Dear Mr. Bharat,
Thank you for your query at DoctorSpring.com

Your son's diagnosis is reasonably clear. He has bilateral hydronephrosis with bilateral PUJ (pelvi-ureteric junction) obstruction.
Your son's ultrasound reports suggest the diagnosis. The concern is the echogenic sediment. Is it infectious? Is it due to microcrystalline aggregation? Is it an imaging artefact? Needs to be checked and confirmed with a renal ultrasound by an alternate expert.
The DTPA scan involves an injection of isotope (in the form of a liquid solution) into a vein in your child's arm or hand (cream is put on an hour before to make the skin numb so that it doesn't hurt). The isotope is an inert substance which is excreted by the kidneys and so shows up the function of the kidneys as well as the degree of the obstruction, when they are scanned using a scanner machine. The scan can take up to an hour as repeated scans are taken to follow the isotope through the kidneys, ureters, and bladder but again is completely painless.
To put it in non-medical terms, ultrasound is like taking a photograph of a person. Looking at the photograph, you can only deduce a few physical aspects. You cannot guess how well the person can work. The same is true for kidneys. We have their ultrasounds (similar to a photograph) but need to find about functional status to decide if this obstruction is causing functional flow problems. In this child DTPA renogram would help assess functional status.
In case of young infants and neonates, some experts prefer alternate renotope scan, MAG-3. This does the same thing but probably slightly better in the younger age group. Both these agents can cause allergic reactions in any age group. Need to be prepared for such an eventuality. You need to get a renal function test also ( blood urea nitrogen and serum creatinine). Otherwise, overall the scan is safe for your son.
Regards