Obstructive vs. nonobstructive dilatation of the renal collecting system in children: Distinction with Duplex sonography. - Kessler,R.M.; Quevedo,H.; Lankau,C.A.; Ramirez-Seijas,F.; Cepero-Akselrad,A.; Altman,D.H.; Kessler,K.M. (AJR, 1993)