Precise control of end-tidal carbon dioxide and oxygen improves BOLD and ASL cerebrovascular reactivity measures.

In-depth investigation of cerebrovascular blood flow and MR mechanisms underlying the blood oxygenation level dependent signal requires precise manipulation of the arterial partial pressure of carbon dioxide and oxygen, measured by their noninvasive surrogates, the end-tidal values. The traditional methodology consists of administering a fixed fractional concentration of inspired CO2, but this causes a variable ventilatory response across subjects, resulting in different values of end-tidal partial pressures of CO2 and O2. In this study, we investigated whether fine control of these end-tidal partial pressures would improve stability and predictability of blood oxygenation level dependent and arterial spin labeling signals for studying cerebrovascular reactivity. In 11 healthy volunteers, we compared the MR signals generated by the traditional fixed fractional concentration of inspired CO2 method to those of an automated feed-forward system, a simpler, safer, and more compact alternative to dynamic end-tidal forcing systems, designed to target constant end-tidal partial pressures of CO2 and O2. We found that near square-wave changes in end-tidal partial pressure of CO2 of 5, 7.5, and 10 mm Hg (61.01 mm Hg within two to three breaths) and constrained changes in the end-tidal partial pressure of O2 (<10 mm Hg) induced cerebral vascular reactivity measurements with faster transitions, together with improved stability and gradation, than those achieved with the traditional fixed fractional concentration of inspired CO2 method. Magn Reson Med 64:749–756, 2010. V C 2010 Wiley-Liss, Inc.