Role of PET and SPECT in the assessment of ischemic cerebrovascular disease.

Functional neuroimaging techniques such as positron and single-photon emission computed tomography (PET and SPECT) have contributed to our knowledge of pathophysiological changes in ischemic stroke. Determinations of cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO2) permit the discrimination of various compensatory mechanisms in occlusive vascular disease, where changes in the CBF/CBV ratio indicate a perfusional reserve and increases in the oxygen extraction fraction (OEF), a metabolic reserve, that prevent ischemic tissue damage during graded flow decreases. Early in the course of acute ischemia, CBF and CMRO2 below a certain threshold (approximately 12 ml/100 g/min and/or 65 mumol/100 g/min, respectively) indicate irreversible tissue damage, while preservation of CMRO2 with decreased flow resulting in increased OEF ("misery perfusion") suggests still viable tissue up to 48 h after the attack, which, however, turns into necrosis in most instances during the following period. In a few instances such tissues can survive, suggesting a potential for effective therapy. Transient ischemic attacks are caused by less severe regional flow disturbances and the consequent metabolic changes are not so significant. In these cases the impact of obstructive vascular changes on hemodynamic reserve can be evaluated by functional tests applying CO2 or acetazolamide. While the regional cerebral metabolic rate of glucose (rCMRglu) in early ischemia is often not coupled to flow or CMRO2 and might even be increased (nonoxidative glycolysis with consequent tissue lactacidosis), this variable is the best indicator of permanent impairment of tissue function. (ABSTRACT TRUNCATED AT 250 WORDS)