Non-contrast CT and stone detection
Native (non-contrast) CT is the method of choice for initial stone detection, surpassing intravenous urography in speed and accuracy. It not only detects calculi but also rules out extraurinary pathology. It is important to remember that certain stones (e.g., mucoid or associated with protease inhibitor therapy) have soft-tissue attenuation, making them difficult to detect.
Indirect CT signs of obstruction by a calculus include:
- Hydronephrosis and hydroureter;
- Ipsilateral renal enlargement;
- Perinephric fat stranding and fluid accumulation;
- The ureter rim sign.
The combination of hydronephrosis, ureteral dilation, and inflammatory fat stranding yields a 90% confidence level for an obstructing calculus. The likelihood of spontaneous stone passage directly depends on size: for 2–4 mm calculi, it is 76%, whereas for sizes greater than 9 mm, it drops below 25%.
CT urography protocols
Standard CT urography includes three main scanning phases:
- Non-contrast phase (baseline attenuation assessment).
- Nephrographic phase (80–100 seconds delay) — provides the highest sensitivity for detecting renal parenchymal masses.
- Excretory (pyelographic) phase (5–15 minutes delay) — allows evaluation of the opacified upper urinary tract and urinary bladder.
To reduce the radiation dose, the split-bolus technique (split-bolus) is actively used. The essence of the method is that the contrast agent is administered in two portions with a pause between them. As a result, a single scan captures a hybrid phase: the renal parenchyma is in the nephrographic stage, while the collecting system is already in the excretory stage. However, dense contrast in the ureter can mask small tumors, so patients are often given diuretics (e.g., furosemide) to dilute the contrast agent.
Evaluation of renal masses
Assessment of neoplasms requires measuring X-ray attenuation in Hounsfield units (HU). Enhancement greater than 10 HU indicates a solid lesion, and greater than 20 HU is highly suspicious for malignancy. If an unenhanced CT incidentally reveals a mass with an attenuation of 20 to 70 HU, this has high specificity for carcinoma.
Different histological types of renal cell carcinoma (RCC) have distinct enhancement patterns:
- Clear cell carcinoma (Clear cell carcinoma): enhances intensively and heterogeneously.
- Papillary carcinoma: characterized by homogeneous and less pronounced enhancement.
- Chromophobe carcinoma and oncocytoma: demonstrate homogeneous peripheral enhancement (distinguishing them by imaging methods is practically impossible).
Dual-energy CT (DECT)
Dual-energy CT allows image acquisition at different tube voltage levels (typically 80 and 140 kV), enabling tissue differentiation based on chemical composition. The main advantage of DECT in urology is the creation of a virtual unenhanced CT by software iodine subtraction from contrast-enhanced images. This eliminates the need for a physical native phase and reduces the radiation dose.
Additionally, DECT helps differentiate uric acid stones from non-uric acid stones (important for deciding on litholytic medical therapy) and allows the use of color overlay for clearer visualization of iodine-containing structures and calculi.
Alternative: MR urography
Magnetic resonance urography (MRU) is used when CT is contraindicated: in pregnant women, children, and patients with renal failure. MRI provides better contrast resolution and does not use ionizing radiation, but it may miss small stones.
MRI effectively handles the "pseudoenhancement" phenomenon that occurs on CT when evaluating small cysts. If signal intensity on MRI increases by more than 15% after contrast administration, this reliably confirms the presence of tumor tissue. To visualize urine on MRI, heavily T2-weighted sequences (static fluid MR urography) or T1-weighted imaging after gadolinium administration with mandatory diuretic use are employed to avoid artifacts.
Briefly: answers to topical questions
What are the absolute contraindications to intravenous administration of iodinated contrast media in CT urography?
Contraindications to the administration of iodinated contrast agents include:
- renal impairment; contrast administration should be avoided when eGFR is below 30 mL/min/1.73 m²;
- history of severe allergies;
- thyrotoxicosis or elevated thyroid hormone levels.
If contraindications are present, consultation with relevant specialists is required.
What types of renal calculi have soft-tissue attenuation and are poorly visualized on unenhanced CT?
Calculi with soft-tissue X-ray attenuation are poorly visualized on non-contrast computed tomography, making them difficult to detect.
Such radiolucent lesions include:
- Indinavir sulfate stones — specific calculi whose formation is associated with protease inhibitor therapy.
- Mucoid matrix stones — structures that also possess soft-tissue attenuation and do not stand out against surrounding tissues.
To avoid confusing phases and timing: the nephrographic phase is measured in seconds (80–100 s, parenchymal assessment), while the excretory phase is measured in minutes (5–15 min, assessment of urine excretion).
Class and exam questions
What is the main advantage of CTU over conventional intravenous urography?
CT urography is significantly better at detecting both internal and external causes of obstruction (tumors, lymphadenopathy, fibrosis) and has a much higher sensitivity in diagnosing malignant urothelial neoplasms.
Why is prone scanning used during CT urography?
Prone imaging improves natural ureteral distension and helps differentiate a fixed pelviureteric junction stone from a mobile calculus.
How to differentiate a phlebolith from a ureteral stone on unenhanced CT?
A phlebolith is indicated by the comet-tail sign — a linear soft-tissue structure extending from the calcification. True ureteral stones do not exhibit this sign.
Is CT urography the definitive method for diagnosing bladder cancer?
No. Although CTU can reveal focal wall thickening, cystoscopy with mandatory histological confirmation remains the gold standard for bladder assessment.
What else to study on this topic
- Technical features of DECT platforms: dual-source systems versus fast kilovoltage-switching.
- MRI semiotics of renal tumors: differentiation of angiomyolipomas and papillary carcinomas on T2-weighted imaging.
- The cyst pseudoenhancement phenomenon on CT and its resolution algorithm.
- Indications and technique for furosemide administration to optimize urinary tract distension.
- The role of diagnostic imaging in screening patients with von Hippel-Lindau syndrome.
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