Quantitative fMRI of rat brain by multi-modal MRI and MRS measurements

Abstract

Quantitative magnetic resonance imaging (MRI) and spectroscopy (MRS) measurements of energy metabolism (i.e., cerebral metabolic rate of oxygen consumption (CMR O 2 )), blood circulation (i.e., cerebral blood flow (CBF) and volume (CBV)), and functional MRI (fMRI) signal over a wide range of neuronal activity were used to interpret the energetic and physiologic basis of blood oxygenation level dependent (BOLD) image-contrast at 7 T in the rat brain. Since each parameter that can influence the BOLD image-contrast is measured quantitatively and separately, multi-modal measurements of changes in CMR O 2 , CBF, CBV, BOLD fMRI signal allow the calibration and validation of the BOLD image-contrast in glutamatergic neurons of the rat cerebral cortex. Good agreement between changes in CMR O 2 calculated from the BOLD theory and measured by 13C MRS reveals that BOLD fMRI signal-changes at 7 T are closely linked with alterations in the neuronal glucose oxidation. Comparisons of CMR O 2 and CBF over a wide dynamic range of neuronal activity provide insight into the regulation of energy metabolism and oxygen delivery in the cerebral cortex. Consequences of these results from rat brain for similar calibrated BOLD fMRI studies in the human brain are discussed. The current results revealed the energetic and physiologic components of the BOLD fMRI signal and indicated the required steps towards mapping the neuronal activity quantitatively by fMRI at steady state.