Inclusion Criteria:  Unilateral renal obstruction confirmed by Nuclear Medicine MAG3 Dynamic Renal Scan (diuretic t (time) ½ >20 minutes) Obstructed kidney with differential function >15% confirmed by Nuclear Medicine Mercaptoacetyltriglycine (MAG3) Dynamic Renal Scan within less or equal (<=) 6 months from enrollment Patient has elected to move forward with pyeloplasty or proximal ureteroureterostomy to correct the obstruction  Exclusion Criteria:  Solitary kidney Any abnormalities of contralateral collecting system (for example (e.g.) hydronephrosis, renal calculi, duplicated collecting system) Known vesicoureteral reflux Ureteral stent in place in the obstructive kidney Patients without a Nuclear Medicine MAG3 Dynamic Renal Scan Previous history of hypersensitivity to Ultrasound Contrast Agents or Definity (perflutren lipid microspheres)) Previous sensitivity to polyethylene glycol Pregnant or breastfeeding females Body mass index (BMI) >=40 without prior ultrasound confirming ability to adequately visualize the kidneys (Elevated BMI is known to decrease ability to image the kidneys) Global renal function with a Glomerular Filtration Rate (GFR) <30 (chronic kidney disease Stage 4 & 5 excluded) Unstable cardiopulmonary conditions (acute myocardial infarction, acute coronary artery syndromes, worsening or unstable congestive heart failure, serious ventricular arrhythmias, presence of a cardiac shunt)