Quantitative measurement of burn depth is of great significance for the clinical assessment of burn degree and treatment plan. Currently, the most widely used assessment method is visual inspection, which places high demands on doctors’ experience and is easily influenced by subjective judgment. Other detection techniques, such as laser Doppler imaging, ultrasound imaging, and fluorescence imaging, have also been used to assess the extent of burns; however, these techniques cannot non-invasively and accurately measure burn depth. Polarization-sensitive optical coherence tomography (PSOCT) has the advantages of non-invasiveness, fast imaging speed and high resolution and can quantitatively measure the burn depth based on the polarization information of the burned tissue. However, the traditional measurement method is based on the accumulated polarization information from the sample surface to a certain depth inside the sample, which cannot accurately characterize the local polarization information at this depth; hence, the burn depth cannot be accurately measured. Therefore, this study proposes a local polarization information extraction algorithm based on spectral domain polarization-sensitive optical coherence tomography (SD-PSOCT) to obtain polarization information at each depth inside the burned biological tissue to quantitatively measure the burn depth of the biological tissue.

A local polarization property extraction algorithm based on the SD-PSOCT system was proposed and used to quantitatively measure the burn depth of biological tissue. All single-mode-fiber-based systems adopt fiber-based polarization controllers to illuminate a sample with a single-input polarization state. A custom-built linear-in-wavenumber spectrometer consisting of a diffraction grating, dispersive prism, Wollaston prism, and a focusing lens was used to realize polarization-sensitive detection (Fig. 2). Then, the local phase retardation and axis orientation of each layer of the sample were calculated by eigenvalue decomposition based on the Jones matrix and layer-by-layer iterative algorithm. To evaluate the measurement accuracy and stability of the system, we used a quarter-wave plate (QWP) as the sample and measured the phase retardation and axis orientation of the QWP under different axis orientations each day for 14 days. To measure the burn depth of the biological tissue, we selected a piece of bovine tendon tissue as experimental sample, burned the same position of the bovine tendon five times for 10 s each, and then reconstructed the local phase retardation images of the bovine tendon unburned and burned for 10 s, 20 s, 30 s, 40 s, and 50 s, respectively. We then considered the full width at half maximum of the local phase retardation versus the imaging depth curve as the burn depth.

From the sensitivity roll-off curves we can see that the sensitivity at the detection depths of 0.2 mm and 1.2 mm are approximately 105 dB and 98 dB, respectively (Fig. 3). The measured average value of the phase retardation of the QWP is 82.9° and the measurement error is 1.9° (Fig. 4). The 14-day measurement results show that the phase retardation varies within a range of -0.42° to + 0.42° and the axis orientation varies within a range of -0.66° to + 0.66°. By comparing the local phase retardation images of the bovine tendon subjected to different burn times (Fig. 6), it is found that the local phase retardation inside the burned bovine tendon increases, and as the burn time increases, the region with a higher local phase retardation extendes to a deeper position. From the depth-resolved local phase retardation, it can be seen that the region with higher phase retardation gradually widens with increasing burn time (Fig. 6). Thus, the measured burn depth of bovine tendon tissue burned for 50 s is 390 μm.

We deduce the local polarization property extraction algorithm based on the Jones matrix in detail and provide the calculation formulas of local phase retardation and axis orientation. The sensitivity roll-off curves of the two orthogonal polarization channels in the linear wavenumber spectrometer are experimentally measured, and the measured sensitivity of the system is 105 dB. The actual phase retardation and axis orientation of the QWP at different axis orientations are measured and it is verified that the system can measure the polarization properties of birefringent samples with high accuracy and maintain good measurement stability. The imaging results of bovine tendon tissue subjected to different burn times show that the SD-PSOCT system can obtain polarization images with higher contrast than traditional OCT images. Additionally, compared with the cumulative phase retardation image, the local phase retardation image obtained by the algorithm can highlight the difference in the bovine tendon after being burned for different times and quantitatively measure the burn depth according to the local phase retardation images. This study provides a new method for quantitatively measuring tissue burn depth, which can be applied to clinical diagnosis and burn treatment in the future.