MAG3 Uptake Above the Kidney: Artifact Identification
MAG3 uptake visualized above the kidney is typically an artifact caused by background activity from the liver, spleen, or bowel, not true renal uptake, since MAG3 is primarily excreted through renal tubular secretion with minimal hepatobiliary excretion. 1, 2
Understanding Normal MAG3 Biodistribution
MAG3 is designed for renal tubular transport and excretion, with approximately 40-50% extraction fraction through active tubular secretion. 3 The tracer should demonstrate:
- Rapid renal uptake with minimal background activity compared to DTPA 3
- Primary excretion through the kidneys via MRP2 and MRP4 transporters on the apical membrane of proximal tubule cells 4
- Superior image quality with less background activity than other renal agents due to efficient tubular extraction 5, 2
Common Sources of Suprarrenal Activity
Activity appearing above the kidney on MAG3 imaging typically represents:
- Hepatic background activity from the small percentage of MAG3 that undergoes hepatobiliary excretion 2
- Splenic uptake appearing as background activity in the left upper quadrant 2
- Bowel activity from hepatobiliary excretion that can overlap with or appear superior to the kidney on planar imaging 2
Technical Considerations to Minimize Artifacts
Adequate hydration is critical to optimize MAG3 excretion and minimize background activity. 6 Dehydration can cause:
- Abnormal continual parenchymal uptake 6
- Minimal excretion into the collecting system 6
- Increased background-to-kidney ratio that may create confusing patterns 6
Image acquisition timing matters: MAG3 demonstrates more rapid time to peak height of renogram curves compared to other agents, so delayed imaging may show more background activity. 5
Distinguishing True Pathology from Artifact
True renal uptake should demonstrate all three phases of renal function: perfusion, excretion, and urine flow. 3, 1 Activity above the kidney that lacks these characteristics is artifact.
Key differentiating features:
- Artifact remains static or follows hepatobiliary/bowel transit patterns rather than renal excretion curves 2
- True renal tissue shows characteristic time-activity curves with uptake and washout phases 1, 7
- Ectopic or abnormal renal tissue would still demonstrate functional phases if viable 3
Clinical Pitfall to Avoid
Do not mistake hepatic or splenic background for accessory renal tissue or renal pathology. 2 If suprarrenal activity is concerning, correlation with anatomic imaging (ultrasound or CT) will definitively exclude ectopic kidney or other structural abnormalities. 3