Technique and basic principles
Scanning of the urinary tract is performed in axial and coronal planes. The spatial resolution of the method is 2–3 mm, which is somewhat inferior to CT urography (CTU). The signal-to-noise ratio can be optimized by using phased-array surface coils (phased-array surface coils).
On T2-weighted images (T2WI), dilatation of the collecting system is well visualized, whereas excretory series after administration of gadolinium preparations (gadolinium) help assess the degree of functional obstruction. Additionally, MRU allows for MR angiography to perform a detailed assessment of the arterial blood supply to both kidneys.
Diagnosis of calculi and masses
Calcified lesions on MRI create a signal drop-out effect (signal drop-out) or appear as filling defects (filling defects). The main difficulty lies in the poor visualization of stones smaller than 5 mm, whose appearance can be mimicked by blood clots, surgical clips, or small tumors.
Spin-echo MRI (Spin-echo MRI) is used for differential diagnosis: unlike stones and clots, a tumor demonstrates enhancement on post-contrast T1WI.
MRI semiotics of renal masses:
- Angiomyolipoma (Angiomyolipoma): On T1WI in-phase, it gives an almost isointense signal, while in-phase/out-of-phase (out-of-phase) shows a signal drop, confirming the presence of intracellular fat.
- Renal cell carcinoma (Renal cell carcinoma): A large tumor demonstrates prominent contrast enhancement (enhancing) on fat-saturated T1WI (fat-saturated) following intravenous enhancement.
Evaluation of urinary tract obstruction
Static MRU sequences outperform CT urography in detecting secondary signs of obstruction, such as renal edema, by detecting hyperintense urine in the renal tubules.
The choice of sequence depends on the degree of urodynamic impairment:
- Contrast-enhanced T1WI: Optimal in incomplete obstruction. They allow visualization of the ureter above and below the site of obstruction, providing an accurate assessment of ureteral stricture.
- T2WI: Sufficient for diagnosis in conditions of high-grade (high-grade) or complete obstruction.
Special clinical situations and radiation dose
The role of magnetic resonance imaging increases in the evaluation of pregnant patients (using HASTE sequences to assess hydronephrosis and calculi), as well as in searching for synchronous and metachronous urothelial tumors.
An important advantage of MRI is the absence of ionizing radiation. For comparison, the standard dose in computed tomography is about 3.63 mSv (which is significantly higher than 0.7 mSv for plain radiography), and 4-phase CT urography can yield an effective dose of up to 25–35 mSv. X-ray exposure is associated with the risk of deterministic effects (which have a dose threshold) and stochastic effects such as cancer (probabilistic, with no safe lower threshold). Replacing CT with MRU where clinically justified completely eliminates these risks, in accordance with the principle of radiation dose minimization.
Briefly: answers to topical questions
What are the absolute and relative contraindications for gadolinium-enhanced MR urography?
When using gadolinium-based contrast agents for MR urography, there are pathological conditions that require strict caution.
- Severe chronic kidney disease — at an estimated glomerular filtration rate (eGFR / eGFR) of less than 30 mL/min/1.73 m².
- Acute kidney injury (acute kidney injury) — contrast administration is limited due to the risk of nephrogenic systemic fibrosis (nephrogenic systemic fibrosis).
What artifacts can occur and impair visualization during MR urography?
In the cited sources for MR urography, specific motion or edge artifacts are not directly indicated, but rather limitations and causes of interpretation difficulties.
- Calcified lesions can create a signal drop-out effect (signal drop-out) or filling defects (filling defects).
- Stones smaller than 5 mm are poorly visualized.
- The MR appearance of stones can be mimicked by blood clots, surgical clips, or small tumors.
- Limitations of urinary tract MRI include difficulties in detecting calcifications and air pockets.
Class and exam questions
How to distinguish a renal pelvis tumor from a blood clot or stone on MRI?
Blood clots, stones, and tumors may appear as filling defects. Post-contrast T1WI are used for differentiation: tumor tissue will demonstrate contrast enhancement, whereas calculi and blood clots will not.
What is the advantage of MRU over CT urography in diagnosing obstruction?
Static MRU sequences better detect secondary signs of obstruction, such as renal parenchymal edema, by visualizing hyperintense urine in the renal tubules.
How to safely assess hydronephrosis in a pregnant patient?
In pregnant patients, the use of heavily T2-weighted HASTE sequences on MRI is an effective and safe method for detecting hydronephrosis and calculi without radiation exposure.
What else to study on this topic
- Split-bolus technique in CT urography
- Principles of automatic tube current modulation (ATM) in CT
- Comparison of stochastic and deterministic radiation effects
- Iterative image reconstruction (ASIR) for CT dose reduction
- Intravenous urography (IVU) and patient skin dose calculation
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