Metabolite ¹H relaxation in normal and hyponatremic brain

Proton spin relaxation rate constants in normal and hyponatremic rat brain were measured to determine the sensitivity of metabolite relaxation properties to cytotoxic edema and to quantify metabolite concentration in normal and edematous brain. Relaxation rate constants for protons of water and spectral regions with dominant contributions from methyl protons of cholines (Cho), creatines (Cr), N-acetylaspartate (NA), and lactate (Lac), and for methylene protons of glutamate (Glu) were measured at 7 T. Changes in metabolite relaxation properties associated with cytotoxic edema were a decrease in the Cr longitudinal rate constant, from 0.63 ± 0.02 s^-1 (mean ± SE) in controls to 0.50 ± 0.03 s^-1 in edematous brain, and an increase in the transverse rate constant of NA from 5.3 ± 0.2 s^-1 in controls to 6.6 ± 0.3 s^-1 in edematous brain. Four hours after induction of hyponatremia, there was a 14% reduction in summed metabolite concentrations of Cho, Cr, and NA, and a 200% increase in Lac signal intensity. It is concluded that changes in both metabolite spin relaxation and detectable spin concentration accompany the cerebral pathology of cytotoxic edema complicated with secondary ischemia.