## 1 Introduction  
Applications of optical coherence tomography (OCT) in brain tissue include structural imaging and functional sensing. Structural imaging includes macroscopic imaging [1, 2] and angiography of the brain tissue [3]. Common detectable brain functional activities are shown in Figure 1[4, 5], and the associated optical processes are also indicated. Among them, scattering and absorption can be obtained by spectroscopic OCT [6, 7], and Doppler shift can be measured by Doppler OCT [8]. This paper focuses on the applications of OCT in brain structural imaging as well as in the measurement of absorption and scattering coefficients of brain tissue. Both numerical and in vivo testbeds are useful for brain tissue imaging. In vivo testbeds provide the ultimate confirmation of performance, while numerical testbeds allow a more controlled evaluation of imaging modalities and optimization of their parameters. Monte Carlo (MC) simulation is the golden means to simulate the interaction between light and tissue [9; 10]. Many efficient MC models have been developed to study OCT signals of different tissues [11; 12; 13; 14] or polarization-sensitive OCT [11; 15]. However, there are few literatures to analyze OCT signals in brain tissue using MC technology. Therefore, as a slight supplement to this field, we detail how to use MC simulation to analyze the applications of OCT in brain tissue structural imaging and functional sensing. A model of mesh-based Monte Carlo [14; 16; 17] for OCT (MMC-OCT) is used in this paper, which is useful for any complex three-dimensional (3D) bio-tissues. This work provides an analysis method and related data for brain simulation as well as help for experimental study. Quantitative accuracy is undoubtedly an important indicator in the measurement of scattering and absorption of brain tissue. In the spectroscopic OCT, extinction coefficients of different wavelengths are obtained by using the short-time Fourier transform (STFT) and exponential fitting [7], and the extinction coefficient is the sum of the absorption coefficient and scattering coefficient \(S\). It is a common choice to use a visible light waveband for detection of scattering and absorption [6; 18], and the main absorption in this range comes from hemoglobin for brain tissue [19; 20]. Thus, the total hemoglobin concentration \(C\) and oxygen saturation \(Y\) of the brain tissue can be calculated by solving linear equations using absorption coefficients of several different wavelengths [4], where \(C\) and \(Y\) are defined in Figure 1. The extinction coefficients of multiple wavelengths can be obtained in spectroscopic OCT, so the results obtained by fitting the oversampled data will undoubtedly be more accurate considering the noise and experimental errors in the actual applications. However, the spectral resolution and spatial resolution are contradictory [21], and the more positions the window function traverses, the longer the calculation time. In addition, the narrower the width of the window function in the spectral domain, the worse the signal-to-noise ratio (SNR). For measurements of blood vessels with high hemoglobin concentrations (\(\sim\)2 mmol) [22], two or three wavelengths can be used to resolve the correct functional parameters [23; 24] while achieving high spatial resolution. In the process of solving \(C\), \(Y\), and \(S\) from the fitted extinction coefficients of three wavelengths, we found that the error of the extinction coefficient has a great influence on the calculated results, and the values of \(C\), \(Y\), and \(S\) calculated using different wavelengths are distinct for the certain error. Therefore, the selection of wavelengths that are used to calculate scattering and absorption is explored in this paper. It is proposed and validated here that the measurement accuracy of \(C\), \(Y\), and \(S\) can be improved by selecting the appropriate wavelengths for calculation. Figure 1: Relationship among common brain functional activities, physiological parameters, and optical processes.