title: Influence of baseline hematocrit and hemodilution on BOLD fMRI activation.
abstract: Current understanding of blood oxygenation level dependent (BOLD) fMRI physiology predicts a close relationship between BOLD signal and blood hematocrit level. However, neither this relationship nor its effect on BOLD percent activation (BPA) has been empirically examined in man. To that end, BPA in primary visual cortex in response to photic stimulation was determined in a group of 24 normal subjects. A positive linear relationship between BPA and hematocrit was seen, particularly in men. To evaluate the effect of change in hematocrit on BPA, 9 men were studied before and following isotonic saline hemodilution, resulting in an average 6% reduction in hematocrit and an 8-31% reduction in BPA. No significant change in the number of activated pixels was seen. A model of predicted BPA as a function of hematocrit and vessel size was developed, and results from this model closely mirrored the empiric data. These results suggest that hematocrit significantly influences the magnitude of BPA and that such baseline factors should be accounted for when comparing BOLD data across groups of subjects, particularly in the many instances in which hematocrit may vary systematically. Such instances include several disease states as well as studies involving sex differences, drug administration, stress and other factors. Finally, the robust agreement between predicted and empiric data serves to validate a semiquantitative approach to the analysis of BOLD fMRI data.
authors.affiliation: Laboratory for Cerebral Blood Flow, Brain Imaging Center, McLean Hospital, Harvard Medical School, Belmont, MA 02478, USA. levin@mclean.harvard.edu
authors.forename: J M
authors.initials: JM
authors.lastname: Levin
authors.forename: B de B
authors.initials: Bde B
authors.lastname: Frederick
authors.forename: M H
authors.initials: MH
authors.lastname: Ross
authors.forename: J F
authors.initials: JF
authors.lastname: Fox
authors.forename: H L
authors.initials: HL
authors.lastname: von Rosenberg
authors.forename: M J
authors.initials: MJ
authors.lastname: Kaufman
authors.forename: N
authors.initials: N
authors.lastname: Lange
authors.forename: J H
authors.initials: JH
authors.lastname: Mendelson
authors.forename: B M
authors.initials: BM
authors.lastname: Cohen
authors.forename: P F
authors.initials: PF
authors.lastname: Renshaw
mesh_terms: D001921:Brain / Q000098:blood supply / Q000502:physiology*; D005260:Female; D006400:Hematocrit; D006438:Hemodilution; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008954:Models, Biological; D010100:Oxygen / Q000097:blood*; D010775:Photic Stimulation; D012044:Regression Analysis
chemical_list: D010100:Oxygen