4616 Reproducibility of BOLD and CBF responses to fixed step changes in inspired O 2 / CO 2 using dual-echo pCASL

Target audience: Researchers interested in calibrated MRI and fMRI measures of cerebrovascular reactivity to respiratory gases. Purpose: Functional MRI of CBF and BOLD responses to blood gas manipulations have been used to characterize reactivity of the cerebral vasculature, and to calibrate physiological models allowing estimation of cerebral oxygen consumption. Such manipulations are usually achieved using breathing maneuvers that induce changes in respiratory end-tidal (ET) levels, which reflect arterial blood gas content. A common way to induce hypercapnia (HC) and hyperoxia (HO) has been to administer air mixtures containing CO2 or supplemental O2 through loosely fitting clinical oxygen masks used in respiratory therapy. A limitation of this approach is the poor control over the composition of gases actually inspired by the participant, which hampers reproducibility of the respiratory stimuli and related fMRI measures. More sophisticated alternatives include the use of mouthpieces + nose clips in place of the oxygen mask, or sealing the edges of standard oxygen masks with adhesive tape, but these solutions can be uncomfortable for the subject. We have designed a breathing circuit that provides improved control over the inspired doses using a comfortable mask that is easily put on and removed, and which provides a backup supply of room air if the flow of medical gases is interrupted. The objective of this study was to assess the test-retest reliability of BOLD and CBF responses to fixed step changes in inspired O2 and CO2 levels, using a dual-echo pCASL sequence to simultaneously image blood flow and oxygenation.