Breaking β: Understanding the β-Value in Calibrated Functional MRI
Avery J.L. Berman1,2 and Bruce Pike2
1Montreal Neurological Institute, McGill University, Montreal, QC, Canada, 2Department of Radiology and Hotchkiss Brain Institute, University of Calgary, Calgary, AB, Canada
In calibrated functional MRI (fMRI), the
parameter β is used to describe the non-linear dependence of the change in the
transverse relaxation rate on the susceptibility offset (Δχ) of blood relative
to tissue. Estimates of β at high field strengths have generally been assumed
or obtained in post-hoc analyses. Using simulations from vessel networks, we
present here a detailed description of β’s dependence on vessel radius. We show
that the estimate of β is highly dependent on the range of Δχ used in the fit.
This could have important implications for methods that propose to measure
β-values in vivo using a contrast agent to alter Δχ.
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