In Vivo Blood Velocity Measurements by Upconversion Correlation Spectroscopy

In this exploratory interdisciplinary project, we aim to develop a new and powerful technique for measuring in vivo blood flow with unprecedented spatial and temporal resolution. The development of the technique proposed here goes far beyond the capabilities of current techniques such as optical micro-angiography, computed tomography, MRI and Doppler ultrasound. While MRI and ultrasound have the advantage of being non-invasive, they offer relatively low spatial resolution. Other techniques such as optical micro-angiography provide higher spatial resolution images of vasculature, including capillaries, but lack the capability to dynamically quantify flow rates. Development of such new tools capable of monitoring blood flow in the microvasculature with high temporal and spatial resolution are urgently needed and will have a game-changing impact on biomedical research. These new tools will allow researchers to quantify and visualize, in the near future, neurovascular dysfunction and provide insight into cerebral blood flow dynamics, blood brain barrier (BBB) disruption, and neurovascular coupling in neurodegenerative diseases. To achieve this goal, we will use a recently introduced concept of molecular based UCF by the main applicant to develop an upconversion fluorescence correlation spectroscopy technique (UCS). Standard Stokes-side FCS is a well-established technique for measuring diffusion, flow rates, and interactions between molecules in solution in vitro. This approach has been applied to soluble proteins in solution and to a more limited extent in live cells. However, all too often the FCS signal can be obscured by auto-fluorescence making the signal of interest hard to quantify which has severely restricted in vivo applications. We will solve this problem by using consecutive photon absorption to generate Anti-Stokes UCF. Our Anti-Stokes signal is background free since the probe emission is on the higher energy side with respect to the excitation laser, allowing easy suppression of auto-fluorescence by use of a short-pass filter. To achieve the UCF, we will employ DNA-stabilized silver nanoclusters emitters embedded in cell derived nanovesicles, which have the capability of generating a strong UCF signal