Intracellular water preexchange lifetime in neurons and astrocytes

Purpose To determine the intracellular water preexchange lifetime, τ i , the “average residence time” of water, in the intracellular milieu of neurons and astrocytes. The preexchange lifetime is important for modeling a variety of MR data sets, including relaxation, diffusion‐sensitive, and dynamic contrast–enhanced data sets. Methods Herein, τ i in neurons and astrocytes is determined in a microbead‐adherent, cultured cell system. In concert with thin‐slice selection, rapid flow of extracellular media suppresses extracellular signal, allowing determination of the transcytolemmal‐exchange‐dominated, intracellular T 1 . With this knowledge, and that of the intracellular T 1 in the absence of exchange, τ i can be derived. Results Under normal culture conditions, τ i for neurons is 0.75 ± 0.05 s versus 0.57 ± 0.03 s for astrocytes. Both neuronal and astrocytic τ i s decrease within 30 min after the onset of oxygen‐glucose deprivation, with the astrocytic τ i showing a substantially greater decrease than the neuronal τ i . Conclusions Given an approximate intra‐ to extracellular volume ratio of 4:1 in the brain, these data imply that, under normal physiological conditions, an MR experimental characteristic time of less than 0.012 s is required for a nonexchanging, two‐compartment (intra‐ and extracellular) model to be valid for MR studies. This characteristic time shortens significantly (i.e., 0.004 s) under injury conditions. Magn Reson Med 79:1616–1627, 2018. © 2017 International Society for Magnetic Resonance in Medicine.