Non-contact scanning time-domain functional optical imaging of the adult human brain

We developed a novel scanning system that relies on gated detection of late photons at short source-detector separation, enabling the recording of absorption changes in deep tissue compartments. The tissue was scanned by a galvanometer scanner from a distance of more than 10 cm, with a fixed separation of the illumination and the detection spot of a few mm. The light source was a supercontinuum laser with an acousto-optic tunable filter that was used to rapidly switch between two wavelength bands centered at 760 nm and 860 nm. A fast-gated single-photon avalanche diode was employed to eliminate the intense early part of the diffusely remitted signal and to detect photons with long times of flight with improved signal-to-noise ratio. A second detection channel contained a non-gated detector. The gated and non-gated time-of-flight distributions of photons were recorded by imaging time-correlated single photon counting synchronized with the movement of the scanner. A tissue area with dimensions of several cm was scanned with 32×32 pixels within a frame time of 1 s. Sensitivity and spatial resolution of the system were characterized by phantom measurements. In-vivo tests included functional brain activation by various tasks and demonstrated the feasibility of non-contact imaging of hemodynamic changes in the cerebral cortex.