| The overall goal of my thesis work is to improve the understanding of the mechanisms of nuclear magnetic resonance (NMR) signal changes in the brain during activation. The NMR techniques, magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS), are extremely versatile methods for studying the structure and function of biological materials. Since its inception in the early 1970s, MRI has become a major diagnostic tool of radiology, providing a noninvasive means of visualizing the detailed anatomy of the human body. In addition to providing anatomical and structural information, MRI methods can be used to probe various physiological processes, such as water diffusion, oxygen consumption, and blood flow and blood volume alterations. Over the last decade, one such method known as functional magnetic resonance imaging (fMRI), has assumed a major role in investigation of human brain activity. Because of its noninvasiveness, and high spatial and temporal resolution, fMRI has been routinely used to map motor, sensory and cognitive functions of the brain.; During neuronal activation, fMRI signal changes reflect changes in cerebral blood flow (CBF), and volume (CBV), and oxygen consumption. All of these physiological alterations are proportional to changes in deoxygenated hemoglobin concentration, which is the basis of the Blood Oxygenation Level Dependent (BOLD) contrast in fMRI. It follows that oxygen saturation alters the magnetic state of hemoglobin in red blood cells, such that oxygenated and deoxygenated hemoglobin are diamagnetic and paramagnetic, respectively. The presence of deoxygenated Hb induces susceptibility based field gradients in and around blood vessels, leading to more efficient relaxation of water protons. During neuronal activation, the transverse relaxation of water protons is affected by changes in magnetic susceptibility, such that deoxygenated hemoglobin acts as an endogenous contrast agent for transverse relaxation time T2 and T2*-weighted MR images. This BOLD contrast mechanism has been applied to a variety of perfused tissues, such as the heart, tumors, and in the brain during hypoxia and after vascular occlusion. Recently, there has been great interest in the use of BOLD-fMRI to quantify hemodynamic responses associated with neuronal activation. (Abstract shortened by UMI.)... |
