Instrument response function acquisition in reflectance geometry for time-resolved diffuse optical measurements

Time-resolved (TR) techniques are exploited in many biomedical applications in order to find absolute values of absorption (mu(a)) and reduced scattering (mu(s)') coefficients that characterize biological tissues chemical and microstructure properties. However, the concomitant acquisition of tissue distribution time-of-flight (DTOF) and instrument response function (IRF) is necessary to perform quantitative measurements. This can be a non-trivial time consuming operation which typically requires to detach the optical fibers from the measurement probe (usually put in a reflectance configuration for in-vivo applications) in order to face them one to each other ("reference" geometry). To overcome these difficulties, a new IRF measurement method that exploit the "reflectance" geometry is here proposed. A practical 3D printed implementation has been carried out for a specific device to test the feasibility of this approach and if the IRF acquired in the "reflectance" geometry is equivalent to the "reference" one. A particular problem addressed is the determination of the temporal shift T-0 that can occur between IRF and sample DTOF. Two different approaches, based respectively on the curves barycenters difference and on a calibration phantom, are proposed. Both methods are valid and indifferently applicable according to specific measurement requirements. This allows "reflectance" IRF acquisition to be eligible as standard methodology for TR measurements.