Estimating blood oxygen saturation through susceptibility source separation: A new standpoint on quantitative BOLD models

MR methods have been proposed to estimate blood oxygen saturation in cerebral tissues by exploiting the magnetic susceptibility difference between oxy and deoxyhemoglobin. These models neglect the contribution of extravascular susceptibility sources leading to inaccurate measurements. Recent approaches combined quantitative BOLD and QSM methods, but these models neglected the impact of the geometrical arrangements of extravascular sources on signal decay. Consequently, we propose a new model to unravel the magnitude and phase of MRI gradient echo sequence signal contributions of intravascular and extravascular compartments to produce blood-oxygen saturation and non-vascular susceptibility maps. This new model and previous MRI approaches are implemented in rat data acquired in healthy and stroke settings. The results are compared across anatomical regions of interest characterized by different extravascular magnetic susceptibilities. Results obtained with the new model improved estimations in myelin-rich white matter regions and delineated oxygen saturation decrease in stroke lesions after 60 min of middle cerebral artery occlusion. Characterization of extravascular susceptibility sources improved the overall results, highlighting its critical role in the translation toward pathological applications.