Q: 38 year old female is admitted via ED for unilateral acute nephrolithiasis. Patient is admitted to ICU because of spiking fever, severe dehydration and azotemia. Renal ultrasound was performed in ED. Radiologist calls you to report "a fluid-fluid level" in renal pelvis. What does it mean? 

Answer:  Pyonephrosis 

"a fluid-fluid level" means urine on top of purulent debris. Ultrasound is a much better indicator in identifying pyonephrosis than CT imaging, which is usually performed without contrast in such situations due to severe dehydration and worsening azotemia. Needless to say, antibiotics are required.

Reference:

Jeffrey RB, Laing FC, Wing VW, Hoddick W. Sensitivity of sonography in pyonephrosis: a reevaluation.AJR Am J Roentgenol. Jan 1985;144(1):71-3.