In vivo Na‐23 MR imaging and spectroscopy of rat brain during TmDOTP⁵⁻ infusion

In vivo sodium‐23 and hydrogen‐1 magnetic resonance (MR) imaging and spectroscopy of the rat brain during infusion of the shift reagent thulium DOTP⁵⁻ (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetra[methylene phosphonate]) was performed to assign the various peaks observed during infusion and to evaluate the shift reagent in discriminating tissue compartments. Na‐23 spectra collected during the infusion showed two shifted peaks that were assigned to intravascular Na⁺ and extracellular muscle Na⁺, respectively, and one unshifted peak assigned to intra‐ and extracellular brain Na⁺ and cerebrospinal fluid Na⁺. These assignments were validated with H‐1 and Na‐23 MR imaging and Na‐23 chemical shift imaging (CSI). The H‐1 and Na‐23 images showed that a surface coil placed on a rat head can detect a substantial amount of signal from muscle surrounding the skull. Na‐23 CSI spectra from successive 1‐mm‐thick coronal sections indicated that the shift reagent did not cross the blood‐brain barrier. The study also showed that bulk susceptibility shifts are quite small with Tm‐DOTP⁵⁻. This reagent may be useful in determining compartmental Na⁺ concentrations and blood flow kinetics in brain and in examining the integrity of the blood‐brain barrier.