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Using carbogen for calibrated fMRI at 7 Tesla: Comparison of direct and modelled estimation of the M parameter

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Krieger,  Steffen
Department Neurophysics, MPI for Human Cognitive and Brain Sciences, Max Planck Society;
Monash Biomedical Imaging, Monash University, Melbourne, Australia;

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Ivanov,  Dimo
Department Neurophysics, MPI for Human Cognitive and Brain Sciences, Max Planck Society;
Maastricht Brain Imaging Centre, Maastricht University, the Netherlands;

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Huber,  Laurentius
Department Neurophysics, MPI for Human Cognitive and Brain Sciences, Max Planck Society;

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Roggenhofer,  Elisabeth
Department Neurology, MPI for Human Cognitive and Brain Sciences, Max Planck Society;

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Sehm,  Bernhard
Department Neurology, MPI for Human Cognitive and Brain Sciences, Max Planck Society;

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Turner,  Robert
Department Neurophysics, MPI for Human Cognitive and Brain Sciences, Max Planck Society;

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Gauthier,  Claudine
Department Neurophysics, MPI for Human Cognitive and Brain Sciences, Max Planck Society;

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Citation

Krieger, S., Ivanov, D., Huber, L., Roggenhofer, E., Sehm, B., Turner, R., et al. (2014). Using carbogen for calibrated fMRI at 7 Tesla: Comparison of direct and modelled estimation of the M parameter. NeuroImage, 84, 605-614. doi:10.1016/j.neuroimage.2013.09.035.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0014-5392-C
Abstract
Task-evoked changes in cerebral oxygen metabolism can be measured using calibrated functional Magnetic Resonance Imaging (fMRI). This technique requires the use of breathing manipulations such as hypercapnia, hyperoxia or a combination of both to determine a calibration factor M. The M-value is usually obtained by extrapolating the BOLD signal measured during the gas manipulation to its upper theoretical physiological limit using a biophysical model. However, a recently introduced technique uses a combination of increased inspired concentrations of O2 and CO2 to saturate the BOLD signal completely. In this study, we used this BOLD saturation technique to measure M directly at 7 Tesla (T). Simultaneous carbogen-7 (7 % CO2 in 93 % O2) inhalation and visuo-motor task performance was used to elevate venous oxygen saturation in visual and motor areas close to their maximum, and the BOLD signal measured during this manipulation was used as an estimate of M. As accurate estimation of M is crucial for estimation of valid oxidative metabolism values, these directly estimated M-values were assessed and compared with M-values obtained via extrapolation modelling using the generalized calibration model (GCM) on the same dataset. Average M-values measured using both methods were 10.4±3.9 % (modelled) and 7.5±2.2 % (direct) for a visual-related ROI, and 11.3±5.2 % (modelled) and 8.1±2.6 % (direct) for a motor-related ROI. Results from this study suggest that, for the CO2 concentration used here, modelling is necessary for the accurate estimation of the M parameter. Neither gas inhalation alone, nor gas inhalation combined with a visuomotor task, was sufficient to completely saturate venous blood in most subjects. Calibrated fMRI studies should therefore rely on existing models for gas inhalation-based calibration of the BOLD signal.