title: Robustly measuring vascular reactivity differences with breath-hold: normalising stimulus-evoked and resting state BOLD fMRI data.
abstract: Inter-subject differences in local cerebral blood flow (CBF) and cerebral blood volume (CBV) contribute to differences in BOLD signal reactivity and, therefore, unmodelled variance in group level fMRI analyses. A simple way of elevating blood CO(2) concentrations to characterise subject differences in vascular reactivity is through breath-holds but two aspects of this measure are often neglected: (1) breath-holds are usually modelled as blocks even though CO(2) accumulates over time and (2) increases in CO(2) differ between subjects. This study demonstrates that the BOLD breath-hold response is best modelled by convolving the end-tidal CO(2) trace with a standard haemodynamic response function and including its temporal derivative. Inclusion of the BOLD breath-hold response as a voxel-dependent covariate in a group level analysis increases the spatial extent of activation in stimulus evoked and resting state datasets. By expressing the BOLD breath-hold response as a percentage signal increase with respect to an absolute change in the partial pressure of CO(2) (expressed in mmHg), the spatial extent of stimulus-evoked activation is further improved. This demonstrates that individual end-tidal CO(2) increases to breath-hold should be accounted for to provide an accurate measure of vascular reactivity resulting in more statistically active voxels in group level analyses.
authors.affiliation: Cardiff University Brain Research Imaging Centre (CUBRIC), School of Psychology, Cardiff University, Cardiff, UK. murphyk2@cardiff.ac.uk
authors.forename: Kevin
authors.initials: K
authors.lastname: Murphy
authors.forename: Ashley D
authors.initials: AD
authors.lastname: Harris
authors.forename: Richard G
authors.initials: RG
authors.lastname: Wise
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001810:Blood Volume; D001921:Brain / Q000502:physiology; D001931:Brain Mapping; D001944:Breath Tests; D002245:Carbon Dioxide / Q000097:blood*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008451:Maximal Voluntary Ventilation / Q000502:physiology; D010100:Oxygen / Q000097:blood; D012044:Regression Analysis; D015656:Respiratory Mechanics / Q000502:physiology; D012146:Rest / Q000502:physiology; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen