Quantification of Oxygen Saturation of Venous Vessels Using Susceptibility Mapping

The regulation of oxygen supply and consumption by the brain is a complex and dynamic process. The determination of the oxygen saturation of venous blood is an indirect means to assess local tissue oxygen saturation. Quantitatively measuring oxygen saturation is important to characterize the physiological or pathological state of tissue function. Noninvasive and reliable measurements can help to better understand the changes in cerebral hemodynamics due to neuronal activation or to improve the characterization and monitoring of treatment of cerebral pathologies, such as stroke, multiple sclerosis (MS) or tumors. There are several methods available to quantify blood or tissue oxygenation. Most methods are invasive requiring the insertion of a catheter into the jugular vein or using radioactive isotopes, such as positron emission tomography (PET). Near-infrared spectroscopy (NIRS), a noninvasive method, can only access surface cortical structures of the brain due to the limited penetration of light into the tissue. MRI has the potential to estimate the blood oxygen saturation level because of the difference of the magnetic properties of oxygenated and deoxygenated blood. Deoxygenated blood in veins is less diamagnetic than oxygenated blood, and relative to the surrounding tissue it appears to be paramagnetic, which makes it possible to detect venous oxygen saturation levels using either susceptibility weighted MRI or susceptibility mapping. In this thesis, we demonstrate the possibility of using susceptibility mapping to noninvasively estimate the venous blood oxygen saturation level. Accurate susceptibility quantification is the key to oxygen saturation quantification. Two approaches are presented in this thesis to generate accurate and artifact free susceptibility maps (SM): a regularized inverse filter and a k-space iterative method. Using the regularized inverse filter, with sufficient resolution, major veins in the brain can be visualized. The usual geometry dependent phase dipole effects can be removed leaving basically images of the veins. We found that different sized vessels show a different level of contrast depending on their partial volume effects; smaller vessels show smaller values due to errors in the methodology and due to partial volume effects; larger vessels show a bias toward a PhD Thesis – Jin Tang McMaster – School of Biomedical Engineering-iv-reduced susceptibility approaching 90% of the expected value. Also, streaking artifacts associated with high susceptibility structures such as veins are obvious in the reconstructed susceptibility map. To further improve susceptibility quantification and reduce the streaking artifacts in the susceptibility maps, we proposed a threshold-based k-space/image domain iterative approach that …