T2 exchange agents: a new class of paramagnetic MRI contrast agent that shortens water T2 by chemical exchange rather than relaxation.

Exchange of water molecules between the frequency-shifted inner-sphere of a paramagnetic lanthanide ion and aqueous solvent can shorten the T(2) of bulk water protons. The magnitude of the line-broadening T(2) exchange (T(2exch)) is determined by the lanthanide concentration, the chemical shift of the exchanging water molecule, and the rate of water exchange between the two pools. A large T(2exch) contribution to the water linewidth was initially observed in experiments involving Eu(3+)-based paramagnetic chemical exchange saturation transfer agents in vivo at 9.4 T. Further in vitro and in vivo experiments using six different Eu(3+) complexes having water exchange rates ranging from zero (no exchange) to 5 × 10(6) s(-1) (fast exchange) were performed. The results showed that the exchange relaxivity (r(2exch)) is small for complexes having either very fast or very slow exchange, but reaches a well-defined maximum for complexes with intermediate water exchange rates. These experimental results were verified by Bloch simulations for two site exchange. This new class of T(2exch) agent could prove useful in the design of responsive MRI contrast agents for molecular imaging of biological processes.