CHARACTERIZING MENISCAL CALCIFICATIONS WITH PHOTON COUNTING-BASED DUAL-ENERGY COMPUTED TOMOGRAPHY

INTRODUCTION

Meniscal calcifications, including basic calcium phosphate (BCP) and calcium pyrophosphate (CPP), are commonly associated with OA and may disrupt meniscal function, contributing to joint degeneration. However, the role of specific calcification types in OA is not fully understood due to the lack of non-invasive imaging techniques that can differentiate them in vivo. While Raman spectroscopy accurately distinguishes BCP from CPP, it is limited to 2D and requires destructive histological processing. In contrast, dual-energy computed tomography (DECT) has shown potential for differentiating calcifications in both in vivo and ex vivo, but its performance varies across previous studies. The integration of photon-counting detectors (PCD) in CT imaging improves spatial resolution and enables multi-energy acquisition, enhancing in vivo calcification characterization.

OBJECTIVE

We evaluated the capability of dual-energy computed tomography with a photon counting detector (PCD-DECT) to differentiate BCP and CPP calcification deposits in the posterior horns of human menisci ex vivo, using Raman spectroscopy as the reference.

METHODS

This study included 82 medial and lateral meniscus samples from 21 deceased donors without known knee OA and 20 TKR patients with medial compartment OA. Samples were imaged using an experimental cone-beam CT setup with PCD, operating at 120 kVp and 0.2 mA. Low energy (LE) data were collected in the 20-50 keV range, and high energy (HE) data in the 50-120 keV range, with a final voxel size of 37 µm. Only calcified samples identified using Raman spectroscopy (n = 36), 8 CPP and 28 BCP samples, were included to the analysis. Calcifications were segmented and divided between BCP and CPP groups. Subsequently, LE, HE, and Dual Energy Index (DEI) values were measured for each calcification. We used linear mixed models to estimate associations between LE and HE variables and the calcification type, and to compare the DEI values between the calcification types. Estimates are presented with 95% confidence intervals.

RESULTS

Figure 1A-C shows a 3D visualization of menisci with and without different calcifications. The results showed that CPP calcifications had consistently lower LE values than BCP for corresponding HE values. The difference increased with higher HE values, peaking at 500 HU with a difference of 166.1 HU (95% CI: 73.4, 258.8), while the smallest difference occurs at -100 HU, where the difference is 33.81 HU (95% CI: -40.38, 107.99) HU. The differences between LE and HE values are shown in Figure 1D-E. Additionally, estimated mean DEI values were higher in BCP calcifications compared to CPP, with an estimated difference of 0.035 (95%CI: 0.011, 0.059). Detailed results are shown in Table 1.

CONCLUSION

Our findings show that BCP and CPP meniscal calcifications differ in LE and HE as well as DEI values measured with PCD-DECT. The method reveals average differences between calcification types, while precise identification of individual calcifications could be improved in the future with more advanced PCD detectors. To conclude, PCD-DECT successfully enabled ex vivo assessment of meniscal calcification types, highlighting its potential in future in vivo applications to better understand calcification mechanisms and evaluate responses to calcification-targeting therapies.