Preparation method for a liquid pharmaceutical formulation of lanthanide chelate comprising a mol/mol excess of free chelate comprised between 0.002 and 0.4%, the chelate being DOTA, the lanthanide being Gadolinium, the lanthanide chelate being the DOTA-Gd complex, and said method comprising the following successive steps: 1) Step 1: mixture of the chelate and the lanthanide to obtain complexation of the lanthanide by the chelate, the complexation solution obtained comprising, in addition to the chelate-lanthanide complex, a quantity X1 of excess free chelate, 2) Step 2: preferably measurement of X1 and possible adjustment of X1 to have X1 comprised between 0.002 and 0.4% mol/mol, 3) Step 3: precipitation or crystallization of the complexation solution obtained in step 1) or step 2) in an organic solvent chosen from among ethanol, methanol, propanol, isopropanol, methyl ethyl acetone, ethyl acetate, and mixtures thereof, including aqueous, according to a ratio of 5 to 20 volumes of solvent for one volume of complexation solution, at a temperature comprised within the range [0; 80°C],so as to obtain a chelate-lanthanide complex powder, the powder containing a quantity X2 of excess free chelate, 4) Step 4: adjustment of X2 so as to obtain: 4.a) X2 comprised between 0.002 and 0.4%, and more especially between 0.02 and 0.3% mol/mol, very advantageously between 0.025 and 0.25% mol/mol, and 4.b) X2 corresponding to between 0.2 and 2 times X1, notably between 0.5 and 1.5 times X1, X2 advantageously belonging to the range: [0.5 - 0.95] ∗ X1, or the range [1.05-1.3] ∗ X1, 5) Step 5: preferably measurement of X2, 6) Step 6: putting the powder obtained in step 4) in solution, leading to a liquid pharmaceutical formulation of lanthanide chelate comprising a mol/mol excess of free chelate comprised between 0.002 and 0.4%. 1) Step 1: mixture of the chelate and the lanthanide to obtain complexation of the lanthanide by the chelate, the complexation solution obtained comprising, in addition to the chelate-lanthanide complex, a quantity X1 of excess free chelate, 2) Step 2: preferably measurement of X1 and possible adjustment of X1 to have X1 comprised between 0.002 and 0.4% mol/mol, 3) Step 3: precipitation or crystallization of the complexation solution obtained in step 1) or step 2) in an organic solvent chosen from among ethanol, methanol, propanol, isopropanol, methyl ethyl acetone, ethyl acetate, and mixtures thereof, including aqueous, according to a ratio of 5 to 20 volumes of solvent for one volume of complexation solution, at a temperature comprised within the range [0; 80°C],so as to obtain a chelate-lanthanide complex powder, the powder containing a quantity X2 of excess free chelate, 4) Step 4: adjustment of X2 so as to obtain: 4.a) X2 comprised between 0.002 and 0.4%, and more especially between 0.02 and 0.3% mol/mol, very advantageously between 0.025 and 0.25% mol/mol, and 4.b) X2 corresponding to between 0.2 and 2 times X1, notably between 0.5 and 1.5 times X1, X2 advantageously belonging to the range: [0.5 - 0.95] ∗ X1, or the range [1.05-1.3] ∗ X1, 4.a) X2 comprised between 0.002 and 0.4%, and more especially between 0.02 and 0.3% mol/mol, very advantageously between 0.025 and 0.25% mol/mol, and 4.b) X2 corresponding to between 0.2 and 2 times X1, notably between 0.5 and 1.5 times X1, X2 advantageously belonging to the range: [0.5 - 0.95] ∗ X1, or the range [1.05-1.3] ∗ X1, 5) Step 5: preferably measurement of X2, 6) Step 6: putting the powder obtained in step 4) in solution, leading to a liquid pharmaceutical formulation of lanthanide chelate comprising a mol/mol excess of free chelate comprised between 0.002 and 0.4%.