Multi-Parameter Imaging Analysis of Pig Skin Burns Based on Fiber Polarization-Sensitive Optical Coherence Tomography

Objective Burns are a common type of skin injury. Diagnosing the degree of the burn is very important for proper treatment. Optical coherence tomography is a non-invasive, non-destructive, and high-resolution optical detection technology. Polarization-sensitive optical coherence tomography (PS-OCT) provides a comparison of birefringence information compared to the conventional structural OCT modality. It can be used for the high-resolution, high-contrast, real-time three-dimensional imaging of damaged skin. In this work, a simple, compact, flexible, and efficient PS-OCT system is developed based on single-mode fiber optics with a circularly polarized single-input state as the swept source. The high-performance swept source enables a high imaging speed and long coherence length for the OCT imaging. The PS-OCT system is based on single-mode fiber optics and features low polarization crosstalk, low polarization mode dispersion, and a compact size. A multiple-parameter analysis shows that the PS-OCT system has the potential to provide accurate clinical assessments of skin burns. Methods We construct a swept-source PS-OCT system with single-mode fiber optics. By tuning the polarization controllers step by step, a single circular polarization input in the sample arm and OCT signal detection with orthogonal polarization channels are achieved. Using straightforward data processing algorithms, the PS-OCT system has the capability to acquire various parameters, including the structural intensity, degree of polarization uniformity (DOPU), cumulative phase retardation (CPR), and Stokes state. Given the anatomical and physiological resemblance between pig skin and human skin, ex vivo pig skin is selected as the imaging subject for the skin burn model in this study. To simulate the burns, eight groups of pig skin samples are subjected to a circular thermal injury with a diameter of 10 mm using a temperature-controlled wound burning device at 90 degrees C for a duration of 30 s. We compare the multi-parameter PS-OCT images of the normal and burned pig skin samples. According to the image histogram, the Bhattacharyya distance is calculated to demonstrate the capability of the PS-OCT system for skin burn evaluation. Results and Discussions In the structural OCT images, the difference between the normal and burned pig skin samples is not obvious (Fig.3). As shown in the cross-sectional structural OCT images, the total scattering intensity has similar values in the regions of the normal and burned skin samples. In the en-face structural images, the boundary of the burned skin region is clear, and the pattern of the skin texture is different. Compared to the structural images, the polarized images show obvious differences between the normal and burned pig skin samples in terms of the Stokes state, DOPU, and cumulative phase retardation (Fig.4). In the region of the burned skin, the color of the Stokes state image becomes relatively uniform, the value of the DOPU image is relatively large, and the CPR value is relatively low. The en-face images demonstrate that the structural intensity values of the normal and burned pig-skin regions are very similar, whereas the DOPU, CPR, and Stokes values have relatively large differences. The histograms of these en-face images further verify that polarized images are more useful in distinguishing normal and burned skin (Fig.5 and Fig.6). Using to the histograms, we calculate the Bhattacharyya distance to quantify the difference in the images between normal and burned pig skin (Fig.7). If the images are very similar, the Bhattacharyya distance is close to 0. If the images are very different, the Bhattacharyya distance is close to 1. For the 8 groups of skin burn experiments, the average Bhattacharyya distance of the structural images is 0.184, while the values for the DOPU images, CPR images, and Stokes images are 0.917,0.744, and 0.839, respectively. A quantification analysis demonstrates that the difference between normal and burned skin in the traditional OCT structural images is small, while the polarized images show a significant difference in the burned skin. The PS-OCT system used in this study adopts a design based on single-mode fiber optics. However, the birefringence characteristics of single-mode fiber optics are easily affected by environmental factors such as bending stress and temperature changes. Therefore, once we have completed the steps to calibrate the polarization state of the PS-OCT system, the optical fibers in the system must not be touched. In an actual work environment, the polarization state of the imaging system can be maintained for several days without significant changes, thus meeting the needs of most clinical and life science applications. In the future, we will further optimize the optical design of the PS-OCT system to improve its polarization stability. In addition, the PS-OCT imaging system shows that a polarized image of pig skin tissue exhibits an obvious change after being burned. The mechanism of the change in the polarization state mainly comes from the irreversible denaturation of the collagen and elastic fibers in the skin tissue after heating. However, in this study, only a small amount of ex vivo pig skin is used as the model for a skin burn. The number and type of experimental samples are insufficient. In addition, the ex vivo skin samples have lost their biological tissue activity characteristics. In the future, we will increase the number and type of skin burn models. Furthermore, we also need to investigate living animal samples and skin burn patients to promote the application of PS-OCT imaging in the diagnosis and treatment of skin burns. Conclusions A flexible and efficient PS-OCT system based on single-mode fiber optics and a single-state input is built to image ex vivo pig skin for skin-burn investigations. The system can provide structural images and three polarized images (DOPU, CPR, and Stokes state) of skin tissue. We compare images of normal and burned skin, and perform histogram statistical analysis to illustrate the distribution of these parameters. Moreover, we calculate the Bhattacharyya distance as a histogram similarity coefficient to further quantify the imaging performance. The results show that there are significant birefringence changes in the burned skin tissue compared to the normal skin tissue, which are mainly due to the denaturation of the collagen and elastic fibers after heating. The changes in burned skin can be clearly observed using the polarization parameters (DOPU, CPR, and Stokes state). These polarized OCT images exhibit enhanced contrast and more pronounced distinctions for burned skin compared to conventional structural OCT images. This research demonstrates the promising potential of PS-OCT technology for skin-burn diagnosis.