Magnetic Resonance Imaging of Cortical Connectivity

Canals, S., et al. "Magnetic resonance imaging of cortical connectivity< i> in vivo." Neuroimage 40.2 (2008): 458-472. Understanding the vast potential of manganese-enhanced MRI as a neuronal tract tracing tool for examining complex neural connections, this study by Canals et. al looked to characterize direct injections of manganese into the cerebral cortex, as well as better define efferent connectivity from of the somatosensory and motor regions. Specifically, a variety of manganese concentrations (0.05-0.80M) and volumes (10-200nl) were injected into the somatosensory and motor cortex using an osmotic pump and cannulae. Following cranial injections (multiple protocols), immunohistochemistry and fluorescence microscopy were performed to determine the extent of neuronal damage/toxicity. An initial study using injections of 200nl .4M and .8M MnCl2 in water revealed that injections of 200nl .8M MnCl2 produced extensive cell death and astrogliosis. This is important because cell viability is critical when performing longitudinal studies. Also, damaged tissue compromises the spread and uptake of acute manganese, effecting acute studies as well. A second protocol using multiple injections with varying manganese concentrations with an array of pH's demonstrated that injections of higher concentrated manganese and greater acidity produced larger neuronal lesions and increased astrogliosis. On the basis of the characterization results, this study proclaimed infusion of 100mM MnCl2 in 80nl at a rate of .5nl/min produced the most optimal results with the lowest toxicity. The later half of the study was then designed to investigate the efferent projections from the somatosensory and motor cortex using the previously defined manganese solution against non-optimal solutions (acidic, hypertonic,and concentrated). Importantly, the results from this second study showed better signal enhancement following an injection of 8nmol 100mM MnCl2 pH 7.3 than 8nmol 800mM MnCl2 pH 5.5 in multiple brain regions the somatosensory cortex projects to, including the thalamus and S2. Also, using the optimal manganese solution also allowed for the visualization of trans-synaptic tracing. With an appropriate manganese solution identified, they then examined the possibility of enhancing multiple neuronal pathways with multiple injection sites. Conclusively, it was determined using 3D analysis that it is possible to not only image multiple neuronal pathways, but also to image and analyze the overlap between these pathways using statistical connectivity maps. Interestingly, they were also able to study and quantify connectivity strength though the corpus callosum using this same 3D statistical mapping analysis. Conclusively, the final study revealed that using a microosmotic pump to slowly infuse large amounts of manganese (24nmol .25ul/hr compared to the 8nmol .5nl/min)allowed for visualization of brain regions previously unidentified using the initial faster injection. However, there are still experiments being done to mearue teh time course of transport and effective injection site viability. Overall, this was a large descriptive study that examined many factors of direct injections of MnCl2 into the brain, that will be an extremely helpful reference for our future studies. ~JI