Age-related changes in brain hemodynamics; A calibrated MRI study

Introduction: Blood oxygenation-level dependent (BOLD) magnetic resonance imaging signal changes in response to stimuli have been used to evaluate age-related changes in neuronal activity. Contradictory results from these types of experiments have been attributed to differences in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO₂). To clarify the effects of these physiological parameters, we investigated the effect of age on baseline CBF and CMRO₂. Materials and Methods: Twenty young (mean±sd age, 28±3 years), and 45 older subjects (66±4 years) were investigated. A dual-echo pseudocontinuous arterial spin labeling (ASL) sequence was performed during normocapnic, hypercapnic, and hyperoxic breathing challenges. Whole brain and regional gray matter values of CBF, ASL cerebrovascular reactivity (CVR), BOLD CVR, oxygen extraction fraction (OEF), and CMRO₂ were calculated. Results: Whole brain CBF was 49±14 and 40±9 ml/100 g/min in young and older subjects respectively (P<0.05). Age-related differences in CBF decreased to the point of nonsignificance (B=-4.1, SE=3.8) when EtCO₂ was added as a confounder. BOLD CVR was lower in the whole brain, in the frontal, in the temporal, and in the occipital of the older subjects (P<0.05). Whole brain OEF was 43±8% in the young and 39±6% in the older subjects (P=0.066). Whole brain CMRO₂ was 181±60 and 133±43 μmol/100 g/min in young and older subjects, respectively (P<0.01). Discussion: Age-related differences in CBF could potentially be explained by differences in EtCO₂. Regional CMRO₂ was lower in older subjects. BOLD studies should take this into account when investigating age-related changes in neuronal activity.