During preventive ultrasound scan of abdomen performed during regular checkup in a school the following was revealed in an 11-year-old student of the 5th grade: the left kidney is 3 cm below the normal position, its shape, size and structure are within the norm, the contralateral kidney cannot be observed at its proper place. The preliminary diagnosis is as follows: congenital anomaly of renal development, dystopic left kidney, right kidney is absent or pelvic dystopic. What X-ray method would be required for making the final diagnosis and determining the functional capacity of both kidneys?