Objective Early-stage screening and treatment of mucosal tissue lesions are particularly important for mucosal cancer prevention. Since most mucosal lesions originate from the superficial epithelium, that is sub-diffusive regime, where the diffusion approximation theory is no longer applicable. In the sub-diffusive regime, the second-order phase function parameter γ has a great effect on the sub-diffusive reflectance, in addition to the absorption coefficient μa and reduced scattering coefficient μ's. Therefore, traditional diffuse reflectance spectroscopy (DRS) technique has some deficiencies in the early detection of mucosal lesions. Sub-diffusive reflectance spectroscopy (sDRS) technique can collect the light at short source-detector separations, which experiences fewer scattering interactions and carries more information about the microstructure of the tissue. Meanwhile, sDRS can achieve quantitative and multi-parameters detection, making it more suitable for mucosal tissue screening. In order to determine the optical properties of mucosal tissue, a spatially resolved reflectance in sub-diffusive regime measurement system was designed and digital lock-in detection technique was adopted to improve measurement speed and suppress random noise.
Methods In this study, the sDRS system is composed of light source module, hand-held optical fiber probe, data acquisition module, central control module, signal processing and human-computer interaction module. By collecting the reflection light of multi-wavelengths (520, 650, 785 and 830 nm) and multi-source-detector separations (220, 440, 660, 880 and 1100 μm), the system realizes the detection of spatially resolved sub-diffusive reflectance spectrum. In this system, digital lock-in detection technique of square-wave modulation is adopted by which the four wavelength light sources are excited simultaneously, and the five detectors are used to detect the reflected light parallelly. In addition, the system noise and the ambient light interference can be suppressed, which is extremely meaningful for detecting weak light. In order to demonstrate the feasibility of the system, a series of experiments are conducted to assess the system’s performances in terms of stability, linearity, ambient light suppression, and anti-crosstalk among multi-frequency channels. To further verify the effectiveness of the proposed system, turbid phantom experiments are carried out. Herein India ink and polystyrene microspheres are used to simulate the absorption and scattering properties of mucosal tissue, respectively. A modified forward Monte Carlo numerical model based on optical fiber probe geometry and Gegenbauer-kernel phase function and 3D lookup-table algorithm are used to predict the three optical parameters (μa, μ's, γ) simultaneously and quantitatively.
Results and Discussions The assessment results show that the fluctuation ratio of the system is less than 1.5% (Fig. 2); the crosstalk ratio among multi-frequency channels is less than 3% (Fig. 3). At different intensities of ambient light, all the voltage amplitudes obtained based on the digital lock-in detection technique are very low and almost unchanged (Fig. 4). When the intensity of the light source increases with a specific step, the correlation coefficients of the corresponding detection values obtained by linear fitting are all close to 1 (Fig. 5 and Table 1). Turbid phantom experiment results show that the predicted values and true values of optical parameters demonstrate satisfactory consistency (Fig. 6 and Table 3), under the condition that the numerical simulation noise cannot exactly match the experimental noise. These experimental results prove the feasibility and effectiveness of the proposed system and its potential application in mucosal lesion recognition.
Conclusions In this paper, towards practical application, a spatially resolved reflectance in sub-diffusive regime measurement system combined with digital lock-in detection technique was designed, which has the advantages of non-invasiveness, portability and speediness. A series of preliminary system evaluation experiments and turbid phantom experiments verify that the system possesses good stability, linearity, anti-frequency crosstalk ability as well as strong anti-ambient interference ability, and can effectively predict the three optical parameters simultaneously, indicating the feasibility and effectiveness of the proposed system. In the future work, we will further study the potential application of sDRS in the detection of mucosal lesions in vivo.
