Effect of respiratory CO2 changes on the temporal dynamics of the hemodynamic response in functional MR imaging

Increasing end-expiratory CO2 levels (PETCO2) increases the dispersion and the time of maximum of the hemodynamic response curve in human primary visual cortex. This was demonstrated using event-related multislice functional magnetic resonance imaging (fMRI) with short repetition time and 3-s flicker light stimulation. Measurements were performed at 5 different PETCO2 levels between 20 and 60 mmHg using hyperventilation or by adding CO2 to the inspired air. Between 30 and 60 mmHg the full-width-at-half-maximum of the hemodynamic response curve induced by visual stimulation increased nearly linearly at 130 ms per mmHg PETCO2. Consistent with previous studies a concomitant decrease of the signal amplitude was observed at PETCO2 values below 40 mmHg and above 50 mmHg. The relevance of these findings for the temporal resolution of fMRI and especially of event-related methods is discussed. (C) 2001 Academic Press.