Brain dead diagnosis is not very scientific based.
Not really. And let me explain.
You can see if there is electrical activity signals , you can do a fMRI scan (shows blood usage which indicates changes in cognitive processing. Most brains show patterns of blood use when doing tasks).
But. When you are unconscious, these always show low activity.
It's not really that much different than someone being under anesthesia. They show the same pattern of brain activity as someone in a coma.
Now sometimes there is specific abnormalities for an injured region. But this just tells you that part of the brain is damaged. It doesn't tell us if the person will regain consciousness
Awareness (consciousness in this definition) is controlled by a specific area in the mid brain (just north of the brain stem) called the renticular activating system.
Damage to this system is considered more conclusive brain death but that's rare because of where it is. But it can still happen.
Actually interesting fact, anesthesia is used to study locked-in and "brain dead," patients. Lots of ongoing research.
Anywho. There is a lot of controversy about what counts as brain dead. And how to distinguish , objectively with tests, from locked-in syndrome. Which is like being aware but being 100% paralyzed.
But one way you can be 100 % sure someone is brain dead is you keep them alive for 6 months to a year. And you compare their white matter scans from a MRI from start to that time.
For those who won't be coming back, they will show pretty fast white matter loss generally everywhere.
Because the brain works like a muscle. Use it or lose it. And a loss that fast means nothing , literally no cognitive or perception processing, is occurring.
But as you can imagine there are ethical issues for that too and , financial ones.
Often if the patient shows no changes in electrical activity and no awareness for a few weeks or so, the general conclusion is they won't be recovering.
Not exactly a reliable conclusion.