Articular cartilage imaging by the use of phase-contrast tomography in a collagen-induced arthritis mouse model

MRI overestimates articular cartilage thickness and volume compared to synchrotron radiation phase-contrast imaging

Accurate evaluation of morphological changes in articular cartilage are necessary for early detection of osteoarthritis (OA). 3T magnetic resonance imaging (MRI) has highly sensitive contrast resolution and is widely used clinically to detect OA. However, synchrotron radiation phase-contrast imaging computed tomography (SR-PCI) can also provide contrast to tissue interfaces that do not have sufficient absorption differences, with the added benefit of very high spatial resolution. Here, MRI was compared with SR-PCI for quantitative evaluation of human articular cartilage. Medial tibial condyles were harvested from non-OA donors and from OA patients receiving knee replacement surgery. Both imaging methods revealed that average cartilage thickness and cartilage volume were significantly reduced in the OA group, compared to the non-OA group. When comparing modalities, the superior resolution of SR-PCI enabled more precise mapping of the cartilage surface relative to MRI. As a result, MRI showed significantly higher average cartilage thickness and cartilage volume, compared to SR-PCI. These data highlight the potential for high-resolution imaging of articular cartilage using SR-PCI as a solution for early OA diagnosis. Recognizing current limitations of using a synchrotron for clinical imaging, we discuss its nascent utility for preclinical models, particularly longitudinal studies of live animal models of OA.

Technical note: Cartilage imaging with sub-cellular resolution using a laboratory-based phase-contrast x-ray microscope

BACKGROUND

Microscopic imaging of cartilage is a key tool for the study and development of treatments for osteoarthritis. When cellular and sub-cellular resolution is required, histology remains the gold standard approach, albeit limited by the lack of volumetric information as well as by processing artifacts. Cartilage imaging with the sub-cellular resolution has only been demonstrated in the synchrotron environment.

PURPOSE

To provide a proof-of-concept demonstration of the capability of a laboratory-based x-ray phase-contrast microscope to resolve sub-cellular features in a cartilage sample.

METHODS

This work is based on a laboratory-based x-ray microscope using intensity-modulation masks. The structured nature of the beam, resulting from the mask apertures, allows the retrieval of three contrast channels, namely, transmission, refraction and dark-field, with resolution depending only on the mask aperture width. An ex vivo equine cartilage sample was imaged with the x-ray microscope and results were validated with synchrotron tomography and histology.

RESULTS

Individual chondrocytes, that is, cells responsible for cartilage formation, could be detected with the laboratory-based microscope. The complementarity of the three retrieved contrast channels allowed the detection of sub-cellular features in the chondrocytes.

CONCLUSIONS

We provide the first proof-of-concept of imaging cartilage tissue with sub-cellular resolution using a laboratory-based x-ray microscope.

Accelerating functional gene discovery in osteoarthritis

Osteoarthritis causes debilitating pain and disability, resulting in a considerable socioeconomic burden, yet no drugs are available that prevent disease onset or progression. Here, we develop, validate and use rapid-throughput imaging techniques to identify abnormal joint phenotypes in randomly selected mutant mice generated by the International Knockout Mouse Consortium. We identify 14 genes with functional involvement in osteoarthritis pathogenesis, including the homeobox gene Pitx1, and functionally characterize 6 candidate human osteoarthritis genes in mouse models. We demonstrate sensitivity of the methods by identifying age-related degenerative joint damage in wild-type mice. Finally, we phenotype previously generated mutant mice with an osteoarthritis-associated polymorphism in the Dio2 gene by CRISPR/Cas9 genome editing and demonstrate a protective role in disease onset with public health implications. We hope this expanding resource of mutant mice will accelerate functional gene discovery in osteoarthritis and offer drug discovery opportunities for this common, incapacitating chronic disease.

