Multipulse sodium magnetic resonance imaging for multicompartment quantification: Proof-of-concept

We present a feasibility study of sodium quantification in a multicompartment model of the brain using sodium (Na-23) magnetic resonance imaging. The proposed method is based on a multipulse sequence acquisition and simulation at 7T, which allows to differentiate the Na-23 signals emanating from three compartments in human brain in vivo: intracellular (compartment 1), extracellular (compartment 2), and cerebrospinal fluid (compartment 3). The intracellular sodium concentration C-1 and the volume fractions alpha(1), alpha(2), and alpha(3) of all respective three brain compartments can be estimated. Simulations of the sodium spin 3/2 dynamics during a 15-pulse sequence were used to optimize the acquisition sequence by minimizing the correlation between the signal evolutions from the three compartments. The method was first tested on a three-compartment phantom as proof-of-concept. Average values of the Na-23 quantifications in four healthy volunteer brains were alpha(1) = 0.54 +/- 0.01, alpha(2) = 0.23 +/- 0.01, alpha(3) = 1.03 +/- 0.01, and C-1 = 23 +/- 3mM, which are comparable to the expected physiological values alpha(theory)(1) similar to 0.6, alpha(theory)(2) similar to 0.2, alpha(theory)(3) similar to 1, and C-1(theory) similar to 10-30mM. The proposed method may allow a quantitative assessment of the metabolic role of sodium ions in cellular processes and their malfunctions in brain in vivo.