1. A method of enhancing a magnetic resonance imaging (MRI) image of a body or body region such as an organ or organ region in a subject, comprising: parenterally administering a composition comprising ascorbate or a pharmaceutically acceptable salt thereof to said subject in an MRI image-enhancing amount, wherein said composition comprises a spin-spin exchange catalyst; and then generating, by MRI of the subject, an image of said body or body region, whereby said composition enhances the MRI image.