Convolutional neuronal networks combined with X-ray phase-contrast imaging for a fast and observer-independent discrimination of cartilage and liver diseases stages

We applied transfer learning using Convolutional Neuronal Networks to high resolution X-ray phase contrast computed tomography datasets and tested the potential of the systems to accurately classify Computed Tomography images of different stages of two diseases, i.e. osteoarthritis and liver fibrosis. The purpose is to identify a time-effective and observer-independent methodology to identify pathological conditions. Propagation-based X-ray phase contrast imaging WAS used with polychromatic X-rays to obtain a 3D visualization of 4 human cartilage plugs and 6 rat liver samples with a voxel size of 0.7 × 0.7 × 0.7 µm³ and 2.2 × 2.2 × 2.2 µm³, respectively. Images with a size of 224 × 224 pixels are used to train three pre-trained convolutional neuronal networks for data classification, which are the VGG16, the Inception V3, and the Xception networks. We evaluated the performance of the three systems in terms of classification accuracy and studied the effect of the variation of the number of inputs, training images and of iterations. The VGG16 network provides the highest classification accuracy when the training and the validation-test of the network are performed using data from the same samples for both the cartilage (99.8%) and the liver (95.5%) datasets. The Inception V3 and Xception networks achieve an accuracy of 84.7% (43.1%) and of 72.6% (53.7%), respectively, for the cartilage (liver) images. By using data from different samples for the training and validation-test processes, the Xception network provided the highest test accuracy for the cartilage dataset (75.7%), while for the liver dataset the VGG16 network gave the best results (75.4%). By using convolutional neuronal networks we show that it is possible to classify large datasets of biomedical images in less than 25 min on a 8 CPU processor machine providing a precise, robust, fast and observer-independent method for the discrimination/classification of different stages of osteoarthritis and liver diseases.

Propagation phase-contrast micro-computed tomography allows laboratory-based three-dimensional imaging of articular cartilage down to the cellular level

OBJECTIVE

High-resolution non-invasive three-dimensional (3D) imaging of chondrocytes in articular cartilage remains elusive. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) permits imaging cells within articular cartilage.

DESIGN

Bovine osteochondral plugs were prepared four ways: in phosphate-buffered saline (PBS) or 70% ethanol (EtOH), both with or without phosphotungstic acid (PTA) staining. Specimens were imaged with micro-CT following two protocols: 1) absorption contrast (AC) imaging 2) propagation phase-contrast (PPC) imaging. All samples were scanned in liquid. The contrast to noise ratio (C/N) of cellular features quantified scan quality and were statistically analysed. Cellular features resolved by micro-CT were validated by standard histology.

RESULTS

The highest quality images were obtained using propagation phase-contrast imaging and PTA-staining in 70% EtOH. Cellular features were also visualised when stained in PBS and unstained in EtOH. Under all conditions PPC resulted in greater contrast than AC (p < 0.0001 to p = 0.038). Simultaneous imaging of cartilage and subchondral bone did not impede image quality. Corresponding features were located in both histology and micro-CT and followed the same distribution with similar density and roundness values.

CONCLUSIONS

Three-dimensional visualisation and quantification of the chondrocyte population within articular cartilage can be achieved across a field of view of several millimetres using laboratory-based micro-CT. The ability to map chondrocytes in 3D opens possibilities for research in fields from skeletal development through to medical device design and treatment of cartilage degeneration.

Quantitative assessment of murine articular cartilage and bone using X-ray phase-contrast imaging

Murine models for rheumatoid arthritis (RA) research can provide important insights for understanding RA pathogenesis and evaluating the efficacy of novel treatments. However, simultaneously imaging both murine articular cartilage and subchondral bone using conventional techniques is challenging because of low spatial resolution and poor soft tissue contrast. X-ray phase-contrast imaging (XPCI) is a new technique that offers high spatial resolution for the visualisation of cartilage and skeletal tissues. The purpose of this study was to utilise XPCI to observe articular cartilage and subchondral bone in a collagen-induced arthritis (CIA) murine model and quantitatively assess changes in the joint microstructure. XPCI was performed on the two treatment groups (the control group and CIA group, n = 9 per group) to monitor the progression of damage to the femur from the knee joint in a longitudinal study (at 0, 4 and 8 weeks after primary injection). For quantitative assessment, morphologic parameters were measured in three-dimensional (3D) images using appropriate image analysis software. Our results showed that the average femoral cartilage volume, surface area and thickness were significantly decreased (P<0.05) in the CIA group compared to the control group. Meanwhile, these decreases were accompanied by obvious destruction of the surface of subchondral bone and a loss of trabecular bone in the CIA group. This study confirms that XPCI technology has the ability to qualitatively and quantitatively evaluate microstructural changes in mouse joints. This technique has the potential to become a routine analysis method for accurately monitoring joint damage and comprehensively assessing treatment efficacy.

