Signal and Measurement Considerations for Human Translation of Diffuse in vivo Flow Cytometry

Significance Diffuse in vivo flow cytometry (DiFC) is an emerging technology for fluorescence detection of rare circulating cells directly in large deep-seated blood vessels in mice. Because DiFC uses highly scattered light, in principle it could be translated to human use. However, an open question is whether fluorescent signals from single cells would be detectable in human-scale anatomies. Aim Suitable blood vessels in a human wrist or forearm are at a depth of approximately 2-4 mm. The aim of this work was to study the impact of DiFC instrument geometry and wavelength on the detected DiFC signal and on the maximum depth of detection of a moving cell. Approach We used Monte Carlo simulations to compute Jacobian (sensitivity) matrices for a range of source-detector separations and tissue optical properties over the visible and near infrared (NIR) spectrum. We performed experimental measurements with three available versions of DiFC (488 nm, 640 nm, and 780 nm), fluorescent microspheres, and tissue mimicking optical flow phantoms. We used both computational and experimental data to estimate the maximum depth of detection at each combination of settings. Results and Conclusions For the DiFC detection problem, our analysis showed that for deep-seated blood vessels, the maximum sensitivity was obtained with NIR light (780 nm) and 3 mm source-and-detector separation. These results suggest that - in combination with a suitable molecularly targeted fluorescent probes - circulating cells and nanosensors could in principle be detectable in circulation in humans.