Large vein blood oxygenation level can be estimated by magnetic susceptibility-based oximetry. Calculation of the local field shift between intravascular blood and surrounding tissue using measurement of signal phase is done with field mapping. Measurement is confounded by background static field inhomogeneity. Correction by either high-pass filtering to remove spatially slow-varying inhomogeneity, or polynomial fitting of inhomogeneity to be subtracted-out require measuring reference tissue phase, imparting bias and constraining signal-to-noise ratio considerations to tissue, not lumen. An objective approach based on inhomogeneity estimation using a smoothness criterion is proposed. Tissue phase is eliminated, allowing measurement of intravascular phase alone.
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