Brain Death: Diagnosis and Imaging Techniques

Brain death (BD) is an irreversible cessation of functions of the entire brain, including the brainstem. The diagnosis of BD is made on clinical grounds and neurologic examination. In the United States, clinical criteria set by the American Academy of Neurology (AAN) emphasize 3 specific clinical findings to confirm BD, which include coma, absence of brainstem reflexes and apnea. Ancillary tests are needed when neurologic examination or apnea test cannot be performed. AAN recommended ancillary tests include electroencephalogram, which confirms electrical activity loss; catheter cerebral angiogram, which confirms loss of cerebral blood flow; as well as transcranial Doppler and nuclear scintigraphy. Digital subtraction angiography remains the gold standard for confirmation of lack of cerebral blood flow. On 99m Techentium hexa methyl propylene amine oxime or 99mTechnetium-ethylene cysteine diethyl ester (99mTc-ethylene cysteine diethyl ester) Nuclear scintigraphy, lack of intracranial radiotracer uptake, correlates with BD. Although imaging studies like computed tomography angiogram (CTA), MR angiogram, CT perfusion, and MR perfusion are frequently used, they are currently not recommended by AAN. However, they hold tremendous promise in future as imaging tools in the armamentarium of a radiologist investigating BD as adjunct imaging to clinical findings. Imaging markers for BD on CTA include nonopacification of the cortical middle cerebral arteries and internal cerebral veins. On CT perfusion, there is lack of cerebral blood flow and blood volume in brainstem. Residual brain perfusion can occur with reduced intracranial pressure as in decompressive craniectomy, ventricular drainage and multiple skull fractures leading to false-negative results. On magnetic resonance imaging, there can be massive brain edema with herniations, poor gray or white matter differentiation, diffuse diffusion restriction, and nonvisualization of intracranial vessels on MR angiogram. On transcranial Doppler, cerebral circulatory arrest is indicated by flow patterns without forward flow progress, progressing from decrease in diastolic flow to disappearance of diastolic flow to oscillating pattern with retrograde flow in diastole, short systolic spikes, and finally absence of Doppler signal. AAN has included neuroimaging explaining coma as one of their prerequisite to be checked before evaluation for BD. Thus, a radiologist can play a critical role by recognizing the initial extensive hypoxic or ischemic damage to the central nervous system including the brainstem on imaging; guiding a neurologist evaluating a potential BD, as well as ruling out other pitfalls. In many cases, the radiologist is often the first person to appreciate the devastating findings of irreversible brain damage. Three most common mimics of BD are hypothermia, locked-in syndrome, and drug intoxication. By judicious usage of the available ancillary tests, cautiously interpreting the findings with awareness of their limitations and pitfalls, a radiologist can provide the support needed to confirm BD.