Abstract
Gas calibrated fMRI in its most common form uses hypercapnia in conjunction with the Davis model to quantify relative changes in the cerebral rate of oxygen consumption (CMRO2) in response to a functional stimulus. It is most commonly carried out at 3T but, as 7T research scanners are becoming more widespread and the majority of clinical scanners are still 1.5T systems, it is important to investigate whether the model used remains accurate across this range of field strengths. Ten subjects were scanned at 1.5, 3 and 7T whilst performing a bilateral finger-tapping task as part of a calibrated fMRI protocol, and the results were compared to a detailed signal model. Simulations predicted an increase in value and variation in the calibration parameter M with field strength. Two methods of defining experimental regions of interest (ROIs) were investigated, based on (a) BOLD signal and (b) BOLD responses within grey matter only. M values from the latter ROI were in closer agreement with theoretical predictions; however, reassuringly, ROI choice had less impact on CMRO2 than on M estimates. Relative changes in CMRO2 during motor tasks at 3 and 7T were in good agreement but were over-estimated at 1.5T as a result of the lower signal to noise ratio. This result is encouraging for future studies at 7T, but also highlights the impact of imaging and analysis choices (such as ASL sequence and ROI definition) on the calibration parameter M and on the calculation of CMRO2.
Highlights
Gas calibrated functional magnetic resonance imaging has emerged as a promising tool to non-invasively measure stimulus evoked changes in the cerebral metabolic rate of oxygen consumption (CMRO2) (Davis et al, 1998; Hoge, 2012)
The results of this study suggest that the Davis model is reassuringly insensitive to field strength, provided that the value for β is adjusted appropriately (Bulte et al, 2009; Davis et al, 1998; Driver et al, 2012)
Changes in CMRO2 during a motor task, as calculated by the Davis model, were consistent between 3 and 7 T and were in close agreement with the results of theoretical simulations. This is encouraging for future studies of calibrated functional magnetic resonance imaging (fMRI) at ultra-high fields, and supports the
Summary
Gas calibrated functional magnetic resonance imaging (fMRI) has emerged as a promising tool to non-invasively measure stimulus evoked changes in the cerebral metabolic rate of oxygen consumption (CMRO2) (Davis et al, 1998; Hoge, 2012). Is this more directly physiologically relevant than measuring only the blood oxygen leveldependent (BOLD) signal, but it has been shown to be more consistent between subjects and between scanning sessions (Krieger et al, 2014). Experimental confirmation of these findings has so far not been performed
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