Abstract
The ability to image through turbid media, such as organic tissues, is a highly attractive prospect for biological and medical imaging. This is challenging, however, due to the highly scattering properties of tissues which scramble the image information. The earliest photons that arrive at the detector are often associated with ballistic transmission, whilst the later photons are associated with complex paths due to multiple independent scattering events and are therefore typically considered to be detrimental to the final image formation process. In this work, we report on the importance of these highly diffuse, "late" photons for computational time-of-flight diffuse optical imaging. In thick scattering materials, >80 transport mean free paths, we provide evidence that including late photons in the inverse retrieval enhances the image reconstruction quality. We also show that the late photons alone have sufficient information to retrieve images of a similar quality to early photon gated data. This result emphasises the importance in the strongly diffusive regime of fully time-resolved imaging techniques.
Highlights
Imaging through a highly diffuse medium such as biological tissue, can be severely hindered by the presence of scattering which causes photons to propagate along a random and complicated trajectory
Photons travelling in scattering media can be divided into three broad categories [1,2,3]: ballistic photons, where there is no interaction with the medium and the photons propagate straight through with a coherent wavefront; snake photons which are weakly scattered, arrive immediately after ballistic photons and maintain some coherence since there are only minor deviations in direction; and diffuse photons which have scattered many times and have a trajectory that no longer corresponds to their initial propagation direction, leading to an incoherent wavefront
We explore the impact of the arrival time of the detected photons on image reconstruction for time-of-flight diffuse optical imaging
Summary
Imaging through a highly diffuse medium such as biological tissue, can be severely hindered by the presence of scattering which causes photons to propagate along a random and complicated trajectory. Other time-domain diffuse optical imaging systems, configured in a reflection geometry, use late arriving photons to selectively isolate information from different depths [25,26,27,28,29] This does not overcome the limitation of detecting only highly scattered photons when gating for very deep regions of the sample. The transmitted photons were detected with a Single Photon Avalanche Diode (SPAD) array with the time-of-flight information provided using Time Correlated Single Photon Counting (TCSPC) at each individual pixel, recording both the spatial and temporal properties of the transmitted light This method uses regularised least-squares optimisation applied to the full 3-dimensional data for image reconstruction and has been demonstrated to resolve feature sizes on the order of 1 mm through more than 80 transport mean free paths. Using the experimental data acquired in [41], we demonstrate that the whole temporal profile of the transmitted photons must be sampled to give the best result and highlight the importance of measuring late photons
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