title: Physiological basis of vascular autocalibration (VasA): Comparison to hypercapnia calibration methods.
abstract: The statistical power of functional MRI (fMRI) group studies is significantly hampered by high intersubject spatial and magnitude variance. We recently presented a vascular autocalibration method (VasA) to account for vascularization differences between subjects and hence improve the sensitivity in group studies. Here, we validate the novel calibration method by means of direct comparisons of VasA with more established measures of baseline venous blood volume (and indirectly vascular reactivity), the M-value.
  
  
  
  Seven healthy volunteers participated in two 7 T (T) fMRI experiments to compare M-values with VasA estimates: (i) a hypercapnia experiment to estimate voxelwise M-value maps, and (ii) an fMRI experiment using visual stimulation to estimate voxelwise VasA maps.
  
  
  
  We show that VasA and M-value calibration maps show the same spatial profile, providing strong evidence that VasA is driven by local variations in vascular reactivity as reflected in the M-value.
  
  
  
  The agreement of vascular reactivity maps obtained with VasA when compared with M-value maps confirms empirically the hypothesis that the VasA method is an adequate tool to account for variations in fMRI response amplitudes caused by vascular reactivity differences in healthy volunteers. VasA can therefore directly account for them and increase the statistical power of group studies. The VasA toolbox is available as a statistical parametric mapping (SPM) toolbox, facilitating its general application. Magn Reson Med, 2016. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, London, U.K.
authors.forename: Samira M
authors.initials: SM
authors.lastname: Kazan
authors.affiliation: Section on Functional Imaging Methods, National Institute of Mental Health, Bethesda, MD, USA.
authors.forename: Laurentius
authors.initials: L
authors.lastname: Huber
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, London, U.K.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Flandin
authors.affiliation: Maastricht Brain Imaging Centre, Maastricht University, Maastricht, The Netherlands.
authors.forename: Dimo
authors.identifier: 0000-0001-9387-5823
authors.initials: D
authors.lastname: Ivanov
authors.affiliation: Section on Functional Imaging Methods, National Institute of Mental Health, Bethesda, MD, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Bandettini
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, London, U.K.|Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
keywords: BOLD calibration; BOLD fMRI; SPM toolbox; VasA; VasA toolbox; autorescaling; vascular reactivity; vascularization differences