Renal Tract and Bladder Ultrasound

Renal tract ultrasound images the kidneys, ureters (when dilated), bladder and prostate (transabdominally), using a 3 to 5 MHz curvilinear probe (higher frequency linear probes in children and thin adults).

The kidneys are scanned in the supine, lateral decubitus and prone positions through the flank or posterior approach, in long and transverse axes, with measurement of length, cortical thickness, and echotexture compared with the liver and spleen.

The collecting systems are examined for dilatation, the bladder is scanned when full (wall thickness, masses, stones, ureteric jets, and volume) and after voiding (post-void residual volume by the ellipsoid formula length x width x depth x 0.52).

Colour and spectral Doppler show renal perfusion, resistive indices (0.5 to 0.7), ureteric jets, and renal artery and vein flow; contrast enhanced ultrasound characterises lesions. The examination takes 15 to 30 minutes, with no radiation or nephrotoxic contrast, so it is the first line test for most renal tract conditions, especially in children, pregnancy and renal failure.

Chronic kidney disease affects 10 to 13% of adults, urinary tract infection is among the commonest infections, kidney stones affect 5 to 10% of people during life, and hydronephrosis, haematuria and renal masses are common reasons for imaging.

Ultrasound detects hydronephrosis with a sensitivity of 90 to 95%, shows kidney size and cortical thickness (showing the chronicity of renal disease), identifies renal cysts and solid masses (sensitivity 85 to 95% for masses over 3 cm), and measures bladder residual urine.

It is less sensitive for ureteric stones (45% sensitivity, 94% specificity), for small renal stones in some cases, and for small urothelial tumours.

Indications. Acute and chronic kidney disease (to separate obstructive from non-obstructive causes, to measure size and cortex, and to find scarring). Flank pain and suspected renal colic or obstruction. Haematuria (with cystoscopy and CT urography as needed). Urinary tract infection (recurrent, febrile or complicated; infants and children; males).

Lower urinary tract symptoms, urinary retention, suspected prostate enlargement, post-void residual volume. Antenatal hydronephrosis and postnatal follow up. Suspected renal mass, cyst, polycystic kidney disease, stones, and abscess. Renal artery stenosis (Doppler), transplant kidney assessment (perfusion, collections, obstruction, rejection), and haemodialysis access.

Hypertension in young patients (kidney size and renal artery). Renal trauma (focused), ultrasound control of biopsy and nephrostomy. Abnormal kidney position or congenital anomalies. Not as a routine in uncomplicated cystitis in adult women.

Contraindications and precautions. None. For bladder views, the bladder should be comfortably full (drink 500 to 750 mL of water 1 hour before; in infants, a feed before the scan). Over distension causes discomfort and can mimic hydronephrosis (physiological), so renal views are repeated after voiding if the system is dilated.

Fasting for 6 hours helps reduce gas for kidney views in some centres. Catheterised patients need the catheter clamped or the bladder filled. Obese patients need low frequency probes.

Preparation and technique. The kidneys are measured in maximum length (pole to pole) and the cortical thickness is noted. The renal sinus fat, pyramids, cortex, and any collecting system dilatation (anteroposterior diameter of the renal pelvis, calyceal dilatation) are assessed. Both kidneys are compared.

The bladder is scanned in transverse and longitudinal views, measuring wall thickness, shape and contents; the ureteric orifices and jets are sought with colour Doppler. The prostate is measured transabdominally (volume and median lobe protrusion). Post-void residual urine is measured within 10 minutes of voiding.

Renal artery Doppler uses peak systolic velocity, renal aortic ratio and the acceleration time of intrarenal waveforms. Transplant kidneys are scanned in the iliac fossa with resistive index measurements.

Normal appearances. Kidneys are bean shaped and lie in the retroperitoneum, with the left slightly higher and longer. Length is 9 to 12 cm in adults (about 1 cm shorter in women, and decreasing with age). Width is 4 to 5 cm and parenchymal thickness 1.5 to 2 cm.

The cortex is less echogenic than the liver and spleen (or equal). The pyramids are prominent and hypoechoic. The central sinus is echogenic from fat and vessels.

Right kidney is slightly smaller than the left, and the difference between the two is under 2 cm. Normal collecting system is not seen, except a small anechoic pelvis (under 10 mm anteroposterior diameter in adults). Ureters are not seen unless dilated.

Bladder: thin wall (under 3 mm when distended, up to 5 mm when empty), smooth, with anechoic urine, and ureteric jets visible on colour Doppler. Post-void residual is under 50 mL (under 100 mL is acceptable in older adults, and under 10% of capacity in children). Prostate volume is under 25 to 30 mL in adult men (rises with age).

