The importance of water exchange rates in the design of responsive agents for MRI

► The rates of water exchange in lanthanide complexes is often overlooked in the design of new imaging agents. ► There exists an optimal water exchange rate for T1-based agents, T2 line-broadening agents (i.e., resulting from chemical exchange), and paramagnetic CEST agents. ► The optimal water exchange rate for each class of imaging agent varies over several orders of magnitude. ► Lanthanide complexes have been reported that have water exchange rates that vary by nearly six orders of magnitude, from 2 ns to 1 ms.