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

TL;DR

This scoping review addresses how advanced X-ray techniques enable high-resolution, non-destructive imaging of articular cartilage (AC) by leveraging absorption, refraction, and scattering signals. It catalogs absorption-based methods (contrast-enhanced CT, dual-energy imaging, and detector-based spectral imaging), refraction-based phase-contrast approaches (propagation-based, analyzer-based imaging, gratings interferometry, and edge-illumination), and scattering-based dark-field imaging, detailing their principles, advantages, and limitations for AC in health and disease. The review emphasizes the tradeoffs between contrast agent use, beam coherence requirements, exposure dose, and hardware (synchrotron vs laboratory) and highlights rheologic applications such as time-resolved DVC under loading and in-situ studies. It concludes that while phase-contrast and dark-field techniques offer cellular- to subcellular-scale insight, practical translation hinges on access to coherent sources or compact alternatives, with detector-based spectral imaging and EI showing promise for lab-based rheology and high-resolution AC imaging.

Abstract

Articular cartilage 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 articular cartilage translates into poor contrast using conventional absorption modality. To overcome this limitation, several approaches can be embraced. X-ray visibility of articular cartilage can be increased with the use of radiopaque contrast agents. Therefore, further discrimination of structures could be provided by spectral techniques, pivoting on either multi-energy acquisitions or photon-counting technology. Alternatively, phase-contrast techniques unveil details typically undetected with conventional approaches. Phase-contrast 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. Additionally, some phase-contrast techniques retrieve the small-angle scattering-based dark-field signal, relatable to sub-pixel structures. This scoping review aims to catalogue the application of these advanced X-ray techniques to articular cartilage imaging, following PRISMA guidelines. It discusses their advantages, limitations, and includes an overview of rheologic applications to articular cartilage.

Figures (8)

• Figure 1: Scheme of AC
• Figure 2: (a) Section from contrast-enhanced microCT of human AC, following the immersion in CA4+ solution (3.2 $\mu$m-pixel size, commercial microCT scanner). Along with the characteristic gradient of radiopacity increasing towards the deep layer of AC, spots with augmented signal are recognizable as chondrocytes' lacunae. Image adapted from the work of Karhula et al. Karhula2017Micro, published under Creative Commons license (CC BY 4.0). (b) Section from contrast-enhanced microCT of rabbit AC, following the immersion in solution of hexaammineruthenium(III) chloride and cacodylic acid (0.65 $\mu$m-pixel size, commercial microCT scanner). The proposed protocol aimed to investigate the collagen fiber orientation in a osteoarthritic AC model (dashed white line and dashed black line mark tissue surface and tidemark, respectively). Image adapted from the work of Ojanen et al. Ojanen2023Micro, published under Creative Commons license (CC BY 4.0). (c) Section from contrast-enhanced microCT of human AC, following the immersion in propidium iodide (2.6 $\mu$m-pixel size, SR-microCT). The direct binding of the selected CA to DNA in chondrocytes allowed the punctual depiction of the cellular pattern, enabling the quantification of chondrocytes' distribution even inside each lacuna. Image adapted from the work of Danalache et al. Danalache2021Exploration, published under Creative Commons license (CC BY 4.0).
• Figure 3: (a) Scheme of DE technique. The procedure includes two distinct X-ray exposures, each at a different beam energy. The low-energy and high-energy components serve as input for decomposition algorithms, to deliver two material maps. (b) Reconstructed section from low-energy E- and high-energy E+ binned photons is reported on blue and red screen, respectively. In the low-energy and high-energy screens are reported sections from reconstructed volume of AC, following the immersion in a gadoteridol/CA4+ mixture. In particular, the contrast in low- and high- energy sections is due mainly to gadoteridol and CA4+, respectively. Images on screens of panel (b) adapted from the work of Honkanen et al. Honkanen2020Synchrotron, published under Creative Commons license (CC BY 4.0).
• Figure 4: (a) Scheme of spectral imaging technique. The procedure includes a single X-ray exposure. The discrimination occurs on-chip, following the selection of an energy threshold. According to the latter, photons are binned in low-energy and high-energy components. Analogously to DE technique, both components are required for decomposition algorithms to provide two material maps. (b) Reconstructed section from low-energy E- and high-energy E+ binned photons of a bovine osteochondral sample is reported on blue and red screen, respectively. (c, d) Section of density maps, as a result of the decomposition algorithm. (c) Iodine density map accounting for iodine-based CA4+ diffused in AC. (d) Hydroxyapatite (HA) density, accounting for calcium-rich bone tissue. Images in panels (b), (c) and (d) adapted from the work of Fantoni et al. Fantoni2024Quantitative, published under Creative Commons license (CC BY 4.0).
• Figure 5: (a) Scheme of PBPC. Unlike absorption modalities, PBPC bases its mechanism on the distance between the sample and the detector. In particular, such distance is selected to produce the edge-enhancement, originating from interfaces between different materials. (b) Three-dimensional rendering of a typical PBPC setup. The edge-enhancement requires a high degree of spatial coherence (i.e., SR). (c) Section from PBPC volume of healthy human AC. Besides the interface AC-air on the top, darker spots in the tissue, attributable to chondrocytes' lacunae, are clearly recognizable. Image adapted from the work of Horng et al. Horng2021Multiscale, published under Creative Commons license (CC BY 4.0).