Mont-Carlo simulation of OCT structural images of subcutaneous blood vessels Petrov D.A., Potlov A.Yu., Proskurin S.G. Biomedical engineering, TSTU, Russia http://bmt.tstu.ru/ http://spros.tamb.ru/ spros@tamb.ru Saratov Fall Meeting 2016

Abstract A method of optical coherence tomography (OCT) structural images simulation using the Monte Carlo method with the help of multithreaded computation is described. Simulated structural images of subcutaneous blood vessel are presented. Biological object inner structure simulated with set of cubic volumetric shapes based on the experimental results. B-scans of the inner structure are demonstrated instead of analytical representation of the boundary geometry. Effectiveness of the described simulation technique is checked by the comparison of the simulated and experimentally acquired A-scans. For this purpose, the biological object is considered as a set of 3D elements that allow simulation of media, structure of which cannot be described analytically. Each element is characterized by its refractive index and anisotropy, scattering and absorption coefficients. The probability of reflection on the interface of elements with different refractive indices is determined using Fresnel equations. The photon mean free path is calculated using total attenuation coefficient. The possibility of simulation and visualization of biological objects with high scattering, such as blood vessels and blood flow was demonstrated.

Voxel representation Biological object inner structure simulation based on voxels and analytical expressions

Object structure simulation Biological object inner structure simulation based on the experimental B-scan

Simulation algorithm

Photon packet launching Photon packet start position: where - beam waist, - azimuthal angle - current A-scan index - uniformly distributed random number between 0 and 1 Photon packet initial position: where - focal distance

Photon packet launching And energy absorption Step size of photon packet : where - scattering coefficient Photon packet energy absorption calculated using Beer-Lambert law: where - absorption coefficient - statistical weight of photon packet

Scattering As a photon travels, free path distance direction of its propagation is changed as where - scattering angle, - azimuthal scattering angle, - directional cosines.

Voxel boundary intersection Voxel – photon packet intersection calculated using Smith algorithm: - distances to voxel boundary plane, - current photon location , - Voxel facet length, Distances are compared, and the minimal value represents the distance to the closest voxel boundary.

Photon detection When photon exits tissue its weight added to intensity of the image according to - structural image pixel intensity - wave length - coherence length - fiber acceptance angle - detector radius - distance from center of detector to point where photon exits tissue - Photon path length in object arm

Subcutaneous vessel structure Experimental subcutaneous blood vessel image Object structure used in simulation

Optical properties of the layers Stratum corneum 320 0,2 0,9 1,49 Upper epidermis 63 0,3 0,93 1,37 Epidermis 120 0,6 0,87 1,38 Dermis 90 0,8 1,36 Lower dermis 110 1.1 1,35 Blood 650 2 0.995 1.37 Vessel wall 2.8 0.9 0.95 1.38

MC simulated images Simulated images of subcutaneous blood vessel with (right) and without (left) speckle influence. Correlation with experimental data is between 0.85 and 0.9 Size of the image is 2x2 mm in all cases

A-scans comparison The biggest difference located at the stratum corneum – air boundary

Conclusion Described simulation technique allows to simulate OCT of biological object with high precision. This is because the structure of an object is reconstructed using information taken from experimental data. High efficiency is provided both for simulation with and without presence of speckles. Some inconsistency may occur due to choose of inaccurate optical properties of the simulated object. Future work will be focused on other types of tissues and light source properties.

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