Evaluation of in-vivo measurement errors associated with micro-computed tomography scans by means of the bone surface distance approach

In vivo microCT-based time-lapse morphometry reveals anatomical site-specific differences in bone (re)modeling serving as baseline parameters to detect early pathological events

The bone structure is very dynamic and continuously adapts its geometry to external stimuli by modeling and remodeling the mineralized tissue. In vivo microCT-based time-lapse morphometry is a powerful tool to study the temporal and spatial dynamics of bone (re)modeling. Here an advancement in the methodology to detect and quantify site-specific differences in bone (re)modeling of 12-week-old BALB/c nude mice is presented. We describe our method of quantifying new bone surface interface readouts and how these are influenced by bone curvature. This method is then used to compare bone surface (re)modeling in mice across different anatomical regions to demonstrate variations in the rate of change and spatial gradients thereof. Significant differences in bone (re)modeling baseline parameters between the metaphyseal and epiphyseal, as well as cortical and trabecular bone of the distal femur and proximal tibia are shown. These results are validated using conventional static in vivo microCT analysis. Finally, the insights from these new baseline values of physiological bone (re)modeling were used to evaluate pathological bone (re)modeling in a pilot breast cancer bone metastasis model. The method shows the potential to be suitable to detect early pathological events and track their spatio-temporal development in both cortical and trabecular bone. This advancement in (re)modeling surface analysis and defined baseline parameters according to distinct anatomical regions will be valuable to others investigating various disease models with site-distinct local alterations in bone (re)modeling.

PTH(1-34) treatment and/or mechanical loading have different osteogenic effects on the trabecular and cortical bone in the ovariectomized C57BL/6 mouse

In preclinical mouse models, a synergistic anabolic response to PTH(1–34) and tibia loading was shown. Whether combined treatment improves bone properties with oestrogen deficiency, a cardinal feature of osteoporosis, remains unknown. This study quantified the individual and combined longitudinal effects of PTH(1–34) and loading on the bone morphometric and densitometric properties in ovariectomised mice. C57BL/6 mice were ovariectomised at 14-weeks-old and treated either with injections of PTH(1–34); compressive loading of the right tibia; both interventions concurrently; or both interventions on alternating weeks. Right tibiae were microCT-scanned from 14 until 24-weeks-old. Trabecular metaphyseal and cortical midshaft morphometric properties, and bone mineral content (BMC) in 40 different regions of the tibia were measured. Mice treated only with loading showed the highest trabecular bone volume fraction at week 22. Cortical thickness was higher with co-treatment than in the mice treated with PTH alone. In the mid-diaphysis, increases in BMC were significantly higher with loading than PTH. In ovariectomised mice, the osteogenic benefits of co-treatment on the trabecular bone were lower than loading alone. However, combined interventions had increased, albeit regionally-dependent, benefits to cortical bone. Increased benefits were largest in the mid-diaphysis and postero-laterally, regions subjected to higher strains under compressive loads.

Reproducibility and Radiation Effect of High-Resolution In Vivo Micro Computed Tomography Imaging of the Mouse Lumbar Vertebra and Long Bone

A moderate radiation dose, in vivo µCT scanning protocol was developed and validated for long-term monitoring of multiple skeletal sites (femur, tibia, vertebra) in mice. A customized, 3D printed mouse holder was designed and utilized to minimize error associated with animal repositioning, resulting in good to excellent reproducibility in most cortical and trabecular bone microarchitecture and density parameters except for connectivity density. Repeated in vivo µCT scans of mice were performed at the right distal femur and the 4th lumbar vertebra every 3 weeks until euthanized at 9 weeks after the baseline scan. Comparing to the non-radiated counterparts, no radiation effect was found on trabecular bone volume fraction, osteoblast and osteoblast number/surface, or bone formation rate at any skeletal site. However, trabecular number, thickness, and separation, and structure model index were sensitive to ionizing radiation associated with the µCT scans, resulting in subtle but significant changes over multiple scans. Although the extent of radiation damage on most trabecular bone microarchitecture measures are comparable or far less than the age-related changes during the monitoring period, additional considerations need to be taken to minimize the confounding radiation factors when designing experiments using in vivo µCT imaging for long-term monitoring of mouse bone.

