CT and MRI of the urinary tract in urothelial cancer

CT urography protocols and scanning phases

Computed tomographic urography (CT urography, CTU) utilizes multiphasic scanning for maximum diagnostic accuracy. Each phase has its specific purpose:

• Unenhanced phase (Unenhanced CT): highly accurate for detecting calculi.
• Nephrographic phase: optimal for detecting mass lesions in the renal parenchyma.
• Excretory phase: best visualizes urothelial cancer within the lumen of the urinary tracts.

To optimize radiation dose and image quality, a split-bolus protocol is frequently employed (split-bolus). The patient drinks water, then receives the first part of the contrast agent. Physical activity (e.g., walking) is subsequently recommended to ensure better mixing of the agent with urine. The combined nephrographic and excretory phase is performed in the prone position (prone position), which improves ureteral opacification.

A triple-bolus protocol (triple-bolus) is not recommended when cancer is suspected, because splitting the dose reduces the volume of contrast available for the excretory phase. A promising approach is dual-energy CT (DECT), which allows for the generation of virtual unenhanced scans directly from the nephrographic phase, eliminating the need for a separate unenhanced scan.

Imaging features of upper tract urothelial carcinoma (UTUC)

Upper tract urothelial carcinoma has characteristic patterns on CT images.

In the kidney, the tumor typically appears as a hypovascular infiltrative mass growing centrally within the pelvicalyceal system. An important diagnostic criterion is that the normal renal contour usually remains preserved. In the excretory phase, a loss of normal enhancement of the affected calyces is noted. The tumor may invade the sinus adipose tissue and parenchyma.

In the ureter, the neoplasm appears as an eccentric or circumferential wall thickening. Due to the intrinsically small luminal diameter, obstruction occurs early, causing hydroureter and hydronephrosis proximal to the lesion. Against the background of opacified urine, the tumor manifests as an irregular filling defect. The distal third of the ureters is most frequently affected.

When UTUC is identified, the mandatory evaluation algorithm includes searching for synchronous multiple lesions (occurring in 2–5% of patients), assessing regional lymph nodes, and searching for distant metastases.

Alternative: Magnetic resonance urography

MR urography (MRU) is inferior to CT in detecting and characterizing urothelial lesions (having lower spatial resolution) and exhibits low sensitivity for calculi. However, it is the method of choice when iodine-based contrast is contraindicated due to allergic reactions, renal impairment, pregnancy, or in pediatric patients.

Optimal tumor detection requires a combination of T2-weighted images and series with intravenous gadolinium administration. In this case, sensitivity for tumors smaller than 2 cm reaches 75%. MRI exceptionally demonstrates the presence and exact level of obstruction.

Important limitation: in patients with severe renal impairment (glomerular filtration rate less than 30 mL/min), gadolinium administration is associated with a theoretical risk of developing nephrogenic systemic fibrosis (nephrogenic systemic fibrosis).

Differential diagnosis and false-positive results

The positive predictive value of CT urography for upper tract carcinoma is limited; therefore, histological confirmation (biopsy via ureteroscopy) is always required prior to surgical treatment. Many conditions can mimic urothelial cancer (appearing as a filling defect or thickening):

• Blood clots and proteinaceous debris.
• Physiological ureteral kinking (especially those accentuated by respiration).
• Benign fibroepithelial polyps and nephrogenic adenoma.
• Inflammatory changes (cystitis cystica, focal fibrosis).
• Post-traumatic changes (following stone passage or stent placement).
• Flow artifacts from contrast.

Bladder cancer: General overview

The majority of urinary tract tumors are epithelial, predominantly urothelial carcinoma. The main risk factor is smoking (50–60% of cases), alongside occupational hazards (diesel exhaust, petrochemicals, dyes) and parasitic infections (schistosomiasis).

Based on growth pattern, bladder cancer is divided into two groups:

1. Non-muscle-invasive (70–80%): do not invade the muscular layer. These include papillary tumors (usually low grade) and flat tumors. Flat neoplasms are classified as carcinoma in situ (CIS), have high grade malignancy, and without treatment invade the muscle in 80% of cases. They are treated with transurethral resection and instillations (BCG, chemotherapy).
2. Muscle-invasive (20–30% at diagnosis): invade deeply into the wall, requiring radical cystectomy.

Up to 40% of tumors may harbor variant histological foci (squamous, glandular, or sarcomatoid components), which dramatically worsens the prognosis and increases the risk of distant metastases.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• A scoring system for evaluating the opacification quality of various urinary tract segments.
• Technical and post-processing strategies for radiation dose optimization during CT examinations.
• WHO classification of urothelial neoplasms by grade of malignancy.
• Specific symptoms on retrograde pyelography: «stippling», «champagne goblet», «phantom calyx».
• The role of diffusion-weighted imaging (DWI) in the detection and monitoring of UTUC.

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

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