CT Urogram & CT-Based Imaging in Reconstructive Urology

Quick Reference

Parameter	Value	Clinical Note
Radiation dose — CTU (full protocol)	5–10 mSv	Comparable to ~500 chest X-rays
Radiation dose — split-bolus CTU	3–5 mSv	Preferred protocol to reduce dose
Contrast nephrotoxicity threshold	eGFR <30 mL/min/1.73 m²	High risk; use with caution or avoid
Contrast caution zone	eGFR 30–45 mL/min/1.73 m²	Hydration + risk/benefit discussion
MAG3 delay after iodinated contrast	48 hours minimum	OAT transporter competition
Excretory phase timing	7–15 min post-injection	Collecting system opacification
Unenhanced phase sensitivity for stones	>95%	Superior to all other modalities
CT-RUG radiation dose	3–8 mSv per acquisition	Varies by pelvic anatomy and FOV

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1. Overview: Role of CT Urogram in Reconstructive Urology

The CT urogram (CTU) provides the highest-resolution anatomic mapping of the entire urinary tract — from renal parenchyma through the ureteral orifices — available in clinical practice. For the reconstructive urologist, CTU occupies a specific and non-interchangeable niche: it excels where fluoroscopic studies and nuclear medicine are fundamentally limited, particularly in characterizing the soft-tissue envelope surrounding the urinary tract, identifying intrinsic urothelial pathology, and delineating complex spatial relationships in the retroperitoneum and pelvis.

What CTU Provides That Other Studies Cannot

Upper tract anatomy in three dimensions. Multiplanar reconstruction and maximum intensity projection (MIP) rendering allow precise localization of ureteral strictures, crossing vessels at the ureteropelvic junction (UPJ), and periureteral fibrosis — information that neither retrograde pyelography nor MAG3 can reliably supply.

Urothelial carcinoma detection. Upper tract urothelial carcinoma (UTUC) is a critical diagnosis to exclude before any urethroplasty or urinary diversion procedure in patients with hematuria. Filling defects, urothelial thickening, and papillary lesions are identified on the excretory phase with sensitivity approaching 80–85% for lesions >5 mm. CTU is the recommended primary imaging modality for hematuria evaluation per AUA guidelines.

Stone disease characterization. Unenhanced CT remains the gold standard for urolithiasis, with >95% sensitivity and specificity. When stone disease coexists with ureteral stricture — a clinically important combination in which stone passage may be the precipitating cause of stricture formation — CTU provides simultaneous assessment of both pathologies.

Fistula tract anatomy. Complex urinary fistulae, particularly vesicovaginal fistula (VVF) with ureteral involvement and rectourethral fistula, require soft-tissue characterization of the fistula tract that retrograde studies cannot supply. CTU with thin-slice reconstruction can trace fistula tracks through pelvic soft tissue, identify ureteral orifice proximity to fistula margins, and assess bowel involvement.

Post-radiation and post-surgical pelvic anatomy. In irradiated pelves — where tissue planes are obliterated, bladder capacity is reduced, and ureteral dilation may reflect radiation fibrosis rather than discrete obstruction — CTU provides anatomic context unavailable from functional studies.

Fundamental Limitations of CTU

:::warning Radiation and Contrast Burden CTU delivers 5–10 mSv effective dose. In patients requiring serial upper tract imaging (e.g., post-urethroplasty surveillance, ureteroscopy follow-up, congenital anomalies in young patients), cumulative radiation burden must be considered. For serial functional surveillance, MAG3 renal scintigraphy (effective dose ~1.5–2.5 mSv) is preferred over repeated CTU. :::

No functional data. CTU demonstrates anatomy, not physiology. It cannot quantify split renal function, measure drainage kinetics, or distinguish a dilated but unobstructed system from a truly obstructed one. This distinction — mechanically critical in deciding whether to reconstruct a renal unit or recommend nephrectomy — requires MAG3 renography with diuretic challenge.

No drainage kinetics. The excretory phase of CTU is a static snapshot, typically acquired 7–15 minutes post-injection. It cannot generate drainage half-time (T½), calculate differential function, or model post-furosemide clearance. Tc-99m MAG3 with F+20 protocol is irreplaceable for these measurements.

Iodinated contrast risks. Contrast-induced nephropathy (CIN) is the primary concern in reconstructive urology patients, who commonly have pre-existing renal impairment from chronic obstruction. Contrast allergy, though less common, must be addressed with premedication protocols (see Section 8).

When CTU adds value over MAG3 and RUG/SUG:

Clinical Question	CTU Required?	Reason
Is there a crossing vessel causing UPJ obstruction?	Yes	Soft-tissue vessel identification requires CT
Is there retroperitoneal fibrosis encasing the ureter?	Yes	Periaortic soft tissue not visible on RUG or MAG3
Does hematuria represent UTUC?	Yes	Urothelial filling defects require excretory CT
What is split renal function?	No — use MAG3	CTU provides no functional quantification
Is the obstruction hemodynamically significant?	No — use MAG3	T½ drainage requires dynamic nuclear imaging
Where is the ureteral stricture transition point?	Yes (or retrograde pyelogram)	Cross-sectional localization with periureteral tissue
Does a stone cause the stricture?	Yes	Gold standard stone imaging
Is a pelvic kidney present before urethroplasty?	Yes	Anatomic variant requiring operative planning

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2. CT Urogram Protocol

Standard Three-Phase Protocol

A complete CTU consists of three sequential acquisitions, each timed to a different phase of contrast handling by the kidney.

