Ultrasound Evaluation of Kidneys and Urinary Bladder
A sonographer systematically evaluates both kidneys in longitudinal and transverse planes, assessing for hydronephrosis, renal parenchymal abnormalities, and measuring renal dimensions, while the bladder is scanned in sagittal and transverse views to assess volume, wall thickness, and detect calculi or masses 1.
Equipment and Technique
Probe Selection
- Primary probe: Curved array abdominal transducer, 2-5 MHz frequency 1
- Alternative: Small footprint or phased array probe for intercostal scanning 1
- Pediatric patients: Higher frequency 5.0-7.0 MHz for better resolution 1
Patient Positioning and Scanning Approach
Right Kidney:
- Anterior subcostal approach using liver as acoustic window
- Patient in left lateral decubitus or prone position
- Deep breath to extend hepatic window 1
- Intercostal approach (between anterior axillary line and midline posteriorly) if bowel gas obscures view
- Probe indicator always toward the head 1
Left Kidney:
- Intercostal approach only (no hepatic window available)
- Same positioning technique as right kidney 1
Bladder:
- Transverse and sagittal planes
- Scan from top to bottom and side to side 1
Key Sonographic Findings to Evaluate
Kidneys - Normal Anatomy
Renal Sinus:
- Homogeneously hyperechoic structure
- Should contain no urine under normal circumstances 1
Renal Parenchyma:
- Assess echogenicity and thickness
- Look for cortical-medullary differentiation 1
Kidneys - Pathological Findings
Hydronephrosis Grading 1:
- Grade I (Mild): Any hydronephrosis up to Grade II
- Grade II (Moderate): Confluent calices creating "bear's paw" appearance
- Grade III (Severe): Extensive hydronephrosis causing parenchymal effacement
Other Abnormalities:
- Cysts
- Masses
- Renal calculi (identified by posterior acoustic shadowing) 1
Bladder Evaluation
Volume Assessment:
- Qualitative assessment for overdistension 2
- Pre-void and post-void measurements when evaluating outlet obstruction 1
Wall Thickness:
- Normal distended bladder: ≤3 mm
- Normal empty bladder: ≤5 mm
- Mean thickness varies: 1.55 mm (distended) to 2.76 mm (empty) 3
Additional Findings:
- Calculi
- Masses
- Evidence of distal ureteral obstruction 1
Normal Measurements
Renal Dimensions
Length and Width:
- Measurements of kidney length and width can be obtained 1
- Note: Specific measurements "rarely relevant in focused examination" per guidelines 1
- Kidney volume correlates better with actual size than length alone 4
Bladder Volume
Simplified Estimation:
- Single sagittal measurement (sagittal length) shows strongest correlation (r = 0.92) 5
- Formula: Bladder volume (mL) = 5 × Sagittal Length (mm) - 222 5
Critical Pitfalls and Caveats
Common Mimics of Hydronephrosis 1:
- Dilated renal vasculature
- Renal sinus cysts
- Bladder distension (can cause artifactual mild hydronephrosis)
- Medullary pyramids (especially in young patients)
Technical Limitations 1:
- Obesity or narrow intercostal spaces
- Bowel gas interference
- Dehydration (may mask obstruction)
- Empty bladder limits examination
Diagnostic Limitations 1:
- Renal stones <3 mm: Usually not identified
- Absence of hydronephrosis: Does NOT rule out ureteral stone
- Many small ureteral stones cause no hydronephrosis
Anatomical Variations 1:
- Duplicated collecting systems
- Unilateral, bipartite, ectopic, or horseshoe kidney
- These are NOT pathologic but may be mistaken for abnormalities
Optimal Scanning Sequence
Ideal Protocol 1:
- Scan bladder before voiding (assess volume, pathology)
- Patient voids
- Scan kidneys after voiding (prevents artifactual hydronephrosis from bladder distension)
- Post-void bladder scan if outlet obstruction suspected
Reality: This ideal sequence is "rarely met with the exigencies of emergency care" 1
Documentation Requirements
Always document 1:
- Indication for examination
- Organs/structures studied
- Interpretation of findings
- Store images when feasible per facility policy 1
Both kidneys must be imaged to identify unilateral kidney or bilateral disease processes 1.