Functional magnetic resonance imaging: imaging techniques and contrast mechanisms

Functional magnetic resonance imaging (fMRI) is a widely used technique for generating images or maps of human brain activity. The applications of the technique are widespread in cognitive neuroscience and it is hoped they wi ll eventually extend into clinical practice. The activation signal measured with fMRI is predicated on indirectly measuring changes in the concentrati on of deoxyhaemoglobin which arise from an increase in blood oxygenation in the vicinity of neuronal firing. The exact mechanisms of this blood oxygen ation level dependent (BOLD) contrast are highly complex. The signal measur ed is dependent on both the underlying physiological events and the imaging physics. BOLD contrast, although sensitive, is not a quantifiable measure of neuronal activity A number of different imaging techniques and parameter s can be used for fMRI, the choice of which depends on the particular requi rements of each functional imaging experiment. The high-speed MRI technique , echo-planar imaging provides the basis for most fMRI experiments. The pro blems inherent to this method and the ways in which these may be overcome a re particularly important in the move towards performing functional studies on higher field MRI systems. Future developments in techniques and hardwar e are also likely to enhance the measurement of brain activity using MRI.