hello dear thanks for your query at HCM urinary infection is common in children, major risk factor for urinary infection is female gender because of there anatomy. positive pus cells and negative culture is called as sterile pyuria, it occurs in partially treated cases, viral infection and in urinary obstruction due to anatomical defect. the goal of imaging studies in children with a urinary infection is to identify anatomic abnormalities that predispose to infection. your doctor might have suspected sterile pyuria because of anatomic defects and ordered for ultrasonogram. you can proceed with that it is a simple painless and no radiation effects and rule out most of renal pathology. regards