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 ( 23 Na) magnetic resonance imaging. The proposed method is based on a multipulse sequence acquisition and simulation at 7 T, which allows to differentiate the 23 Na 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 α 1 , α 2 , and α 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 23 Na quantifications in four healthy volunteer brains were α 1 = 0.54 ± 0.01, α 2 = 0.23 ± 0.01, α 3 = 1.03 ± 0.01, and C 1 = 23 ± 3 mM, which are comparable to the expected physiological values α _1^theory α 1 theory ∼ 0.6, α _2^theory α 2 theory ∼ 0.2, α _3^theory α 3 theory ∼ 1, and C_1^theory C 1 theory ∼ 10–30 mM. 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 .