What findings and etiologies can an Ultrasound (US) of the kidney reveal?

Kidney Ultrasound Findings and Etiologies

Ultrasound of the kidney can reveal numerous pathological conditions including hydronephrosis, renal stones, masses, infections, and structural abnormalities, making it an invaluable first-line imaging tool for evaluating urinary tract diseases. 1

Normal Findings

• Kidneys appear as paired structures that lie obliquely to every anatomical plane 1
• Normal renal length is typically >10 cm in the third decade of life, with size correlating with height, sex, age, and weight 1
• Renal parenchyma should have uniform echogenicity 2

Pathological Findings

Obstructive Uropathy

• Hydronephrosis - dilation of the renal collecting system, a key finding in urinary obstruction 1
  • Appears as anechoic fluid-filled spaces within the renal sinus 1
  • Grayscale ultrasound is highly sensitive (>90%) for detecting hydronephrosis 1
• Potential pitfalls:
  • Hydronephrosis may be mimicked by dilated renal vasculature, renal sinus cysts, and bladder distension 1
  • Medullary pyramids may mimic hydronephrosis, especially in young patients 1
  • Dehydration may mask the presence of obstruction 1
  • Absence of hydronephrosis does not rule out a ureteral stone, especially small ones 1

Urolithiasis

• Renal stones appear as hyperechoic foci with posterior acoustic shadowing 1
• Detection limitations:
  • Stones smaller than 3 mm are usually not identified by current sonographic equipment 1
  • Stones are usually identified by the shadowing they cause, as their echogenicity is similar to surrounding renal sinus fat 1
  • US sensitivity for stone detection ranges from 24% to 57% compared to CT 1
  • The addition of color Doppler and assessment of twinkling artifact can improve sensitivity, particularly for small renal stones 1

Renal Failure

• Acute kidney injury (AKI):
  • Normal kidney size suggests AKI rather than chronic kidney disease (CKD) 1
  • Increased renal echogenicity may be present but is nonspecific (only 30-40% of AKI cases show increased echogenicity) 1
• Chronic kidney disease:
  • Small echogenic kidneys are diagnostic of CKD 1
  • Both kidney size and parenchymal thickness decrease in CKD 1

Infections and Abscesses

• Pyelonephritis:
  • May appear as focal or multifocal decreased parenchymal enhancement 1
  • Can lead to complications including renal or perirenal abscess 1
• Renal abscess:
  • Appears as a hypoechoic or anechoic collection within the renal parenchyma 1

Structural Abnormalities

• Anatomical variations that may be mistaken for pathology:
  • Reduplicated collection systems 1
  • Unilateral, bipartite, ectopic and horseshoe kidneys 1
• Cystic disease:
  • Simple cysts appear as anechoic, round lesions with posterior acoustic enhancement 2, 3
  • Polycystic kidney disease shows bilaterally enlarged kidneys containing multiple cysts of various sizes 2

Renal Masses

• Solid masses may represent tumors and require further evaluation 1
• Ultrasound can differentiate between solid and cystic renal lesions 3
• Renal tumors may present with gross hematuria and are likely to be found with ultrasound 1

Vascular Abnormalities

• Color Doppler can assess:
  • Global renal perfusion 1
  • Arterial and venous patency 1
  • Renovascular disease (arterial stenosis or venous thrombosis) 2, 3

Technical Limitations

• Examination may be limited by:
  • Patient habitus (obesity, narrow intercostal spaces) 1
  • Bowel gas 1
  • Abdominal or rib tenderness 1
  • An empty bladder (for complete urinary tract assessment) 1

Clinical Applications

• Primary indications:
  • Evaluation of obstructive uropathy 1
  • Assessment of acute urinary retention 1
• Extended applications:
  • Investigation of acute hematuria 1
  • Evaluation of acute renal failure 1
  • Detection of infections and abscesses 1
  • Screening for chronic kidney disease 4, 2
  • Assessment of renal transplants 4

Advantages of Renal Ultrasound

• Non-invasive and uses non-ionizing radiation 4
• Rapidly deployed at bedside 1
• Does not require removal of patient from resuscitation area 1
• Avoids delays, costs, specialized technical personnel, contrast agents, and radiation exposure 1
• Generates real-time images 4

Ultrasound remains the ideal initial radiographic test for patients with suspected kidney disease, providing valuable information about both structural and functional aspects of the kidneys 4, 5.