<title>Compact time-resolved reflectance system for dual-wavelength multichannel assessment of tissue absorption and scattering</title>

Abstract

We developed a dual-wavelength multichannel system for time-resolved reflectance measurements based on the time- correlated single-photon counting (TCSPC) technique. The output pulses of two laser diodes (672 and 818 nm, 80 MHz pulse repetition rate, 1 mW average power, 100 ps FWHM) are delayed and coupled into a multimode graded-index fiber (50/125 ?m) and injected into the tissue. The reflectance photons are collected by 1 mm fibers and detected by a multianode photomultiplier. The output signals of the photomultiplier are redirected by a router to different memory blocks of a TCSPC PC board, allowing the parallel acquisition of the curves. The solution of the time-dependent diffusion equation for the reflectance through a semi infinite homogeneous medium is best-fitted to the experimental curves, leading to the simultaneous estimate of the reduced scattering and absorption coefficients at two wavelengths in four different positions. The minimum acquisition time is 100 ms, fixed by the maximum count rate of the TCSPC PC board (1 MHz) and by the required S/N ratio. The performances of the system were tested on phantoms in terms of stability and accuracy in the determination of absolute values of the absorption and reduced scattering coefficients. Preliminary in vivo measurements were performed on healthy volunteers to estimate absorption and reduced scattering in different physiological conditions and to derive tissue functional parameters such as tissue oxygenation.