0.5 Billion Counts per Second Enable High Speed and Penetration in Time-Domain Diffuse Optics

We present the application to time-domain diffuse optics of a high-speed 8 x 256 array of single-photon avalanche diodes with integrated 256 parallel time-to-digital converters. Thanks to the high light harvesting capability granted by the overall 0.85 mm(2) active area combined with a high throughput (i.e., saturated photon counting and timing rate of 512 million of counts per second), it has been possible for the first time to reconstruct histograms of photons time-of-flight in diffusive media using pulsed illumination at photon counting rate of about 450 million of counts per second even using a source-detector distance of 2 cm. This has been achieved both on tissue-mimicking phantoms as well as in-vivo, permitting high accuracy with extremely low acquisition times (down to 5 ms). This approach has been systematically validated on phantoms using established performance assessment protocols in the field of diffuse optics covering both homogeneous (demonstrating high linearity in the recovering of the absorption coefficient) and heterogeneous (demonstrating high penetration inside scattering media) paradigms. Two preliminary in-vivo proof-of-concept applications on healthy volunteer are shown, specifically, the detection of the heartbeat pattern in the brachioradialis muscle during an arterial cuff occlusion and of the same pattern acquired on the forehead during resting state.