Oxidative metabolism and the detection of neuronal activation via imaging

Recent years have witnessed a great growth of interest in non-invasive imag ing methods, such as functional magnetic resonance imaging (fMRI) and posit ron emission tomography (PET), permitting identification of brain structure s that mediate specific cognitive and behavioural tasks in humans. Because these techniques use physiological responses such as increased perfusion or metabolism as surrogate indicators of evoked neuronal electrical activity, understanding the role of these processes in sustaining the information pr ocessing function of neurons is vital to the proper interpretation Of funct ional neuroimaging data. An ultimate goal of these non-invasive techniques is to approach the sensitivity and spatial resolution of earlier autoradiog raphic methods, which have repeatedly demonstrated exquisitely detailed del ineations of neuronal response patterns using metabolic glucose uptake as a physiological tag. Although glucose is generally metabolized ill conjuncti on with oxygen, technical challenges in imaging tissue oxygen Consumption i n vivo have limited the use of this complementary process in the detection of neuronal activation. In this article we review concepts linking cerebral blood flow and metabolism to neuronal activation, and compare functional i maging techniques that exploit these relationships. We also describe recent ly introduced MRI based methods for measurement of oxygen consumption and a ssess the relative contributions of different metabolic pathways during neu ronal activation. Our calculations suggest that the bulk of the energy dema nd evoked during stimulation of neurons in visual cortex is met through Oxi dative metabolism of glucose, supporting the use of oxygen uptake as a mark er for increased neuronal electrical activity. (C) 2001 Elsevier Science B. V. All rights reserved.