# CT Urography and Urothelial Carcinoma Imaging

## 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.

![CT urography excretory phase demonstrating normal contrast opacification of bilateral collecting systems and ureters](images/ctu-excretory-phase.jpg)

## 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).

![Axial CT showing a filling defect in the left renal pelvis consistent with urothelial carcinoma](images/utuc-filling-defect.jpg)

## 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.

![Coronal CT urography showing ureteral wall thickening with proximal hydroureteronephrosis](images/ureteral-thickening-ctu.jpg)

## 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

1. ACR Appropriateness Criteria: Hematuria. American College of Radiology, 2024.
2. Cowan NC. CT urography for hematuria. *Radiol Clin North Am*. 2017;55(1):1-17.
3. Park SY, et al. Imaging of upper tract urothelial carcinoma. *AJR Am J Roentgenol*. 2017;209(4):W231-W242.
4. Verma S, et al. Urinary tract imaging: basic principles. *Radiology*. 2020;295(1):7-22.