Phase 1 — Unenhanced (Non-Contrast)

Acquired before contrast administration. The unenhanced phase detects:

• Urolithiasis (sensitivity >95%; identifies stones not visible on subsequent contrast phases)
• Renal parenchymal calcifications (medullary nephrocalcinosis, cortical nephrocalcinosis)
• Ureteral wall calcification
• Baseline density of soft tissue masses for enhancement calculation
• Pelvic phleboliths (distinguished from ureteral stones on non-contrast CT by their smooth, rounded morphology and eccentric calcification)

Reconstruction: 1–2.5 mm axial slices, soft-tissue and bone windows. No oral or IV contrast.

Phase 2 — Corticomedullary / Nephrographic Phase

Timed to ~80–100 seconds post-IV contrast injection (nephrographic phase preferred for reconstructive evaluation). This phase assesses:

• Renal cortical enhancement and parenchymal thickness (surrogate for functional reserve)
• Renal masses (solid vs. cystic; enhancement quantification)
• Ureteral wall enhancement and thickening
• Renal vascular anatomy (crossing vessels at UPJ, aberrant renal arteries relevant to pyeloplasty)
• Retroperitoneal soft tissue (lymphadenopathy, fibrosis, tumor encasement)

Corticomedullary phase (25–40 seconds) provides superior vascular detail for crossing vessel identification. Nephrographic phase (80–100 seconds) provides superior parenchymal characterization and is preferred when the primary indication is mass evaluation or parenchymal thickness assessment.

Phase 3 — Excretory Phase

Acquired 7–15 minutes post-injection, after contrast has been excreted into the collecting system. The excretory phase is the diagnostic heart of the CTU for reconstructive urologists:

• Opacification of renal pelvis, calyces, ureters, and bladder
• Urothelial filling defects (UTUC, blood clot, fungal ball)
• Stricture localization (transition point from opacified to non-opacified ureter)
• Collecting system duplication, aberrant insertions, ureterocele
• Ureteral dilation and tortuosity
• Periureteral inflammation vs. fibrosis vs. tumor encasement
• Bladder wall thickening, diverticula, capacity estimation

A full-length ureter must be visualized in the excretory phase. IV furosemide (10 mg) administered concurrently with or shortly after contrast injection promotes ureteral distension and reduces the problem of non-opacified mid-ureter segments — a common technical failure point that can be mistaken for obstruction.

:::tip Furosemide-Assisted Ureteral Opacification Administering 10 mg IV furosemide at the time of contrast injection (the "Furo-CTU" or "diuretic CTU") significantly improves mid-ureteral opacification. This is particularly important in patients with mild-to-moderate ureteral dilation, where contrast may pool in the pelvis without adequately filling the ureter distally. In patients with suspected obstruction, standard non-diuretic excretory phase is appropriate to preserve the obstructive anatomy. :::

Split-Bolus Protocol

To reduce total radiation exposure, the split-bolus technique combines phases into a single acquisition:

1. First contrast bolus administered (30–50% of total dose)
2. Wait 8–12 minutes to allow excretion into collecting system
3. Second contrast bolus administered (remaining 50–70%)
4. Single CT acquisition acquired ~80 seconds after second bolus

The resulting scan simultaneously shows excretory-phase opacification of the collecting system (from first bolus) and nephrographic-phase parenchymal enhancement (from second bolus). This eliminates one of the three standard phases, reducing effective dose from ~8–10 mSv to ~3–5 mSv — approaching MAG3 levels for reconstructive planning contexts.

Split-bolus is appropriate when:

• Stone disease is not the primary indication (no dedicated unenhanced phase)
• Renal mass characterization is not required
• Primary goal is urothelial and collecting system assessment

Standard three-phase is preferred when:

• Stone detection is the primary or co-primary indication
• Renal mass enhancement quantification is needed
• Precise pre-pyeloplasty vascular mapping is required

Patient Preparation

IV contrast:

• Renal function: obtain serum creatinine and calculated eGFR before contrast administration
• Hydration: normal saline 100–150 mL/hour for 4–6 hours pre- and post-procedure in patients with eGFR 30–45 mL/min/1.73 m²
• Contrast volume: typically 100–150 mL iohexol or iodixanol (300–350 mg I/mL), weight-adjusted
• Non-ionic low-osmolality contrast reduces CIN risk compared to high-osmolality agents

Oral contrast: Not routinely used for CTU — oral contrast can obscure ureteral filling and confuse pelvic anatomy. Water (negative oral contrast, 500–1000 mL) improves bowel delineation without obscuring the urinary tract.

Patient positioning: Supine. Prone positioning can improve opacification of ureterovesical junction and reduce posterior-dependency of contrast.

Radiation Dose in Context

Imaging Study	Approximate Effective Dose	Equivalent Chest X-Rays
Chest X-ray (PA)	0.02 mSv	1
Plain abdominal X-ray (KUB)	0.7 mSv	~35
Fluoroscopic RUG (standard)	0.5–1.5 mSv	~25–75
MAG3 renography	1.5–2.5 mSv	~75–125
IVP (intravenous pyelogram)	1.5–3 mSv	~75–150
CT pelvis (non-contrast)	3–5 mSv	~150–250
Split-bolus CTU	3–5 mSv	~150–250
Standard three-phase CTU	5–10 mSv	~250–500
CT-RUG hybrid	3–8 mSv	~150–400

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3. Indications in Reconstructive Urology

3.1 Exclusion of Upper Tract Urothelial Carcinoma

:::warning Mandatory Before Urethroplasty with Hematuria Any patient presenting with gross or persistent microscopic hematuria in the context of a urethral stricture must undergo CTU to exclude upper tract urothelial carcinoma (UTUC) before urethroplasty proceeds. Urethroplasty in the presence of undiagnosed UTUC introduces diagnostic delay and may compromise oncologic staging. :::

UTUC has an incidence of approximately 1–2 per 100,000 per year, but risk is markedly elevated in patients with history of bladder urothelial carcinoma (~15–25% develop upper tract disease over time), Lynch syndrome, Balkan nephropathy, aristolochic acid exposure, or analgesic nephropathy. CTU excretory phase identifies filling defects, urothelial thickening, and hydroureteronephrosis above a discrete intraluminal lesion.