Renal artery peak systolic velocity is under 180 to 200 cm each second, renal aortic ratio under 3.5, and intrarenal resistive index 0.5 to 0.7.

Interpretation.

• Hydronephrosis: grades 1 (mild pelvic dilatation) to 4 (marked calyceal dilatation with thinned cortex). Causes include stones, pelviureteric or vesicoureteric junction obstruction, tumours, prostate enlargement, stricture, pregnancy, and vesicoureteric reflux.
• A dilated ureter points to a distal cause. A dilated pelvis with a normal ureter points to pelviureteric junction obstruction.
• A non-obstructive dilatation exists with reflux and full bladder. Raised resistive index (over 0.7) and absent ureteric jets support obstruction.
• Stones: echogenic foci with posterior acoustic shadowing in the calyces, pelvis or ureter (at the vesicoureteric junction through the bladder window). Twinkling artefact on colour Doppler helps.
• Staghorn calculi fill the collecting system. Ureteric stones are best seen in the dilated proximal ureter or the vesicoureteric junction, with absent ureteric jet.
• Chronic kidney disease: small (under 9 cm), echogenic kidneys with thin cortex, and loss of corticomedullary differentiation.
• Large kidneys with chronic disease occur in diabetic nephropathy, amyloidosis, HIV nephropathy, polycystic kidney disease and acute causes (acute glomerulonephritis, acute tubular necrosis, infiltration).
• Acute pyelonephritis: enlarged kidney with focal swelling and decreased echogenicity or reduced perfusion on Doppler (often normal).
• Abscess is a complex hypoechoic mass with debris and a thick wall. Pyonephrosis shows echogenic debris in the dilated system. Emphysematous pyelonephritis shows gas with dirty shadowing.
• Cysts and cystic disease:
  • Simple cysts: anechoic, smooth, thin wall, posterior enhancement, in 30 to 50% of people over 50.
  • Complex cysts: septa, calcification, wall thickening; classified by features that predict malignancy.
  • Autosomal dominant polycystic kidney disease: enlarged kidneys with multiple cysts of different sizes, liver cysts.
  • Multicystic dysplastic kidney.
• Solid masses:
  • Renal cell carcinoma: solid isoechoic or hypoechoic mass, may have necrosis and vascularity, renal vein extension.
  • Angiomyolipoma: very echogenic mass with shadowing, fat containing, tuberous sclerosis.
  • Oncocytoma, lymphoma and metastases.
  • Transitional cell carcinoma of the pelvis: soft tissue mass in the sinus.
• Contrast enhanced ultrasound or CT is used to characterise solid masses.
• Congenital anomalies: horseshoe kidney, ectopic or pelvic kidney, duplex system (two separate pelvicalyceal systems, upper pole hydronephrosis and ureterocele), renal agenesis (with compensatory hypertrophy), and hypoplasia.
• Renal scarring: focal cortical thinning with retracted contour (reflux nephropathy).
• Renal artery stenosis: peak systolic velocity over 180 to 200 cm each second, renal aortic ratio over 3.5, tardus parvus waveform, delayed acceleration time over 0.07 seconds, resistive index asymmetry over 0.05.
• Renal vein thrombosis: enlarged kidney, absent venous flow. Infarct: wedge shaped avascular area.
• Transplant kidney: perinephric collections (haematoma, urinoma, lymphocele, abscess), hydronephrosis, arterial stenosis, renal vein thrombosis, and arteriovenous fistula after biopsy.
• Raised resistive index in a transplant (over 0.8 to 0.9) is non-specific for rejection, acute tubular necrosis, or obstruction.
• Bladder: wall thickening and trabeculation (obstruction, neuropathic bladder, cystitis), diverticula, foreign body, and catheter balloon. Cystitis shows wall thickening with debris. Ureterocele has a "cobra head" appearance.
• Bladder stones are mobile echogenic foci with shadow. Tumour is a fixed polypoid or irregular mass projecting into the lumen, vascular on Doppler, not mobile. Blood clots are mobile and avascular.
• Raised post-void residual (over 100 to 150 mL) indicates retention.
• Prostate: enlargement (volume over 30 mL) with a median lobe protruding into the bladder base; calcifications; hypoechoic nodules need further tests.
• Retroperitoneal and perirenal: collections, haematoma, retroperitoneal fibrosis (hypoechoic mass encasing the aorta and ureters), nodes, and adrenal masses.

Critical values and findings needing immediate action.

