Intracerebral Reserve Assessment with SPECT: Reactivity to Acetazolamide and Cerebral Blood Volume Measurement

Abstract

Measurement of cerebrovascular reserve is useful for clinicians in order to investigate the effects of severe carotid lesions on intracerebral circulation. It is postulated that patients with impairment of intracerebral reserve represent a group with high risk of stroke and that in these cases carotid endarterectomy might be of interest. The measurement of cerebral blood flow (CBF), cerebral blood volume (CBV), and oxygen extraction ratio (OER) are the basis of hemodynamic and metabolic studies with positron emission tomography (PET) [5,9]. The routine application of these measurements is limited by the scarcity and expense of PET facilities. A fall in perfusion pressure is followed by an increase in CBV and a subsequent decline in CBF when the limit of autoregula-tion is reached. Consequently, the relationship between the two physiological parameters expressed by the CBF/CBV ratio is a sensitive index of hemodynamic reserve. Until now the direct and quantitative assessment of hemodynamic compensatory responses to a fall in perfusion pressure has been performed only with PET [5,9]. Intracerebral vascular reserve has been already studied with single photon emission computerized tomography (SPECT) but it was indirect and nonquantitative. It is assumed that patients with impaired hemodynamic reserve have a reduced capacity to increase CBF in response to a vasodilatatory agent (CO₂ or acetazolamide). Thus, reactivity of CBF to CO₂ and acetazolamide indirectly reflects cerebral vascular reserve.