Residency · Residency · Diagnostic Radiology
CT Urography and Urothelial Carcinoma Imaging
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Alright, intern, you're looking a little green around the gills. Probably another long night on call. Let's make this quick and digestible because you'll be seeing a lot of these in your career. Imagine you're on service, and a computed tomography urography (CTU) pops up for review. It's 3 AM, and the clinical indication is "hematuria." Your eyes are struggling to focus, but you know you need to find something. The trick here isn't just seeing a mass; it's understanding where to look, how to look, and what not to miss, especially when a diagnosis like urothelial carcinoma is on the line.
Think of the urinary tract like a plumbing system: from the kidneys, down two long pipes (the ureters), to the holding tank (the bladder), and then out. Urothelial carcinoma is a cancer that can pop up anywhere along this lining. Your CTU, which typically has an unenhanced phase, a nephrographic phase, and an excretory phase, is your best tool for seeing these hidden surprises. The unenhanced phase is crucial for spotting stones, while the nephrographic phase highlights the kidney tissue itself, and the excretory phase, where the contrast fills the plumbing, is your money shot for finding those sneaky tumors.
So, when that case comes up, remember the patient likely has painless gross hematuria, a classic sign. Your job is to meticulously trace the contrast as it fills the collecting systems and ureters. Look for any filling defects, any focal thickening, anything that doesn't belong in that smooth, contrast-filled tube. Don't just glance; scrutinize every segment. A small, enhancing bump could be the difference between an early diagnosis and a much more advanced cancer down the line. We're looking for subtle changes that indicate something is growing inside those tubes.
Introduction
CT urography (CTU) is the primary imaging modality for evaluating the entire urothelial tract, from the renal calyces to the urinary bladder. It has largely replaced conventional intravenous urography due to superior sensitivity for detecting urothelial carcinoma, the most common malignancy of the urinary tract.
Epidemiology and Clinical Context
Urothelial carcinoma accounts for approximately 90% of bladder cancers and the majority of upper tract malignancies. Risk factors include smoking, occupational aromatic amine exposure, chronic inflammation, and aristolochic acid. The classic presentation is painless gross hematuria. Upper tract urothelial carcinoma (UTUC) represents 5-10% of all urothelial malignancies.
CTU Protocol and Technique
Multi-Phase Acquisition
The CTU protocol includes an unenhanced phase to detect calculi and establish baseline attenuation, a nephrographic phase (90-100 seconds) that is optimal for renal parenchymal lesion detection, and an excretory phase (7-15 minutes) for opacification of collecting systems and ureters. Split-bolus techniques reduce radiation dose by combining phases into fewer acquisitions.
Patient Preparation
Adequate hydration and optional IV saline bolus promote ureteral distention. Prone positioning or an abdominal compression band improves ureteral opacification. IV furosemide (10 mg) can enhance collecting system distention.
Imaging Findings of Urothelial Carcinoma
Upper Tract Lesions
The hallmark finding is a filling defect within an opacified collecting system or ureter on the excretory phase. Focal urothelial thickening or mural enhancement on the nephrographic phase is another important sign. The goblet sign describes dilation of the ureter distal to an intraluminal mass. Circumferential ureteral wall thickening suggests invasive disease. Secondary signs include hydronephrosis, perinephric stranding, and lymphadenopathy.
Bladder Lesions
Bladder urothelial carcinoma typically appears as an intraluminal polypoid mass with enhancement on the nephrographic phase or as focal asymmetric wall thickening. Perivesical fat stranding suggests extravesical extension (stage T3b or higher).
Staging
| T Stage | Description |
|---|---|
| Ta | Non-invasive papillary carcinoma confined to urothelium |
| T1 | Invasion into lamina propria |
| T2 | Invasion into muscularis propria |
| T3 | Extension into perivesical or periureteral fat |
| T4 | Invasion into adjacent organs |
Ta is non-invasive papillary carcinoma confined to the urothelium. T1 indicates invasion into the lamina propria. T2 indicates invasion into the muscularis propria. T3 indicates extension into perivesical or periureteral fat. T4 indicates invasion into adjacent organs. CT cannot reliably differentiate Ta, T1, and T2, and MRI with VI-RADS is increasingly used for bladder staging.
