Radiological testing for brain death

Radiological confirmation of brain death is required when the preconditions for clinical brain death testing cannot be met. The objective of these tests is to demonstrate an absence of blood flow above the foramen magnum. The college love to ask candidates about the preconditions of clinical testing, as well as the specific radiological modalities available. Again, the ANZIC statement on Brain Death and Organ Donation is my primary resource for this summary.  At the time of writing, the recent edition is Version 4.1 (2021).

Previous SAQs involving the radiological diagnosis of brain death include the following:

• Question 14 from the second paper of 2018
• Question 15 from the first paper of 2016
• Question 12.2 from the second paper of 2010
• Question 28 from the first paper of 2009
• Question 20 from the second paper of 2007

Indications for radiological diagnosis of brain death

Broadly speaking, these are almost the same as the contraindicatiosn to clinical testing.

• Inability to perform clinical brain death testing
  • No access to at least one eye and one ear
  • Severe hypoxia
  • Haemodynamic instability
  • High spinal cord injury
• Presence of persisting confounding factors which invalidate the clinical testing for brain death (eg. severe metabolic disturbances, organ system dysfunction)
• Absence of a clear cause for the coma, which - for the puspose of brain death testing - is defined as "sufficient intracranial pathology" to explain brain death.

Four-vessel Intra-Arterial Angiography

• Gold standard; injection of contrast into both carotids and both vertebrals
• Blood flow should not be demonstrated above the level of the carotid 
  siphon in the anterior circulation, or above the foramen magnum in the posterior circulation

Nuclear Medicine Scans

• This  refers specifically to the Tc-99 HMPAO SPECT scan, which (after the four-vessel DSA) is viewed by the ANZICS Statement on Death and Organ Donation as the second best way of confirming that there is no blood flow to the noggin.
• Tc-99 HMPAO compound gets absorbed by the brain; its lipophilic, and when it crosses the blood-brain barrier, it becomes hydrophilic, and is thus retained.
• Tc-99m pertechnetate is not a useful radionuclide, as it does not cross the blood-brain barrier.
• SPECT is the modality of choice (single photon emission computerized tomography).

The indications for choosing a radionuclide scan, rather than a DSA, may be as follows:

• Allergy to IV contrast
• Injury to the carotids or vertebral arteries, which precluded fluoroscopic access

Advantages of the HMPAO-SPECT:

• Equivalent to DSA in terms of false positive rate (0%)
• Does not require the precodnitions for brain death to be met
• Rapid return of results
• Safe non-toxic contrast agent
• Visually effective representation of absent brain perfusion

Disadvantages of HMPAO-SPECT:

• Expensive
• Requires specialised equipment
• Not available in all but the best-equipped centers
• Portable gamma-cameras are infrequently available, exposing the patient to the risk of transport.
• In very young infants with open cranial sutures, HMPAO-SPECT may return false positive results if used in a single imaging plane.
• In adults, it may return false positives due to minimal flow in meningeal vessels.

Other imaging

CT angiography

• There must be contrast enhancement of the external carotid arteries, but not of the peripheral intracranial arteries and central veins
• There are no false positives, but there may be many false negatives.
• The college says CT angio "may be acceptable" in their answer to  Question 15 from the first paper of 2016, but in fact it is now completely accepted, as long as the results arescored according to the four-point scale which consists of "absent enhancement of both middle cerebral artery (MCA) cortical branches (i.e. beyond the Sylvian branches); andabsent enhancement of both internal cerebral veins"
• The argument againt CT angiography was the gated timing of the scan. Whereas DSA watches contrast wash in over time in a series of still images, the CT offers a momentary snapshot of that contrast in the vessels. One might argue that the CT was mis-timed, and the contrast did not yet have time to move into the brain (in this way, perfectly healthy people could be made to appear brain-dead by adjusting the timing of the scan). Obviously, when one is going to declare brain death, there needs to be no argument about the validity of the confirmatory test. The 2014 Cochrane review (Taylor et al) could not support the use of CTA at that stage.

MRI

• There is potential risk of false positives, and so this is not recommended.

Transcranial Doppler

• Inaccurate and operator-dependent, but can be used for screening.