Kidney and Bladder Ultrasound Findings
Kidney and bladder ultrasound will show hydronephrosis (kidney obstruction), kidney stones, bladder volume and retention, structural abnormalities, masses, cysts, and signs of infection or renal failure. 1, 2
Primary Diagnostic Capabilities
Obstructive Uropathy (Hydronephrosis)
- Hydronephrosis appears as anechoic (black) fluid-filled spaces within the renal sinus, representing dilation of the collecting system 2
- Grayscale ultrasound has >90% sensitivity for detecting hydronephrosis 2
- Classification includes: Mild/Grade I (minimal dilation), Moderate/Grade II ("bear's paw" appearance with confluent calices), or Severe/Grade III (parenchymal effacement) 1
- Important pitfall: A distended bladder can cause artifactual mild hydronephrosis in normal adults, so ideally scan kidneys after the patient voids 1
- Other mimics include dilated renal vasculature and renal sinus cysts 2
Kidney Stones (Urolithiasis)
- Stones appear as hyperechoic (bright white) foci with posterior acoustic shadowing 2
- Critical limitation: Stones smaller than 3 mm are usually not identified by current sonographic equipment 1
- Ultrasound sensitivity for stone detection is only 24-57% compared to CT 2
- Stones may be missed because their echogenicity is similar to surrounding renal sinus fat—they are typically identified by the shadowing they produce 1
Bladder Assessment
- The bladder should always be imaged as part of kidney ultrasound, as many urinary tract conditions are identifiable in the bladder 1
- Ultrasound evaluates bladder capacity, wall thickness, mural and luminal masses, and urinary retention 1, 3
- Scan bladder from top to bottom and side to side in transverse and sagittal planes 1
Extended Diagnostic Applications
Renal Failure
- Acute kidney injury (AKI) may show normal kidney size with increased but nonspecific renal echogenicity 2
- Chronic kidney disease presents as small echogenic kidneys with decreased parenchymal thickness 2
Infections and Abscesses
- Pyelonephritis appears as focal or multifocal areas of decreased parenchymal enhancement 2
- Renal abscess shows as hypoechoic or anechoic collections within the renal parenchyma 2
Structural Abnormalities
- Anatomical variations commonly mistaken for pathology include: reduplicated collection systems, unilateral kidney, bipartite kidney, ectopic kidney, and horseshoe kidney 1, 2
- Normal renal length is typically >10 cm in the third decade of life, correlating with height, sex, age, and weight 2
Masses and Tumors
- Solid masses may represent tumors requiring further evaluation 2
- Renal tumors often present with gross hematuria and are detectable by ultrasound 2
- Cysts appear as anechoic fluid-filled structures 1
Vascular Assessment
- Color Doppler can assess global renal perfusion, arterial and venous patency, and renovascular disease 2
Technical Considerations
Examination Technique
- Both kidneys must be imaged in longitudinal and transverse planes for comparison and to exclude absence of either kidney 1
- Use a 2-5 MHz curved array abdominal probe; a small footprint or phased array probe facilitates intercostal scanning 1
- Kidneys are retroperitoneal and lie obliquely to every anatomic plane, with inferior poles anterior and lateral to superior poles 1, 2
Limitations
- Examination may be limited by patient habitus, bowel gas, abdominal/rib tenderness, and empty bladder 2
- Parts or entire kidney may not be visible due to interposed bowel loops—use intercostal approach with patient in decubitus position 1
Clinical Context
Ultrasound is ideal for rapid, non-invasive evaluation without radiation, contrast agents, or removing patients from the resuscitation area 1, making it valuable for time-sensitive conditions like acute renal colic and urinary retention 1. However, ultrasound is a focused examination that does not identify all urinary tract abnormalities and must be interpreted in the full clinical context 1. When findings are equivocal, additional diagnostic studies are warranted 1.