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DTPA Scan - Complete Overview

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1. Radiopharmaceutical (RP) and Mechanism of Action (MOA)

• Radionuclide: ⁹⁹ᵐTc - half-life of 6 hours, gamma energy 140 keV (ideal for gamma camera imaging)
• Dose: 5 to 10 mCi (185-370 MBq) IV push
• MOA: After IV injection, ⁹⁹ᵐTc-DTPA is extracted from the bloodstream entirely through glomerular filtration (not tubular secretion). It then passes through the renal tubules and is excreted in urine without tubular reabsorption.
  • Because clearance is 100% glomerular, it directly mirrors GFR
  • Extraction efficiency is approximately 20% per renal pass (lower than MAG3's ~60%)
  • This low extraction efficiency causes high background activity and poorer image quality in patients with impaired renal function - the major limitation vs MAG3

• Campbell Walsh Wein Urology, p. 123

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2. Protocol

Patient Preparation

• Patient must be well hydrated on the day of the scan (ensures optimal radiotracer delivery to kidneys)
• Note all medications - especially diuretics and antihypertensives, which affect interpretation
• Note anatomic variants: horseshoe kidney, renal transplant, duplication of urinary tract
• If neurogenic bladder or bladder outlet obstruction: place a Foley catheter before the study (prevents retrograde pressure)
• If percutaneous nephrostomy tubes present: cap the tubes to assess native ureter patency
• For obstruction studies, furosemide (frusemide) is co-administered (diuretic renogram - see interpretation)

Acquisition Protocol (Two-Phase Dynamic Imaging)

• Images acquired every 1-2 seconds immediately after IV radiotracer injection
• Regions of interest (ROI) drawn over: aorta, each renal unit, and background (just outside each kidney for subtraction)
• Data plotted as Time-Activity Curve (TAC): time (x-axis) vs. counts/activity (y-axis)
• Activity should appear in kidneys within seconds of appearing in aorta

• Images acquired at 1 frame per minute
• Peak cortical uptake occurs at 3-5 minutes post-injection
• Radiotracer seen in collecting system shortly after peak
• Bladder visualizes by 10-15 minutes
• Campbell Walsh Wein Urology, p. 124

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3. Indications

• Campbell Walsh Wein Urology, p. 123; Grainger & Allison's Diagnostic Radiology, p. 1836

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4. Interpretation

Normal Renogram Curve (TAC)

1. Phase 1 (Vascular/Perfusion, 0-30 sec): Sharp upstroke - reflects renal blood flow
2. Phase 2 (Tubular/Cortical, 1-3 min): Continued rise to peak - reflects tubular uptake/GFR
3. Phase 3 (Excretory, 3-20 min): Gradual downslope as tracer is excreted into urine

• Split function difference up to 10% is within normal limits
• Half-clearance time (T½) from collecting system: 15-20 minutes = nonobstructed

Abnormal Patterns

The Diuretic Renogram (F+20 Protocol)

• Furosemide 0.5 mg/kg IV given at 20 minutes post-radiotracer (F+20), or at -15 min (F-15 protocol)
• Washout (T½ <10 min) after furosemide = no obstruction
• No washout (T½ >20 min) = obstructed
• T½ 10-20 min = equivocal

Captopril Renogram (for Renovascular Hypertension)

• Captopril (ACE inhibitor) given 1 hour before study
• Blocked angiotensin II effect unmasks reduced GFR in a stenotic kidney
• Positive study: marked reduction in uptake on affected side vs. baseline = renovascular hypertension

Renal Artery Stenosis Pattern (Non-captopril)

• Delayed peak time, prolonged time to peak cortical uptake
• Delayed circulation and excretion of isotope on the affected side
• Brenner and Rector's The Kidney, p. 365

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• DTPA is the cheapest dynamic renal agent but has lower image quality than MAG3 due to only 20% extraction efficiency
• Not suitable for neonates or patients with significantly impaired renal function (poor image quality; MAG3 preferred)
• DTPA cannot characterize a renal mass (wrong tool - use CT/MRI)
• T½ of ⁹⁹ᵐTc itself is 6 hours, making same-day imaging practical

Sources: Campbell Walsh Wein Urology, pp. 123-124; Grainger & Allison's Diagnostic Radiology, p. 1836; Brenner and Rector's The Kidney, p. 365