In patients with contraindications to contrast (eGFR <30), retrograde pyelogram with ureteroscopy and biopsy is the diagnostic alternative.

3.2 Complex Stone Disease Concurrent with Stricture

Stone passage is one of the documented etiologies of ureteral stricture. When a patient presents with obstruction and concurrent nephrolithiasis, CTU simultaneously:

• Identifies all stones by size and location
• Localizes the stricture transition point
• Quantifies hydronephrosis and ureteral dilation
• Assesses residual cortical thickness as a surrogate for functional reserve
• Identifies urinary infection complications (perinephric stranding, abscess)

This combined assessment avoids the scheduling and coordination burden of separate stone CT and retrograde pyelogram. For surgical planning — whether ureteroscopic stone fragmentation, concurrent ureteroscopy + dilation, or formal open/laparoscopic ureteroplasty — a single CTU provides the operative road map.

3.3 Ureteral Stricture Characterization

Transition point localization. CTU with excretory phase and MIP reconstruction identifies the precise level of ureteral narrowing — critical for planning the surgical approach (flank vs. transperitoneal vs. pelvic). Stricture length can be measured when both the proximal dilated segment and the distal collapsed segment are visible.

Crossing vessel identification for UPJ obstruction. The anterior crossing vessel — most commonly an aberrant lower-pole arterial branch — is the etiology of UPJ obstruction in ~40–60% of cases. CTU corticomedullary phase identifies the vessel crossing the UPJ, confirming intrinsic vs. extrinsic etiology and guiding the Anderson-Hynes dismembered pyeloplasty approach. The vessel is typically a lower-pole artery arising from the main renal artery, lying anterior to the ureter at the UPJ level.

:::info Crossing Vessel Detection CT angiography-quality images (thin-slice, arterial-phase acquisition, 3D reconstruction) provide the highest sensitivity for crossing vessel identification. Standard CTU corticomedullary phase is adequate in most cases, but for high-complexity UPJ cases — large redundant renal pelvis, secondary crossing vessels, horseshoe kidney — dedicated CT angiography or MRI angiography may be preferred. :::

Periureteral fibrosis. The periureteral soft-tissue cuff is directly visualized on CTU. Stranding, thickening, and enhancement around the ureter indicate active inflammation. Dense fibrous encasement with medial ureteral deviation suggests retroperitoneal fibrosis (see Section 7). Post-radiation fibrosis appears as diffuse mesodermal thickening in the irradiated field, often with associated pelvic soft-tissue changes.

3.4 Complex Fistula Mapping

Vesicovaginal fistula with ureteral involvement. Large or complex VVF may involve the ureteral orifices or extend to the ureterovesical junction, requiring ureteral reimplantation concurrent with fistula repair. CTU excretory phase demonstrates:

• Fistula tract opacification (contrast extravasation into vaginal vault on delayed images)
• Ureteral orifice proximity to fistula margins
• Bilateral ureteral dilation if trigonal distortion is present
• Residual bladder capacity (relevant for deciding between primary repair vs. augmentation cystoplasty)

Rectourethral fistula. Following radical prostatectomy or after radiation, rectourethral fistula is among the most challenging reconstructive problems. CTU with concurrent CT cystogram (see Section 4.5) can delineate:

• Fistula communication between rectum and urinary tract
• Extent of radiation fibrosis in surrounding tissue
• Associated ureteral or bladder involvement
• Presence of pelvic abscess

Post-radiation fistula in the irradiated pelvis. Radiation-induced fistulae in the bladder, ureter, or urethra are surrounded by devascularized tissue that does not support primary repair. CTU characterizes:

• Bladder wall thickness and capacity (post-radiation cystitis features)
• Ureteral dilation (radiation ureteritis, stricture)
• Fistula tract in relation to surrounding non-irradiated tissue for interposition flap planning
• Bowel segment proximity and viability (for omental or bowel interposition planning)

3.5 Congenital Anomalies Before Reconstruction

Horseshoe kidney. The isthmus of a horseshoe kidney creates anomalous vascular supply, altered ureteral course, and abnormal UPJ orientation. CTU with 3D reconstruction prior to pyeloplasty maps:

• Isthmus position relative to the aorta and mesenteric vessels
• Multiplicity and origin of aberrant renal arteries supplying the isthmus
• Ureteral course (typically anterior, crossing the isthmus)
• Need for isthmus division concurrent with pyeloplasty

Pelvic kidney. A pelvic kidney requires identification of its anomalous vascular supply and the spatial relationship of the ureter to pelvic structures before any ureteral or reconstructive pelvic procedure. CTU provides this mapping in a way that RUG cannot.

