Improving qBOLD based measures of oxygen extraction fraction using hyperoxia-BOLD derived measures of blood volume

Streamlined-qBOLD (sqBOLD) is a recently proposed refinement of the quantitative BOLD (qBOLD) technique capable of producing non-invasive and quantitative maps of oxygen extraction fraction (OEF) in a clinically feasible scan time. However, sqBOLD measurements of OEF have been reported as being systematically lower than expected in healthy brain. Since the qBOLD framework infers OEF from the ratio of the reversible transverse relaxation rate (R2’) and deoxygenated blood volume (DBV), this underestimation of OEF has been largely attributed to an overestimation of DBV made using this technique.This study proposes a novel method, hyperoxia-constrained qBOLD (hqBOLD), to improve sqBOLD estimates of OEF. This method circumvents difficulties associated with inferring DBV from the qBOLD model by replacing it with a separate measurement of blood volume derived from hyperoxia-BOLD contrast. In a group of ten healthy volunteers, hqBOLD produced measurements of OEF in cortical grey matter (OEFhqBOLD = 44.7 ± 11.9 %) that were in better agreement with global oximetry measures (OEFTRUST = 40.4 ± 7.7 %), compared to sqBOLD derived measures (OEFsqBOLD = 13.1 ± 4.0 %).However, in the same group hqBOLD measures of OEF were found to be outside the physiological range in white matter regions (> 100%). By deriving maps of simulated R2’ from TRUST and hyperoxia-BOLD imaging data, the hqBOLD overestimation of OEF in white matter was hypothesised to originate from additional sources of magnetic susceptibility beyond deoxyhaemoglobin that are present in white matter.