Ultrasound scan for kidney
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Kidney Ultrasound (Renal Sonography)
Ultrasound is the first-line imaging modality for the kidney - it is non-invasive, radiation-free, widely available, and provides real-time structural information. Here is a complete overview.
Indications
Renal ultrasound is used to evaluate:
- Acute kidney injury (AKI) - to exclude urinary obstruction as a cause
- Chronic kidney disease (CKD) - assess kidney size, echogenicity, and chronicity
- Hydronephrosis - grading and identifying the level of obstruction
- Renal colic / suspected stones - detect nephrolithiasis
- Urinary tract infection (UTI) - especially in children; detect congenital anomalies, abscess, or pyonephrosis
- Haematuria - detect masses, cysts, or stones
- Hypertension - evaluate kidney size and detect renal artery abnormalities
- Transplant kidney - monitor for rejection, perinephric collections, and vascular complications
- Guidance for biopsy and percutaneous procedures
(Comprehensive Clinical Nephrology, 7th Ed.)
Technique
Probe and Patient Position
- A curvilinear (convex) transducer is the standard choice. A phased array probe can be used as a substitute but offers inferior parenchymal detail.
- Patient is typically supine, though lateral decubitus, oblique, or prone positions may be needed (especially for the left kidney, where overlying bowel gas is a challenge).
- Acoustic windows:
- Right kidney - imaged through the liver
- Left kidney - imaged through the spleen
- This avoids bowel gas interference.
Probe Positioning
- Place the transducer in the mid- or anterior axillary line at the 10th rib interspace, orientation marker pointing toward the patient's head, angled slightly posteriorly.
- Fan (tilt) anteriorly and posteriorly to image the kidney completely in longitudinal view.
- Rotate 90 degrees counterclockwise for the short axis (transverse) view, then fan superiorly and inferiorly to image from pole to pole.
- Color Doppler should always be added for global perfusion assessment and to differentiate vascular structures from dilated collecting systems.
Normal Sonographic Appearances
| Feature | Normal Finding |
|---|---|
| Shape | Well-defined, bean-shaped with echogenic capsule |
| Size | 9-12 cm in longitudinal plane (varies with body height) |
| Cortex | Hypoechoic or isoechoic relative to liver/spleen |
| Medullary pyramids | Anechoic or hypoechoic compared to cortex; prominent in children |
| Columns of Bertin | Cortical tissue separating pyramids (can hypertrophy and mimic a mass) |
| Sinus fat | Echogenic; occupies inner kidney; encases collecting system |
| Collecting system | Not visible when undilated; any anechoic area in sinus fat raises suspicion for hydronephrosis |
| Transverse (short axis) view | C-shaped at midportion; circular at poles |
Key Pathological Findings
1. Hydronephrosis
Appears as anechoic branching, interconnected areas in the collecting system (backlogged urine).
| Grade | Features |
|---|---|
| Mild | Dilation of renal pelvis and calyces; parenchymal architecture intact |
| Moderate | Medullary pyramids begin to flatten; increasing pelvicalyceal dilation |
| Severe | Ballooned pelvis and calyces; corticomedullary differentiation lost; thin parenchyma |
- If internal echoes are present within the hydronephrotic area → consider pyonephrosis (pus in the collecting system), especially with fever and flank pain.
- Pitfall: Color Doppler differentiates blood vessels (show flow) from hydronephrosis (no flow). Parapelvic cysts can mimic hydronephrosis but are round, well-circumscribed, and not connected to the ureter.
2. Kidney Stones (Nephrolithiasis)
- Appear as hyperechoic structures with posterior acoustic shadow (signal void beyond the stone).
- On color Doppler: exhibit the "twinkling sign" - a rapidly alternating color Doppler signal that is more pronounced with rough-surfaced stones.
3. Echogenicity Changes (CKD / AKI)
- Normal cortex is hypoechoic compared to liver.
- Increased cortical echogenicity (cortex brighter than liver) = parenchymal disease (CKD, glomerulonephritis, diabetic nephropathy, etc.)
- In CKD: kidneys become small and echogenic, with loss of corticomedullary differentiation.
- In AKI: kidneys are often normal or slightly enlarged; echogenicity alone cannot distinguish cause.
4. Renal Cysts
- Appear as smooth, round, anechoic structures with posterior acoustic enhancement.
- Simple cysts: thin walls, no internal echoes = benign.
- Complex cysts: internal echoes, septations, or thick walls = require further evaluation (CT or MRI).
5. Masses
- Solid renal masses appear as hyper- or isoechoic lesions disturbing normal architecture.
- Ultrasound is a screening tool; CT or MRI is needed for characterization.
6. Renal Vein Thrombosis
- Kidney enlargement, loss of corticomedullary differentiation, linear echogenicity radiating from the renal hilum (from venous clot).
- Doppler shows absent or reversed diastolic flow.
Doppler Assessment
| Application | Finding |
|---|---|
| Color Doppler | Global perfusion; differentiates vessels from hydronephrosis/cysts |
| Spectral Doppler (RI) | Resistive Index (RI) = (Peak systolic velocity - End diastolic velocity) / Peak systolic velocity |
| Normal RI | < 0.70 |
| Elevated RI (>0.70) | Suggests obstruction, rejection (transplant), or parenchymal disease |
| Renal artery stenosis | Peak systolic velocity > 180-200 cm/s at stenosis; dampened waveform ("tardus-parvus") distally |
Limitations of Renal Ultrasound
- Operator-dependent - image quality and interpretation vary with experience.
- Bowel gas - can obscure the left kidney.
- Body habitus - difficult in obese patients.
- Not sensitive for acute pyelonephritis - abnormalities found in only 20-69% vs. 40-92% with DMSA scintigraphy.
- Not diagnostic for vesicoureteral reflux (VUR) or kidney scarring.
- False negatives for hydronephrosis - can occur with acute/partial obstruction, volume depletion, or retroperitoneal fibrosis.
- Cannot assess function - functional studies (DMSA, MAG3 scintigraphy) or GFR measurement needed for functional information.
Clinical Decision Points
| Clinical Question | Role of Ultrasound |
|---|---|
| AKI - is it obstructive? | Rule out hydronephrosis (high sensitivity) |
| CKD - how chronic? | Small, echogenic kidneys = chronic; normal size with AKI = acute |
| Renal colic | Detect stones and hydronephrosis; CT KUB more sensitive for small stones |
| Haematuria | Screen for mass, cyst, stone |
| Paediatric UTI | Detect congenital anomalies (CAKUT), abscess, pyonephrosis |
| Transplant monitoring | Perinephric collections, Doppler for rejection |
| Pre-biopsy | Confirm kidney position, size, and guide needle placement |
Sources: Comprehensive Clinical Nephrology, 7th Edition (Chapter 5 - Kidney and Bladder Ultrasound, p. 80-82); Brenner and Rector's The Kidney, 2-Volume Set (Imaging Tests chapter)
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