Iodixanol impurity H5-[Acetyl-[3-[acetyl[3-[[3-[3-[acetyl[3,5-bis-[(2,3-dihydroxypropyl)-carbamoyl]-2,4,6-triiodphenyl]-amino]-2-hydroxypropoxy]-2-hydroxypropyl]-carbamoyl]-5-[(2,3-dihydroxypropyl)-carbamoyl]-2,4,6-triiodphenyl]-amino]-2-hydroxypropyl]-amino]-N,N′-bis-(2,3-dihydroxypropyl)-2,4,6-triiodobenzol-1,3-dicarboxamide