Background: Reliable methods used to measure cerebral hemodynamics, metabolism and autoregulation are time-consuming and invasive.
Objectives: We sought to measure cerebral blood flow (CBF), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2), in the whole brain, gray and white matter and detect physiological changes in humans under normal and ≈10% reductions in mean arterial pressure (MAP).
Methods: CBF was measured using pseudo continuous arterial spin labeling (pCASL). OEF was measured using a MRI asymmetric spin echo-planar imaging (EPI). The arterial content of oxygen (CaO2) was computed. CMRO2 was calculated using the equation CMRO2=CaO2xCBFxOEF. Cerebral autoregulation was determined by measuring changes in CBF, OEF and CMRO2 in response to ≈10% reductions in MAP in stroke-free controls induced by IV Nicardipine infusion. Autoregulation was measured in the whole brain, gray matter and white matter. Autoregulatory Index (AI) was computed using the equation AI = %CBF change/% MAP change supplemented by CBF associated OEF changes.
Results: A total of 30 subjects were consented (Mean age 31, 93% male, 40% white, 43% black and 17% others). At baseline, CBF was measured in the whole brain (52.5±9.5 ml/100g/min), gray matter (72.0±13.0 ml/100g/min) and white matter (30.2±8.4 ml/100g/min). OEF was measured in the whole brain (0.32±0.02), gray matter (0.33±0.02) and white matter (0.30±0.01). CMRO2 was measured in the whole brain (3.2±0.8 ml/100g/min), gray matter (4.6±1.1 ml/100g/min) and white matter (1.8±0.5 ml/100g/min). A target of 10% MAP lowering was successfully achieved in 18 subjects, resulting in a change of mean CBF in the whole brain (52 vs. 50, p=0.1), gray matter (72 vs. 69, p=0.2) and white matter (30 vs. 28, p=0.08). This translated to a AI of 0.3 ± 0.9 in the whole brain, 0.2±1.0 in the gray matter and 0.5±1.7 in the white matter. These CBF changes corroborated with no significant changes in OEF and CMRO2.
Conclusions: MR measurement of CBF, OEF and CMRO2, in the whole brain, gray and white matter was comparable to measures obtained by invasive methods. Differences in these measures, as well as AI in the gray and white matter, may help understand cerebrovascular diseases that affect white matter such as lacunar strokes.