There is a physical limit to what a scanner can do.  In standard
photography, your resolution is limited by the grain size of your film
emulsion.  But in scanning, resolution is limited by the dpi (dots per inch)
capability of the scanner, which is a much coarser matrix than a film
emulsion.  A 1-mm object is only 1/25 of an inch long, so an image of such
an object scanned at 1200 dpi is made up of less than 50 dots linearly, and
you can't get any usable resolution working with so few dots.

Paul M.