Since its inception in 1973, magnetic resonance imaging (MRI) has been a revolutionary development in its scope and utility. The range of parameters that may be mapped using nuclear magnetic resonance has continued to increase and currently spans such phenomena as proton density measurement, nuclear magnetic relaxation times T1 (longitudinal relaxation time) and T2 (transverse relaxation time), flow in large vessels, diffusion, perfusion, temperature, blood volume, and blood oxygenation. All of the above-mentioned parameters have applications of clinical relevance, and many of them are in routine use. Clinical MRI is likely to move on from its current role simply as a structural technique for visualizing pathology, since researchers have now developed methods used to measure dynamic or functional aspects of human physiology. One of the significant applications of dynamic/functional MRI is in the visualization of localized neuronal activity, inferred through its physiological correlates, such as accompanying changes in cerebral blood volume, cerebral blood perfusion, and cerebral blood oxygenation.
The investigation of neuronal activity via fMRI is based on three main principles: (1) blood volume imaging techniques, (2) blood flow imaging techniques, and (3) blood oxygenation imaging techniques—the last of these being the most commonly used.
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