Improved neuronal tract tracing using manganese enhanced magnetic resonance imaging with fast T 1 mapping

There has been growing interest in using manganese‐enhanced MRI (MEMRI) to detect neuronal activation, neural architecture, and neuronal connections. Usually Mn 2+ produces a very wide range of T 1 change. In particular, in neuronal tract tracing experiments the site of Mn 2+ injection can have very short T 1 while distant regions have small T 1 reductions, primarily due to dilution of Mn 2+ . Most MEMRI studies use T 1 ‐weighted sequences, which can only give optimal contrast for a narrow range of T 1 changes. To improve sensitivity to the full extent of Mn 2+ concentrations and to optimize detection of low concentrations of Mn 2+ , a fast T 1 mapping sequence based on the Look and Locker technique was implemented. Phantom studies demonstrated less than 6.5% error in T 1 compared to more conventional T 1 measurements. Using center‐out segmented EPI, whole‐brain 3D T 1 maps with 200‐μm isotropic resolution were obtained in 2 h from rat brain. Mn 2+ transport from the rat olfactory bulb through appropriate brain structures could be detected to the amygdala in individual animals. The method reliably detected less than 7% reductions in T 1 . With this quantitative imaging it should be possible to study more extensive pathways using MEMRI and decrease the dose of Mn 2+ used. Magn Reson Med, 2006. Published 2006 Wiley‐Liss, Inc.