The Relationship Between M in “Calibrated fMRI” and the Physiologic Modulators of fMRI

Hanzhang Lu*, 1, 3, Joanna Hutchison2, 3, Feng Xu1, Bart Rypma2, 3
1 Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
2 Center for BrainHealth, University of Texas at Dallas, Dallas, Texas 75235, USA
3 Department of Psychiatry, University of Texas Southwestern Medical Center, Dallas, Texas 75390

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© Lu et al; Licensee Bentham Open

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

* Address correspondence to this author at the Advanced Imaging Research Center, UT Southwestern Medical Center, 5323 Harry Hines Blvd. Dallas, TX 75390, USA; Tel: 214-645-2761; Fax: 214-645-2744; E-mail:


The “calibrated fMRI” technique requires a hypercapnia calibration experiment in order to estimate the factor “M”. It is desirable to be able to obtain the M value without the need of a gas challenge calibration. According to the analytical expression of M, it is a function of several baseline physiologic parameters, such as baseline venous oxygenation and CBF, both of which have recently been shown to be significant modulators of fMRI signal. Here we studied the relationship among hypercapnia-calibrated M, baseline venous oxygenation and CBF, and assessed the possibility of estimating M from the baseline physiologic parameters. It was found that baseline venous oxygenation and CBF are highly correlated (R2=0.77, P<0.0001) across subjects. However, the hypercapnia-calibrated M was not correlated with baseline venous oxygenation or CBF. The hypercapnia-calibrated M was not correlated with an estimation of M based on analytical expression either. The lack of correlation may be explained by the counteracting effect of venous oxygenation and CBF on the M factor, such that the actual M value of an individual may be mostly dependent on other parameters such as hematocrit. Potential biases in hypercapnia-based M estimation were also discussed in the context of possible reduction of CMRO2 during hypercapnia.

Keywords: Calibrated fMRI, venous oxygenation, cerebral blood flow, cerebral metabolic rate of oxygen, brain.