Kidney Ultrasound Findings and Etiologies

Normal Findings

• The American College of Radiology suggests that normal renal length is typically greater than 10 cm in the third decade of life, with size correlating with height, sex, age, and weight 3
• Kidneys appear as paired structures that lie obliquely to every anatomical plane 2

Pathological Findings

Obstructive Uropathy

• Hydronephrosis is characterized by dilation of the renal collecting system, a key finding in urinary obstruction, and appears as anechoic fluid-filled spaces within the renal sinus 1, 2
• Grayscale ultrasound is highly sensitive (>90%) for detecting hydronephrosis 3
• Potential pitfalls in diagnosing hydronephrosis include dilated renal vasculature, renal sinus cysts, and bladder distension, which may be mimicked by hydronephrosis 2

Urolithiasis

• Renal stones appear as hyperechoic foci with posterior acoustic shadowing 4
• The detection of renal stones is limited by size, with stones smaller than 3 mm usually not identified by current sonographic equipment 2
• The sensitivity of ultrasound for stone detection ranges from 24% to 57% compared to CT 4

Renal Failure

• Acute kidney injury (AKI) may present with normal kidney size, and increased renal echogenicity may be present but is nonspecific 3
• Chronic kidney disease is characterized by small echogenic kidneys, and both kidney size and parenchymal thickness decrease in CKD 3

Infections and Abscesses

• Pyelonephritis may appear as focal or multifocal decreased parenchymal enhancement 5
• Renal abscess appears as a hypoechoic or anechoic collection within the renal parenchyma 1, 5

Structural Abnormalities

• Anatomical variations that may be mistaken for pathology include reduplicated collection systems and unilateral, bipartite, ectopic, and horseshoe kidneys 2

Renal Masses

• Solid masses may represent tumors and require further evaluation 6
• Renal tumors may present with gross hematuria and are likely to be found with ultrasound 6

Vascular Abnormalities

• Color Doppler can assess global renal perfusion, arterial and venous patency, and renovascular disease 3

Technical Limitations

• The examination may be limited by patient habitus, bowel gas, abdominal or rib tenderness, and an empty bladder 2

Clinical Applications

• The primary indications for renal ultrasound include evaluation of obstructive uropathy and assessment of acute urinary retention 1
• Extended applications include investigation of acute hematuria, evaluation of acute renal failure, and detection of infections and abscesses 1, 3, 6