Activation Procedures

Both PET and SPECT may be used to measure a change in CBF. By injecting only a portion of the HMPAO tracer during one condition (baseline) and injecting another dose during a second condition (e.g., following carbon dioxide inhalation or acetazolamide infusion), the brain images obtained during these two conditions may be compared. The same technique (split-dose injection) may be used in PET scanning. In PET it is more common to utilize a short-lived radionuclide such as 15O so that many separate images of CBF may be obtained during a single scanning session. Each of these CBF maps can be compared, but usually mean CBF images are compared to measure CBF changes from one condition (e.g., rest) to a second condition (e.g., activation). In clinical practice, HMPAO SPECT is often used to distinguish vascular dementia from degenerative dementia (as described previously) or to demonstrate impaired CBF distal to an occluded vessel, as in vasospasm following subar-achnoid hemorrhage. HMPAO SPECT demonstrates CBF changes in response to acetazolamide or carbon dioxide, and impaired reactivity is associated with greater stroke risk. Thus, HMPAO SPECT provides clinically useful information about cerebral physiology and may supplement the information obtained from TCD.

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