Brain death is defined as a condition wherein there is irreversible cessation of all brain functions. It is of paramount medico legal significance in the context of more widespread organ transplantation programmes. It is mostly a clinical diagnosis; imaging and neurophysiological studies are not mandatory. However, in certain situations, clinical evaluation may be indeterminate or difficult to perform as in ocular trauma. In such cases, imaging has an important role to play.

Pathophysiology of brain death- in cases of severe brain injury in form of extensive SAH, trauma, Stroke, Cardiopulmonary arrest, there occurs increase in intracranial pressure (ICP) due to presence of brain edema. This results in reduction in diastolic flow of brain. When there is further increase in ICP to the extent that it exceeds the diastolic flow, there is complete stoppage of cerebral perfusion, resulting in brain infarction and ultimately brain death. So it is the combination of ischemia and hypoxia which results in brain death.

Clinically, diagnosis of brain death is made in presence of three main findings of coma, apnea and absent brainstem reflexes. 

Ancillary tests can be done which can either detect electrical inactivity of the brain or absent vascular supply. Amongst the former, electroencephalogram and somatosensory evoked potentials are included. EEG is the commoner one used which would depict absence of any electrical activity of brain.

Digital Subtraction Angiography is the gold standard, depicting absent cerebral blood flow. However as this procedure is invasive, requires trained neuroradiologist and DSA suite with substantial expertise, hence is not preferable diagnostic imaging choice. Hence modalities like CT and MRI are preferred as these are quick, non invasive and easy to perform. However, in current practice, CT or MRI are not recommended by American academy of Neurology to form the diagnosis of brain death as it can give fallacious results when there is reduced cerebral flow due to other conditions. However in certain conditions like hypothermia, locked in syndrome, drug intoxication where clinically patient can appear to be brain dead, imaging modalities like CT/MR angiography studies can be very useful.