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 3.2 (2013).
Previous SAQs involving radiological diagnosis of brain death include the following:
- 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:
- 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.
- 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 aceptable" in their answer to Question 15 from the first paper of 2016, but in fact it is not. The argument againt CT angiography is 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.
- There is potential risk of false positives, and so this is not recommended.
- Inaccurate and operator-dependent, but can be used for screening.