• Hydronephrosis with fever, sepsis or pyonephrosis: urgent decompression (nephrostomy or ureteric stent) and antibiotics.
• Bilateral hydronephrosis or hydronephrosis of a solitary kidney with acute kidney injury: urgent relief of obstruction (catheter for bladder outlet obstruction, nephrostomy or stent).
• Acute urinary retention with large bladder volume: catheterisation, monitoring for post-obstructive diuresis and electrolyte disturbance.
• Renal trauma with perinephric haematoma and instability: resuscitation, CT and urological care.
• Emphysematous pyelonephritis: emergency drainage or nephrectomy in a diabetic patient.
• Renal vein thrombosis or acute renal artery occlusion: anticoagulation and specialist care.
• Transplant kidney with obstruction, collection, vascular thrombosis or absent flow: urgent transplant team assessment (graft salvage depends on hours).
• Suspected renal cell carcinoma with venous extension: urgent staging CT and urology referral.
• Bladder tumour with haematuria and clot retention: catheter irrigation and urology referral.
• Antenatal severe bilateral hydronephrosis with oligohydramnios: foetal medicine referral.

Limitations and artefacts.

• Hydronephrosis is not always obstruction. Other causes are full bladder, reflux, pregnancy, extrarenal pelvis, post-obstructive dilatation, diuresis, and congenital megacalycosis.
• Absence of dilatation does not exclude acute obstruction (early colic, dehydration, rupture of a calyceal fornix, or staghorn). Isotope renography, CT or MR urography are needed to confirm.
• Small stones (under 5 mm), stones in the mid-ureter, and stones in an obese patient or overlying bowel gas are missed (sensitivity of 45% for ureteric stones). Low dose non-contrast CT is the test for renal colic when needed.
• Small renal masses (under 2 to 3 cm) in the sinus or isoechoic tumours can be missed. Ultrasound cannot always distinguish solid from complex cystic masses. Enhancing masses are characterised by contrast enhanced ultrasound, CT or MR.
• Urothelial tumours, small bladder tumours and wall lesions are better detected by cystoscopy.
• Kidney size measurement varies with technique (oblique axis underestimates length), hydration and the observer. The kidney length is shorter in the elderly and in women.
• Normal variants mimic disease: foetal lobulation, dromedary hump, column of Bertin (prominent cortex between pyramids, mimicking a mass), duplicated collecting system, extrarenal pelvis, and sinus fat.
• Echogenicity is relative to the liver and spleen, which may themselves be abnormal (fatty liver makes normal kidneys look hypoechoic).
• Post-void residual estimates have an error of 20 to 30%, and are higher if the bladder was not full or if repeated.
• Doppler for renal artery stenosis depends on the operator and patient (obesity, bowel gas), with sensitivity of 60 to 90%. CT or MR angiography confirms.
• Over distended bladder produces false bladder wall measurements.

Special groups.

• Infants and children: ultrasound is the first line test for urinary tract infection (renal size, scarring, hydronephrosis, bladder wall), antenatal hydronephrosis (postnatal scan after the first 48 hours, ideally at 3 to 7 days), haematuria and renal masses (Wilms tumour). The kidney length is compared with centile charts for age.
• Pregnancy: mild right sided physiological hydronephrosis is common; ultrasound is first line for flank pain.
• Older men: prostate enlargement, bladder outlet obstruction, residual urine, stones, and hydronephrosis.
• Patients with chronic kidney disease: baseline kidney size and echogenicity, exclusion of obstruction, and polycystic kidneys.
• Renal transplant recipients: routine post-operative Doppler and surveillance for complications.
• Patients with spinal cord injury or neuropathic bladder: surveillance for hydronephrosis, stones and bladder wall changes.
• Patients with haematuria: ultrasound is part of a multimodal workup with cystoscopy and CT urography.
• Regions: urinary schistosomiasis (bladder wall thickening, calcification, polypoid lesions, ureteric strictures, hydronephrosis, bladder cancer), renal tuberculosis (scarring, calcification, hydronephrosis), obstructive uropathy from prostate disease and cervical cancer, stones, and chronic kidney disease of unknown cause are common.
• Ultrasound is often the only imaging available and is the main tool for deciding on dialysis or catheter drainage.

Follow up tests.

• Non-contrast low dose CT of the urinary tract for stones.
• CT urography or MR urography for haematuria and masses.
• Cystoscopy for bladder lesions and haematuria.
• Isotope renography (technetium mercaptoacetyltriglycine) for obstruction and split function.
• Dimercaptosuccinic acid scan for scarring.
• Micturating cystourethrogram for reflux.
• Contrast enhanced ultrasound, and renal artery CT or MR angiography.
• Urine microscopy, culture, protein and albumin to creatinine ratio.
• Serum creatinine, estimated glomerular filtration rate and electrolytes.
• Urine cytology, prostate specific antigen, and urodynamic studies.
• Tests for schistosomiasis and tuberculosis.
• Renal biopsy under ultrasound for unexplained kidney disease.
• Referral to nephrology or urology.