Duplex collecting systems. In duplex systems, CTU identifies:

• Upper moiety insertion (typically ectopic, below and medial to lower moiety orifice — Weigert-Meyer rule)
• Upper moiety ureterocele and degree of bladder neck involvement
• Lower moiety UPJ obstruction (common coexisting finding)
• Relevant anatomy before upper pole heminephrectomy or common sheath ureteral reimplantation

3.6 Post-Urethroplasty Upper Tract Surveillance

Hydronephrosis that predated urethroplasty should resolve following successful relief of bladder outlet obstruction, typically over 6–12 weeks. Serial CTU or MAG3 is used to confirm resolution of upper tract dilation. CTU is preferred over MAG3 when anatomic clarification is needed (e.g., persistent hydronephrosis after technically successful urethroplasty, suggesting an independent ureteral or UPJ-level process). MAG3 is preferred when the question is purely functional: is obstruction hemodynamically significant?

:::tip Post-Urethroplasty Imaging Sequence For patients with pre-operative hydronephrosis: MAG3 at 3 months post-urethroplasty to assess functional resolution is preferred. If MAG3 shows persistent obstruction pattern, CTU at 6 months to exclude a contributing anatomic lesion at the ureteral level. Serial CTU alone is radiation-inefficient for purely functional surveillance. :::

3.7 Pre-Radical Cystectomy Evaluation and Diversion Planning

Before radical cystectomy with urinary diversion, CTU provides:

• Upper tract anatomy: bilateral ureteral caliber, position, and any pre-existing stricture
• Renal function estimation (cortical thickness, degree of hydronephrosis)
• Presence of UTUC (critical to identify pre-operatively — changes surgical extent)
• Ureteral length and course for neobladder or conduit anastomosis planning
• Lymph node assessment in the retroperitoneum
• Exclusion of ureteral involvement by primary bladder tumor

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4. CT Pelvis in Pelvic Fracture Urethral Injury (PFUI)

4.1 Role of CT in Acute Trauma Evaluation

CT of the pelvis with IV contrast is the standard of care imaging study in the multiply-injured trauma patient with suspected pelvic fracture and urethral injury. In the acute setting, CT is performed as part of the primary survey and provides simultaneous assessment of pelvic ring stability, vascular injury, bladder laceration, and urethral displacement.

:::info CT Before Retrograde Urethrogram in Acute Trauma In the hemodynamically unstable patient or as part of the standard trauma CT ("pan-scan"), CT pelvis with cystographic phase is obtained before or concurrently with retrograde urethrogram. The CT findings drive immediate resuscitative priorities (pelvic packing, angioembolization) and simultaneously characterize the urethral injury pattern for subsequent reconstruction planning. :::

4.2 What CT Shows in PFUI

Pelvic ring disruption pattern. The pattern of pelvic fracture predicts the likelihood and severity of posterior urethral injury:

• Anterior arch fractures alone (pubic rami fractures without posterior ring disruption): low risk for posterior urethral distraction defect (PUDD)
• Posterior ring involvement (sacral fractures, sacroiliac joint disruption): substantially elevated risk
• Complete ring disruption (Tile C / Young-Burgess APC-III or VS): highest risk; nearly all patients with complete ring disruption and urethral injury have PUDD

The "pie-in-the-sky" bladder. Superior displacement of the bladder above the pelvic inlet on CT sagittal or coronal reconstruction is pathognomonic for complete posterior urethral disruption with a large pelvic hematoma. The bladder, no longer tethered by the intact urethra and periurethral attachments, floats superiorly in the expanding hematoma. This sign reliably identifies complete urethral transection and indicates suprapubic catheter placement is required — attempts at transurethral catheterization in this setting risk false passage formation and should not be undertaken without urethrographic guidance.

Pelvic hematoma characterization. CT demonstrates:

• Volume and distribution of pelvic hematoma
• Active extravasation (contrast blush on arterial phase) — indicates need for angioembolization
• Posterior hematoma extent (relevant to subsequent perineal approach for delayed urethroplasty — large posterior hematoma predicts dense scar requiring inferior pubectomy)
• Superior spread of hematoma above the peritoneal reflection (intraperitoneal component)

Bone fragment position. Retropubic bone fragments from pubic arch fractures can:

• Penetrate or lacerate the bladder neck or urethra at time of injury
• Become incorporated into the fibrous obliterative scar, requiring excision during delayed anastomotic urethroplasty
• Obscure the urethrographic anatomy on RUG/SUG

Associated bladder injury. Bladder lacerations (intraperitoneal or extraperitoneal) occur in ~10–29% of PFUI cases. CT cystogram — obtained by retrograde bladder filling with diluted contrast (300–400 mL) or by passive filling from IV contrast on delayed CT — demonstrates:

• Intraperitoneal rupture: contrast surrounding bowel loops in the peritoneal cavity
• Extraperitoneal rupture: contrast tracking in the perivesical space, pelvic fat, or along fascial planes
• Bladder neck integrity

Rectal injury. High-energy PFUI, particularly open-book pattern fractures, may lacerate the rectum. CT characterizes rectal wall integrity, perirectal fat stranding, and free pelvic air — critical findings before any surgical exploration, as unrecognized rectal injury significantly complicates urethroplasty and diversion decisions.

