Kidney and bladder ultrasound: hydronephrosis and residual volume

What it is

Ultrasound is the first imaging test for most kidney and bladder questions: it has no radiation, needs no contrast, works at the bedside and is safe in pregnancy and in kidney failure. The two findings a general doctor must recognise are hydronephrosis and a raised post-void residual. Hydronephrosis is a dilated renal pelvis and calyces: urine is black (anechoic), so the normally bright central sinus of the kidney is split by branching black spaces that join each other at the pelvis. It means the flow of urine is blocked or slowed somewhere downstream, from a stone in the ureter to an enlarged prostate. The post-void residual is the urine left in the bladder straight after the patient voids; the bladder is measured in three planes and its volume estimated, by the operator or by a portable bladder scanner. A large residual means the bladder does not empty, which explains overflow incontinence, recurrent infection and, when both kidneys are dilated, kidney injury from bladder outlet obstruction. The same scan also shows kidney size, cortical brightness, stones, cysts and masses.

What it looks like

• Normal adult kidney: about 10 to 12 cm long, cortex as dark as or darker than the liver or spleen, bright central sinus fat.
• Hydronephrosis: anechoic, branching spaces in the renal sinus that connect with each other at a dilated renal pelvis.
• Graded mild to severe: mild is a dilated pelvis with normal calyces; severe has ballooned calyces and a thinned cortex.
• A dilated ureter (hydroureter) seen behind the bladder or near the kidney places the block lower down.
• A stone is a bright focus with a black acoustic shadow behind it; colour Doppler may show a twinkling artifact.
• Ureteric jets: colour Doppler shows urine squirting into the bladder; a jet on the affected side argues against complete obstruction.
• Post-void residual: the bladder is measured just after voiding (length × width × height × a correction factor) or with a bladder scanner.

Why it looks that way

• Urine is fluid, so it transmits sound with no echoes and looks black; the fat and vessels of the renal sinus reflect sound and look bright.
• Pressure from a downstream block dilates the collecting system first; long-standing pressure thins and finally destroys the cortex.
• Small, bright kidneys with a thin cortex mean chronic kidney disease, which tells the clinician the injury is old, not acute.

Differential and pitfalls

• Bilateral hydronephrosis with a full bladder points to bladder outlet obstruction (enlarged prostate, urethral stricture, blocked catheter, pelvic mass).
• Unilateral hydronephrosis points to a ureteric cause: stone, tumour, stricture, pelvi-ureteric junction obstruction or extrinsic compression.
• False positives: an overfull bladder, an extrarenal pelvis, parapelvic cysts (they do not connect), prominent renal vessels (check with Doppler).
• Pregnancy causes mild physiological dilatation, more often on the right; it is not by itself proof of obstruction.
• False negatives: very early obstruction (first hours), dehydration and encasing tumour or fibrosis can block without much dilatation.
• A large residual in an older man suggests prostatic obstruction; in diabetes or spinal disease, a neurogenic bladder; also drugs with anticholinergic effects.

Next step

• Acute kidney injury: request a kidney and bladder ultrasound early to exclude obstruction, the cause that can be reversed fastest.
• Full bladder with bilateral hydronephrosis: place a urinary catheter, record the volume drained and repeat the scan once it is decompressed.
• Hydronephrosis with fever or sepsis (infected obstructed kidney): call urology at once for drainage (stent or nephrostomy) alongside antibiotics.
• Suspected stone: non-contrast CT is the usual next test in adults; ultrasound first in pregnancy, children and young patients.
• Unexplained unilateral hydronephrosis without a stone: ask the radiologist for CT or CT urography to look for a tumour or extrinsic mass.

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Elsewhere in DurgiPedia

• Bladder examination: the distended bladder
• Kidney palpation: bimanual palpation and ballottement

Connections

← All of radiology

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: Ultrasound of the Urinary Tract (StatPearls (NCBI Bookshelf), 2025); Hydronephrosis and Hydroureter (StatPearls (NCBI Bookshelf), 2023); Bladder Post Void Residual Volume (StatPearls (NCBI Bookshelf), 2024).

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