. "The basic composition of a solution according to the invention comprises: a) a gadolinium salt of a carboxylic acid of general formula ##STR3## where R is an alkyl of not less than four carbon atoms, and R1 and R2 are selected from the group consisting of hydrogen and an alkyl group; b) a free branched-chain carboxylic acid; and c) a non-polar solvent." . . .