Neuroimaging with calibrated fMRI

The conventional functional MRI (fMRI) map offers information indirectly about localized changes in neuronal activity because it reflects changes in blood oxygenation, not actual neuronal activity. To provide a neurophysiological basis of fMRI, researchers have used electrophysiology to show correlations of fMRI and electric signals. However, quantitative interpretation of the degree to which neuronal activity has changed still cannot be made from conventional fMRI data. The fMRI signal has 2 parts: one describes the correlation between oxidative metabolism (cerebral metabolic rate of oxygen [CMRO2]) and cerebral blood flow (CBF), which supports the bioelectric work to sustain neuronal excitability; the other is the requisite dilation of blood vessels (cerebral blood volume [CBV]), which is the mechanical response involved in removal of waste while providing nutrients. Since changes in energy metabolism are related to bioelectric work, we tested whether spiking frequency of a neuronal ensemble (nu) is reflected by local energy metabolism (CMRO2) in rat brain. We used extracellular recordings to measure Deltanu/nu and calibrated fMRI (ie, using fMRI signal, CBF, and CBV maps) to measure DeltaCMRO2/CMRO2 during sensory stimulation. We found that DeltaCMRO2/CMRO2 is approximately Deltanu/nu, which suggests efficient energy use during brain work. Thus, calibrated fMRI provides data on where and by how much the neuronal activity has changed. Possibilities of utilizing calibrated fMRI as a neuroimaging method are discussed.