Which imaging finding is most characteristic of pyonephrosis: mobile, position dependent debris within the collecting system or other features?

Which imaging finding is most characteristic of pyonephrosis: mobile, position dependent debris within the collecting system or other features?

• A. Unilateral enlarged kidney with dilation of calyces
• B. Mobile, position dependent debris within the collecting system
• C. Stones present within the kidney
• D. Dirty shadow or ring-down artifact from gas

Answer: (B)

Pyonephrosis is hydronephrosis that is infected, with purulent material filling the collecting system. On ultrasound the telling feature is echogenic debris within the dilated collecting system that moves with gravity and changes position when the patient is repositioned. This mobile, gravity-dependent debris reflects pus rather than just clear urine, making it the most characteristic sign of infection atop obstruction. While dilation of the collecting system shows obstruction, it isn’t specific for infection. Stones may be present but don’t by themselves confirm pyonephrosis. Gas-related artifacts can occur with certain gas-forming infections, but they are not the defining finding of pyonephrosis and are less consistently seen.

Pyonephrosis is hydronephrosis that is infected, with purulent material filling the collecting system. On ultrasound the telling feature is echogenic debris within the dilated collecting system that moves with gravity and changes position when the patient is repositioned. This mobile, gravity-dependent debris reflects pus rather than just clear urine, making it the most characteristic sign of infection atop obstruction. While dilation of the collecting system shows obstruction, it isn’t specific for infection. Stones may be present but don’t by themselves confirm pyonephrosis. Gas-related artifacts can occur with certain gas-forming infections, but they are not the defining finding of pyonephrosis and are less consistently seen.