Scanning techniques: CT urography and MR urography
Endoscopic examination remains the gold standard of diagnosis; however, diagnostic imaging is essential for a comprehensive evaluation. Conventional excretory urography misses up to 40% of tumors, which is why CT urography (with iodine-based contrast) and MR urography (with gadolinium-based agents) have become the methods of choice.
Selecting the correct scanning phases is of key importance:
- Corticomedullary phase (60 seconds after injection): small urothelial neoplasms abnormally enhance brightly against the background of unenhanced urine.
- Excretory phase (5–15 minutes): contrast fills the bladder lumen. To prevent small defects from being "lost" against the dense contrast, images must be reviewed using wide-windowing settings.
To reduce radiation dose in CT, a split-bolus technique is frequently used (split-bolus): the agent is administered twice with an interval of 5–10 minutes, followed by a single scan. This allows for the simultaneous acquisition of the nephrographic and excretory phases. Before scanning, patients are advised to change body position so that the contrast mixes evenly with the urine.
Radiological semiotics of urothelial cancer
Most patients are referred for examination presenting with hematuria as the leading symptom. On tomograms, urothelial cancer most commonly appears as focal asymmetrical wall thickening or a nodular, frond-like mass that protrudes into the bladder lumen and actively enhances with contrast.
Important diagnostic signs:
- Calcifications: detected within the structure of the cancerous tumor in approximately 5% of cases (more frequently along the anterior and lateral walls).
- Attenuation: in the excretory phase of CT, tumor attenuation blends with the attenuation of the wall, making early phases critical for differentiation from blood clots, mucus, or fungal balls (which do not enhance).
- Multiplicity: urothelial cancer tends toward multicentric growth; a thorough check of the upper urinary tract for additional lesions is mandatory.
Note: Diffuse, circular, and symmetrical thickening of the urinary bladder wall is extremely rarely cancer; as a rule, it is a sign of cystitis, bladder outlet obstruction, or neurogenic bladder.
Staging assessment (TNM) and advantages of MRI
Local staging of bladder cancer by CT has low accuracy (40–60%). CT allows for the suspicion of invasion into the perivesical adipose tissue (stage T3b, irregular outer contour), but is completely incapable of determining the depth of penetration into the muscle layer (distinguishing superficial T2a from deep T2b invasion).
MRI has significantly higher accuracy (73–96%) and allows for a more reliable differentiation of non-muscle-invasive (T1) tumors from invasive (>T1) ones:
- On T1-weighted images, the tumor is isointense to the normal wall.
- On T2-weighted images, the intact wall produces a low signal. If the tumor is invasive, this continuous hypointense rim is interrupted.
- The use of diffusion-weighted imaging (DWI) dramatically increases specificity. The apparent diffusion coefficient (ADC / ADC) decreases in proportion to the tumor grade.
DWI is also a lifesaver in postoperative evaluation (following transurethral resection). Standard MRI struggles to distinguish residual cancer from reactive inflammation and fibrosis, whereas DWI reliably differentiates these conditions.
Blind spots, limitations, and PET imaging
Even ideal CT or MR urography can miss pathology. False-negative results are probable in the presence of:
- Flat neoplasms, such as carcinoma in situ.
- Lesion sizes less than 1 cm.
- Localization in the bladder base (due to the volume averaging effect with prostate and perineal tissues). The use of an endorectal coil in MRI can partially resolve this problem.
This is precisely why a normal tomographic appearance does not rule out cancer. Patients with hematuria and high risk still require flexible cystoscopy.
Regarding positron emission tomography (18F-FDG-PET/CT), its role in primary diagnosis is minimal, as the radiopharmaceutical is physiologically excreted in the urine, masking the bladder wall. The method is used exclusively to search for distant metastases in equivocal CT results or to differentiate reactive from metastatic lymph nodes.
Briefly: answers to topical questions
Which regional lymph node groups are affected during bladder cancer metastasis?
During bladder cancer metastasis, the true pelvis lymph nodes and common iliac lymph nodes are affected.
- True pelvis lymph nodes (true pelvis) — can be involved singly (stage N1) or multiply (stage N2).
- Common iliac lymph nodes — their involvement (or more central nodes) corresponds to stage N3.
Involvement of nodes on both sides of the bladder (bilateral) is also distinguished, in which metastases may remain mobile (freely moving) or become fixed (immobile).
What stages of bladder cancer according to the TNM system are distinguished based on the depth of wall invasion?
Depending on the depth of invasion of the primary tumor into the bladder wall, the following stages are distinguished:
- Stage Ta — papillary neoplasms limited strictly to the urothelium.
- Stage Tis — flat tumors or carcinoma in situ (carcinoma in situ).
- Stage T1 — invasion into the lamina propria (lamina propria) or submucosal layer.
- Stage T2 — extension into the muscularis propria (T2a — superficial invasion into the inner half, T2b — deep invasion into the outer half).
- Stage T3 — extension into the perivesical tissue (T3a — microscopic, T3b — macroscopic).
- Stage T4 — invasion into adjacent organs, the pelvic wall, or the abdominal wall.
What complications and side effects are possible with the administration of gadolinium-based contrast agents?
The administration of gadolinium-based contrast agents carries the following described risks:
- Nephrogenic systemic fibrosis (nephrogenic systemic fibrosis) — the use of gadolinium requires strict caution in patients with severe chronic kidney disease (eGFR / eGFR < 30 mL/min/1.73 m²) or acute kidney injury due to the risk of developing this condition.
- Gadolinium deposition (gadolinium deposition) — possible in brain structures following contrast-enhanced MRI.
MRI is more accurate than CT for staging: "MRI — Muscles Resolved Intactly". It is precisely on T2-weighted images that the integrity of the muscle layer can be assessed and stages T1 and T2 differentiated.
Class and exam questions
Why doesn't a normal CT scan rule out cystoscopy when bladder cancer is suspected?
A normal CT or MR scan does not exclude the presence of small (less than 1 cm) or flat tumors (such as carcinoma in situ) that are not visible on imaging. At-risk patients require endoscopic visualization.
Why is contrast administration necessary for bladder CT?
In the corticomedullary phase, small tumors enhance more intensely than urine, whereas in the excretory phase they are visualized as filling defects. Unenhanced (non-contrast) CT does not reliably distinguish cancer from blood clots or benign structures.
What is the main advantage of DWI in bladder MRI?
DWI has a sensitivity of up to 100% in detecting cancer and makes it possible to distinguish residual tumor tissue from post-inflammatory fibrosis following chemotherapy or surgical resection, which standard MRI handles poorly.
Is it practical to perform PET/CT to detect the primary bladder tumor itself?
No, standard FDG-PET/CT is not suitable for primary diagnosis because the radiopharmaceutical is actively excreted in the urine into the bladder lumen, completely obscuring the signal from tumors in the wall. The method is only needed for assessing lymph nodes and metastases.
What else to study on this topic
- Virtual CT cystoscopy: technique of bladder distension with air or CO2
- Radiation dose reduction in CT urography using iterative reconstruction
- Alternative radiopharmaceuticals for PET (11C-choline, 11C-methionine)
- Differences in imaging findings between papilloma, leiomyoma, and carcinoma
- Assessment of the upper urinary tract in multicentric urothelial carcinoma
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