 Longitudinal relaxation refers to the return of longitudinal spin polarization, which occurs at a rate described by T1.While the number of hydration water molecules may be insufficient to produce an observable signal, exchange of water molecules between the hydration and bulk population allows characterization of the hydration population, and measurement of the rate at which molecules are exchanging between bulk and bound sites.Such experiments are often termed saturation transfer or chemical exchange saturation transfer (CEST), because the signal of the bulk water is observed to decrease when the hydration population is saturated.