4.3 Pelvic Fracture Classification and Urethral Injury Correlation

Tile Classification

Tile Class	Description	Pelvic Ring Stability	Urethral Injury Risk
A	Stable — minor fractures, posterior arch intact	Stable	Low (<5%)
A1	Avulsion fractures	Stable	Very low
A2	Iliac wing / minimally displaced rami	Stable	Low
A3	Sacral fractures, transverse	Stable	Low
B	Rotationally unstable, vertically stable	Partial instability	Moderate (15–30%)
B1	Open-book (external rotation)	Rotationally unstable	Moderate
B2	Lateral compression (internal rotation)	Partially unstable	Moderate
B3	Bilateral B-type	Bilateral instability	High
C	Rotationally AND vertically unstable	Completely unstable	High (40–60%+)
C1	Unilateral complete disruption	Completely unstable	High
C2	Bilateral, one side C, one side B	Completely unstable	Very high
C3	Both sides complete, acetabular involvement	Completely unstable	Very high

Tile C injuries correlate with the highest rates of posterior urethral distraction defect. The vertical shear component displaces the prostate and bladder base superiorly, stretching and ultimately avulsing the membranous urethra at the apex of the prostate.

Young-Burgess Classification

Complementary to Tile, the Young-Burgess system classifies pelvic fractures by injury mechanism:

Type	Mechanism	Urethral Injury Risk
APC-I	Anterior-posterior compression, <2.5 cm symphysis diastasis	Low
APC-II	APC with anterior SI ligament disruption	Moderate
APC-III	Complete hemipelvis dislocation, all SI ligaments	High
LC-I	Lateral compression, sacral impaction	Low-moderate
LC-II	LC with iliac wing fracture (crescent fracture)	Moderate
LC-III	LC-II with contralateral APC (windswept pelvis)	High
VS	Vertical shear — complete hemipelvis displacement	Very high
CM	Combined mechanism	Variable, often high

APC-III and VS patterns carry the highest urethral injury burden and are most likely to produce complete posterior urethral distraction defects requiring subsequent formal perineal anastomotic urethroplasty.

4.4 CT Pelvimetry for Reconstruction Planning

For delayed anastomotic urethroplasty of PUDD, preoperative CT pelvimetry guides surgical strategy:

Posterior pubic arch displacement. The degree of superior and posterior displacement of the fractured pubic arch determines whether a standard perineal approach will provide adequate exposure to the obliterated membranous urethra. Significant posterior displacement (pubic arch positioned >1–2 cm posterior to the expected anatomic position) may require inferior pubectomy.

Inferior pubectomy planning. CT coronal and sagittal reconstruction identifies:

• Residual thickness of the inferior pubic arch available for osteotomy
• Proximity of the osteotomy plane to the superior neurovascular bundle
• Degree of fibrous obliteration in the retropubic space
• Whether combined perineal-abdominal (transpubic) approach is needed for very long (>3–4 cm) defects

Puboprostatic ligament and prostate position. The prostate position on CT — identified by its characteristic CT density and relationship to the bladder neck — allows prediction of defect length before formal RUG/SUG is performed. A prostate significantly displaced superiorly from the perineal bulbar urethra (visible on CT) predicts a long defect.

Pelvic floor muscle integrity. CT with bone windows assesses ischiopubic rami integrity and levator ani muscle continuity — relevant to the quality of perineal tissue available for dissection and mobilization during urethroplasty.

4.5 CT Cystogram

CT cystogram is performed by instilling 300–400 mL of 3–5% iodinated contrast solution directly into the bladder via Foley or suprapubic catheter, then acquiring a CT pelvis. Alternatively, passive filling from IV contrast on 7–10 minute delayed images can be used when direct instillation is not feasible.

Indications in reconstructive urology:

• Suspected bladder laceration concurrent with PFUI
• Post-operative anastomotic leak assessment after bladder reconstruction
• Complex fistula mapping (concurrent with CTU)
• Bladder diverticulum characterization before surgical correction
• Assessment of bladder neck integrity after trauma or prior surgery

CT cystogram has replaced conventional fluoroscopic cystogram in most centers for complex cases due to superior sensitivity for small extraperitoneal leaks and concurrent assessment of surrounding structures.

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5. CT-Retrograde Urethrogram (CT-RUG)

Technique and Rationale

The CT-retrograde urethrogram (CT-RUG) is an emerging hybrid technique combining conventional retrograde urethral contrast injection with simultaneous or immediately sequential CT acquisition. Rather than the two-dimensional fluoroscopic image of the conventional RUG, CT-RUG generates cross-sectional and full three-dimensional reconstruction of the urethra and its periurethral tissue relationships.

Acquisition protocol:

1. Patient positioned supine (occasionally decubitus for specific anatomy)
2. Penile clamp or occlusive catheter balloon seated at the fossa navicularis
3. Diluted iodinated contrast (15–30%) injected under gentle pressure to opacify the anterior urethra and, ideally, across the stricture into the posterior urethra
4. CT pelvis acquired during active contrast injection or immediately after urethral opacification confirmed on fluoroscopic scout
5. 3D volume-rendered and multiplanar reconstructions performed at workstation

:::info Coordination Between Urology and Radiology CT-RUG requires active urology-radiology coordination at the time of scanning. The urologist manages contrast injection and urethral filling in the CT suite, while the radiologist controls acquisition timing. This cannot be delegated to a technologist alone. Pre-procedure briefing with the radiology team regarding injection technique, contrast concentration, and desired reconstructions is mandatory. :::

What CT-RUG Provides Over Conventional Fluoroscopic RUG

Periurethral tissue characterization. The corpus spongiosum, corpus cavernosum, bulbospongiosus muscle, and perineal body are visualized in cross-section, allowing direct assessment of spongiofibrosis extent — information unavailable from fluoroscopic imaging. Dense spongiofibrosis on CT-RUG correlates with the need for wider excision margins and spongioplasty during excision and primary anastomosis (EPA) or substitution urethroplasty.

