Abstract
The cerebral metabolic rate of oxygen (CMRO~2~) was dynamically evaluated on a pixel‐by‐pixel basis in isoflurane‐anesthetized and spontaneously breathing rats following graded electrical somatosensory forepaw stimulations (4, 6, and 8mA). In contrast to α‐chloralose, which is the most widely used anesthetic in forepaw‐stimulation fMRI studies of rats under mechanical ventilation, isoflurane (1.1–1.2%) provided a stable anesthesia level over a prolonged period, without the need to adjust the ventilation volume/rate or sample blood gases. Combined cerebral blood flow signals (CBF) and blood oxygenation level‐dependent (BOLD) fMRI signals were simultaneously measured with the use of a multislice continuous arterial spin labeling (CASL) technique (two‐coil setup). CMRO~2~ was calculated using the biophysical BOLD model of Ogawa et al. (Proc Natl Acad Sci USA 1992;89:5951–5955). The stimulus‐evoked BOLD percent changes at 4, 6, and 8mA were, respectively, 0.5% ± 0.2%, 1.4% ± 0.3%, and 2.0% ± 0.3% (mean ± SD, N = 6). The CBF percent changes were 23% ± 6%, 58% ± 9%, and 87% ± 14%. The CMRO~2~ percent changes were 14% ± 4%, 24% ± 6%, and 43% ± 11%. BOLD, CBF, and CMRO~2~ activations were localized to the forepaw somatosensory cortices without evidence of plateau for oxygen consumption, indicative of partial coupling of CBF and CMRO~2~. This study describes a useful forepaw‐stimulation model for fMRI, and demonstrate that CMRO~2~ changes can be dynamically imaged on a pixel‐by‐pixel basis in a single setting with high spatiotemporal resolution. Magn Reson Med 52:277–285, 2004. © 2004 Wiley‐Liss, Inc.