What information does a renal ultrasound yield in patients with suspected kidney disease?

Renal Ultrasound Diagnostic Information

Renal ultrasound provides comprehensive structural and functional information including detection of hydronephrosis (>90% sensitivity), kidney stones, bladder pathology, kidney size and echogenicity, masses and cysts, anatomical variants, and vascular patency—making it the ideal first-line imaging modality for suspected kidney disease. 1

Primary Structural Information

Hydronephrosis and Obstruction

• Hydronephrosis detection is the most reliable ultrasound finding, with sensitivity exceeding 90%, appearing as anechoic (fluid-filled) spaces within the renal sinus representing collecting system dilation 1, 2
• Grading includes mild, moderate (characteristic "bear's paw" appearance), and severe classifications, with severe hydronephrosis showing extensive dilation and parenchymal effacement 1, 3
• Critical pitfall: Absence of hydronephrosis does NOT rule out ureteral stones—many stones, especially small ones, cause no dilation 4, 3
• Dehydration can mask obstruction by preventing hydronephrosis development 4, 3
• False-positive hydronephrosis can result from dilated renal vasculature, renal sinus cysts, bladder distension, or normal medullary pyramids (especially in young patients) 4, 3

Kidney Stones

• Stones appear as hyperechoic foci with posterior acoustic shadowing 1, 2
• Significant limitation: Stones <3mm are usually NOT detected by current equipment, and ultrasound sensitivity for stones is only 24-57% compared to CT 4, 1, 2
• Stones may be missed because their echogenicity resembles surrounding renal sinus fat 4

Kidney Size and Parenchymal Assessment

• Normal renal length is typically >10cm in the third decade of life, correlating with height, sex, age, and weight 1, 2
• Acute kidney injury typically shows normal-sized kidneys with increased (but nonspecific) echogenicity 1, 2
• Chronic kidney disease presents as small, echogenic kidneys with decreased parenchymal thickness 1, 2
• Ultrasound displays excellent anatomic detail for determining kidney site, size, and detecting local lesions 5

Bladder Evaluation

• The bladder must always be imaged as part of renal ultrasound—many urinary tract conditions are identifiable in the bladder 4, 1
• Bladder assessment includes capacity, wall thickness, mural and luminal masses, and urinary retention detection 1
• Ideally, kidneys should be scanned after voiding since a distended bladder can cause artifactual mild hydronephrosis 1

Masses and Cysts

• Solid masses may represent tumors requiring further evaluation 2
• Renal tumors may present with gross hematuria and are detectable with ultrasound 2
• Ultrasound enables differentiation between solid and cystic renal lesions 6

Infectious and Inflammatory Processes

• Pyelonephritis appears as focal or multifocal areas of decreased parenchymal enhancement 1, 2
• Renal abscess shows as hypoechoic or anechoic collections within the renal parenchyma 1, 2

Anatomical Variants (Common Pitfalls)

• Both kidneys must be imaged to identify unilateral kidney absence or bilateral disease 4, 1, 3
• Anatomical variations commonly mistaken for pathology include: reduplicated collecting systems, unilateral kidney, bipartite kidney, ectopic kidney, and horseshoe kidney 4, 1, 3, 2

Vascular Information

• Color Doppler assesses global renal perfusion, arterial and venous patency, and renovascular disease 2
• Doppler ultrasound allows detection and quantification of renal artery stenosis 5
• Increased resistance index values may indicate irreversible disease or predict allograft failure in transplant kidneys 5

Technical Limitations to Document

• Examination may be limited by: patient habitus (obesity, narrow intercostal spaces), bowel gas, abdominal/rib tenderness, and empty bladder 4, 1, 3, 2
• When technical factors prevent complete scanning, limitations must be documented and may mandate alternative imaging methods 4, 3
• Parts or entire kidney may not be visible due to interposed bowel loops 1

Clinical Advantages

• Ultrasound is ideal for rapid, non-invasive evaluation without radiation, contrast agents, or removing patients from the resuscitation area, making it valuable for time-sensitive conditions like acute renal colic and urinary retention 1
• It uses nonionizing radiation, is noninvasive, generates real-time images, and permits bedside examination 5, 7
• However, ultrasound is a focused examination that does not identify all urinary tract abnormalities—when findings are equivocal, additional diagnostic studies (CT urography or MR urography) are warranted 1, 3