In a patient with a complete duplicated collecting system, which is the preferred first‑line imaging study: magnetic resonance urography (MRU) or contrast‑enhanced CT urography (CTU)?

MR Urography vs CT Urography for Complete Duplicated Collecting System

For a patient with a complete duplicated collecting system, MR urography (MRU) is the preferred first-line imaging study when detailed anatomic and functional assessment is required, particularly in younger patients, those requiring repeated imaging, or when radiation exposure is a concern. 1, 2

Primary Imaging Recommendation

MRU is the study of choice when upper urinary tract anatomy is complex or when functional evaluation is needed in duplicated systems. 1 This recommendation is based on MRU's superior ability to:

• Provide comprehensive morphologic and functional evaluation that allows better separation of the renal poles and more accurate calculation of differential renal function compared to renal scintigraphy 1, 2
• Assess barely or non-functioning renal poles and ectopic ureters, which are common complications in duplicated systems 2
• Evaluate congenital anomalies including hydroureteronephrosis, ureteroceles, and ectopic ureteral insertions without radiation exposure 3, 1

When CTU Remains Appropriate

CTU should be selected in specific clinical scenarios:

• When urolithiasis is suspected, as CT has 100% sensitivity for detecting calculi versus only 69% for MRU 3
• When rapid imaging is essential and MRU's longer examination time (typically 45-60 minutes) is prohibitive 3, 4
• In patients with contraindications to MRI (pacemakers, severe claustrophobia, metallic implants) 3
• When MRU expertise or technology is unavailable, as there remains variability in access to high-quality MRU and lack of protocol standardization 3

Technical Advantages of MRU in Duplicated Systems

MRU provides several critical advantages for duplicated collecting systems:

• Static-fluid T2-weighted sequences image the urinary tract without contrast, particularly successful in dilated or obstructed collecting systems 3, 2
• Excretory MRU with gadolinium demonstrates nondilated systems and provides functional information about each moiety 3, 2
• Functional analysis per moiety allows assessment of calyceal transit time and renal transit time for each pole separately 2
• Superior soft-tissue contrast resolution enables detection of small tumors and better characterization of parenchymal abnormalities 3, 5

Clinical Context for Imaging Selection

Pediatric and Young Adult Patients

MRU is strongly preferred to avoid cumulative radiation exposure from repeated imaging, as duplicated systems often require surveillance 6, 1

Pregnant Patients

MRU without gadolinium is the only appropriate advanced imaging option for evaluating duplicated systems during pregnancy 3, 6

Patients with Renal Insufficiency

MRU is preferred over CTU to avoid iodinated contrast nephrotoxicity, though gadolinium carries risk of nephrogenic systemic fibrosis in severe renal dysfunction (GFR <30 mL/min) 3, 6

Patients Requiring Functional Assessment

MRU is definitively superior when differential function of upper versus lower pole moieties must be determined for surgical planning 1, 2

Critical Pitfalls to Avoid

• Do not assume CTU is always first-line simply because it's more widely available; duplicated systems represent a specific indication where MRU's functional capabilities provide critical information 1, 4
• Missing ectopic ureteral insertion is common with CTU alone, as absence of hydronephrosis does not exclude ureteral pathology in duplicated systems 7, 2
• Inadequate hydration before functional MRU studies may mask obstruction and yield false-negative results 7
• Failing to use diuretic administration (furosemide) during excretory phase MRU limits visualization of nondilated systems 3

When Advanced Imaging is Indicated

Based on surveillance protocols for duplicated systems 7:

• New hydronephrosis on ultrasound requires either CTU or MRU to evaluate for obstruction 7
• Recurrent UTIs (≥2 in 6 months) necessitate upper tract imaging with either modality 7
• Hematuria requires comprehensive upper tract evaluation, where CTU has traditionally been preferred but MRU with gadolinium achieves 80% sensitivity for upper tract malignancies 3

Practical Algorithm

1. First, determine if advanced imaging is needed beyond ultrasound based on clinical presentation 7
2. If functional assessment of moieties is required: Choose MRU 1, 2
3. If stones are suspected: Choose CTU 3
4. If patient is <30 years old or requires repeated imaging: Strongly favor MRU 6, 1
5. If rapid diagnosis is critical and MRU unavailable: Use CTU 4
6. If both modalities contraindicated: Combine ultrasound with retrograde pyelograms 3, 7