A process for the preparation of the gadolinium complex of N-(1-hydroxymethyl-2,3- dihydroxypropyl)-1,4,7-triscarboxymethyl- 1,4,7,10- tetraazacyclododecane, by reacting cyclen with 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]octane and dimethylformamide dimethyl acetal, hydrolyzing the formyl intermediate by addition of lithium hydroxide, adding chloro- or bromoacetic acid and reacting lithium hydroxide or N-methylimidazole, adjusting to an acidic pH with hydrochloric acid or hydrobromic acid, subsequently determining the butrol ligand content and adding the stoichiometric amount of a gadolinium salt.
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