title: Quantitative β mapping for calibrated fMRI.
abstract: The metabolic and hemodynamic dependencies of the blood oxygenation level-dependent (BOLD) signal form the basis for calibrated fMRI, where the focus is on oxidative energy demanded by neural activity. An important part of calibrated fMRI is the power-law relationship between the BOLD signal and the deoxyhemoglobin concentration, which in turn is related to the ratio between oxidative demand (CMRO2) and blood flow (CBF). The power-law dependence between BOLD signal and deoxyhemoglobin concentration is signified by a scaling exponent β. Until recently most studies assumed a β value of 1.5, which is based on numerical simulations of the extravascular BOLD component. Since the basal value of CMRO2 and CBF can vary from subject-to-subject and/or region-to-region, a method to independently measure β in vivo should improve the accuracy of calibrated fMRI results. We describe a new method for β mapping through characterizing R2' - the most sensitive relaxation component of BOLD signal (i.e., the reversible magnetic susceptibility component that is predominantly of extravascular origin at high magnetic field) - as a function of intravascular magnetic susceptibility induced by an FDA-approved superparamagnetic contrast agent. In α-chloralose anesthetized rat brain, at 9.4 T, we measured β values of ~0.8 uniformly across large neocortical swathes, with lower magnitude and more heterogeneity in subcortical areas. Comparison of β maps in rats anesthetized with medetomidine and α-chloralose revealed that β is independent of neural activity levels at these resting states. We anticipate that this method for β mapping can help facilitate calibrated fMRI for clinical studies.
authors.affiliation: Department of Biomedical Engineering, Yale University, New Haven, CT, USA. Electronic address: christina.shu@yale.edu.
authors.forename: Christina Y
authors.initials: CY
authors.lastname: Shu
authors.affiliation: Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
authors.forename: Basavaraju G
authors.initials: BG
authors.lastname: Sanganahalli
authors.affiliation: Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Coman
authors.affiliation: Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Herman
authors.affiliation: Department of Biomedical Engineering, Yale University, New Haven, CT, USA; Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Rothman
authors.affiliation: Department of Biomedical Engineering, Yale University, New Haven, CT, USA; Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA. Electronic address: fahmeed.hyder@yale.edu.
authors.forename: Fahmeed
authors.initials: F
authors.lastname: Hyder
mesh_terms: D000818:Animals; D002138:Calibration; D059907:Functional Neuroimaging / Q000379:methods* / Q000592:standards; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000592:standards; D019579:Neocortex / Q000502:physiology*; D000069336:Neurovascular Coupling / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
keywords: Blood volume; Feraheme; Neurometabolic coupling; Neurovascular coupling; SPIO