Time-Resolved MRI Oximetry for Quantifying CMRO2 and Vascular Reactivity

Felix W. Wehrli, Zachary B. Rodgers, Varsha Jain, Michael C. Langham, Cheng Li, Daniel J. Licht, Jeremy Magland
2014 Academic Radiology  
This brief review of magnetic resonance susceptometry summarizes the methods conceived in the authors' laboratory during the past several years. It is shown how venous oxygen saturation is quantified in large draining veins by field mapping and how this information, in concert with simultaneous measurement of cerebral blood flow, yields CMRO 2 , the brain's rate of oxygen consumption. The accuracy of this model-based approach in which the blood vessel is approximated as a long, straight
more » ... , for which an analytical solution for the induced field exists, is discussed. It is shown that the approach is remarkably robust, allowing for time-resolved quantification of whole-brain metabolism at rest and in response to stimuli, thereby providing detailed information on cerebral physiology in health and disease not previously amenable by noninvasive methods. Brain Oxygen Metabolism The human brain accounts for only 2% of the body's weight but 20% of the body's total energy requirement (see, for example, [1]). This enormous energy demand, necessary to maintain the default mode of brain activity, is incompletely understood. Oxygen consumption, typically quantified as the cerebral metabolic rate of oxygen (CMRO 2 ), has been measured in various ways, most typically by positron emission tomography (PET), which yielded values on the order of 130 μmol/min/100g [2] in the resting awake state. Even during light sleep CMRO 2 is only insignificantly lower [3] , and only in deep sleep [4] , and more so during anesthesia [5], is energy demand substantially lower. Further, incremental CMRO 2 increases in response to mental tasks have been found to be minor relative to the brain's baseline demand. The brain's default-mode energy demand is satisfied by oxidation of glucose to water and carbon dioxide. The resulting free energy generates ATP, the body's universal currency of energy. In contrast, incremental demand during task activation is supplied via the glycolytic pathway of glucose oxidation. The ratio of the oxygen consumed over the oxygen supplied by blood flow (oxygen extraction fraction, OEF) has been found to be largely independent of the region of the brain in which it is measured [2] . Even though oxygen consumption of gray matter is four to five times greater than that of white matter, the greater oxygen demand is offset by a commensurate increase in the rate of delivery in the form of cerebral blood flow. Once OEF
doi:10.1016/j.acra.2013.11.001 pmid:24439334 pmcid:PMC3896886 fatcat:swmqkjn4rzatjcjdoel6wjapye