Renal Ultrasound

Renal ultrasound is the first-line imaging test for evaluating the kidneys and urinary bladder without ionizing radiation or contrast. It reliably detects hydronephrosis, renal size and parenchymal changes, cysts, masses, and calculi, and is essential in the workup of renal failure and hematuria. Doppler adds assessment of renal and, in transplants, vascular perfusion.

Indications

Acute or chronic kidney injury to assess size, echogenicity, and obstruction
Suspected hydronephrosis or urinary tract obstruction
Hematuria evaluation and detection of renal or bladder masses
Nephrolithiasis and follow-up of known stones
Characterization of a renal mass or cyst
Renal transplant assessment of perfusion, collecting system, and perinephric collections
Suspected renovascular disease or renal artery stenosis with Doppler
Guidance for renal biopsy or nephrostomy

Contraindications & Cautions

No absolute contraindications; radiation-free and contrast-free
Limited sensitivity for small or mid-ureteric stones compared with CT
Body habitus and bowel gas limit visualization, especially of the ureters
Non-distended bladder limits assessment of bladder wall and distal ureters
Follow ALARA principles for Doppler thermal exposure

Patient Preparation

No strict fasting required; a moderately full bladder aids bladder and distal ureter evaluation
Explain the examination; no consent needed for diagnostic scanning
Positioning supine, decubitus, and prone as needed; transplant kidneys imaged over the iliac fossa
Deep inspiration to bring kidneys below the ribs
Document relevant history including creatinine, hematuria, and prior stones

Technique & Parameters

Curvilinear 2-5 MHz transducer; higher frequency for thin patients and transplants
Both kidneys imaged in longitudinal and transverse planes with maximal bipolar length measured (normal 9-12 cm)
Assess cortical thickness, echogenicity relative to liver and spleen, and corticomedullary differentiation
Evaluate collecting system for hydronephrosis, grading mild to severe
Scan for calculi with attention to echogenic foci, posterior shadowing, and twinkle artifact
Bladder assessed for volume, wall thickness, masses, and ureteric jets with color Doppler
Doppler assessment of main renal arteries and intrarenal arteries with resistive index (normal <0.70)
Transplant protocol adds evaluation of anastomoses, resistive indices, and perinephric collections

Systematic Review

Kidney size and bipolar length bilaterally
Cortical thickness and echogenicity versus adjacent liver or spleen
Corticomedullary differentiation and pyramids
Collecting system for dilation and grade of hydronephrosis
Renal calculi with size, location, and shadowing
Focal renal lesions: cystic versus solid characterization
Bladder volume, wall, masses, and ureteric jets
Doppler: arterial waveforms, resistive index, venous patency (especially transplants)

Key Findings & Significance

Small echogenic kidneys with thinned cortex indicate chronic kidney disease
Increased cortical echogenicity with preserved size suggests medical renal disease
Dilated calyces and renal pelvis indicate hydronephrosis; degree grades severity
Echogenic focus with posterior shadowing and twinkle artifact confirms a calculus
Anechoic thin-walled lesion with posterior enhancement is a simple (Bosniak I) cyst
Complex cyst with septa, calcification, or a solid enhancing component raises malignancy concern
Elevated resistive index (>0.80) in a transplant suggests rejection, ATN, or obstruction

Differential Considerations

Bilateral small echogenic kidneys: chronic glomerulonephritis, hypertensive nephrosclerosis, diabetic nephropathy
Unilateral enlarged kidney: obstruction, pyelonephritis, renal vein thrombosis, infiltrative tumor
Cystic renal lesion: simple cyst, complex cyst, ADPKD, cystic RCC, multilocular cystic nephroma
Solid renal mass: renal cell carcinoma, angiomyolipoma (echogenic), oncocytoma, lymphoma, metastasis
Hydronephrosis: obstructing stone, UPJ obstruction, tumor, extrinsic compression, reflux, pregnancy
Transplant dysfunction: ATN, rejection, obstruction, renal artery stenosis, collection

Pearls & Pitfalls

Pelvicalyceal dilation is not always obstruction; extrarenal pelvis, reflux, and a full bladder can mimic hydronephrosis
Twinkle artifact on color Doppler improves detection of small calculi
An echogenic renal lesion is most often an angiomyolipoma but a small RCC can also be echogenic; consider CT/MRI
Parapelvic cysts can mimic hydronephrosis; they do not communicate and lack a connecting infundibulum
Measure resistive indices in transplant kidneys at upper, mid, and lower poles
Column of Bertin and dromedary hump are normal variants that can simulate a mass
Assess the bladder before it is emptied to capture ureteric jets and wall pathology

Structured Report

Report bilateral kidney sizes, cortical thickness, and echogenicity
Describe hydronephrosis presence and grade, and identify obstructing lesion if seen
Characterize renal lesions with size, cystic versus solid, and Bosniak category where applicable
Report bladder findings and Doppler resistive indices, especially in transplants
State technical limitations
Impression: concise summary highlighting obstruction, masses requiring CT/MRI, or transplant complications with recommendations

References

ACR-AIUM-SRU Practice Parameter for the Performance of an Ultrasound Examination of the Kidneys and Urinary Bladder
Bosniak Classification of Cystic Renal Masses, version 2019
ACR Appropriateness Criteria: Renal Failure and Hematuria
Rumack CM, Levine D. Diagnostic Ultrasound, 5th edition

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

Renal Ultrasound

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References