Hypercapnic normalization of BOLD fMRI: Comparison across field strengths and pulse sequences

Eric R. Cohen, Egill Rostrup, Karam Sidaros, Torben E. Lund, Olaf B. Paulson, Kamil Ugurbil, Seong Gi Kim

Research output: Contribution to journalArticlepeer-review

81 Scopus citations


The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV v) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B 0). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapnia-induced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO 2 gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided.

Original languageEnglish (US)
Pages (from-to)613-624
Number of pages12
Issue number2
StatePublished - Oct 2004

Bibliographical note

Funding Information:
Supported by the National Institutes of Health (EB00201, EB00337, EB00332, EB00331, and RR08079), the Keck Foundation and the MIND Institute.


  • Brain mapping
  • High magnetic fields
  • Hypercapnia
  • fMRI


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