Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Silva, Afonso C., et al. "Manganese‐enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations." NMR in Biomedicine 17.8 (2004): 532-543. The development of manganese-enhanced magnetic resonance imaging (MeMRI) has led to three different functional applications including systemic MnCl2 accumulation for anatomical mapping, activity-induced manganese-enhanced MRI (AIM-MRI), and manganese-enhancement neuronal tract tracing. The third of these techniques, MeMRI neuronal tract tracing, is particularly useful for visualizing neural circuits in vivo. In recent years it has been shown that manganese is not only transported throughout a neuron, but can also cross synapses as well (Pautler et al.). In Pautler’s experiment 2.4 M MnCl2 solutions were injected into the nose and eyes of mice and imaged 48 hours. Significant traces of manganese were then found in the olfactory bulb as well as the olfactory cortex, indicating the manganese is able to go trans-synaptic. It has also been proven that acute injections into the brain can produce significant visualization of neural connections. So far small doses of manganese chloride (10-1000nl of 5mM-.8M) have been injected into the brains of birds, monkeys, and mice. As promising as these results seem for future neuronal tract tracing techniques, it has also been shown that manganese does not cross synapses along other pathways, such as the visual circuit. Unfortunately, these results suggest that manganese behaves differently amongst different neural pathways and needs to be further examined. It is promising however, that with direct applications of manganese, much smaller concentrations can be used to produce significant results. The smaller the amount of manganese needed, the more likely that MeMRI can be used as a diagnostic tool in humans. ~JI