Non-contact time-domain scanning brain imager: results of proof of principle tests

We report on the development of a scanning non-contact brain imager, based on a novel technique in time-resolved near-infrared spectroscopy, i.e. the null source-detector distance approach. Our concept is designed to image an area of about 10 cm(2) with small adjustable scanning steps, i.e. a high density of mapping points can be realized. The feasibility of the proposed method was tested with a single-point confocal optical setup without beam scanning so far. A set of test measurements was performed on a liquid phantom with a small black polyvinyl chloride (PVC) cylinder as a target, which was translated in X direction to emulate the optical scanning and estimate lateral spatial resolution, and in Z direction to estimate the depth sensitivity of the instrument. The problem of dominance of early photons at null source-detector separation was solved by applying a fast time-gated detector to detect late only photons. Two fast-gated detectors, a newly developed state-of-art time-gated single-photon avalanche photodiode (tgSPAD) and commercially available fast-gated intensified CCD (iCCD) camera, were compared against each other. It was shown that, due to its better dynamic range, the tgSPAD is capable to detect later photons than the iCCD camera, and hence, a scanning system equipped with the time-gated SPAD has better depth sensitivity. Thus the time-gated SPAD is the detector of choice for further development of the non-contact confocal brain scanner.