The present invention relates to a process for preparing a liquid pharmaceutical formulation containing a complex of macrocyclic chelate with a lanthanide and a mol/mol amount of free macrocyclic chelate of between 0.002% and 0.4%, said macrocyclic chelate being DOTA and said lanthanide being gadolinium, said process comprising the following successive steps:b) preparing a liquid pharmaceutical composition containing:- the complex of macrocyclic chelate with a lanthanide,- free macrocyclic chelate that is not under the form of an excipient X[X',L] in which L is the macrocyclic chelate and X and X' are a metal ion, in particular chosen independently from calcium, sodium, zinc and magnesium, and/or- free lanthanide,by mixing a solution of free macrocyclic chelate and of free lanthanide so as to obtain complexation of the lanthanide by the macrocyclic chelate, wherein there is a difference between the amounts of free macrocyclic chelate and of free lanthanide added and the stoichiometric proportions；c) measuring, in the pharmaceutical formulation obtained in step b), the concentration of free macrocyclic chelate Cch l and/or free lanthanide Clan l；d) adjusting, in the pharmaceutical formulation obtained in step b), Cch l and Clan l so as to obtain Clan l = 0 and to obtain Cch l = Ct ch l, wherein Ct ch l is the target concentration of the free macrocyclic chelate in the final liquid pharmaceutical formulation and is selected in the range of between 0.002 % and 0.4 % mol/mol,said adjustment of Cch l and of Clan l being performed as a function of the Clan l and Cch l measured in step c),wherein the amount of free macrocyclic chelate in the final liquid pharmaceutical formulation corresponds to the proportion of free macrocyclic chelate relative to the amount of complexed macrocyclic chelate DOTA-Gd in the final liquid pharmaceutical formulation in mol/mol.