A new method to monitor bone geometry changes at different spatial scales in the longitudinal in vivo μCT studies of mice bones

Longitudinal studies of bone adaptation in mice using in vivo micro-computed tomography (μCT) have been commonly used for pre-clinical evaluation of physical and pharmacological interventions. The main advantage of this approach is to use each mouse as its own control, reducing considerably the sample size required by statistical power analysis. To date, multi-scale estimation of bone adaptations become essential since the bone activity that takes place at different scales may be associated with different bone mechanisms. Measures of bone adaptations at different time scales have been attempted in a previous study. This paper extends quantification of bone activity at different spatial scales with a proposition of a novel framework. The method involves applying level-set method (LSM) to track the geometric changes from the longitudinal in vivo μCT scans of mice tibia. Bone low- and high-spatial frequency patterns are then estimated using multi-resolution analysis. The accuracy of the framework is quantified by applying it to two times separated scanned images with synthetically manipulated global and/or local activity. The Root Mean Square Deviation (RMSD) was approximately 1.5 voxels or 0.7 voxels for the global low-spatial frequency or local high-spatial frequency changes, respectively. The framework is further applied to the study of bone changes in longitudinal datasets of wild-type mice tibiae over time and space. The results demonstrate the ability for the spatio-temporal quantification and visualisation of bone activity at different spatial scales in longitudinal studies thus providing further insight into bone adaptation mechanisms.

Effect of parathyroid hormone on the structural, densitometric and failure behaviors of mouse tibia in the spatiotemporal space

Parathyroid hormone (PTH) is an anabolic bone drug approved by the US Food and Drug Administration (FDA) to treat osteoporosis. However, previous studies using cross-sectional designs have reported variable and sometimes contradictory results. The aim of the present study was to quantify the localized effect of PTH on the structural and densitometric behaviors of mouse tibia and their links with the global mechanical behavior of bone using a novel spatiotemporal image analysis approach and a finite element analysis technique. Twelve female C57BL/6J mice were divided into two groups: the control and PTH treated groups. The entire right tibiae were imaged using an in vivo micro-computed tomography (μCT) system eight consecutive times. Next, the in vivo longitudinal tibial μCT images were rigidly registered and divided into 10 compartments across the entire tibial space. The bone volume (BV), bone mineral content (BMC), bone tissue mineral density (TMD), and tibial endosteal and periosteal areas (TEA and TPA) were quantified in each compartment. Additionally, finite element models of all the tibiae were generated to analyze the failure behavior of the tibia. It was found that both the BMC and BV started to increase in the proximal tibial region, and then the increases extended to the entire tibial region after two weeks of treatment (p < 0.05). PTH intervention significantly reduced the TEA in most tibial compartments after two weeks of treatment, and the TPA increased in most tibial regions after four weeks of treatment (p < 0.05). Tibial failure loads significantly increased after three weeks of PTH treatment (p < 0.01). The present study provided the first evidence of the localized effect of PTH on bone structural and densitometric properties, as well as their links with the global mechanical behaviors of bone, which are important pieces of information for unveiling the mechanism of PTH intervention.

Variability in strain distribution in the mice tibia loading model: A preliminary study using digital volume correlation

It is well known that bone has an enormous adaptive capacity to mechanical loadings, and to this extent, several in vivo studies on mouse tibia use established cyclic compressive loading protocols to investigate the effects of mechanical stimuli. In these experiments, the applied axial load is well controlled but the positioning of the hind-limb between the loading endcaps may dramatically affect the strain distribution induced on the tibia. In this study, the full field strain distribution induced by a typical in vivo setup on mouse tibiae was investigated through a combination of in situ compressive testing, µCT scanning and a global digital volume correlation (DVC) approach. The precision of the DVC method and the effect of repositioning on the strain distributions were evaluated. Acceptable uncertainties of the DVC approach for the analysis of loaded tibiae (411 ± 58µɛ) were found for nodal spacing of approximately 50 voxels (520 µm). When pairs of in situ preloaded and loaded images were registered, low variability of the strain distributions within the tibia were seen (range of mean differences in principal strains: 585-1800µɛ). On contrary, larger differences were seen after repositioning (range of mean differences in principal strains: 2500-5500µɛ). To conclude, these preliminary results on thee specimens showed that the DVC approach applied to the mouse tibia can be precise enough to evaluate local strain distributions under loads, and that repositioning of the hind-limb within the testing machine can induce large differences in the strain distributions that should be accounted for when modelling this system.

