Diffusion‐weighted MRS of acetate in the rat brain

Acetate has been proposed as an astrocyte‐specific energy substrate for metabolic studies in the brain. The determination of the relative contribution of the intracellular and extracellular compartments to the acetate signal using diffusion‐weighted magnetic resonance spectroscopy can provide an insight into the cellular environment and distribution volume of acetate in the brain. In the present study, localized ¹H nuclear magnetic resonance (NMR) spectroscopy employing a diffusion‐weighted stimulated echo acquisition mode (STEAM) sequence at an ultra‐high magnetic field (14.1 T) was used to investigate the diffusivity characteristics of acetate and N‐acetylaspartate (NAA) in the rat brain in vivo during prolonged acetate infusion. The persistence of the acetate resonance in ¹H spectra acquired at very large diffusion weighting indicated restricted diffusion of acetate and was attributed to intracellular spaces. However, the significantly greater diffusion of acetate relative to NAA suggests that a substantial fraction of acetate is located in the extracellular space of the brain. Assuming an even distribution for acetate in intracellular and extracellular spaces, the diffusion properties of acetate yielded a smaller volume of distribution for acetate relative to water and glucose in the rat brain.