New advances in imaging and their impact on cognitive neurosciences

摘要

In the last decade, ability to measure and image functional, physiological, and metabolic parameters in the human brain using magnetic resonance (MR) methods has evolved significantly. MR based functional maps in the brain can be generated based on deoxyhemoglobin or perfusion alterations originating from secondary metabolic and hemodynamic responses to increased neuronal activity. However, to date, direct imaging of electrical activity by MR is not feasible. Because the MR based functional methods (referred to as fMRI) rely on the secondary and tertiary events induced by the neuronal activity, spatial specificity of the MR functional maps and the ultimate resolution that can be achieved is determined by several parameters: These include: 1) the nature of the coupling between neuronal activity and the secondary metabolic and hemodynamic responses, 2) coupling of fMRI signal changes to these metabolic and hemodynamic responses through the vasculature, and 3) signal-to-noise ratio (SNR) in fMRI images. Because of the magnetic field dependence of SNR and the deoxyhemoglobin-based blood oxygen level dependent (BOLD) contrast (which is the most commonly used fMRI approach), high magnetic fields have been utilized extensively in our laboratory to achieve improvements in specificity (i.e. accuracy) and resolution in functional maps. As a result, recently such studies have been extended for the first time to 7 Tesla in the human brain and 9.4 Tesla in animal brains. In these efforts, understanding the complex field dependence of BOLD mechanism has been imperative. These ultra-high field studies have already provided unique results regarding human cognitive functions that have not been achievable at lower magnetic fields due to limitations in accuracy of the maps.