False passage identification. Prior catheterization attempts, prior failed urethroplasty, or traumatic instrumentation can create false passages — channels tracking through the periurethral tissue that may not be visible on fluoroscopic RUG or may be misidentified as the true urethra. CT-RUG's cross-sectional view identifies the true lumen versus periurethral tracking with high reliability.

Complex fistula anatomy. In rectourethral fistula, CT-RUG demonstrates the fistula tract in three dimensions, identifies the rectal communication point, and characterizes the interposing tissue — critical for planning gracilis flap or omental transposition. Similarly, urethrocutaneous fistulae with complex branching tracts are fully mapped before reoperation.

Prior failed urethroplasty. In patients with recurrent stricture after previous urethroplasty — particularly where peri-anastomotic scarring, incorporated grafts, or distorted anatomy are anticipated — CT-RUG provides the preoperative "road map" that standard RUG cannot.

Current Limitations and Appropriate Use

• Higher radiation dose than fluoroscopic RUG (3–8 mSv vs. 0.5–1.5 mSv)
• Requires formal radiology suite access with CT capability at the time of urethral injection
• Not yet standardized for acquisition protocol, contrast concentration, or reporting
• Limited published evidence base (mostly single-center series); not yet incorporated into major society guidelines
• 3D reconstructions require post-processing time and workstation software

:::warning Selective Use of CT-RUG CT-RUG is not a replacement for standard fluoroscopic RUG + simultaneous urethrogram (SUG). It is reserved for cases where standard RUG and SUG are inconclusive or insufficient for operative planning. Routine use would substantially increase radiation burden and resource utilization without proportionate diagnostic gain in straightforward anterior urethral stricture cases. :::

Current appropriate indications for CT-RUG:

• Complex rectourethral fistula mapping when standard studies are inconclusive
• Re-do urethroplasty with suspected false passage or complex periurethral anatomy
• Complex posterior urethral distraction defect where prostate position and defect length are unclear on RUG/SUG
• Urethral duplication or complex congenital urethral anomaly
• Urethral injury with suspected associated perirectal or perineal abscess

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6. CT Urogram vs. Other Modalities — Comparison Table

Feature	CTU	MAG3 Renography	MRI Urography	IVP	Retrograde Pyelogram
Anatomic detail	Excellent (3D)	Poor (functional map only)	Excellent (3D, superior soft tissue)	Moderate (2D)	Good (collecting system only)
Functional data	None	Excellent (SRF, T½, ERPF)	Moderate (DCE-MRI)	Limited (qualitative)	None
Radiation	5–10 mSv	1.5–2.5 mSv	None	1.5–3 mSv	0.5–1.5 mSv
IV contrast required	Yes (iodinated)	No (Tc-99m)	Yes (gadolinium) or No	Yes (iodinated)	Yes (iodinated, retrograde)
Urothelial Ca detection	Good (filling defects, wall thickening)	Cannot detect	Good (wall thickening, MRI mass)	Moderate (filling defects)	Good (filling defects, requires scope for biopsy)
Stone detection	Excellent (gold standard)	Poor	Poor	Moderate	Poor (radiolucent stones missed)
Collecting system detail	Excellent	Functional outline only	Excellent	Good	Excellent (retrograde distension)
Crossing vessel (UPJ)	Good (arterial phase)	Cannot detect	Excellent (MRA)	Cannot detect	Cannot detect
Periureteral soft tissue	Good	Cannot assess	Excellent	Cannot assess	Cannot assess
Retroperitoneal fibrosis	Good (enhancing soft tissue mantle)	Cannot characterize	Excellent (T2, DWI)	Suggests (medial deviation)	Suggests (medial deviation)
Renal function impairment impact	Non-diagnostic excretory phase if eGFR <30	Reduced curve quality; MAG3 secretion preserved	Non-diagnostic if no gadolinium; reduces MRU quality	Non-diagnostic if eGFR <20	Unaffected (retrograde)
Cost	High	Moderate–High	High	Low–Moderate	Moderate (OR/procedure suite)
Radiation sensitivity (pediatric/young)	Significant concern	Lower concern	No radiation	Concern	Low concern
Bladder/lower tract assessment	Good (delayed phase)	Cannot assess	Good	Limited	No (antegrade only)

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7. Retroperitoneal Fibrosis on CT Urogram

CT Appearances of Retroperitoneal Fibrosis

Retroperitoneal fibrosis (RPF) is a fibro-inflammatory disorder producing a periaortic and pericaval soft-tissue mantle that encases the retroperitoneal structures, most critically the ureters. CTU is the primary imaging modality for diagnosis, staging, and treatment response monitoring.

Characteristic CT features:

• Homogeneous soft-tissue density mass surrounding the infrarenal aorta and IVC
• Extends from the renal hila to the sacral promontory (variable, but typically L4–S1 dominant)
• Medial deviation of the mid-ureters (at or below the level of L4–L5) — pathognomonic for RPF when bilateral; the ureters are pulled medially into the fibrotic mass rather than being displaced laterally as by lymphadenopathy or retroperitoneal tumor
• Variable ureteral encasement with intrinsic or extrinsic obstruction — may produce bilateral hydronephrosis
• Enhancement on post-contrast CTU varies with disease activity: active inflammatory phase enhances avidly; chronic fibrotic phase enhances minimally or not at all

Active vs. Chronic RPF on CT:

• Active phase: heterogeneous, avid enhancement; elevated ESR/CRP; IgG4-related disease subtype may show bilateral lacrimal/submandibular gland enlargement on concurrent imaging
• Chronic/fibrotic phase: homogeneous low-density mantle; minimal enhancement; ureters densely incorporated

