Digital phantom for time-domain near-infrared spectroscopy of tissue: concept and proof-of-principle experiments

In the clinically relevant field of tissue oximetry, there is an urgent need to develop phantom-based methods for validation. Physical phantom approaches based on solid or liquid turbid media containing hemoglobin with variable oxygenation have limited capabilities to represent real tissues regarding optical properties, structure and variability. Digital phantoms are an alternative with high flexibility. Rather than physically simulating the process of light propagation, they provide the detector with light signals mimicking the signals detected in vivo. We present a technique to produce digital phantoms for time-domain diffuse optical spectroscopy that mimic arbitrary photon time-of-flight distributions (DTOFs) by creating a time-dependent attenuation. The setup contains a spatial light modulator (SLM) and a set of optical fibers of different lengths corresponding to a stepwise delay. The light pulse entering the arrangement is spatially dispersed and illuminates the SLM which controls the intensity at each pixel. The SLM array is imaged onto the entrance faces of the delay fibers. The amount of photons received by each individual fiber can be adjusted. Finally, the light transmitted through all fibers are combined and fed to the detector of the timedomain instrument under test. In this way, DTOFs of any desired shape can be obtained. For first proof-of-principle experiments to demonstrate the general feasibility of the concept we used a liquid-crystal SLM and a set of four graded-index fibers differing in length by about 100 mm. The tests were performed with a timedomain instrument based on time-correlated single photon counting, with picosecond diode and supercontinuum laser sources and a single-photon avalanche diode as well as a hybrid photomultiplier as detectors. This large separation in fiber lengths allowed the performance regarding amplitude and temporal shape to be assessed for each delay independently. The generation of arbitrary DTOFs was simulated by realizing various patterns of target amplitudes. Temporal position and width of the measured pulse profiles for all fibers were in agreement with the expectations. Amplitude linearity was reasonable while the contrast between highest and lowest amplitude values was not yet satisfactory. Steps of further development are discussed.