Evaluating Brain Activation: A Methodological Perspective

Abstract

Commonly used methods for measuring cerebral blood flow (CBF) stem from the nitrous oxide method. The method developed by Kety and Schmidt [1] first provided a means for measuring CBF and rates of glucose and oxygen consumption in the human brain [2]. The Kety and Schmidt method is based on the Fick Principle. Given a compartment (W) with one entrance and one exit, the relationship among the instantaneous (at any time t) concentration values of a tracer in the in-flowing (Ca) and out-flowing (Cv) liquid (blood), the flow rate of the blood perfusing the compartment, and the concentration of the dye within the compartment (Ci) is expressed by the following equation:

$$ dCi/dt = F/W\left( {Ca - Cv} \right) $$

The equation defines the parameters to be measured in order to calculate flow, expressed per unit of mass of tissue (F/W). The principle is applied to the brain, which satisfies the requirements imposed by the model. Assessments of the blood concentrations of a suitable indicator are performed in samples drawn from the in-coming arterial and out-coming jugular blood. The tissue concentrations of the indicator are inferred from the venous concentrations of a freely diffusible tracer according to the blood-tissue exchange theory [1]. Blood flow values, however, refer to the brain as a whole. The Kety and Schmidt method is ineffective in resolving the heterogeneity of the brain.