Assessing Cerebrovascular Responsiveness: Comparing Functional Magnetic Resonance Imaging With Doppler Ultrasound

Cerebrovasculature responsiveness (CVR) to alterations in arterial carbon dioxide (PCO2) content is a common test to assess brain health. Traditionally, higher CVR is associated with higher aerobic fitness, while natural aging and brain-related diseases are associated with lower CVR. However, recent findings challenge some of these relations, but may be related to inconsistent neuroimaging methodologies. PURPOSE: 1) to examine fitness effects on CVR between active and sedentary individuals using functional magnetic resonance imaging (fMRI) and transcranial Doppler (TCD), and 2) compare different stimulus concentrations for CVR measures between fMRI and TCD. METHODS: Fourteen volunteers participated (26 ± 7 yrs; 8 active, 6 sedentary), with 10 pair-matched for age and sex to examine fitness effects (5 active, 5 sedentary). Following medical screening, participants completed an aerobic fitness test (VO2 max) and the CVR protocol familiarisation. Participants then completed two experimental sessions on separate days (randomized and counter-balanced). For both sessions, CVR was assessed using two concentrations of CO2 via the same Douglas bag open circuit (4-min cycles of room air, 5% CO2, room air, 7% CO2). CVR was measured using fMRI (EPI-based sequence allowing simultaneous acquisition of blood-oxygen level dependent and perfusion responses) and TCD (middle cerebral artery flow velocity). CVR measures were correlated (Pearson’s) with fitness and differences between stimulus concentration within and between approaches compared. RESULTS: 1) Higher VO2 max was associated with higher CVR derived from 5% and 7% CO2 stimuli, but were higher for fMRI (5%: r=.640, p=.06; 7%: r=.690, p=.04) than TCD (5%: r=.209, p=.30; r=.365, p=.17). 2) When comparing CVR between fMRI and TCD, values obtained from the 5% stimulus correlated (r=.626, p< .05); while there was no correlation between values obtained from the 7% stimulus (r=.252, p=.46). Paired t-tests comparing CO2 concentration (5% vs 7% CO2) revealed no difference for fMRI-derived CVRs (p=.93), but a trend between TCD-derived CVRs (p=.10). CONCLUSION: Both fMRI-and TCD-CVR approaches differentiated active and sedentary groups. The CVR measure between and within neuroimaging approaches was differentially influenced by CO2 concentration.