What is the purpose of a renal scan?

Purpose of Renal Scan

A renal scan is primarily used to evaluate renal function and urinary tract drainage by providing information on split renal function and detecting potential obstructions, which is essential for assessing kidney health and guiding clinical management decisions. 1

Types of Renal Scans and Their Functions

MAG3 Renal Scan

• Tc-99m MAG3 (mercaptoacetyltriglycine) is primarily excreted through active renal tubular transport with an extraction fraction of 40-50%, providing superior imaging with less background activity 1
• Preferred for patients with suspected obstruction or impaired renal function due to its rapid renal clearance and primary excretion by tubules on which furosemide acts 1
• Provides critical information on split renal function and urinary tract drainage based on renal washout curves 1
• Used for diagnosis of obstructive conditions, including primary obstructing megaureter 1
• Helps monitor renal function over time, with decreases in differential renal function often indicating need for intervention 1

DTPA Renal Scan

• Tc-99m DTPA (diethylenetriamine pentaacetic acid) is excreted primarily by glomerular filtration with approximately 20% extraction fraction 1
• Provides information on renal function and urinary tract drainage based on split renal function and renal washout curves 1
• Less preferred than MAG3 in patients with suspected obstruction or impaired renal function due to lower extraction fraction 1
• Often delayed until at least 2 months of age in newborns due to their lower glomerular filtration rate 1

DMSA Renal Scan

• Ideal for functional renal cortical imaging 1
• Most useful for detection of focal renal parenchymal abnormalities and scars in acute or chronic pyelonephritis 1
• Provides accurate assessment of differential renal function 1
• Helpful for monitoring renal cortical scarring through serial imaging 1

Clinical Applications of Renal Scans

Evaluation of Hydronephrosis

• Essential for assessing moderate to severe (SFU grade 3 or 4) hydronephrosis, especially in infants with antenatal diagnosis 1
• Helps determine if hydronephrosis is obstructive or non-obstructive 1
• Surgical intervention decisions are based on evidence from renal scans, including T1/2 of time activity curve (T1/2 >20 minutes), decreased renal function (<40% differential function), or deteriorating function (>5% change on consecutive scans) 1

Assessment of Renal Function in Transplants

• Provides noninvasive evaluation of renal transplant function and screens for surgical complications 1
• Offers functional information when serum creatinine levels lag behind actual function 1
• Can detect perfusion issues in transplanted kidneys 1

Diagnosis of Renovascular Hypertension

• Captopril renal scintigraphy can be used to evaluate renovascular hypertension 1
• Assesses functional changes in renal perfusion and function rather than just anatomical abnormalities 1
• Particularly useful in appropriately selected patients, though sensitivity decreases in bilateral stenosis and impaired renal function 1

Detection of Acute Pyelonephritis and Renal Scarring

• Several studies have shown renal scintigraphy to be equivalent or superior to other imaging modalities for detecting pyelonephritis and renal scarring 1
• Can distinguish between acute pyelonephritis, renal scarring, and renal dysplasia 1

Advantages and Limitations

Advantages

• Provides functional information that anatomical imaging alone cannot offer 2, 3
• Non-invasive assessment of split renal function 1
• Particularly valuable when information on the function of each kidney is necessary 2
• Can detect obstruction and monitor changes in renal function over time 1

Limitations

• Scintigraphic parameters alone do not yield sufficient diagnostic power for specific diagnosis of some conditions 1
• Like ultrasound resistive indices, renogram changes may not contribute to differential diagnosis between acute rejection, ATN, and cyclosporine toxicity in transplants 1
• Limited in early stages of renal disease when functional changes may be minimal 4

Clinical Decision Making

• Ultrasound is typically the first-line imaging modality for initial kidney evaluation due to its accessibility, safety, and ability to assess both structure and function 5, 6
• Renal scans are often used as a second-line test when functional assessment is needed beyond what ultrasound can provide 2
• In cases of suspected obstruction, MAG3 is preferred over DTPA, especially in patients with impaired renal function 1
• For transplant monitoring, renal scans may be performed before patient discharge to establish baseline for future comparison 1

In summary, renal scans play a crucial role in the comprehensive evaluation of kidney function, providing valuable information that guides clinical management decisions and helps monitor disease progression or response to treatment.