Our overall goal is to develop a quantitative understanding of the coupling of blood flow and energy metabolism in the human brain, and to develop quantitative methods for assessing this coupling in health and disease. Functional magnetic resonance imaging (fMRI) has revolutionized the study of the working human brain by providing a sensitive, non-invasive tool for mapping brain activity. The method exploits the sensitivity of the MR signal to local changes in deoxy-hemoglobin content, called the Blood Oxygenation Level Dependent (BOLD) effect. The central physiological phenomenon underlying the BOLD effect is that cerebral blood flow (CBF) increases more than the cerebral metabolic rate of oxygen (CMRO2) during increased brain activity. Yet despite its success as a mapping tool, quantitative interpretation of the magnitude of the BOLD response as a reflection of the magnitude of underlying physiological changes is problematic because of our poor understanding of the variability of CBF/CMRO2 coupling. During the previous period of support we implemented and evaluated a calibrated-BOLD approach, measuring local CBF and BOLD responses to mild hypercapnia in addition to neural activation, to measure the coupling of CBF and CMRO2. Our work highlighted the importance of the CBF/CMRO2 coupling ratio for interpreting BOLD responses across brain regions and in disease, and also demonstrated that the calibrated-BOLD approach provides a powerful tool for quantitatively assessing brain physiology for both basic research and potentially in clinical settings. A central problem for the interpretation of the BOLD response is that we do not know to what degree CBF/CMRO2 coupling varies in the healthy human brain, and the primary goal of this proposal is to determine that variability. Our previous results are consistent with the hypothesis that the CBF/CMRO2 coupling ratio increases for stronger stimuli, which is consistent with current ideas that CBF is driven by the input neural activity to a region while CMRO2 responds to the total energy needs of the full evoked activity. We will test this hypothesis in the healthy human brain with experimental paradigms designed to manipulate the types of neural activity involved and test for a dissociation of the CBF and CMRO2 responses. The proposed experiments exploit contrast sensitivity and temporal frequency tuning effects (Aim 1), adaptation effects (Aim 2), and inhibitory effects and negative BOLD signals (Aims 1 and 3). In addition, we will improve the current calibrated-BOLD methodology by developing a more complete mathematical model for the BOLD response that includes effects of intravascular signal change and arterial blood volume changes, and test an alternative hyperoxia technique for calibration as an alternative to hypercapnia (Aim 4). Completion of these goals will lay a solid physiological foundation for both basic science studies with BOLD-fMRI and clinical applications of fMRI.
A calibrated-fMRI methodology has the potential to provide a quantitative probe of brain physiology by measuring blood flow and oxygen metabolism changes. This tool can serve as a """"""""stress-test"""""""" to evaluate brain function for early detection of dysfunction and for monitoring the progression of disease or the response to drugs and therapy. Our goal is to lay the groundwork for these applications by extending and improving the methodology and by gaining a better understanding of how flow and metabolism are coupled in the healthy brain.
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