Abstract
We present a system for non-contact time-resolved diffuse reflectance imaging, based on small source-detector distance and high dynamic range measurements utilizing a fast-gated single-photon avalanche diode. The system is suitable for imaging of diffusive media without any contact with the sample and with a spatial resolution of about 1 cm at 1 cm depth. In order to objectively assess its performances, we adopted two standardized protocols developed for time-domain brain imagers. The related tests included the recording of the instrument response function of the setup and the responsivity of its detection system. Moreover, by using liquid turbid phantoms with absorbing inclusions, depth-dependent contrast and contrast-to-noise ratio as well as lateral spatial resolution were measured. To illustrate the potentialities of the novel approach, the characteristics of the non-contact system are discussed and compared to those of a fiber-based brain imager.
Highlights
We present a system for non-contact time-resolved diffuse reflectance imaging, based on small source-detector distance and high dynamic range measurements utilizing a fast-gated single-photon avalanche diode
When a photon causes an avalanche within the detector, a low-jitter standard nuclear instrumentation module (NIM) pulse is provided by the high-speed comparator of the single-photon avalanche diode (SPAD) module and is fed to the “start” input of the time-correlated single-photon counting (TCSPC) module
In order to characterize the setup, we adopted two standardized protocols for performance assessment of timedomain optical brain imagers: the Basic Instrumental Performance protocol29 and the nEUROPt protocol,30 in particular, the tests related to the Instrument Response Function (IRF) and the responsivity of the detection system as well as several tests of the nEUROPt protocol related to contrast, contrast-to-noise ratio, and lateral spatial resolution
Summary
B)Present address: Group of Applied Physics, University of Geneva, Geneva 1211, Switzerland. The main disadvantage of this technique is the huge increase (with respect to larger source-detector distance) of the peak of early photons causing saturation of the detection electronics. To overcome this problem, we use a single-photon avalanche diode (SPAD) in fast-gated mode to extract late photons by rejecting the peak of early ones. Coupling the fast-gated SPAD to a time-correlated single-photon counting (TCSPC) board and after appropriate post-processing analysis (see Ref. 21 for further details), we could reconstruct the whole distribution of times of flight (DTOF) with a dynamic range of up to 8 decades.21 In another previous work, we have shown the first proofof-principle of non-contact time-resolved diffuse reflectance measurements on phantoms using a quasi-null source-detector distance approach and a fast-gated SPAD.
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