Stimulation of cerebral vasculature using hypercapnia has been widely used to study cerebral vascular reactivity (CVR), which can be expressed as the quantitative change in cerebral blood flow (CBF) per mm Hg change in end-tidal partial pressure of CO 2 (P ET CO 2 ). We investigate whether different respiratory manipulations, with arterial spin labeling used to measure CBF, lead to consistent measures of CVR. The approaches included: (1) an automated system delivering variable concentrations of inspired CO 2 for prospective targeting of P ET CO 2 , (2) administration of a fixed concentration of CO 2 leading to subject-dependent changes in P ET CO 2 , (3) a breath-hold (BH) paradigm with physiologic modeling of CO 2 accumulation, and (4) a maneuver combining breathhold and hyperventilation. When CVR was expressed as the percent change in CBF per mm Hg change in P ET CO 2 , methods 1 to 3 gave consistent results. The CVR values using method 4 were significantly lower. When CVR was expressed in terms of the absolute change in CBF (mL/100 g per minute per mm Hg), greater discrepancies became apparent: methods 2 and 3 gave lower absolute CVR values compared with method 1, and the value obtained with method 4 was dramatically lower. Our findings indicate that care must be taken to ensure that CVR is measured over the linear range of the CBF-CO 2 dose-response curve, avoiding hypocapnic conditions.

Breathing a mixture of 10% CO(2) with 90% O(2) (referred to here as carbogen-10) increases blood flow due to the vasodilatory effect of CO(2), and raises blood O(2) saturation due to the enriched oxygen level. These effects both tend to reduce the level of deoxygenated hemoglobin in brain tissues, thereby reducing the potential for further increases in BOLD contrast. In the present study, blocks of intense visual stimulation (60s) were presented amid longer blocks (180s) during which subjects breathed various fractional concentrations (0-100%) of carbogen-10 diluted with medical air. When breathing undiluted carbogen-10, the BOLD response to visual stimulation was reduced below the level of noise against the background of the carbogen-10 response. At these concentrations, the total (visual+carbogen) BOLD response amplitude (7.5±1.0%, n=6) converged toward that seen with carbogen alone (7.5±1.0%, n=6). In spite of the almost complete elimination of the visual BOLD response, pseudo-continuous arterial spin-labeling on a separate cohort indicated a largely preserved perfusion response (89±34%, n=5) to the visual stimulus during inhalation of carbogen-10. The previously discussed observations suggest that venous saturation can be driven to very high levels during carbogen inhalation, a finding which has significant implications for calibrated MRI techniques. The latter methods involve estimation of the relative change in venous O(2) saturation by expressing activation-induced BOLD signal increases as a fraction of the maximal BOLD signal M that would be observed as venous saturation approaches 100%. While the value of M has generally been extrapolated from much smaller BOLD responses induced using hypercapnia or hyperoxia, our results suggest that these effects could be combined through carbogen inhalation to obtain estimates of M based on larger BOLD increases. Using a hybrid BOLD calibration model taking into account changes in both blood flow and arterial oxygenation, we estimated that inhalation of carbogen-10 led to an average venous saturation of 91%, allowing us to compute an estimated M value of 9.5%.

BackgroundRespiratory manipulations modulating blood flow and oxygenation levels have become an important component of modern functional MRI applications. Manipulations often consist of temporarily switching inspired fractions of CO2 and O2; and have typically been performed using simple oxygen masks intended for applications in respiratory therapy. However, precise control of inspired gas composition is difficult using this type of mask due to entrainment of room air and resultant dilution of inspired gases. We aimed at developing a gas delivery apparatus allowing improved control over the fractional concentration of inspired gases, to be used in brain fMRI studies.FindingsThe breathing circuit we have conceived allowed well controlled step changes in FiO2 and FiCO2, at moderate flow rates achievable on standard clinical flow regulators. In a two run test inside the scanner we demonstrate that tightly controlled simple gas switching manipulations can afford good intra-subject reproducibility of induced hyperoxia/hypercapnia responses. Although our approach requires a non-vented mask fitting closely to the subject’s face, the circuit ensures a continuous supply of breathable air even if the supply of medical gases is interrupted, and is easily removable in case of an emergency. The apparatus we propose is also compact and MRI compatible, allowing subject placement in confined spaces such as an MRI scanner for brain examinations.ConclusionsWe have reported a new approach for the controlled administration of medical gases, and describe an implementation of the breathing circuit that is MRI compatible and uses commercially available parts. The resultant apparatus allows simple, safe and precise manipulations of FiO2 and FiCO2.

The current generation of calibrated MRI methods goes beyond simple localization of task-related responses to allow the mapping of resting-state cerebral metabolic rate of oxygen (CMRO2) in micromolar units and estimation of oxygen extraction fraction (OEF). Prior to the adoption of such techniques in neuroscience research applications, knowledge about the precision and accuracy of absolute estimates of CMRO2 and OEF is crucial and remains unexplored to this day. In this study, we addressed the question of methodological precision by assessing the regional inter-subject variance and intra-subject reproducibility of the BOLD calibration parameter M, OEF, O2 delivery and absolute CMRO2 estimates derived from a state-of-the-art calibrated BOLD technique, the QUantitative O2 (QUO2) approach. We acquired simultaneous measurements of CBF and R2* at rest and during periods of hypercapnia (HC) and hyperoxia (HO) on two separate scan sessions within 24 hours using a clinical 3 T MRI scanner. Maps of M, OEF, oxygen delivery and CMRO2, were estimated from the measured end-tidal O2, CBF0, CBFHC/HO and R2*HC/HO. Variability was assessed by computing the between-subject coefficients of variation (bwCV) and within-subject CV (wsCV) in seven ROIs. All tests GM-averaged values of CBF0, M, OEF, O2 delivery and CMRO2 were: 49.5 ± 6.4 mL/100 g/min, 4.69 ± 0.91%, 0.37 ± 0.06, 377 ± 51 μmol/100 g/min and 143 ± 34 μmol/100 g/min respectively. The variability of parameter estimates was found to be the lowest when averaged throughout all GM, with general trends toward higher CVs when averaged over smaller regions. Among the MRI measurements, the most reproducible across scans was R2*0 (wsCVGM = 0.33%) along with CBF0 (wsCVGM = 3.88%) and R2*HC (wsCVGM = 6.7%). CBFHC and R2*HO were found to have a higher intra-subject variability (wsCVGM = 22.4% and wsCVGM = 16% respectively), which is likely due to propagation of random measurement errors, especially for CBFHC due to the low contrast-to-noise ratio intrinsic to ASL. Reproducibility of the QUO2 derived estimates were computed, yielding a GM intra-subject reproducibility of 3.87% for O2 delivery, 16.8% for the M value, 13.6% for OEF and 15.2% for CMRO2. Although these results focus on the precision of the QUO2 method, rather than the accuracy, the information will be useful for calculation of statistical power in future validation studies and ultimately for research applications of the method. The higher test-retest variability for the more extensively modeled parameters (M, OEF, and CMRO2) highlights the need for further improvement of acquisition methods to reduce noise levels.