Synopsis
Select renal imaging by the clinical question, recognise modality limitations and escalate obstructed infection, stones, masses and contrast-related risk without delaying necessary diagnosis.
- Name the question before naming the scan: retention, hydronephrosis, calculus, renal mass, urothelial lesion, vascular event, differential function and cortical scarring require different techniques.
- Ultrasound is radiation-free and shows kidney size, parenchymal appearance, gross masses, cysts, collecting-system dilatation and bladder residual, but a normal study does not exclude early or intermittent obstruction.
- For adults with suspected renal colic, NICE recommends urgent low-dose non-contrast CT within 24 hours; pregnancy changes the first-line investigation to ultrasound.
- In acute kidney injury with no identified cause or a risk of obstruction, NICE recommends urgent urinary-tract ultrasound within 24 hours; suspected pyonephrosis shortens that target to six hours.
- Non-contrast CT KUB is optimised for stones, whereas multiphase contrast CT or CT urography evaluates enhancement, masses, vessels and urothelium. One protocol cannot answer every renal question.
- Iodinated contrast is not simply 'forbidden' in kidney impairment. Balance diagnostic benefit against current AKI, eGFR, volume status and alternatives with radiology, and do not postpone life-saving imaging on the basis of an unexamined creatinine alone.
- MAG3 renography assesses perfusion, drainage and relative renal function; DMSA imaging maps functioning renal cortex and scarring. Neither substitutes for rapid anatomical imaging in an unstable patient.
- MRI avoids ionising radiation and can characterise selected masses or vascular and soft-tissue abnormalities, but gadolinium choice in advanced kidney disease requires the local radiology policy.
- Kidney length and echogenicity support chronicity but are not histological diagnoses. Diabetes, infiltrative disease and acute swelling can produce large kidneys despite impaired function.
- Always read the report alongside the images available, clinical time course and biochemical trend, then confirm who will act on incidental masses, complex cysts, hydronephrosis or an unexpected solitary kidney.
Key red flags
Fever or hypothermia, rigors, flank pain, pyuria and AKI with hydronephrosis suggest pyonephrosis or an infected obstructed kidney. Antibiotics do not penetrate or drain a pressurised system reliably; decompression is source control.
Sudden severe loin-to-groin pain, restlessness, nausea and haematuria supports ureteric calculus, but aortic, ovarian, testicular, gastrointestinal and musculoskeletal emergencies remain alternatives. Pain severity does not predict stone size or kidney function.
Hesitancy, weak stream, incomplete emptying, suprapubic discomfort, overflow incontinence or a catheter that has stopped draining points toward retention. A bedside bladder scan can redirect the pathway before formal upper-tract imaging.
Sudden flank pain with haematuria, embolic risk, severe hypertension, recent instrumentation or significant trauma can indicate infarction, arterial injury or collecting-system disruption and usually needs urgent contrast-enhanced specialist imaging.
Investigation priorities
Assess kidney size and symmetry, echogenicity, cysts, larger masses, collecting-system dilatation and bladder volume without ionising radiation.
Management branches
Sepsis or systemic illness with flank pain, AKI, anuria, stone history or suspected upper-tract blockage.
- Begin sepsis assessment, cultures, renal-adjusted antimicrobials, lactate and haemodynamic care while alerting urology; do not wait for imaging to start resuscitation.
- Arrange immediate ultrasound within the NICE six-hour target when pyonephrosis is suspected, or the fastest locally agreed cross-sectional imaging if ultrasound will not answer the urgent question.
- When obstruction is present, obtain urgent source control by ureteric stent or percutaneous nephrostomy according to anatomy, local expertise and patient stability, then monitor post-obstructive output.