Akuroma Tolvanen

Our study demonstrates that a compact, low-cost, phantom-calibrated diffuse reflectance spectroscopy system can provide realistic estimates of tissue optical properties, supporting its progression toward in situ cartilage diagnostics. We developed and validated a fiber-based DRS system capable of quantitative estimation of absorption coefficient (µ _a ) and reduced scattering coefficient (µ _s ′) in cartilage using a phantom-derived calibration model at wavelengths of 660, 780, and 850 nm. Diffuse reflectance spectra were acquired using a custom-built multi-distance probe, corrected using wavelength-specific calibration coefficients derived from optical phantoms, and fitted using diffusion theory with extrapolated boundary correction to recover µ _a and µ _s ′. Our system was validated using phantoms with known optical properties, calibrated to correct system response, and applied to articular cartilage. Theoretical and measured reflectance showed an excellent agreement (bias ∼0, 95% limits of agreement ±0.0114). After calibration, µ _a and µ _s ′ were extracted from bovine patellar cartilage at the same wavelengths. The estimated µ _a values (0.07-0.24 cm ⁻¹ ) were characteristic of weakly absorbing, hydrated soft tissue, while µ _s ′ values (9.7-15.8 cm ⁻¹ ) showed the expected consistent decrease with wavelength. We found our estimated optical properties were consistent with literature trends, especially µ _s ′ values, which were between the range reported for µ _s ′ values from integrating-sphere and Monte Carlo-based analysis for bovine cartilage. Phantom-calibrated DRS enables accurate, reproducible estimation of cartilage optical properties, providing a validated framework for future translation toward arthroscopic optical assessment of joint health.

Osteoarthritis (OA) is a debilitating joint disease in which early microstructural and compositional changes in articular cartilage (AC) are challenging to detect with current diagnostic tools. We hypothesized that multispectral imaging (MSI) could capture optical signatures that predict key AC properties. To test this, we developed and evaluated an MSI-based approach for estimating tissue thickness, proteoglycan (PG) content, collagen fiber orientation, and cell morphological properties (area and circularity). Reflectance images of bovine patellar AC were acquired using a custom-built MSI system operating at six wavelengths (550–970 nm), and machine learning models were trained to predict the targeted markers. The models yielded reliable estimates for AC thickness, whereas for PG content, collagen fiber orientation, and cell circularity, they achieved moderate accuracy. This work suggests that MSI holds promise as a label-free tool for characterizing AC and detecting early degenerative changes.

Objective: The key characteristics of light propagation are the average penetration depth, average maximum penetration depth, and average path length of photons. These parameters depend on tissue optical properties and, thus, on the pathological state of the tissue. Hence, they could provide diagnostic information on tissue integrity. This study investigates these parameters for articular cartilage which has a complex structure. Methods: We utilize Monte Carlo simulation to simulate photon trajectories in articular cartilage and estimate the average values of the light propagation parameters (penetration depth, maximum penetration depth, maximum lateral spread, and path length) in the spectral band of 400-1400 nm based on the optical properties of articular cartilage zonal layers and bulk tissue. Results: Our findings suggest that photons in the visible band probe a localized small volume of articular cartilage superficial and middle zones, while those in the NIR band penetrate deeper into the tissue and have larger lateral spread. In addition, we demonstrate that a simple model of articular cartilage tissue, based on the optical properties of the bulk tissue, is capable to provide an accurate description of the light-tissue interaction in articular cartilage. Conclusion: The results indicate that as the photons in the spectral band of 400-1400 nm can reach the full depth of articular cartilage matrix, they can provide viable information on its pathological state. Therefore, diffuse optical spectroscopy holds significant importance for objectively assessing articular cartilage health. Significance: In this study, for the first time, we estimate the light propagation parameters in articular cartilage.