In previous work at 4.7 T, the individual components of biexponential 7Li transverse (T2) spin relaxation in rat brain in vivo were tentatively identified with intra- and extracellular Li. The goal in this work was to estimate Li’s compartmental distribution as a function of total Li concentration in brain from the biexponential decays. Here a localized, biexponential 7Li T2 MR spin relaxation study with isotopically enriched 7LiCl is reported in rat brain in vivo at 7 T. Additionally a simple linear interpolation using the biexponential T2s to estimate intracellular Li from individual monoexponential T2 decays was assessed. Intracellular T2 was 14.8±4.3 ms and extracellular T2 was 295±61 ms. The fraction of intracellular brain Li ranged from 37.3 to 64.8% (mean 54.5±6.7%) and did not correlate with total Li concentration. The estimated intracellular Li concentration ranged from 47 to 80% (mean 68.3±8.5%) of the total brain Li concentration and was highly correlated with it. The monoexponential estimates of the intracellular-Li fractions and derived concentrations averaged about 15% higher than the corresponding biexponential estimates. This work supports the previous conclusion that a large fraction of Li in the brain is within the intracellular compartment.
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