Differential Diagnosis of Filling Defects
| Filling Defect | Key Features | Enhancement |
|---|---|---|
| Blood clot | May change shape on follow-up | None |
| Calculus | Hyperdense on unenhanced images | None |
| Fungal ball | Immunocompromised patients | None |
| Urothelial carcinoma | Soft tissue mass, may cause obstruction | Yes |
| Fibroepithelial polyp | Rare, smooth elongated morphology | Yes |
A blood clot is non-enhancing and may change shape on follow-up. A calculus is hyperdense on unenhanced images with no enhancement. A fungal ball occurs in immunocompromised patients and does not enhance. A fibroepithelial polyp is a rare benign lesion with smooth elongated morphology.
Pitfalls and Limitations
Underdistended ureteral segments can mimic wall thickening. A collapsed bladder may obscure small lesions. Flat lesions (carcinoma in situ) are frequently occult on CTU. Post-procedural changes from stents can obscure or mimic pathology.
Key Clinical Pearls
CTU sensitivity exceeds 95% for UTUC larger than 1 cm but drops below 60% for sub-5 mm lesions. Always evaluate the entire urothelial tract because synchronous multifocal disease occurs in up to 40% of UTUC patients. An enhancing filling defect warrants ureteroscopy and biopsy, while non-enhancing defects favor clot or stone. Report hydronephrosis, fat stranding, and nodal disease as these directly affect surgical planning.
References
- ACR Appropriateness Criteria: Hematuria. American College of Radiology, 2024.
- Cowan NC. CT urography for hematuria. Radiol Clin North Am. 2017;55(1):1-17.
- Park SY, et al. Imaging of upper tract urothelial carcinoma. AJR Am J Roentgenol. 2017;209(4):W231-W242.
- Verma S, et al. Urinary tract imaging: basic principles. Radiology. 2020;295(1):7-22.
For the intern
At 3am, prioritize reviewing the unenhanced phase for obstructing stones and check the excretory phase for filling defects—these need immediate attention if there's hydronephrosis or concern for obstruction. Formal measurement of masses, lymph node assessment, and detailed staging can wait until morning rounds when you can discuss findings with your attending. If you see an enhancing filling defect in a patient with hematuria, document it clearly in your preliminary report and ensure urology is aware, as ureteroscopy may be needed. Most CTU reads are not emergent unless there's concern for acute obstruction or active bleeding requiring intervention.
Common pitfalls
- Missing flat lesions: Carcinoma in situ appears as subtle mucosal irregularity without a visible mass, often invisible on CT. Always correlate with cystoscopy findings.
- Overcalling collapsed segments: An underdistended ureter or bladder mimics wall thickening. Look for enhancement and obstruction—true urothelial carcinoma usually shows both.
- Confusing blood clots with tumor: Clots don't enhance on nephrographic phase imaging. When in doubt, check Hounsfield units on pre-contrast images (clots are typically 40-70 HU).
- Forgetting synchronous disease: Check the entire urothelial tract systematically. Multifocal tumors occur in 40% of cases, so a filling defect in one ureter means carefully scrutinizing the contralateral system and bladder.
Make it stick (one 80-word vignette)
A 72-year-old male presents with painless gross hematuria. His computed tomography urography (CTU) shows a focal enhancing soft tissue mass within the left renal pelvis causing mild hydronephrosis, with another enhancing polypoid lesion in the urinary bladder. Your preliminary report highlights these findings. During multidisciplinary rounds, the urologist confirms bilateral urothelial carcinoma and plans for flexible ureteroscopy and transurethral resection of bladder tumor. Your accurate initial read directly influences timely diagnosis and management, underscoring the importance of systematically evaluating the entire urothelial tract.
If you remember one thing
Systematically evaluate all phases of the CT Urography, focusing on enhancing filling defects or wall thickening throughout the entire collecting system and bladder. Always correlate with clinical history, especially hematuria, to prioritize actionable findings.