The present invention relates to a liquid pharmaceutical composition including a complex of formula (I), in which M is an ion of a paramagnetic metal and R1 to R3, X1 to X3 and K1 to K12 are such as defined in claim 1, said composition also including a calcium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. The invention also relates to the preparation method thereof and to an imaging method involving said composition.