disclosed are contrast agents of the formula ## str1 ## contained in aqueous compositions and methods for their use in diagnostic radiology of the gastrointestinal tract wherein z = h , halo , c 1 - c 20 alkyl , cycloalkyl , lower alkoxy , cyano , where the alkyl and cycloalkyl groups can be substituted with halogen or halo - lower - alkyl groups ; r = c 1 - c 25 alkyl , cycloalkyl , or halo - lower - alkyl , optionally substituted with halo , fluoro - lower - alkyl , aryl , lower - alkoxy , hydroxy , carboxy , lower - alkoxy carbonyl or lower - alkoxy - carbonyloxy , p -- m q , or p -- c . tbd . c -- q ; r 1 , r 2 , r 3 and r 4 are independently lower - alkyl , optionally substituted with halo ; x is 1 - 4 ; n is 1 - 5 ; m is 1 - 15 ; p is 1 - 10 ; and q is h , lower - alkyl , lower - alkenyl , lower - alkynyl , lower - alkylene , aryl , or aryl - lower alkyl .