IS Case 96: Brain Death
Imaging Sciences URMC 2008
Publication Date: 2009-05-20
A 10-month-old female post-respiratory arrest of uncertain etiology. Cerebral perfusion study is requested for assessment of prognosis.
Sequential flow images from a brain perfusion scan (Fig. 1) at the time of injection demonstrate radiotracer within the right subclavian vein with cardiac circulation into the arterial system; however radiotracer activity does not extend above the level of the aortic arch (arrows). No radiotracer activity is present within the middle and anterior cerebral artery circulation. Coronal (Fig. 2A) and sagittal (Fig. 2B) delayed images (obtained 5-10 minutes following injection) also fail to demonstrate expected tracer within the intracranial venous system. Non-contrast enhanced CT shows global cerebral edema with relative increased cerebellar attenuation (Fig. 3). The findings of absent cerebral perfusion are consistent with a clinical diagnosis of brain death.
Brain death is defined as complete, irreversible cessation of brain function. Cerebral perfusion imaging serves as a diagnostic adjunct, but does not substitute the clinical criteria for establishing brain death.
Patients typically undergo a neurologic consultation with CT and/or MR imaging which may demonstrate diffuse cerebral edema and absent intracranial vascular enhancement. Such findings are suggestive, but not diagnostic. CT and MR provide anatomic information, whereas a Tc-99m-hexamethylpropyleneamineoxime (Tc-99m-HMPAO) or ethyl cysteinate dimer (ECD) cerebral perfusion scan provides a functional assessment, as these agents are lipid soluble and cross the blood-brain barrier into nerve cells when arterial perfusion is present.
Diagnostic findings on cerebral perfusion imaging include no intracranial arterial flow or accumulated tracer within the venous sinuses. Coronal images may show a "hot nose", representing focal radiotracer accumulation within the nasal mucosa, and/or a "light bulb" sign (Fig. 2), representing absent intracranial perfusion. Non-contrast enhanced CT may show diffuse cerebral edema with the attenuation "reversal sign" (Fig. 3) -- the normal appearance of cerebral > cerebellar attenuation is reversed (cerebellar > cerebral hemispheres).
These findings can also be seen in the presence of diffuse cerebral edema from reversible causes including drug toxicity, status epilepticus, and global cerebral infarction; therefore interpretation must occur in conjunction with appropriate clinical correlation.
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