kidney isn't just infected — it's infected and something else is wrong. Our job is to find the "and."
What's actually happening
Think of the urinary tract as a one-way river that's supposed to flow downhill: kidney to ureter to bladder to exit. The trouble starts when bacteria decide to be salmon and swim upstream from the bladder back into the kidney. That's the usual route, called ascending infection, and it's why a simple bladder infection that gets ignored can graduate into a full kidney infection.
The medical name, pyelonephritis, just bolts together "pyelo" (the renal pelvis, the funnel where urine collects) and "nephritis" (inflamed kidney). So: inflamed kidney plus inflamed funnel. The kidney swells, gets angry, and parts of it stop working as smoothly as they should.
When do we even image?
Here's the honest version: an otherwise healthy adult with classic symptoms gets antibiotics, not a CT. Imaging earns its keep when the picture is murky or worrying — and the people we worry about are fairly predictable.
| Image when... | Because we're hunting for... |
|---|---|
| Not better after a couple days of antibiotics | A walled-off pocket of pus (abscess) |
| Known or suspected kidney stones | An obstruction trapping infected urine |
| Diabetes, immunosuppression, or looking septic | Severe, complicated, or gas-forming infection |
| Diagnosis genuinely unclear | Something else entirely (appendicitis, etc.) |
An infected kidney behind an obstruction — say, a stone wedged in the ureter with pus building up above it — is an emergency. That's a closed, pressurized space full of bacteria, and it can tip someone into sepsis fast. It usually needs drainage, not just antibiotics. If you see a stone plus a swollen, dilated collecting system plus a fever, say so loudly.
What it looks like on the scan
The workhorse is contrast-enhanced CT. The trick is that infected, inflamed kidney tissue doesn't take up contrast as eagerly or as evenly as healthy tissue. Imagine pouring dye into a sponge that's partly waterlogged — the dye seeps in slowly and unevenly. On CT that shows up as striated nephrogram: alternating light-and-dark stripes fanning out from the center, like someone dragged a comb through the kidney. The affected zones are often wedge-shaped, pointing inward toward the renal pelvis, because that follows the kidney's plumbing.
The kidney may also look swollen, and the fat around it can look dirty and streaky (the radiology word is "stranding," which just means inflammation smudging up the normally crisp fat).
Plenty of pyelonephritis looks completely normal on imaging — especially uncomplicated cases scanned early. A clean CT does not rule out a kidney infection. The scan is there to find complications, not to confirm the diagnosis you already made at the bedside.
Ultrasound is often the first test in children and pregnancy because it skips the radiation, but it's far less sensitive for the early changes — it shines mainly at spotting obstruction (a backed-up, dilated collecting system) and larger abscesses.
The complications that change the plan
This is the part worth memorizing, because it changes what happens to the patient.
| Complication | What you see | Why it matters |
|---|---|---|
| Renal abscess | A rounded, fluid-density pocket that doesn't enhance in the center | May need drainage, not just antibiotics |
| Obstruction + infection | Stone plus a dilated, backed-up collecting system | Emergency — needs decompression |
| Emphysematous pyelonephritis | Gas (pockets of air-density) within the kidney itself | Surgical emergency, often in diabetics |
Gas inside the kidney parenchyma is the one that should make your stomach drop. Emphysematous pyelonephritis is a severe, gas-forming infection that classically strikes people with diabetes, and it carries real mortality. Air where air has no business being — that's the alarm bell. (Don't confuse it with a little gas in the collecting system from a recent catheter or procedure, which is mundane.)
What's left behind
Most infections clear and the kidney recovers. But repeated or severe scarring can leave permanent dents in the kidney's surface — relevant later when a scarred kidney gets confused for something else, or when you're sorting out a renal mass. A focal, mass-like bacterial infection can even mimic a tumor; the history of fever and infection is what keeps you honest.
The one-sentence version: pyelonephritis is a clinical diagnosis, and the scanner's real job is to catch the obstruction, the abscess, or the gas before they catch the patient.