Intra renal Duplex Doppler ultra sonography (DU) has been suggested as a possible adjunctive test in distinguishing obstructed from non-obstructed dilated kidneys. The use of duplex doppler imaging during ultrasonographic evaluation of the kidney allows the determination of the renal resistive index, which is a sensitive and highly specific tests that can contribute significantly to the diagnosis of acute unilateral renal obstruction, particularly in situations in which I.V.U is undesirable.