  We report in this paper a new MRI contrast agent based on magnetic
nanodiamonds fabricated by Fe ion implantation. The Fe atoms that are implanted
into the nanodiamonds are not in direct contact with the outside world,
enabling this new contrast agent to be free from cell toxicity. The image
enhancement was shown clearly through T2 weighted images. The concentration
dependence of the T2 relaxation time gives a relaxivity value that is about
seven times that of the regular non-magnetic nanodiamonds. Cell viability study
has also been performed. It was shown that they were nearly free from
cytotoxicity independent of the particle concentration used. The imaging
capability demonstrated here adds a new dimension to the medical application of
nanodiamonds. In the future one will be able to combine this capability of
magnetic nanodiamonds with other functions through surface modifications to
perform drug delivery, targeted therapy, localized thermal treatment and
diagnostic imaging at the same time.
