Dual energy computed tomography cannot effectively differentiate between calcium pyrophosphate and basic calcium phosphate diseases in the clinical setting

Abstract

Background: Recent reports suggested that dual-energy CT (DECT) may help discriminate between different types of calcium phosphate crystals in vivo, which would have important implications for the characterization of crystal deposition occurring in osteoarthritis.

Purpose: Our aim was to test the hypothesis that DECT can effectively differentiate basic calcium phosphate (BCP) from calcium pyrophosphate (CPP) deposition diseases.

Methods: Discarded tissue after total knee replacement specimens in a 71 year-old patient with knee osteoarthritis and chondrocalcinosis was scanned using DECT at standard clinical parameters. Specimens were then examined on light microscopy which revealed CPP deposition in 4 specimens (medial femoral condyle, lateral tibial plateau and both menisci) without BCP deposition. Regions of interest were placed on post-processed CT images using Rho/Z maps (Syngo.via, Siemens Healthineers, VB10B) in different areas of CPP deposition, trabecular bone BCP (T-BCP) and subchondral bone plate BCP (C-BCP).

Results: Dual Energy Index (DEI) of CPP was 0.12 (SD ​= ​0.02) for reader 1 and 0.09 (SD ​= ​0.03) for reader 2, The effective atomic number (Z_eff) of CPP was 10.83 (SD ​= ​0.44) for reader 1 and 10.11 (SD ​= ​0.66) for reader 2. Nearly all DECT parameters of CPP were higher than those of T-BCP, lower than those of C-BCP, and largely overlapping with Aggregate-BCP (aggregate of T-BCP and C-BCP).

Conclusion: Differentiation of different types of calcium crystals using DECT is not feasible in a clinical setting.

Keywords: CT; Calcium phosphate; Crystal; Dual energy; Osteoarthritis.

Fig. 1

(A) Preoperative anteroposterior knee radiograph of the study's participant showing meniscal calcinosis with more subtle mineralization of the hyaline cartilage. (B)Ex vivo DECT axial reformat of the lateral meniscus with an ROI placed in a focus of CPP, as proven by microscopy. (C)Ex vivo DECT coronal reformat of the lateral tibial plateau, ROI placement in the subchondral cortical bone plate (C-BCP) and within the subchondral trabecular bone (T-BCP). (D) Histologic examination of the lateral meniscus. Hematoxylin and eosin staining shows presence of CPP crystals.

Fig. 2

Box and whisker plots for A. CT attenuation values at 80 ​KVp and B. 150 ​KVp, C. dual energy index (DEI), D. Electron density (Rho) and E. Effective atomic numbers (Z_eff) of BCP (which aggregates both BCP within trabecular bone and BCP within cortical bone), and foci of calcium pyrophosphate (CPP) from both readers. Of note figure D shows the DEI range if 0.0160–0.036 (dashed lines) as proposed in the final definition of CPP.