12.1 Renal and Genitourinary Imaging
Key Takeaways
- Tc-99m MAG3 is a tubular-secretion (ERPF-type) renogram agent preferred when function is reduced; Tc-99m DTPA is filtered and supports GFR-oriented imaging/non-imaging clearance; Tc-99m DMSA binds cortex for scars and relative function
- Diuretic renography (furosemide/Lasix®) distinguishes obstructed from dilated non-obstructed collecting systems by washout timing relative to the F+0 / F+20 (or lab-specific) protocol
- Captopril (ACE-inhibitor) renography screens for functionally significant renovascular hypertension by comparing baseline and post-ACE curves or uptake patterns
- Direct radionuclide cystography detects vesicoureteral reflux with low radiation dose; hydrate, empty/catheterize the bladder as ordered, and avoid infiltrated doses that ruin quantitative renography
- Interpretation keys: delayed peak and poor washout after diuretic → obstruction pattern; asymmetric drop after ACE inhibitor → renovascular HTN pattern; cortical photopenic defects on delayed DMSA → scar
12.1 Renal and Genitourinary Imaging
Quick Answer: Match agent → physiology → question. MAG3 (tubular secretion) and DTPA (filtration) drive flow + renogram; add Lasix® for obstruction and captopril for renovascular HTN. DMSA maps cortex/scars. Direct cystogram finds VUR. Prep = hydrate, empty or catheterize bladder, secure IV, and never quantify an infiltrated dose.
Domain V renal items test procedure logic, not kit chemistry alone. Know what each study answers, how curves are built, and which prep error invalidates the result.
Radiopharmaceutical Roles at a Glance
| Agent | Primary physiology | Functional readout | Typical adult activity (order of magnitude) | Flagship use |
|---|---|---|---|---|
| Tc-99m MAG3 (mertiatide) | Proximal tubular secretion | ERPF-type renogram; high extraction | ~5–10 mCi (185–370 MBq) IV | Renography, obstruction, transplant, poor function |
| Tc-99m DTPA | Almost pure glomerular filtration | GFR imaging/curves; clearance methods | ~5–10 mCi IV (imaging); lower for some plasma-clearance protocols | GFR, renogram when filtration focus, selected diuretic studies |
| Tc-99m DMSA (succimer) | Cortical binding in proximal tubular cells | Morphology, relative cortical function, scars | Often ~1–5 mCi IV | Pyelonephritis scars, congenital anomalies, split function |
ERPF vs GFR: MAG3/hippurate-class agents estimate plasma delivery and tubular handling (ERPF-type); DTPA tracks filtration (GFR). Camera renography and plasma-sample clearance are complementary—both need correct timing and ROIs.
Rule of thumb: low function → MAG3; pure GFR → DTPA or formal clearance; scars → DMSA.
Renal Flow and Baseline Renogram
Flow (perfusion) phase
After a bolus IV (no infiltrate), acquire rapid frames (often 1–3 s/frame for ~60 s). Symmetric prompt renal activity mirrors arterial delivery. A flat or delayed kidney with preserved contralateral flow suggests arterial compromise, transplant rejection patterns, or bolus failure—check the injection site.
Function/excretion renogram
Continue dynamic imaging (e.g., 15–60 s frames for 20–30+ min) with soft-tissue and bladder background ROIs.
| Curve feature | Rough meaning |
|---|---|
| Tmax (time to peak) | Delayed peak/rising curve → obstruction or stasis |
| Uptake slope / early integral | Relative function / extraction |
| Washout T½ / 20-min residual | Drainage vs retained activity |
Split function uses early uptake integrals before excretion contaminates the ROI—watch background, spleen/liver overlap, and prior contrast.
