Three‐dimensional MRI of cerebral projections in rat brain in vivo after intracortical injection of MnCl2

Abstract

In this study we investigated the potential of in vivo MRI detection of axonal Mn²⁺ transport for tracing neuronal projections originating in the sensorimotor cortex in healthy and lesioned rat brains. Special attention was given to the potential of visualizing neuronal sprouting of central nervous system across the midline. After injecting unchelated MnCl₂ into the forelimb area of sensorimotor cortex of 18 healthy and 10 lesioned rats corticofugal projections could be traced through the internal capsule to the cerebral peduncle and the pyramidal decussation. Although the neuronal tract was visible as early as 6 h after MnCl₂ injection, best contrast was achieved after 24–48 h. Beside the cortico‐spinal tract, the cortico‐thalamic fibres were also visualized by anterograde Mn²⁺ transport. Cortico‐striatal fibres were partially masked by the very high signal near the MnCl₂ injection site but could be discerned as well. Slight, diffuse signal enhancement of cortical tissue contralateral to the MnCl₂ injection site in healthy rat brains suggests interhemispheric connections or passive diffusion of Mn²⁺. However, enhanced fibre tract contrast connecting both hemispheres was visible 16 weeks after onset of focal photothrombotic cortical injury. In conclusion our study has shown that we were able to visualize reproducibly the main descending corticofugal projections and interhemispheric connections by non‐invasive MRI after localized injection of MnCl₂. The appearence of interhemispheric Mn²⁺‐enhanced fibres after photothrombotic focal injury indicates that the method may bear potential to follow non‐invasively gross plastic changes of connectivity in the brain after injury. Copyright © 2001 John Wiley & Sons, Ltd.