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DTPA Scan - Complete Overview
Your note covers four topics: RP & MOA, Protocols, Indications, and Interpretation. Here is a detailed breakdown of each.
1. Radiopharmaceutical (RP) and Mechanism of Action (MOA)
Agent: ⁹⁹ᵐTc-DTPA (Technetium-99m Diethylenetriaminepentaacetic Acid), also known as pentetic acid.
- 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
Comparison with other agents:
| Agent | Mechanism | Best Use |
|---|---|---|
| ⁹⁹ᵐTc-DTPA | Glomerular filtration only | GFR calculation, obstruction in normal function |
| ⁹⁹ᵐTc-MAG3 | Tubular secretion (~60% extraction) | Preferred for obstruction, poor function, neonates |
| ⁹⁹ᵐTc-DMSA | Glomerular filtration + tubular binding | Static cortical imaging, scarring |
- Campbell Walsh Wein Urology, p. 123
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)
Phase 1 - Perfusion Phase (0-1 minute)
- 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
Phase 2 - Functional Phase (1-20+ minutes)
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Images acquired at 1 frame per minute
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Peak cortical uptake occurs at 3-5 minutes post-injection
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Radiotracer seen in collecting system shortly after peak
-
Bladder visualizes by 10-15 minutes
-
Campbell Walsh Wein Urology, p. 124
3. Indications
| Indication | Details |
|---|---|
| Split (differential) renal function | Determines relative contribution of each kidney (normal: 45-55% each; >10% difference = abnormal) |
| GFR calculation | Best agent for measuring glomerular filtration rate due to exclusive glomerular clearance |
| Functional urinary obstruction | Diuretic renogram (F+20 or F-15 protocol) to detect UPJ obstruction, UVJ obstruction |
| Renovascular hypertension | Captopril renogram - ACE inhibitor given before study; asymmetric response suggests renal artery stenosis |
| Renal transplant evaluation | Assess perfusion and function post-transplant |
| Ectopic/horseshoe kidney | Assess differential function in anatomic variants |
| Post-obstruction recovery | Monitor renal function recovery after relief of obstruction |
| Vesicoureteral reflux (VUR) | Indirect radionuclide cystography |
| Pre-abdominal radiotherapy | Baseline differential function when kidneys are in therapy field |
| Renal artery stenosis | Delayed uptake and excretion on affected side |
- Campbell Walsh Wein Urology, p. 123; Grainger & Allison's Diagnostic Radiology, p. 1836
4. Interpretation
Normal Renogram Curve (TAC)
A typical normal renogram has three phases:
- Phase 1 (Vascular/Perfusion, 0-30 sec): Sharp upstroke - reflects renal blood flow
- Phase 2 (Tubular/Cortical, 1-3 min): Continued rise to peak - reflects tubular uptake/GFR
- Phase 3 (Excretory, 3-20 min): Gradual downslope as tracer is excreted into urine
Normal values:
- Split function difference up to 10% is within normal limits
- Half-clearance time (T½) from collecting system: 15-20 minutes = nonobstructed
Abnormal Patterns
| Pattern | Appearance | Significance |
|---|---|---|
| Poor perfusion | Slow rise to peak in Phase 1; curve does not match aortic shape | Poor renal blood flow, impaired function |
| Obstruction | Gradually upsloping TAC with no peak; retention of tracer in collecting system | Functional obstruction |
| Patulous/non-obstructed dilated system | Delayed clearance, but washes out with furosemide (T½ <10 min post-diuretic) | Previous UPJ obstruction repair, normal variant |
| Equivocal | T½ 10-20 min post-furosemide | Indeterminate; further evaluation needed |
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
Key Points to Remember
- 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
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