Global intravascular and local hyperoxia contrast phase-based blood oxygenation measurements

The measurement of venous cerebral blood oxygenation (Y(v)) has potential applications in the study of patient groups where oxygen extraction and/or metabolism are compromised. It is also useful for fMRI studies to assess the stimulus-induced changes in Y(v), particularly since basal Y(v) partially accounts for inter-subject variation in the haemodynamic response to a stimulus. A range of MRI-based methods of measuring Y(v) have been developed recently. Here, we use a method based on the change in phase in the MR image arising from the field perturbation caused by deoxygenated haemoglobin in veins. We build on the existing phase based approach (Method I), where Y(v) is measured in a large vein (such as the superior sagittal sinus) based on the field shift inside the vein with assumptions as to the vein's shape and orientation. We demonstrate two novel modifications which address limitations of this method. The first modification (Method II), maps the actual form of the vein, rather than assume a given shape and orientation. The second modification (Method III) uses the intra and perivascular phase change in response to a known change in Y(v) on hyperoxia to measure normoxic Y(v) in smaller veins. Method III can be applied to veins whose shape, size and orientation are not accurately known, thus allowing more localised measures of venous oxygenation. Results demonstrate that the use of an overly fine spatial filter caused an overestimation in Y(v) for Method I, whilst the measurement of Y(v) using Method II was less sensitive to this bias, giving Y(v) = 0.62 ± 0.03. Method III was applied to mapping of Y(v) in local veins across the brain, yielding a distribution of values with a mode of Yv = 0.661 ± 0.008.