The 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%, advantageously between 0.02% and 0.3% and very advantageously between 0.025% and 0.25%, the macrocyclic chelate advantageously being chosen from DOTA, NOTA, DOTAGA, D03A, BT-D03A, HP-D03A and PCTA, and is preferably DOTA, the said process comprising the following successive steps: b) preparation of a liquid pharmaceutical composition containing, firstly, the complex of macrocyclic chelate with a lanthanide, and, secondly, free macrocyclic chelate and/or free lanthanide; c) measurement in the pharmaceutical formulation obtained in step b) of the concentration of free macrocyclic chelate Cchl and/or of free lanthanide Ciani; d) adjustment of Cchl and/or of Ciani so as to obtain Cchl=Ct chi and Ciani=0, wherein Ct chi is the target concentration of free macrocyclic chelate in the final liquid pharmaceutical formulation.