title: Measurement of parenchymal extravascular R2* and tissue oxygen extraction fraction using multi-echo vascular space occupancy MRI at 7 T.
abstract: 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 7 T. 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 7 T for R2* measurement at a spatial resolution of 2.5 × 2.5 × 2.5 mm(3) 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 ± 1.51 s(-1) (mean ± standard error of the mean, n = 6) at rest and activation, respectively, in human visual cortex at 7 T. 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.
authors.affiliation: F. M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, USA; Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Cheng
authors.forename: Peter C M
authors.initials: PC
authors.lastname: van Zijl
authors.forename: Jun
authors.initials: J
authors.lastname: Hua
mesh_terms: D000818:Animals; D001808:Blood Vessels / Q000378:metabolism*; D002417:Cattle; D066128:Gray Matter / Q000378:metabolism*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010100:Oxygen / Q000097:blood / Q000378:metabolism*; D013997:Time Factors
chemical_list: D010100:Oxygen
keywords: BOLD; CBV; OEF; R2*; VASO; extravascular; fMRI; high field