Distinguishing Left Kidney from Spleen on Imaging
The left kidney is differentiated from the spleen by identifying the hyperechoic renal sinus (central collecting system), the distinct corticomedullary differentiation, and the retroperitoneal location of the kidney versus the intraperitoneal position of the spleen. 1
Key Distinguishing Features
Internal Architecture
- Kidney: Displays a characteristic hyperechoic central renal sinus (representing fat and collecting system) surrounded by hypoechoic parenchyma with visible corticomedullary differentiation 1
- Spleen: Shows homogeneous echotexture without a central echogenic structure; lacks the kidney's distinctive internal architecture
Anatomic Position
- Kidney: Retroperitoneal location in the costovertebral angle, typically imaged through an intercostal approach from the posterior axillary line or more posteriorly 1
- Spleen: Intraperitoneal organ that provides a limited acoustic window compared to the liver; located more anteriorly 1
Shape and Orientation
- Kidney: Bean-shaped with identifiable hilum containing vessels; long axis approximates the intercostal spaces with inferior pole positioned anterior and lateral to superior pole 1
- Spleen: Crescentic shape without a central hilum structure
Critical Imaging Pitfalls
Renal Duplication Artifacts
Sound beam refraction between the spleen and adjacent fat can create false duplication artifacts of the left kidney's upper pole 2. These occur in approximately 28% of cases when the spleen is partially in the image, particularly when the splenic lower pole is rounded rather than wedge-shaped. The artifact can manifest as:
- Complete or incomplete upper pole duplication
- "Bayonet" appearance
- Apparent cortical thinning due to geometric compression
Retrorenal Spleen
The spleen can be positioned behind the upper lateral fifth and superior fourth of the left kidney in 21% of supine patients and 15% of prone patients 3. This anatomic variant:
- Occurs predominantly in patients over age 50 (80% of cases)
- Can make the spleen appear as a renal mass on imaging
- Poses risk for percutaneous procedures approaching the upper pole
Splenic Lobulations
Prominent splenic lobulations can flatten the upper pole of the left kidney, simulating a suprarenal mass 4, 5. CT is the most sensitive method to distinguish this normal variant from true pathology.
Optimal Imaging Approach
Ultrasound Technique
- Use intercostal approach with probe on posterior axillary line or more posteriorly to avoid gas-filled bowel 1
- Scan in longitudinal plane first (parallel to intercostal spaces) with probe indicator toward the head 1
- Identify the renal hilum by rotating 90° counter-clockwise to transverse plane and moving superiorly/medially or inferiorly/laterally 1
- Adjust gain settings so renal sinus fat appears white (hyperechoic) and known hypoechoic structures appear black 1
When Ultrasound is Inadequate
CT with IV contrast is the definitive modality when ultrasound cannot reliably distinguish between kidney and spleen, particularly for:
- Evaluating splenic lobulations versus renal masses 5
- Confirming retrorenal spleen positioning 3
- Assessing complex anatomic relationships in the left upper quadrant 6
Clinical Context Considerations
The distinction becomes particularly important when:
- Planning percutaneous renal procedures (risk of splenic injury with retrorenal spleen) 3
- Evaluating left upper quadrant masses
- Assessing for splenosis following splenic trauma, which can appear inseparable from the kidney 7
- Investigating acute left upper quadrant pain where both organs may be involved 6