Diuretic Renography (Lasix® / Furosemide)
Goal: separate mechanical obstruction from a dilated but non-obstructed system (e.g., megaureter, post-pyeloplasty anatomy).
| Protocol label (examples) | Concept |
|---|---|
| F+20 | Furosemide ~20 min after tracer when collecting system filled |
| F+0 / F−15 | Diuretic with or before tracer (lab-specific; used to optimize drainage challenge) |
| Post-void / gravity-assisted views | Empty bladder artifact and reassess washout |
Interpretation pattern (teaching): after adequate diuresis, prompt washout favors non-obstructive dilatation; persistent retention with poor T½ favors obstruction—provided hydration, dose of diuretic, and bladder emptying were adequate. A full bladder can back-pressure the ureters and mimic obstruction; catheter drainage is often required in infants, neurogenic bladder, or when residual urine is large.
Adult diuretic dose is protocol-dependent (often ~40 mg IV furosemide in adults, adjusted for renal failure—follow the order). Document response when relevant.
ACE-Inhibitor Renography (Captopril)
Goal: detect functionally significant renovascular hypertension (usually renal-artery stenosis with angiotensin-dependent GFR on the affected side).
| Step | Teaching points |
|---|---|
| Baseline renogram | Off ACE/ARB per protocol (or compare to prior) |
| ACE challenge | Oral captopril (classic teaching ~25–50 mg) or IV enalaprilat per protocol; monitor BP |
| Post-ACE renogram | Same agent/geometry |
| Positive pattern | Affected kidney shows worsened uptake, delayed peak, or cortical retention after ACE vs baseline |
Prep traps: continued ACE/ARB invalidates the challenge; volume depletion exaggerates hypotension; bilateral disease and poor function reduce specificity. This is a physiologic screen, not an angiogram.
DMSA Cortical Imaging (Planar and SPECT)
Image ~2–4 hours after injection when cortical binding dominates. Posterior ± oblique (anterior if horseshoe/ectopia) planar views; SPECT/SPECT-CT improves scar detection. Persistent photopenic defects = scar/dysplasia (acute pyelonephritis can look similar—timing matters). DMSA is not a first-line dynamic obstruction agent.
Direct Radionuclide Cystography
Direct cystogram: instill Tc-99m sulfur colloid or DTPA via bladder catheter; image fill, void, and post-void for VUR. Versus fluoroscopic VCUG: continuous monitoring and typically lower gonadal dose. Indirect cystography uses IV MAG3/DTPA during voiding after renography—less sensitive for low-grade reflux but avoids catheterization.
Patient Prep Checklist (High-Yield)
| Prep item | Why it matters |
|---|---|
| Hydration | Supports diuresis and clearance; dehydrated patients show sluggish curves |
| Void before imaging / catheter | Bladder activity hides pelvic ureters and creates back-pressure |
| Secure IV, no infiltrate | Infiltrate destroys quantification and flow symmetry |
| Hold/give ACE or diuretic only as ordered | Wrong drug timing ruins captopril or Lasix studies |
| Pregnancy screen; pediatric dose by weight | Standard NM safety |
| Note recent IV contrast, obstruction history, transplant side | Interprets unexpected photopenia or surgical anatomy |
Pattern Recognition Snapshot
| Clinical question | Study package | Classic positive teaching pattern |
|---|---|---|
| UPJ/UVJ obstruction vs baggy system | MAG3/DTPA + Lasix | Poor washout despite diuretic + empty bladder |
| Renovascular HTN screen | Baseline + captopril renogram | Unilateral functional deterioration after ACE |
| Cortical scar after UTI | Delayed DMSA ± SPECT | Persistent cortical photopenic defect |
| VUR | Direct radionuclide cystogram | Activity ascends into ureter/pelvis on fill/void |
| GFR estimate | DTPA imaging and/or plasma clearance | Reduced clearance / flat uptake |
Bottom line: name the physiology, pick the agent, add Lasix or captopril when indicated, and defend prep (hydrate, bladder, no infiltrate).
Which pairing best matches renal radiopharmaceutical physiology to the clinical question?
On diuretic renography, which technical factor most commonly creates a false appearance of obstruction?
What is the physiologic rationale for captopril renography in suspected renovascular hypertension?