Methodologies, practicalities and pitfalls in functional MR imaging
© Cambridge University Press 2010. Introduction As this book attests, MRI is a hugely versatile technique that is able to provide a wealth of structural, physiological, metabolic, biochemical, and biophysical information. In this sense, much of the information that MRI provides can be viewed as being "functional." Nevertheless, over the past (almost) two decades, the term functional MRI (fMRI) has come to imply the imaging of neuronal activity, also known as brain mapping. Within this narrowed definition of what is meant by fMRI a further implicit assumption is often that the phenomenon of blood oxygenation level dependent (BOLD) contrast is the imaging approach that is used. Certainly, images that are weighted by BOLD contrast, in which regional blood hyperoxygenation associated with local neuronal activity leads to a subtle signal increase in T 2-weighted and T 2*-weighted images, are the most prevalent type of acquisition in studies performed to date. The BOLD approach is a rather complex contrast mechanism that when used in isolation has the potential to yield misleading conclusions in patient groups. Therefore, the scope of "functional MRI" that will be assumed in this chapter will be the intermediate definition in which the imaging of neuronal activity or cerebral metabolism is assumed to be the goal, but where the MRI tools available to achieve this are not restricted to the BOLD contrast mechanism.