KUB Ultrasound - Common Abnormalities

Understanding the Basics First

• Normal kidney: bean-shaped, 9-12 cm long, hypoechoic cortex, anechoic/hypoechoic medullary pyramids, echogenic central sinus fat. Undilated collecting system is not visible as a distinct structure.
• Normal bladder: anechoic (black), fluid-filled, ovoid to rectangular when full.
• Key echogenicity terms:
  • Hyperechoic (bright/white) = stones, calcium, fibrous tissue
  • Hypoechoic (dark gray) = renal cortex, soft tissue
  • Anechoic (black) = urine, blood, simple cysts, bile

KIDNEY ABNORMALITIES

1. Hydronephrosis (Obstruction)

Pitfall: Vascular structures and cysts can mimic hydronephrosis (both appear anechoic). Use color Doppler - blood flow confirms vasculature, not hydronephrosis. Parapelvic cysts are round and circumscribed vs. hydronephrosis which is irregular and branching.

Pyonephrosis: If the anechoic hydronephrotic area has internal echoes (debris/pus), suspect pyonephrosis - a urological emergency.

2. Kidney Stones (Nephrolithiasis / Urolithiasis)

• Appear as hyperechoic (bright) foci with posterior acoustic shadowing (black shadow behind the stone)
• On color Doppler: show the "twinkling sign" - rapidly alternating color signals at the stone, more pronounced with rougher surfaces
• Can be incidental or cause obstruction with secondary hydronephrosis
• Small ureteral stones are often missed on ultrasound (CT KUB is more sensitive)

3. Renal Cysts

• Simple cysts: Round/oval, anechoic (black), thin smooth walls, posterior acoustic enhancement. Usually benign and common with aging.
• Complex cysts: Thick walls, internal septations, calcifications, or solid components - may need further workup (CT/MRI) to exclude malignancy (Bosniak classification).
• Parapelvic cysts: Located near the renal pelvis - can closely mimic hydronephrosis.

4. Chronic Kidney Disease (CKD)

• Small kidneys (< 9 cm) - bilateral
• Increased echogenicity of the cortex (hyperechoic vs. normal liver/spleen)
• Loss of corticomedullary differentiation - cortex and medulla become isoechoic to each other
• Thinned cortex
• Together these indicate chronicity rather than acute injury

Grading of echogenicity:

• Grade 1: Cortex = echogenicity of liver (normal liver > kidney)
• Grade 2: Cortex = echogenicity of spleen
• Grade 3: Cortex > spleen
• Grade 4: Hyperechoic cortex with shadowing

5. Acute Pyelonephritis

• Often normal on ultrasound (sensitivity only 20-69%)
• May show: enlarged kidney, focal areas of increased/decreased echogenicity, loss of normal architecture
• If complicated: renal abscess (hypoechoic/complex fluid collection with thick walls) or pyonephrosis
• DMSA scintigraphy is the gold standard for acute pyelonephritis

6. Nephrocalcinosis

• Calcium deposits in the renal parenchyma (not in the collecting system)
• Most common in the medullary pyramids - appear as hyperechoic spots/bands in the medulla
• Dense calcifications cause posterior acoustic shadowing
• Causes: hyperparathyroidism, medullary sponge kidney, renal tubular acidosis, hypercalciuria

7. Renal Masses / Tumors

8. Structural / Congenital Abnormalities

• Horseshoe kidney: Both kidneys fused at lower poles across midline
• Ectopic kidney: Kidney not in normal retroperitoneal position (may be pelvic, crossed)
• Crossed fused renal ectopia: Both kidneys on same side, fused
• Duplex collecting system: Two separate renal pelves and ureters
• Renal agenesis: Absent kidney (confirm no ectopic position before diagnosing)

9. Vascular Abnormalities

• Renal artery stenosis: Requires Doppler (resistive index, peak systolic velocity)
• Arteriovenous malformation (AVM): Appears as anechoic area mimicking hydronephrosis; color Doppler shows turbulent flow
• Renal vein thrombosis: Enlarged, echogenic kidney; absent flow on Doppler

URETER ABNORMALITIES

The normal ureter is not visible on ultrasound. If you can see it, something is wrong.

• Hydroureter: Dilated ureter visible as an anechoic tubular structure - implies obstruction (stone, stricture, extrinsic compression)
• Ureteral stone: Seen at the ureterovesical junction (UVJ) or proximal ureter as a hyperechoic focus with shadowing; may show secondary hydronephrosis
• Ureteral jet: Normal finding on Doppler - pulsatile jets of urine entering the bladder bilaterally; absent jet on one side suggests ureteral obstruction

BLADDER ABNORMALITIES

1. Bladder Outlet Obstruction / Urinary Retention

• Large post-void residual (PVR) volume - normally < 50 mL is complete emptying; > 200 mL is significant
• Causes: BPH (enlarged prostate in males), urethral stricture, neurogenic bladder
• Bladder wall thickening (normal < 3-5 mm when full) - trabeculation pattern in chronic obstruction
• Bladder diverticula: Outpouchings of the bladder wall

2. Bladder Stones

• Hyperechoic foci with posterior acoustic shadowing
• Move with patient repositioning (distinguishes from fixed wall masses)

3. Bladder Tumors

• Focal wall thickening or echogenic projections into the bladder lumen
• Vascular on color Doppler
• Cannot definitively characterize - cystoscopy required for diagnosis

4. Cystitis

• Bladder wall thickening
• Debris or echogenic material within the bladder (pus, blood)
• Bladder may appear irregular

5. Bladder Debris / Hematuria

• Echogenic material layering in the dependent portion of the bladder
• Settles with gravity (shifts with repositioning)

PROSTATE (Males - assessed during KUB)

• Benign Prostatic Hyperplasia (BPH): Enlarged prostate (> 30 mL volume), often causing elevated PVR
• Prostate cancer: May appear hypoechoic on transabdominal view; transrectal ultrasound (TRUS) is more sensitive

Quick Reference Summary Table

Note: Ultrasound has limitations. Small ureteral stones (< 5 mm) and early acute pyelonephritis are frequently missed. CT urography remains the gold standard when clinical suspicion is high despite a normal or inconclusive KUB ultrasound.