"When" is death?

As our understanding of medical science is expands and grows, doctors are coming to a question that most would think would be very simple to answer. When is someone considered dead? What was once as simple as checking for a pulse isint so simple anymore. A doctor recently injected cats with the substance that thay use to kill death row inmates. Using an MRI he atempted to pinpoint the moment of death of the brain. He saw decomposition, but couldent find when the brain "died". With all the technology we have today, we still cant tell when death is truely "irraversable" or weather that even has any meaning at this point, when interviewed, the doctor said “I’m not sure MRI can prove that someone who is dead (or a mummy) won’t come back to life. As a scientist, you simply have to say such events are extraordinarily rare. As a believer, you can say whatever you’d like; I’m a believer, so I do believe that people will live again … but I won’t try to use MRI to convince you of that position.”