:::info PET/CT for Active Inflammatory Phase FDG-PET/CT provides metabolic activity assessment of the RPF mantle that standard CTU cannot. Avid FDG uptake indicates active inflammation and predicts response to corticosteroid therapy (prednisone 40–60 mg/day tapering over 4–6 months). Prior to ureterolysis surgery, PET/CT-confirmed active disease is an indication for steroid trial — a successful biochemical and imaging response avoids the morbidity of retroperitoneal exploration. PET/CT is also valuable for surveillance after ureterolysis to identify disease recurrence. :::

Relevance Before Ureterolysis

The decision between steroid therapy and surgical ureterolysis in RPF is guided by:

1. Degree of ureteral obstruction (MAG3 T½, differential function)
2. Disease activity (PET/CT, IgG4-RD serology, ESR/CRP)
3. Failed response to steroids
4. Secondary RPF from aortic aneurysm, malignancy, or drug exposure (not responsive to steroids)

CTU before ureterolysis documents:

• Level and length of ureteral encasement (planned dissection extent)
• Whether unilateral or bilateral ureterolysis is required
• Ureteral integrity within the mass (perfusion on nephrographic phase)
• Whether stenting has adequately decompressed the collecting system before definitive surgery
• Omental wrap planning (requires assessment of omental vascularity and pedicle reach)

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8. Contrast Considerations

Contrast-Induced Nephropathy Risk Stratification

eGFR (mL/min/1.73 m²)	Risk Category	Recommendation
>60	Low	Proceed with standard IV contrast; routine hydration not required
45–60	Low-moderate	Hydration before and after contrast; no dose restriction
30–45	Moderate	Mandatory IV hydration (NaCl 1 mL/kg/hour, 6–12 hours pre/post); consider alternative (non-contrast CT + retrograde)
<30 (not on dialysis)	High	Avoid iodinated contrast if possible; if essential, nephrology consultation, IV hydration, minimum contrast volume
Dialysis-dependent	Special consideration	Contrast may be used; schedule dialysis within 24 hours post-exposure if possible; CIN risk is less relevant but contrast may affect residual function

N-acetylcysteine (NAC). Earlier protocols recommended NAC 600–1200 mg orally twice daily the day before and day of contrast. Current evidence — including the PRESERVE trial (N Engl J Med, 2018) — does not support routine NAC use for CIN prevention. IV hydration with isotonic saline remains the only intervention with consistent evidence for CIN prevention.

Metformin. Hold metformin for 48 hours after iodinated contrast administration in patients with eGFR <60 mL/min/1.73 m² or acute kidney injury risk, due to rare risk of lactic acidosis from contrast-associated acute kidney injury.

Contrast Allergy Premedication

Standard premedication protocol for contrast allergy (prior mild-moderate allergic reaction to iodinated contrast):

• Prednisone 50 mg orally at 13 hours, 7 hours, and 1 hour before contrast
• Diphenhydramine 50 mg IV/IM/PO 1 hour before contrast
• Non-ionic low-osmolality or iso-osmolality contrast agent

For patients with prior severe anaphylactoid reaction to iodinated contrast: consider whether contrast CT is truly necessary vs. alternative imaging (MRI urography, retrograde pyelogram, non-contrast CT + functional nuclear imaging).

Critical: MAG3 Delay After CTU

:::warning Delay MAG3 by 48 Hours After CTU Iodinated contrast agents and Tc-99m MAG3 share the same proximal tubular secretory pathway via organic anion transporters (OAT1 and OAT3). Residual iodinated contrast in the renal tubules competitively inhibits MAG3 uptake, producing falsely low extraction efficiency, flattened renogram curves, and inaccurate differential function measurements. This results in an uninterpretable or misleading MAG3 study if performed within 48 hours of CTU.

Clinical rule: Always sequence CTU before MAG3, with a minimum 48-hour interval between studies. If both are needed urgently (rare), perform MAG3 first, then CTU. :::

The 48-hour interval applies to all iodinated contrast agents, including low-volume retrograde contrast (RUG, retrograde pyelogram). In practice, a 48-hour gap is straightforward to achieve in elective outpatient evaluation; in urgent inpatient settings (acute obstruction with sepsis), MAG3 is typically not required emergently and can be deferred.

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9. Post-Operative CTU

Role of CTU in the Post-Operative Period

CTU after ureteral reconstruction, renal pelvis surgery, or urinary diversion is used to assess anastomotic integrity, detect complications, and confirm expected anatomic outcomes.

Anastomotic Leak Detection

Extravasation of contrast into the perinephric or retroperitoneal space on the excretory phase identifies urinary leak. Findings:

• High-density fluid tracking along the course of the ureter or around the renal pelvis
• Perinephric urinoma: well-circumscribed fluid collection adjacent to the kidney, homogeneous low density, increasing density after contrast injection (contrast accumulates within the collection over time)
• Comparison of pre- and post-contrast density of perinephric collections confirms urinary vs. hematoma vs. lymphocele origin

Urinoma

A post-operative urinoma may develop from anastomotic leak, inadvertent ureteral injury, or drain malfunction. CTU identifies:

• Size and location of the collection
• Communication with the collecting system (contrast opacification of the collection on excretory phase)
• Proximity to adjacent structures (bowel, major vessels)
• Drainage catheter position if already placed

Ureteral Stricture at Anastomosis

Recurrent hydronephrosis after ureteral reconstruction warrants CTU to determine whether stricture has developed at the anastomosis. CTU demonstrates:

