Hydronephrosis refers to a kidney with a dilated pelvis and collecting system. It can be caused by obstruction of the ureters or bladder outlet. Hydronephrosis can also result from reflux (retrograde leakage of urine from the bladder up the ureters to the renal pelvis. Rarely, some children have hydronephrosis without either obstruction or reflux. This is thought to result form abnormal smooth muscles of the renal pelvis or ureter causing ectasia.
*Black arrow = renal capsule
*Black arrowhead = sinus fat
*White arrow = dilated calyx
*White arrowhead = renal cortex