The detailed theoretical background to the techniques positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are beyond the scope of this chapter. In brief however, both techniques rely on the assumption that neuronal activity is closely coupled to a local increase in cerebral blood flow (CBF) secondary to an increase in metabolism. PET relies on mapping the distribution of inert, freely diffusible radioactive tracers deposited in tissue as a function of regional perfusion (rCBF). Functional MRI comprises different methods, but the studies described below use blood oxygen level-dependent (BOLD) imaging techniques. During an increase in neuronal activation there is an increase in local CBF, but only a small proportion of the greater amount of oxygen delivered locally to the tissue is used. There is a resultant net increase in the tissue concentration of oxyhaemoglobin and a net reduction in paramagnetic deoxyhaemoglobin in the local capillary bed and draining venules. The magnetic properties of haemoglobin depend on its level of oxygenation so that this change results in an increase in local tissue derived signal intensity on T2*-weighted MR images.
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