CT imaging biomarker for evaluation of emodin as a potential drug on LPS-mediated osteoporosis mice

RATIONALE AND OBJECTIVES

To identify micro-computed tomography (CT) imaging biomarkers for evaluating the effects of emodin, a potential drug to treat osteoporosis, in the mouse model of lipopolysaccharide (LPS)-mediated osteoporosis.

MATERIALS AND METHODS

Forty male imprinting control region (ICR) mice with LPS-induced bone resorption were equally divided into four experimental groups: phosphate-buffered saline-treated (control), emodin-treated, LPS-treated, and LPS + emodin-treated groups. Emodin (50 mg/kg) was administered orally on alternate days for 8 days, and LPS (5 mg/kg) was injected intraperitoneally on days 1 and 4. After 8 days, the mice were sacrificed, and micro-CT images of the left proximal femurs were obtained. Three-dimensional images were analyzed by using commercial software to measure the bone volume to total volume fraction (BV/TV), trabecular number (Tb-N), trabecular thickness (Tb-Th), and trabecular separation (Tb-Sp) as CT imaging biomarkers. Histologic analyses of the femurs were performed using hematoxylin and eosin and tartrate-resistant acid phosphatase (TRAP) immunohistochemical staining.

RESULTS

The LPS + emodin-treated group demonstrated marked suppression of LPS-induced bone resorption compared to the LPS-treated group (BV/TV, 28.84% vs. 40.76%; Tb-N, 2.65 vs. 3.45 mm(-1); Tb-Sp, 300.81 vs. 212.31 μm; Tb-Th, 116.94 vs. 131.25 μm). TRAP immunohistochemical analysis showed fewer osteoclasts per field of tissue in the LPS + emodin-treated group than in the LPS-treated group (27.8 vs. 41.8). The BV/TV, Tb-N, and Tb-Sp data correlated well with the histomorphometric findings.

CONCLUSIONS

The findings reveal a novel effect of emodin on bone remodeling in the LPS-mediated osteoporotic mouse model. The ex vivo micro-CT imaging is a promising tool for assessing the therapeutic effects of potential drugs on osteoporosis.

Synchrotron- and laboratory-based X-ray phase-contrast imaging for imaging mouse articular cartilage in the absence of radiopaque contrast agents

The mouse model of osteoarthritis (OA) has been recognized as the most promising research tool for the identification of new OA therapeutic targets. However, this model is currently limited by poor throughput, dependent on the extremely time-consuming histopathology assessment of the articular cartilage (AC). We have recently shown that AC in the rat tibia can be imaged both in air and in saline solution using a laboratory system based on coded-aperture X-ray phase-contrast imaging (CAXPCi). Here, we explore ways to extend the methodology for imaging the much thinner AC of the mouse, by means of gold-standard synchrotron-based phase-contrast methods. Specifically, we have used analyser-based phase-contrast micro-computed tomography (micro-CT) for its high sensitivity to faint phase changes, coupled with a high-resolution (4.5 μm pixel) detector. Healthy, diseased (four weeks post induction of OA) and artificially damaged mouse AC was imaged at the Elettra synchrotron in Trieste, Italy, using the above method. For validation, we used conventional micro-CT combined with radiopaque soft-tissue staining and standard histomorphometry. We show that mouse cartilage can be visualized correctly by means of the synchrotron method. This suggests that: (i) further developments of the laboratory-based CAXPCi system, especially in terms of pushing the resolution limits, might have the potential to resolve mouse AC ex vivo and (ii) additional improvements may lead to a new generation of CAXPCi micro-CT scanners which could be used for in vivo longitudinal pre-clinical imaging of soft tissue at resolutions impossible to achieve by current MRI technology.

Quantitative imaging of murine osteoarthritic cartilage by phase-contrast micro-computed tomography

OBJECTIVE

The mouse is an optimal model organism in which gene-environment interactions can be used to study the pathogenesis of osteoarthritis (OA). The gold standard for arthritis research in mice is based on histopathology and immunohistochemistry, which are labor-intensive, prone to sampling bias and technical variability, and limited in throughput. The aim of this study was to develop a new technique that assesses mouse cartilage by integrating quantitative volumetric imaging techniques.

