Immediately following a stimulus, the BOLD signal and the oxyhemoglobin concentration have been shown in some studies to decrease briefly, often called the вЂњinitial dip,вЂќ which is thought to reflect an increase in oxygen metabolism and oxygen extraction before blood flow increases to the area ( Malonek and Grinvald, 1996 Yacoub and Hu, 2001).

An HRF models the typical change in BOLD signal over time following a stimulus or an epileptiform discharge measured with electroencephalography (EEG). The temporal evolution of BOLD signal changes is in the form of a characteristic curve called the hemodynamic response function (HRF) ( Fig. Currently, it is believed that the magnitude of the BOLD signal reflects more the synaptic input and intracortical processing of a given volume (i.e., its total/overall synaptic activity) than the spiking output of the neurons in that area ( Logothetis et al., 2001), and hence the increase in synaptic activity accompanying epileptiform activity is believed to give rise to a BOLD signal. These events reduce local magnetic field inhomogeneities, and ultimately increase MR image voxel (a three-dimensional volume element) intensity when the brain is imaged using specific MR sequences (e.g., gradient echo imaging). When an area of the brain is activated, there is a regional increase in neuronal activity that results in an increase in cerebral blood flow and cerebral blood volume which exceeds the rate of oxygen extraction from the blood, resulting in a decrease in the local deoxyhemoglobin concentration. Oxyhemoglobin is diamagnetic (nonmagnetic), whereas deoxyhemoglobin is paramagnetic, creating minor distortions in the local magnetic field that reduce the MR signal from the vessel and immediately adjacent tissues. It utilizes differences in the paramagnetic properties of oxyhemoglobin and deoxyhemoglobin, known as blood oxygen level-dependent (BOLD) contrast, to generate images of cerebral activity ( Ogawa and Lee, 1990). Resting-state fMRI represents an important paradigm shift, focusing attention on functional connectivity within intrinsic cognitive networks.ДЇunctional magnetic resonance imaging (fMRI) allows the exploration of the local vascular response, and indirectly the coupled neural response to particular stimuli. Finally, we highlight several basic principles of resting-state fMRI and its emerging translational clinical applications. Integration of BOLD fMRI and diffusion tensor white-matter tractography provides a road map for presurgical planning and intraoperative navigation that helps to maximize the extent of lesion resection while minimizing the risk of postoperative neurologic deficits. Second, we illustrate the clinical application of task-based fMRI to presurgical motor, language, and memory mapping in patients with lesions near eloquent brain areas. These principles inform a nuanced interpretation of the BOLD fMRI signal, along with its neurophysiologic significance and pitfalls. First, we review key physiologic, biophysical, and methodologic principles that underlie BOLD fMRI, regardless of its particular area of application. In the clinical realm, it has become an established tool for presurgical functional brain mapping. Since its inception in 1991, blood oxygen level-dependent (BOLD) fMRI has rapidly become a vital methodology in basic and applied neuroscience research. Buchbinder, in Handbook of Clinical Neurology, 2016 AbstractДЇunctional magnetic resonance imaging (fMRI) maps the spatiotemporal distribution of neural activity in the brain under varying cognitive conditions.

Functional mri