Small volume of mouse cortex is already successfully scanned at 3x3x20nm voxel resolution, with smallest synaptic details being visible http://www.cell.com/abstract/S0092-8674(15)00824-7
The tool is called ATLUM. The process could be scaled up.
Yeah, it's fundamentally not "this is impossible" but "this is very hard and expensive", and very hard and expensive tasks have a remarkable way of becoming cheaper and easier over the long term.