Spectral micro-CT for quantitative analysis of calcification in fibrocartilage

We describe a method for the quantitative analysis of calcification structures in fibrocartilage samples from human hip acetabular labrum, using a lab-based spectral micro-computed tomography ( $\mu \mathrm{CT}$ ) system.

Tissue samples from 26 patients with osteoarthritis and femoroacetabular impingement were analyzed using a spectral $\mu \mathrm{CT}$ system featuring photon-counting detector with a $62\mu m$ pixel size operating at two energy thresholds. Material decomposition was applied to the $\mu \mathrm{CT}$ images to separate calcium and water-like components, resulting in the 3D spatial distribution and quantification of calcium deposits within the tissue. Results were compared with histological analysis using Alizarin Red™ staining. Two calcium crystal deposition scores on a four-grade scale were assigned based on histological and $\mu \mathrm{CT}$ analysis, respectively.

This method provides 3D images with a voxel size of $20\mu m$ over a sample size up to 3 cm. The minimum detectable calcium concentration is $0.24g/{\mathrm{cm}}^3$ , and calcified structures as small as $49.6\mu m$ in diameter were identified. The quantification accuracy was validated using a hydroxyapatite calibration phantom. Across the complete sample set, spectral µCT grading demonstrated significant correlation (Spearman correlation coefficient $\rho =0.67$ ) and substantial agreement (weighted Cohen's coefficient $\kappa =0.70$ ) with histological assessment. Moreover, the calcification spatial distribution exhibited very good consistency with histological results, supporting the method validity.

This approach offers a nondestructive and reproducible alternative to histology for assessing calcification in joint tissue, providing quantitative 3D mapping of calcium concentration in intact samples without the need for staining or physical sectioning.