NUCLEAR-MAGNETIC-RESONANCE IMAGING AND SPECTROSCOPY OF HUMAN BRAIN-FUNCTION

The techniques of in vivo magnetic resonance (MR) imaging and spectros copy have been established over the past two decades. Recent applicati ons of these methods to study human brain function have become a rapid ly growing area of research. The development of methods using standard MR contrast agents within the cerebral vasculature has allowed measur ements of regional cerebral blood volume (rCBV), which are activity de pendent. Subsequent investigations linked the MR relaxation properties of brain tissue to blood oxygenation levels which are also modulated by consumption and blood now (rCBF). These methods have allowed mappin g of brain activity in human visual and motor cortex as well as in are as of the frontal lobe involved in language. The methods have high eno ugh spatial and temporal sensitivity to be used in individual subjects . MR spectroscopy of proton and carbon-13 nuclei has been used to meas ure rates of glucose transport and metabolism in the human brain. The steady-state measurements of brain glucose concentrations can be used to monitor the glycolytic flux, whereas subsequent glucose metabolism- i.e., the flux into the cerebral glutamate pool-can be used to measure tricarboxylic acid cycle flux. Under visual stimulation the concentra tion of lactate in the visual cortex has been shown to increase by MR spectroscopy. This increase is compatible with an increase of anaerobi c glycolysis under these conditions as earlier proposed from positron emission tomography studies. It is shown how MR spectroscopy can exten d this understanding of brain metabolism.