Hi, Thanks for writing in. A mild hydronephrosis in kidneys can be due to the following causes. (i) It can be that your bladder was completely full at the of ultrasound scan (ii) Your kidney pelvicalyceal system is appearing full by itself (extrarenal pelvis) and is an extension of normal appearance. (iii) Rarely an infection of urinary system and kidneys (recent or old) can cause such a finding but this is likely only if you have symptoms like pain in the region of kidney and problems like burning urination or discoloration of urine or any kidney infections in the past. (iv) Stones in kidney or ureter can also cause fullness of pelvicalyceal system and since you do not have any stone therefore this does not apply to you. If your doctor wants a CT scan to see only stones then giving contrast is not required. Stones can be seen well on CT scan without contrast. Please discuss this matter with your doctors and the radiologists before getting the CT scan done. Most stones are made up of calcium and appear bright on non contrast CT scans. That is sufficient to make a diagnosis. If an infection is suspected then urine culture and sensitivity test is required.