Functional Imaging via Diffuse Correlation Spectroscopy

Functional neuroimaging plays a key role in understanding the neural circuitry and fundamental mechanisms of the brain. Advancements in diffuse optics, Light Detection and Ranging (LiDAR) and astronomy imaging systems are fostering the development of new instruments and techniques for non-invasive real-time optical and acoustical neuroimaging. Ozana etc. al. recently introduced the advantage of illumination at 1064nm, demonstrating improved performances with respect to the standard NIRS wavelengths range via the following methods. The first method is based on superconducting nanowire single photon detectors (SNSPDs), which offer superior photon efficiency, timing resolution, dark count rate, broad wavelength sensitivity, and low after-pulsing probability. Initial human results show a 16-fold improvement in SNR and 20% improvement in depth sensitivity. The second approach, a fully custom multichannel system for time-gated diffuse correlation spectroscopy (DCS) at 1064nm is based on: i) pulsed quasi-transform-limited laser emitting at 1064nm; ii) multi-channel FPGA correlator; iii) novel time-gated 32×32 InP/InGaAs based Single Photon Avalanche Diode (SPAD) array. The custom-made pulsed laser operates from 1 to 75MHz and allows for real-time pulse-shaping due to electro-optic modulation, providing up to 100mW average output optical power. Detector performance features sub-ns gate OFF-ON transition for late photons selection and up to 10MHz gate repetition frequency, with SPADs grouped 4×4, providing up to 64 independent fiber-coupled detection channels. This time-gated system allow rejection of short pathlength photons that travel mostly in extracerebral layers i.e. scalp and skull and allow selection of longer pathlength photons that travel in the deeper brain layer. The last method is based on interferometric diffuse correlation spectroscopy (iDCS) heterodyne detection which enables high speed (>50Hz) and high SNR measurements of cerebral blood flow. Monte Carlo simulations results, together with experimental phantom characterization and human functional measurements of these approaches will be presented.