This is true, imaging a die can help but it's not enough for full assurance. See e.g. this work on inserting hardware trojans through changing the dopant levels

This is true, imaging a die can help but it's not enough for full assurance. See e.g. this work on inserting hardware trojans through changing the dopant levels on transistors http://sharps.org/wp-content/uploads/BECKER-CHES.pdf http://sharps.org/wp-content/uploads/BECKER-CHES.pdf