Advanced X-ray techniques for research-oriented high-resolution imaging of articular cartilage: a scoping review

Articular cartilage (AC) is a musculoskeletal soft tissue renowned for its unique mechanical properties. Understanding both its hierarchical structure and the interplay between its constituents could shed light on the mechanical competence of the tissue. Therefore, rheologic approaches based on high-resolution non-destructive imaging techniques are desired. In this context, X-ray imaging could ideally accomplish this task. Nevertheless, the nature of AC translates into poor contrast using conventional absorption modality. To overcome this limitation, several approaches can be embraced. X-ray visibility of AC can be increased with the use of radiopaque contrast agents (CA). Therefore, further discrimination of structures could be provided by spectral techniques, pivoting on either multi-energy acquisitions or photon-counting technology. Alternatively, phase-contrast (PC) techniques unveil details typically undetected with conventional approaches. Phase-contrast (PC) imaging, based on the intrinsic decrement in the refractive index of the tissue, can be achieved with different configurations and implementations, including distinct X-ray sources and optical elements. In addition, some phase-contrast (PC) techniques retrieve the small-angle scattering-based dark-field (DF) signal, relatable to sub-pixel structures. This scoping review aims to catalogue the application of these advanced X-ray techniques to AC imaging, following PRISMA guidelines. It discusses their advantages, limitations and includes an overview of rheologic applications to AC.