• Transition point at the level of the anastomosis (abrupt caliber change)
• Length of recurrent stricture
• Peianastomotic fibrosis or scar tissue
• Upstream ureteral dilation and renal pelvis size

Timing of Post-Operative CTU

:::info Allow Adequate Time Before Post-Operative CTU Post-operative edema, stent-related urothelial reaction, and physiologic dilation from intraoperative manipulation all contribute to false-positive findings on early post-operative CTU. Routine post-operative CTU for anastomotic assessment should be deferred to:

• 3–6 months after pyeloplasty (after stent removal and edema resolution)
• 4–6 weeks for suspected anastomotic leak (earlier if clinical signs of leak are present)
• 3–6 months after ureteral reimplantation or ureteroplasty

Urgent CTU (within days) is appropriate if post-operative fever, flank pain, or drain output suggests active leak or urinoma formation. :::

Hydronephrosis Resolution After Urethroplasty

The timeline for hydronephrosis resolution after successful urethroplasty varies:

• Mild hydronephrosis (<15 mm renal pelvis diameter): typically resolves within 6–12 weeks
• Moderate hydronephrosis: 3–6 months
• Severe, long-standing hydronephrosis: may persist or partially persist even after successful reconstruction, with some degree of "persistent" collecting system dilation from prior compliance changes

CTU at 3 months post-urethroplasty for pre-existing hydronephrosis is a reasonable early assessment point; MAG3 at the same time provides functional confirmation. Persistent hydronephrosis without worsening on CTU, with normalizing T½ on MAG3, represents residual dilation rather than ongoing obstruction and does not require reoperation.

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10. Clinical Pearls

:::tip Pearl 1 — Always Exclude UTUC Before Urethroplasty with Hematuria Any patient with gross hematuria and concurrent urethral or ureteral stricture must have CTU before urethroplasty proceeds. Upper tract urothelial carcinoma presenting as hematuria can co-exist with stricture disease. Missing UTUC before reconstruction results in oncologic delay and may preclude curative intervention. If CTU is contraindicated (contrast allergy, eGFR <30), retrograde pyelogram + ureteroscopy is the alternative. :::

:::tip Pearl 2 — Sequence CTU Before MAG3 with 48-Hour Gap When both CTU and MAG3 are needed in the same evaluation (the most common reconstructive scenario for upper tract obstruction), always obtain CTU first. Wait 48 hours minimum before MAG3 to avoid OAT transporter competition and false renogram curves. If only one study can be scheduled first in an urgent evaluation, obtain MAG3 first — functional data is more immediately actionable for intervention decisions. :::

:::tip Pearl 3 — CT-RUG for Complex Fistulae and Re-do Surgery Reserve CT-RUG for cases where standard fluoroscopic RUG + SUG are insufficient for operative planning. High-yield indications include: rectourethral fistula with suspected perirectal extension, re-do urethroplasty with anticipated false passage, complex PFUI where prostate position and defect length are unclear. The 3D cross-sectional periurethral tissue map changes the operative strategy in a meaningful proportion of complex cases. :::

:::tip Pearl 4 — The "Pie-in-the-Sky" Bladder Means Suprapubic Catheter First Superior displacement of the bladder on CT pelvis in the acute trauma setting indicates complete posterior urethral disruption with large pelvic hematoma. Do not attempt blind transurethral catheterization. Insert a suprapubic catheter under ultrasound guidance. Retrograde urethrogram can be deferred until the patient is stabilized. Document the bladder displacement and hematoma volume, as these predict operative difficulty for delayed urethroplasty. :::

:::tip Pearl 5 — RPF on CT: Consider Steroid Trial Before Ureterolysis When CTU demonstrates periaortic soft tissue with medial ureteral deviation and the clinical picture is consistent with IgG4-related RPF (elevated serum IgG4, elevated ESR/CRP, bilateral hydronephrosis in a middle-aged man), order FDG-PET/CT to confirm active metabolic disease. Active RPF responds to prednisone 40–60 mg/day with imaging reassessment at 6–8 weeks. A biochemical and imaging response avoids a morbid retroperitoneal ureterolysis. Reserve surgical ureterolysis for failed steroid therapy, secondary RPF (malignancy, drug-induced), or irreversible ureteral obstruction. :::

:::tip Pearl 6 — Split-Bolus CTU Reduces Dose for Urothelial Assessment When the primary indication is urothelial carcinoma exclusion or collecting system assessment without concurrent stone evaluation, use the split-bolus protocol (3–5 mSv vs. 8–10 mSv). In younger patients and in patients likely to require serial imaging, the dose reduction is clinically meaningful. Document protocol choice and radiation dose in the report for longitudinal dose tracking. :::

:::tip Pearl 7 — Pelvic Fracture Pattern Predicts Urethral Injury Severity Tile C and Young-Burgess APC-III/VS fracture patterns on CT predict complete posterior urethral distraction defect and should prompt early urologic consultation, suprapubic catheter placement, and deferred anastomotic urethroplasty planning (typically at 3 months post-injury after hematoma resolution). Do not attempt primary realignment in complete ring disruption with large hematoma — it does not reduce stricture rate and increases immediate surgical risk. :::

:::tip Pearl 8 — Post-Operative CTU: Wait 3–6 Months for Anatomic Assessment Early post-operative CTU (within 6 weeks of ureteral reconstruction or pyeloplasty) reflects post-operative edema and stent dilation, not true anatomic outcomes. Persistent dilation at 3 months after stent removal is the earliest reliable assessment point for anastomotic patency. Urgent CTU within days is appropriate only for suspected leak, urinoma, or hematoma. :::

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