Noninvasive Measurements of Cerebral Blood Flow. (CBF) and Cerebral Oxygenation Various noninvasive techniques have been attempted to monitor CBFV or CBF in human including TCD [31], xenon-enhanced computer tomography (XeCT) [32], dynamic perfusion computed tomography (PCT) [33], single photon emission computed tomography (SPECT) [34], positron emission tomography (PET) [35], arterial spin labeled MRI (ASL-MRI) [36], and dynamic susceptibility contrast MRI (DSC-MRI) [37]. TCD is widely used, but limited to measure CBFV in large vessels, which may not be consistent with CBF in microvasculature [38]. However, many cerebral diseases such as stroke and neurocognitive impairment are associated with abnormal microvasculature and tissue dysfunction in brain [39, 40]. Moreover, TCD measurements are not applicable for approximately 9% of adults due to their having poor acoustic windows [41]. Other techniques mentioned above use either endogenous (e.g., ASL-MRI) or exogenous tracers (e.g., PET, SPECT, XeCT, PCT, DSC-MRI) to evaluate CBF in microvasculature. However, those large imaging modalities are expensive and have relatively low temporal resolutions and poor mobility. Noninvasive techniques for measuring cerebral oxygenation in human include blood oxygen level dependent MRI (BOLD-MRI) [42] and near-infrared spectroscopy (NIRS) tissue oximeter [43]. Both techniques are sensitive to cerebral oxygenation in microvasculature. Again, although BOLD-MRI can image cerebral hemodynamics with high spatial resolution, the high cost, low temporal resolution, and poor mobility limit its frequent use in clinic. By contrast, the inexpensive, portable, and easy-to-use NIRS allows for fast and longitudinal measurements at the bedside of clinic, although its spatial resolution is relatively low. Since cerebral hemodynamic parameters (e.g., CBF and cerebral oxygenation) are usually coupled and interactive, it is desirable to simultaneously measure multiple hemodynamic variables and investigate their complex relationships. Combination measurements have been reported previously using NIRS, PET [44], ASL-MRI [45], and BOLD-MRI [46]. Nevertheless, inexpensive, portable, fast, and easy-to-use devices are needed for continuous and simultaneous measurements of both CBF and cerebral oxygenation at the bedside of clinic.
