Method of spatial scanning of modulated laser radiation for outline imaging of interphalangeal joints

Significance: studies of centimeter-thick biological organs. Aim: The development of a technique that overcomes the limitation imposed by scattering will significantly expand the scope of the application of transillumination imaging, allowing the examination or diagnosis of biological objects of centimeter thickness, in particular, interphalangeal joints. Approach: The approach used is based on a previously developed method of highly sensitive registration of predominantly ballistic and snake photons when scanning modulated laser radiation. In addition, to obtain a contour image, a technique of selective registration across the profile of a Gaussian laser beam was used, which made it possible to obtain high spatial resolution with a large diameter of a collimated laser beam. An optical clearing agent was used to mitigate the scattering imposed by the skin. Results: The method was tested on a model object and applied to obtain the contour of the interphalangeal joint. The resulting joint contour matches the actual shape, but its width in the image is larger than the actual width. Calculations show that the size of the structure in the image also depends on the intensity and diameter of the laser beam, as well as the extinction coefficient of the sample. It is shown that, given the other parameters, the extinction coefficient can be determined from the joint image space. The effect of the clearing agent on the clarity and contrast of the image is quantitatively analyzed, and a reduction in the scattering coefficient (by 9% to 14%) is estimated. Conclusions: The suggested method has been shown to be efficient for deep-tissue imaging of biological objects up to several centimeters thick, particularly joints, revealing the outline of structural features. We have shown that the use of an optical clearing agent allows reproducible images to be obtained with noticeably higher contrast and clarity, sufficient for preliminary express diagnostics of pathologies.