Technetium-99m HM-PAO (Hexamethylpropyleneamineoxime) is a lipophilic radiopharmaceutical that is able to cross an intact blood-brain barrier. Its activity is proportional to regional perfusion.
Tc-99m HM-PAO is injected as a bolus intravenously and then a single photon emission computed tomography (SPECT) scan is performed at a later time.
If no uptake is seen (the skull appears "hollow") then the patient is brain dead.
Any uptake (may be limited to the brainstem) indicates that the patient cannot be declared brain dead by this method.
Recommendations of Larar and Nagel to optimize its use:
(1) Delay testing as long as possible; it should be done only after there is unequivocal brain death by clinical criteria.
(2) Use the minimal dose for adequate imaging (< 10 mCi). This allows for a followup study if the results are equivocal.
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