CBF, BOLD, CBV, and CMRO₂ fMRI signal temporal dynamics at 500-msec resolution

Purpose: To investigate the temporal dynamics of blood oxygenation level-dependent (BOLD), cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO₂) changes due to forepaw stimulation with 500-msec resolution in a single setting. Materials and Methods: Forepaw stimulation and hypercapnic challenge on rats were studied. CBF and BOLD functional MRI (fMRI) were measured using the pseudocontinuous arterial spin-labeling technique at 500-msec resolution. CBV fMRI was measured using monocrystalline ironoxide particles following CBF and BOLD measurements in the same animals. CMRO₂ change was estimated via the biophysical BOLD model with hypercapnic calibration. Percent changes and onset times were analyzed for the entire forepaw somatosensory cortices and three operationally defined cortical segments, denoted Layers I-III, IV-V, and VI. Results: BOLD change was largest in Layers I-III, whereas CBF, CBV, and CMRO₂ changes were largest in Layers IV-V. Among all fMRI signals in all layers, only the BOLD signal in Layers I-III showed a poststimulus undershoot. CBF and CBV dynamics were similar. Closer inspection showed that CBV increased slightly first (P < 0.05), but was slow to peak. CBF increased second, but peaked first. BOLD significantly lagged both CBF and CBV (P < 0.05). Conclusion: This study provides important temporal dynamics of multiple fMRI signals at high temporal resolution in a single setting.