They're much, much too small for MRI[0], but we do know a crazy amount about the structure of C. elegans: there's a complete wiring diagram of all the physical

They're much, much too small for MRI[0], but we do know a crazy amount about the structure of C. elegans: there's a complete wiring diagram of all the physical connections between its 302 neurons, a fate map of the embryonic origin and final role of every single cell, and so on. [0] Each voxel (='volume element') in a "standard" MRI is cube 0.5-1mm on a side. A C. elegans is about a millimeter long, but only 100 µm wide, so you could fit about a hundred into one voxel. The resulting scan therefore wouldn't tell you much spatially, though I suppose you could do spectroscopy if you wanted to know what a worm was made of.