“…In essence, the exchange of water between the two compartments provides an additional relaxation pathway for intracellular water. Indeed, in the limit at which the exchange process completely overwhelms the inherent spin-lattice relaxation process, i.e., when [2] the observed relaxation time for intracellular water is identically the exchange lifetime, [3] In what follows, we show for a perfused, microbead-adherent HeLa cell system that: (i) the apparent relaxation of extracellular water is dominated by flow effects, (ii) the apparent relaxation is sufficiently rapid to completely suppress signal from extracellular water, (iii) the intracellular longitudinal relaxation is slow relative to exchange of water between the intracellular and extracellular compartments, being nearly described by Eq. [3], and, thus, (iv) this method, when combined with an inversion recovery pulse sequence, yields the intracellular water lifetime.…”