Design, development and characterization of a compact time-resolved system for functional NIR studies

We developed a compact two-channel time-resolved system for functional near infrared spectroscopy studies of muscle and brain. The system is based on picosecond semiconductor lasers (1 mW average power, 80 MHz repetition rate, operating at 690 and 829 nm), graded-index multimodal fibers and step-index fiber bundle (3 mm diameter, 0.5 NA) for light delivery to and collection from the sample, cooled photomultiplier tubes, and time correlated single photon counting modules interfaced with a notebook. We implemented a novel approach based on space-multiplexing of the two wavelengths, which are alternatively injected in the two channels by means of an optical switch. With this method we can acquire, in each detection line, one wavelength at a time, in order to exploit the full temporal dynamic range of the acquisition boards, thus increasing the signal to noise ratio and avoiding wavelength cross-talk with respect to the typical approach based on time-multiplexing. The instrument was characterized on tissue phantoms and tested during preliminary in vivo measurements performed both on muscle and brain tissues.