Academic News Report: Study on the Optical Transmission Characteristics of Cartilage

Introduction

Cartilage is a complex biological tissue composed of cells and a substantial extracellular matrix. Its main components include collagen fibers, proteoglycans, and water, which form layered structural partitions at the microscopic level. The propagation of light in such a tissue is influenced by its intrinsic optical properties, such as absorption coefficient (𝜇𝑎), scattering coefficient (𝜇𝑠), scattering anisotropy factor (𝑔), and refractive index (𝑛). These optical properties can vary due to changes in the tissue’s microstructure or pathological state. Understanding how these properties affect light propagation can reveal the structural and biochemical characteristics of the tissue. Thus, studying the light transmission characteristics in cartilage is of significant importance for diagnosis and treatment.

Background

This study, authored by I. Kafian-Attari, E. Nippolainen, and F. Bergmann, comes from the University of Eastern Finland and the Institute for Laser Technology in Medicine and Metrology at the University of Ulm. It was published in IEEE Transactions on Biomedical Engineering, aiming to investigate average penetration depth (𝑃𝐷𝑎𝑣𝑔), maximum penetration depth (𝑃𝐷𝑚𝑎𝑥), maximum lateral spread (𝐿𝑆𝑚𝑎𝑥), and path length (𝑃𝐿𝑎𝑣𝑔) of light in cartilage using Monte Carlo simulations. These parameters can provide diagnostic information regarding the health status of joint cartilage.

Research Methods

The study employed multiple methods, including sample preparation, optical property measurement, and Monte Carlo simulation. The specific processes are as follows:

Sample Preparation

Researchers extracted 22 joint cartilage samples from the knees of seven cows. These samples were divided into two groups: Group A (n=11) and Group B (n=11). The samples were collected using a 15 mm diameter punch and preserved in PBS solution to prevent drying.

Micro-CT Imaging

Group A samples were immediately subjected to micro-CT imaging after extraction to estimate the axial thickness and lateral diameter of the samples. These physical characteristics were subsequently used to create digital models of the cartilage tissue for Monte Carlo simulations.

Cryosectioning

Group B samples underwent cryosectioning to estimate the scattering coefficient (𝜇𝑠) and scattering anisotropy factor (𝑔) of each cartilage layer and were calibrated using Beer-Lambert’s law.

Optical Measurement

The study used integrating sphere methods to measure the absorption coefficient (𝜇𝑎) and reduced scattering coefficient (𝜇𝑠’) of Group A samples and used parallel transmission measurement methods to obtain the extinction coefficient (𝜇𝑡), scattering coefficient (𝜇𝑠), and scattering anisotropy factor (𝑔) of Group B.

Polarized Light Microscope Imaging

Group A samples were subjected to polarized light microscope imaging to estimate the axial arrangement of the collagen fibers. The samples were fixed, dehydrated, and embedded in paraffin before obtaining depth-angle distributions of the collagen fibers through polarized light microscope imaging.

Monte Carlo Simulation

Using Monte Carlo simulation tools (such as pyxopto software), the study simulated light propagation in cartilage across the 400-1400 nm spectral range. For each scenario, different wavelengths were analyzed to obtain 𝑃𝐷𝑎𝑣𝑔, 𝑃𝐷𝑚𝑎𝑥, 𝑃𝐿𝑎𝑣𝑔, and 𝐿𝑆𝑚𝑎𝑥. These simulations considered various combinations of optical properties to comprehensively evaluate the light propagation characteristics in single-layer and multi-layer cartilage tissues.

Research Results

The study found that within the 400-1400 nm spectral range, 𝑃𝐷𝑎𝑣𝑔 and 𝑃𝐷𝑚𝑎𝑥 exhibited an increasing trend, while 𝑃𝐿𝑎𝑣𝑔 showed a decreasing trend. Particularly in the visible light range (400-700 nm), the increase in 𝑃𝐷𝑎𝑣𝑔 and 𝑃𝐷𝑚𝑎𝑥 occurred more rapidly, but this increase slowed in the near-infrared range (700-1400 nm). The study pointed out that:

1. Visible light primarily probes the superficial and middle layers of joint cartilage, while near-infrared light can penetrate deeper tissues.
2. Developing new standard models for optical diagnostic devices can aid in the design and optimization of future medical equipment.

Conclusion and Significance

This study is the first to estimate the light transmission parameters in joint cartilage and highlight the different behaviors of light under various spectra. The findings demonstrate that light in the 400-700 nm range mainly probes superficial areas, whereas light in the 700-1100 nm range can penetrate and evaluate the entire cartilage layer. The study concludes that the near-infrared spectrum is applicable for detecting and assessing joint cartilage health, particularly providing real-time information during orthopedic surgeries to enhance diagnostic accuracy.

Highlights

The highlights of this study include: 1. Detailed analysis of cartilage light transmission parameters for the first time, filling a research gap in this area. 2. A comprehensive analytical framework combining multiple technical approaches. 3. Monte Carlo simulation analysis of light propagation in cartilage, providing a theoretical basis for the design of new medical devices.

This research enhances the understanding of light propagation characteristics in cartilage and its application value in diagnosis and treatment, laying a foundation for subsequent studies.