The present invention relates to a composition for use as a contrast agent in medical imaging. The composition comprises a metal ion containing compound comprises at least one metal ion adapted to bind with one or more metal ion chelating agents； wherein said one or more metal ion chelating agents comprise at least one first chelating agent adapted to target a region of interest being imaged, and at least one second chelating agent having at least one functional group adapted to dissociate in an aqueous medium to thereby allow or enhance contrast of the region of interest under the medical imaging. The present invention further relates to a diagnostic agent for use in amyloid (Aβ) protein detection under magnetic resonance imaging (MRI) comprising the above described composition； and a method of preparing thereof.