METHODS

A novel mouse model of OA was generated by cruciate ligament transection (CLT) and evaluated by histopathology and immunohistochemistry. Knee joint specimens were then imaged using a new technique that combines high-resolution micro-computed tomography (micro-CT) and phase-contrast optics followed by quantitative analyses. A comparative analysis was also performed in a previously established mouse model of OA generated by destabilization of the medial meniscus (DMM).

RESULTS

Phase-contrast micro-CT achieved cellular resolution of chondrocytes and quantitative assessment of parameters such as articular cartilage volume and surface area. In mouse models of OA generated by either CLT or DMM, we showed that phase-contrast micro-CT distinguished control and OA cartilage by providing quantitative measures with high reproducibility and minimal variability. Features of OA at the cellular or tissue level could also be observed in images generated by phase-contrast micro-CT.

CONCLUSION

We established an imaging technology that comprehensively assessed and quantified the 2-dimensional and 3-dimensional changes of articular cartilage. Application of this technology will facilitate the rapid and high-throughput assessment of genetic and therapeutic models of OA in mice.

X-ray phase-contrast imaging: from pre-clinical applications towards clinics

Phase-contrast x-ray imaging (PCI) is an innovative method that is sensitive to the refraction of the x-rays in matter. PCI is particularly adapted to visualize weakly absorbing details like those often encountered in biology and medicine. In past years, PCI has become one of the most used imaging methods in laboratory and preclinical studies: its unique characteristics allow high contrast 3D visualization of thick and complex samples even at high spatial resolution. Applications have covered a wide range of pathologies and organs, and are more and more often performed in vivo. Several techniques are now available to exploit and visualize the phase-contrast: propagation- and analyzer-based, crystal and grating interferometry and non-interferometric methods like the coded aperture. In this review, covering the last five years, we will give an overview of the main theoretical and experimental developments and of the important steps performed towards the clinical implementation of PCI.

Visualization of small lesions in rat cartilage by means of laboratory-based x-ray phase contrast imaging

Being able to quantitatively assess articular cartilage in three-dimensions (3D) in small rodent animal models, with a simple laboratory set-up, would prove extremely important for the development of pre-clinical research focusing on cartilage pathologies such as osteoarthritis (OA). These models are becoming essential tools for the development of new drugs for OA, a disease affecting up to 1/3 of the population older than 50 years for which there is no cure except prosthetic surgery. However, due to limitations in imaging technology, high-throughput 3D structural imaging has not been achievable in small rodent models, thereby limiting their translational potential and their efficiency as research tools. We show that a simple laboratory system based on coded-aperture x-ray phase contrast imaging (CAXPCi) can correctly visualize the cartilage layer in slices of an excised rat tibia imaged both in air and in saline solution. Moreover, we show that small, surgically induced lesions are also correctly detected by the CAXPCi system, and we support this finding with histopathology examination. Following these successful proof-of-concept results in rat cartilage, we expect that an upgrade of the system to higher resolutions (currently underway) will enable extending the method to the imaging of mouse cartilage as well. From a technological standpoint, by showing the capability of the system to detect cartilage also in water, we demonstrate phase sensitivity comparable to other lab-based phase methods (e.g. grating interferometry). In conclusion, CAXPCi holds a strong potential for being adopted as a routine laboratory tool for non-destructive, high throughput assessment of 3D structural changes in murine articular cartilage, with a possible impact in the field similar to the revolution that conventional microCT brought into bone research.

Three dimensional quantification of biological samples using micro-computer aided tomography (microCT)

MicroCT is increasingly being used to observe soft animal and plant tissues. Conventional electron and light microscope staining protocols used to enhance the contrast of soft tissues have the potential to be adapted for use in microCT. This would increase the versatility of the microCT beyond improving qualitative observations to facilitating quantitative analysis of soft tissues. This paper describes the development of a culture system and staining protocol which has successfully been used to obtain three dimensional (3-D) quantitative data of filamentous and zoosporic soil fungi. The fungi were grown in an artificial matrix that was developed to simulate the particulate nature of soil. The combination of high contrast staining protocol and use of an X-ray translucent matrix allowed for 3-D qualitative and quantitative analysis of fungal growth. A salient point raised by this study is that the effectiveness of a protocol is reliant on the tissue or cell culture system which includes the composition of the sample, the sampling vessel, the depth of a sample and the combination of stains used. The potential use of this method extends to other fields where distribution and growth patterns in 3-D need to be quantified.