MAGNETIC RESONANCE IMAGING OF HUMAN BRAIN ACTIVATION USING ENDOGENOUS SUSCEPTIBILITY CONTRAST by

Local hemodynamic changes that accompany changes in brain activity cause local magnetic resonance imaging (MRI) signal changes. Functional MRI (fMRI) is the name used for the group of MRI techniques used to detect these changes to map human brain activation. In this dissertation, eight studies were carried out which included the dissection of activation–induced MRI signal change components and characterization of their correlation with the magnitude, timing, and extent of neuronal activation and subsequent hemodynamic events; post processing method development; and fMRI applications. Study one involved the characterization of activation–induced signal change locations, dynamics, and magnitudes using echo–planar imaging (EPI) sequences with combinations of different hemodynamic sensitizations. Study two involved the development and implementation of correlation and Fourier analysis for robust brain activation image creation. Study three involved the development and use of a susceptibility contrast model that incorporated repeated smoothing function convolution and phase rotation to simulate spin diffusion in the presence of magnetic field perturbers. Simulated parameters included diffusion coefficient, vessel radii and orientation, blood volume, field strength, echo time, pulse sequence, and blood oxygenation. Study four involved the assessment of relative susceptibility–related and non–susceptibility–related changes by the use of either separate or combined spin–echo and gradient–echo EPI sequences along with systematic echo–time incrementation. Study five involved the measurement of the dependence of activation–induced transverse relaxation rate changes and functional contrast to noise ratios on field strength (0.5T, 1.5T, and 3T). Study six involved characterization of the dependence of the magnitude of activation–induced MRI signal changes on changes in voxel volume, outer volume saturation, TR, and flip angle. Study seven involved the measurement of the effects that respiratory stresses (hypoxia and hypercapnia) have on resting MRI signal and activation–induced MRI signal changes. A normalization method was also developed to enhance functional resolution and interpretability. Study eight involved fMRI applications. Regions associated with simple, complex, and imagined complex finger tapping tasks were mapped in the entire human brain. Also, brain regions associated with hearing, reading, and generating words were mapped.