Measurement of parenchymal extravascular R₂* and tissue oxygen extraction fraction using multi-echo vascular space occupancy MRI at 7T

Parenchymal extravascular R₂* is an important parameter for quantitative blood oxygenation level-dependent (BOLD) studies. Total and intravascular R₂* values and changes in R₂* values during functional stimulations have been reported in a number of studies. The purpose of this study was to measure absolute extravascular R₂* values in human visual cortex and to estimate the intra- and extravascular contributions to the BOLD effect at 7T. Vascular space occupancy (VASO) MRI was employed to separate out the extravascular tissue signal. Multi-echo VASO and BOLD functional MRI (fMRI) with visual stimulation were performed at 7T for R₂* measurement at a spatial resolution of 2.5×2.5×2.5mm³ in healthy volunteers (n=6). The ratio of changes in extravascular and total R₂* (ΔR₂*) was used to estimate the extravascular fraction of the BOLD effect. Extravascular R₂* values were found to be 44.66±1.55 and 43.38±1.51s^-1 (mean±standard error of the mean, n=6) at rest and activation, respectively, in human visual cortex at 7T. The extravascular BOLD fraction was estimated to be 91±3%. The parenchymal oxygen extraction fraction (OEF) during activation was estimated to be 0.24±0.01 based on the R₂* measurements, indicating an approximately 37% decrease compared with OEF at rest.