Lithium compartmentation in brain by 7 Li MRS: effect of total lithium concentration

In previous work at 4.7 T, the individual components of biexponential 7 Li transverse ( T 2 ) 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 7 Li T 2 MR spin‐relaxation study with isotopically enriched 7 LiCl is reported in rat brain in vivo at 7 T. Additionally, a simple linear interpolation using the biexponential T 2 values to estimate intracellular Li from individual monoexponential T 2 decays was assessed. Intracellular T 2 was 14.8 ± 4.3 ms and extracellular T 2 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. Copyright © 2013 John Wiley & Sons, Ltd.