I've read studies where fixatives (eg glutaraldehyde) have been perfused into the brain while it is still inside the skull. I would have thought that this could also be done after the brain is removed, provided all the veins/arteries are connected to a perfusion machine. Is this possible, and are there any major obstacles?
Of course it is possible to perfuse an organ once it has been removed from the body. It is probably unwise to do so with the brain as the delicacy of the structures and high levels of lipids make rapid fixation necessary and difficult. I think the major obstacle is the speed at which the brain could be removed with arteries intact and then perfused as ultrastructural changes occur rapidly once ischemia (oxygen deprivation) begins.
The main obstacle would be connecting all of the arteries to a perfusion machine. The neurovasculature is complex, and begins branching prior to entering the brain, therefore it is easier to do in situ. With a larger brain, the meninges and skull are also important for supporting the brain stucture. If unfixed brain is removed from the skull, it will begin to deform and collapse under its own weight. For fixation, especially, it is much safer to fix in situ and remove post-fixation.