Brain death refers to the irreversible end of all brain activity and is usually assessed clinically. Radiographic testing may be used as additional support for a clinical diagnosis of brain death, such as when clinical tests are impossible to perform, e.g. ocular trauma, precluding brainstem function assessment.
As the diagnosis of brain death is considered equivalent with cardiac death in many jurisdictions , and it allows organ donation for transplantation or withdrawal of life support, most countries have specific and varied related legal standards and practice guidelines .
Most imaging tests for brain death rely on the absence of cerebral blood flow as a surrogate for brain death. This can be assessed by a number of modalities including CT, MRI, ultrasound, nuclear medicine examinations, and catheter angiography.
It is important to note that not all modalities and examinations are approved for the legal determination of brain death and that this will vary from country to country.
While brain death is ultimately a clinical diagnosis, transcranial Doppler sonography (TCD) may be used as an ancillary modality to provide further clinical support by detecting the presence of cerebral circulatory arrest, features of which include:
- reverberant or oscillating flow in the MCA vessels has been reported
- loss or reversal of diastolic flow in the MCA/ICA
- low acceleration time in the MCA
- on transcranial Doppler
- diffuse cerebral edema with effacement of the grey-white matter borders
- CT angiography: non-opacification of the cortical middle cerebral arteries and internal cerebral veins are the most sensitive and specific markers on CTA
- considered gold standard imaging test
- no forward flow above the terminal internal carotid arteries
- T1: hypointense, with lost grey-white matter differentiation
- swollen gyri with hyperintense cortex
- expected flow void may be absent in vessels
- DWI: hemispheric high signal, severe ADC drop
- MR angiography: may be interpreted similarly to CT angiography, with non-visualization of the intracranial vessels
- BOLD: absence of long-range functional connectivity
- cerebral perfusion