Brain death imaging

Abstract

Brain death is defined as the irreversible cessation of functioning of the entire brain, including the brain stem. Brain death is principally established using clinical criteria including coma, absence of brain stem reflexes, and loss of central drive to breathe, assessed with apnea test. In situations in which clinical testing cannot be performed or when uncertainty exists about the reliability of its parts due to confounding conditions, ancillary tests (i.e., imaging studies) may be useful. The objective of ancillary tests in the diagnosis of brain death is to demonstrate the absence of cerebral electrical activity (EEG and evoked potentials) or cerebral circulatory arrest. In clinical practice catheter cerebral angiography, perfusion scintigraphy, transcranial Doppler sonography, CT angiography, and MR angiography are used. Other methods, like perfusion CT, xenon CT, MR spectroscopy, diffusion-weighted MRI, and functional MRI, are being studied as potentially useful in the diagnosis of brain death. Among these tests, blood flow and perfusion studies are most commonly applied. The diagnosis of brain death can be confirmed when a lack of blood flow or tissue perfusion in the whole brain is documented. However, the application of blood flow and perfusion tests to the diagnosis of brain death is limited by variable vulnerability of individual brain structures to ischemia. Besides there are known factors predisposing to dissociation between brain death and total cessation of cerebral circulation. These factors include skull decompression, infratentorial localization, and hypoxic-ischemic mechanism of brain injury.