Comparison of propagation‐based phase contrast tomography and full‐field optical coherence tomography on bone tissue

Abstract

The current huge development of new 3D microscopic techniques (synchrotron microtomography, optical coherence tomography, light sheet microscopy, …) opens a large variety of new perspectives for life sciences. The contrasts of these new microscopies are mostly well understood on samples of known material content such as those used in physics or instrumentation studies. The situation is different when it comes to the interpretation of the contrasts observed with complex heterogeneous media found in biology. Therefore determining which 3D microscopy technique is suited for which biological question is a topic of current interest (see [1,2] for instance in our group). In this communication, we propose a comparison of the contrast observed with full‐field optical coherence tomography (OCT) and propagation‐based phase contrast tomography (PCT) on bone tissue at similar spatial resolution. A first comparison of OCT with standard absorption microtomography was given in [3] for bones and we extend this comparison to PCT which is known to provide enhanced contrast on bones at multiple scales [4]. The contrast of both these techniques are a priori interesting to be compared since they both rely on discontinuities of refraction index. This produces phase shift in PCT which operates in the X‐ray domain with a monochromatic beam (generated by a synchrotron) while this generates direct intensity reflexion with OCT which only resorts to white light in the visible domain. As visible in Figure 1, we specifically focussed our attention on the contrast observed in both techniques around the same bone structural unit, a so‐called osteon, at a microscopic scale with images of same spatial resolution (voxel size 3.5µm). It happens that the osteons are visible in PCT while they are not perceptible with conventional absorption micro computed tomography. Also, concentric lamellae, corresponding to the so‐called Harvers system, appear clearly visible in OCT while they are not perceptible with PCT at this spatial resolution. The contrast between the osteon and the surrounding bone tissue, is found in terms of homogeneous regions in PCT. However, this less spatially resolved contrast in PCT is constant throughout the sample while it is spatially variable in OCT where a continuous degradation of the contrast is observed along the direction Z of the propagation of light. We found, as given in Figure 2, that a certain spatial average of some 30 µm along Z was able to improve optimally the contrast across the concentric lamellae when inspected at the surface (up to 500 µm depth) of the sample with OCT. This contributes to establish quantitatively the complementarity of OCT and PCT for the characterization of bones at the microscopic scale.