Lung disease is a local or systemic disease characterized by pulmonary manifestations. The importance of this organ, the lung, to the human body cannot be overstated, and a diseased or damaged lung can seriously affect human health. According to the World Health Organization, global cancer incidence and mortality rates have shown a continuous increase in recent years, with lung cancer topping the list. With increasing air pollution, a growing smoking and aging population, and the emergence of drug-resistant pathogens, tuberculosis and pneumoconiosis have become the top two infectious diseases in China, accounting for 90% of occupational disease cases. The diagnosis and treatment of lung diseases are becoming increasingly important, and many patients with lung diseases such as lung cancer, severe pneumonia, emphysema, and pulmonary embolism in intensive care units require noninvasive, continuous, and immediate monitoring. Thus, noninvasive real-time lung monitoring methods are important for the prevention and treatment of lung diseases.

The VCH slice specimens used in this study were from an adult male that was sliced horizontally at regular intervals in a standing position. Each slice was a digital color photograph with distinguishable tissues. The lung was divided into five lobe regions for simulations. Two hundred such images were combined with image processing methods to construct a three-dimensional matrix to present the entire lung tissue structure. According to the location of the lung lobes, five lung lobe models of 420 voxel×436 voxel×200 voxel were segmented. The simulation results showed the specific situation of the five lung lobes. Each voxel was a 0.04 cm×0.04 cm×0.04 cm^{cube. The Monte Carlo method was used to simulate the migration of light in the lungs. This study used the MCVM software, which targets 3D voxelized media. In the simulation, the light source was set to an 800 nm point light source, and the optical properties of 10 tissues were in the near-infrared band at 800 nm. The light source was initially located near the fourth right rib near the sternum body. Additionally, in a preliminary experiment, light intensity signals were measured in the lungs of 13 young healthy volunteers. The 13 volunteers included 4 women and 9 men (25-35 years old). During the experiment, the light sources were placed at the corresponding positions of the five lung lobes in turn. The volunteers’ prothorax detected light intensity at three locations. The back of the volunteer detected light intensity at two locations. All subjects were asked to lie down quietly and rest for 2-3 min before the measurements to allow their breathing to stabilize. The parameters for the experiment were set based on near-infrared device applications. The main parameters are the wavelength of the light source and the collection frequency. These parameters were transmitted to the near-infrared device using bluetooth. Then, the light intensity signals at each position were collected by the near-infrared device. The parameters collection time was 2 min.}

Based on the VCH lung model (Fig. 1), this study used the Monte Carlo simulation method to quantitatively analyze the photon migration characteristics in the human lung. The light fluence distribution in the lungs and the changes in the light fluence intensity indicated that photons could reach the lungs from the light source (Fig. 3). The photon penetration depth from the skin to the lung was 32-36 mm, while the photon penetration depth in the lung was 6-8.4 mm. The SSD for the five lobes was 0.0235%-0.0368%. The average photon absorption of five lung lobes was approximately 9% (Fig. 4). The differences in photon migration in the five lung lobes could be reflected. Additionally, we proposed an optimization plan for the source-detector distance (L_SD). According to the two path length factors, PPF and DPF, combined with the PPF/DPF index, the L_SD in the lungs could be the optimal topological location between 2.8 and 3.6 cm; however, the distance from the superior lobe of the right lung was 3.3-3.5 cm. Using VCH-based Monte Carlo simulations of human lungs, the optical migration characteristics of the lungs were quantitatively visualized, the feasibility of noninvasive optical detection of the lungs was proven, and the optimal L_SD for the optical detection of the lungs was found. This study performed optimization to determine the important parameter L_SD for optical noninvasive testing of the lungs. The optimal lung L_SD was 2.8-3.6 cm, but the optimal spacing of the superior lobe of the right lung was 3.3-3.5 cm. Compared to previous studies on the detection of other parts of the human body in the biomedical optics community, the optimal distance between the lungs was significantly smaller than the optimal distance in other body parts. The optimal spacing for these different body parts has been reported to be approximately 3 cm. In this case, a greater separation distance did not a better result. On the contrary, with an increase in the spacing of the source-detector, the energy density of the transmitted photon rapidly decreased, which seriously affected the quality of the detection signal. The optimal separation preference depended on the detection of light density and the path length factors, PPF and DPF. In a preliminary lung light intensity experiment, the L_SD spacing was 2.8 cm, which was within the optimal L_SD range for the lung. A better light intensity signal could be obtained by performing detection in the optimal L_SD range. The experimental data showed that the light intensity values calculated from the detection were much smaller than the simulated light intensity values [Fig.7(a) and Fig.7(c)]. The overall light intensity trend of the five locations indicated a correlation between the experimental and simulated values with P<0.05 [Fig. 7(b) and Fig. 7(d)]. This implied that the detected light intensity signals at the five locations may come from five lung lobes. This further proves the feasibility of the noninvasive optical detection of human lungs.

In summary, this study used Monte Carlo simulations to visualize the propagation of near-infrared light in a VCH lung model. Photon migration in the lungs was studied. Photon absorption of each lobe was shown to be approximately 9%. The optimal source-detector distance in the superior lobe of the right lung was found to be 2.8-2.9 cm, while the optimal distance for the other lung lobes was 3.0-3.6 cm. A preliminary experiment demonstrated the feasibility of optical detection of the lungs. This study also provided theoretical support for using functional near-infrared spectroscopy for lung detection and a reference for the optimal L_SD. It is hoped that the results of this study will promote research on lung diseases in the biomedical optics community.