Improved neuronal tract tracing using manganese enhanced magnetic resonance imaging with fast T1 mapping

There has been growing interest in using manganese-enhanced MRI (MEMRI) to detect neuronal activation, neural architecture, and neuronal connections. Usually Mn2+ produces a very wide range of T-1 change. In particular, in neuronal tract tracing experiments the site of Mn2+ injection can have very short T-1 while distant regions have small T-1 reductions, primarily due to dilution of Mn2+. 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 Mn2+ concentrations and to optimize detection of low concentrations of Mn2+, 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-mu m isotropic resolution were obtained in 2 h from rat brain. Mn2+ 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 Mn2+ used. Magn Reson Med 55:604-611, 2006. Published 2006 Wiley-Liss, Inc.