Water exchange between intra and extracellular compartments studied by nuclear magnetic resonance relaxometry and optical microscopy

Nuclear magnetic resonance (NMR) techniques, in particular H-1 relaxometry, enable the acquisition of information on the water exchange between different compartments in real-time and in a non-invasive and non-destructive way. Therefore, these are currently the methods of choice for studying the diffusion of water in living tissues and cells. The multi-compartmental nature of these systems may generate multi-exponential decays of the nuclear magnetization vector components to be analyzed in terms of discrete or continuous distributions of the so-called relaxation times. We have evaluated the sensitivity of the continuous distributions of the transverse component relaxation time (T-2) to detect and monitor, over time, the water diffusion caused by osmotic pressure variations in tumor cells suspended in isotonic and hypotonic/hypertonic culture media. Our results indicated that the T-2 distribution shapes were capable of describing the water compartmentalization and exchange, with evolutions that strongly depended on the degree of tonicity alterations. In order to validate the NMR results we also monitored the samples by phase contrast optical microscopy finding consistency between the two methods. Moreover, a study of T-2-T-2 correlation function allowed us to observe the exchange between cytoplasmic water and intracellular H-1 spin groups. Using a two-site exchange model we also discussed, as a function of the uncertainties on the intra-to-extra water fraction and on the T-2 values, the computed intracellular average water lifetime at equilibrium, an important parameter recently proposed as indicator of tumor aggressiveness. The results support the idea that the information conveyed by the T-2 parameter might be useful for an MRI diagnostic approach to diseases in which the fluid tonicity or the cellular membranes permeability are altered.