Brain death is the determination of human death by showing the irreversible cessation of the clinical functions of the brain. The concept of brain death is based on death being defined as the irreversible loss of functioning of the organism as a whole. The criterion satisfying this definition is the irreversible cessation of functioning of a critical mass of neurons in the cerebral hemispheres, diencephalon, and brain stem. But how many and which neurons are necessary and sufficient to satisfy the criterion of death remains a point of controversy. I propose that the only test that can confidently satisfy the criterion of death must prove the cessation of all intracranial blood flow. Procedures of contrast angiography, radionuclide angiography, and transcranial Doppler ultrasound reliably demonstrate absent intracranial blood flow and document brain death. The demonstration of absent intracranial blood flow by of these tests should become mandatory in brain death determination.

Selected references:
Bernat JL. The biophilosophical basis of whole-brain death. Soc Philos Policy. 2002; 19:324-42.
Bernat JL. How much of the brain must die in brain death? J Clin Ethics. 1992; 3:21-6
Bernat JL. Ethical issues in neurology. Butterwoth-Heineman, Boston, 2002