MR Urography in Children: When Is It Needed, What Does It Show?

MR urography (MRU) is a scan that uses a powerful magnet and an intravenous contrast agent, without radiation, to produce three-dimensional images of the kidneys, the collecting system, the ureters, and the surrounding blood vessels. It has two components: fluid-sensitive (static) images map the widened collecting system like a chart, while contrast-enhanced dynamic images track the contrast passing through the kidneys to provide information on function and drainage.

• In this sense, MRU can offer, in a single session, both a more detailed version of ultrasound's anatomical information and data similar to MAG3's functional information. But it is expensive, takes a long time, and requires sedation or anesthesia in young children; for these reasons it is not a first-line test.

In most children, ultrasound, and if needed VCUG and MAG3 diuretic renography (a kidney scan), are enough to decide. MR urography is considered in these selected situations:

• Suspected crossing vessel: especially in an older child or adolescent with episodic flank pain (Dietl's crisis), showing the lower-pole vessel compressing the UPJ from outside changes the surgical plan. A crossing vessel is reported in roughly 20–40% of children with UPJ obstruction in the literature, and more often in older children with intermittent pain.
• Complex anatomy: in situations such as a duplex collecting system, horseshoe kidney, an ectopic or rotated kidney, or high ureteral insertion, to see the exact location of the narrowing and the course of the ureter.
• Conflicting ultrasound and MAG3 results: when there is marked widening on ultrasound but function and drainage cannot be interpreted on the scan, or it is unclear whether the narrowing is at the UPJ or lower down.
• Planning a redo operation: when narrowing recurs after a previous pyeloplasty, the length of scarring, the size of the pelvis, and the vessel relationships determine the surgical approach.
• When it is necessary to evaluate the quality of the parenchyma (kidney tissue) and the condition of the other kidney together in a kidney with very low function.

• This is not a fixed rule; the center's experience with MRU, access to the equipment, and the child's age all influence the decision.

The scan takes longer than half an hour, and staying still is required throughout. For this reason, sedation or brief general anesthesia is most often used in preschool-age children; older children can often be scanned awake after preparation, with a parent in the room and headphones.

MRU shows, at high resolution, the exact location of the narrowing, the shape and volume of the pelvis, the caliber of the ureter, and the surrounding blood vessels; it is clearly superior to ultrasound and the renal scan for showing a crossing vessel. The thickness and quality of the kidney tissue — scarring, cysts, dysplasia — are seen in detail.

MRU is not done for every child, because the burden of sedation and its cost are not worthwhile in a child where the result will not change the decision. In an infant with mild-to-moderate, stable hydronephrosis and preserved function, MRU has no role; serial ultrasound is enough.