From bed to bench: How in silico medicine can help ageing research

Driven by the raising ethical concerns surrounding animal experimentation, there is a growing interest for non-animal methods, in vitro or in silico technologies that can be used to reduce, refine, and replace animal experimentation. In addition, animal experimentation is being critically revised in regard to its ability to predict clinical outcomes. In this manuscript we describe an initial exploration where a set of in vivo imaging based subject-specific technologies originally developed to predict the risk of femoral strength and hip fracture in osteoporotic patients, were adapted to assess the efficacy of bone drugs pre-clinically on mice. The CT2S technology we developed generates subject-specific models based on Computed Tomography that can separate fractured and non-fractured patients with an accuracy of 82%. When used in mouse experiments the use of in vivo imaging and modelling was found to improve the reproducibility of Bone Mineral Content measurements to a point where up to 63% less mice would be required to achieve the same statistical power of a conventional cross-sectional study. We also speculate about a possible approach where animal-specific and patient-specific models could be used to better translate the observation made on animal models into predictions of response in humans.

Validation of finite element models of the mouse tibia using digital volume correlation

The mouse tibia is a common site to investigate bone adaptation. Micro-Finite Element (microFE) models based on micro-Computed Tomography (microCT) images can estimate bone mechanical properties non-invasively but their outputs need to be validated with experiments. Digital Volume Correlation (DVC) can provide experimental measurements of displacements over the whole bone volume. In this study we applied DVC to validate the local predictions of microFE models of the mouse tibia in compression. Six mouse tibiae were stepwise compressed within a microCT system. MicroCT images were acquired in four configurations with applied compression of 0.5 N (preload), 6.5 N, 13.0 N and 19.5 N. Failure load was measured after the last scan. A global DVC algorithm was applied to the microCT images in order to obtain the displacement field over the bone volume. Homogeneous, isotropic linear hexahedral microFE models were generated from the images collected in the preload configuration with boundary conditions interpolated from the DVC displacements at the extremities of the tibia. Experimental displacements from DVC and numerical predictions were compared at corresponding locations in the middle of the bone. Stiffness and strength were also estimated from each model and compared with the experimental measurements. The magnitude of the displacement vectors predicted by microFE models was highly correlated with experimental measurements (R² >0.82). Higher but still reasonable errors were found for the Cartesian components. The models tended to overestimate local displacements in the longitudinal direction (R² = 0.69-0.90, slope of the regression line=0.50-0.97). Errors in the prediction of structural mechanical properties were 14% ± 11% for stiffness and 9% ± 9% for strength. In conclusion, the DVC approach has been applied to the validation of microFE models of the mouse tibia. The predictions of the models for both structural and local properties have been found reasonable for most preclinical applications.

Investigating the Longitudinal Effect of Ovariectomy on Bone Properties Using a Novel Spatiotemporal Approach

Osteoporosis is the most common bone disease. However, the mechanism of osteoporosis-induced alterations in bone is still unclear. The aim of this study was to investigate the effects of osteoporosis on the structural, densitometric and mechanical properties of the whole tibia using in vivo μCT imaging, spatiotemporal analysis and finite element modeling. Twelve C57Bl/6 female mice were adopted. At 14 weeks of age, half of the mice were ovariectomized (OVX), and the other half were SHAM-operated. The whole right tibia was scanned using an in vivo μCT imaging system at 14, 16, 17, 18, 19, 20, 21 and 22 weeks. The image datasets were registered in order to precisely quantify the bone properties. The results showed that OVX led to a significant increase in the endosteal area across the whole tibia 4 weeks after OVX intervention but did not have a significant influence on the periosteal area. Additionally, the bone volume and mineral content significantly decreased only in the proximal regions, but these decreases did not have a significant influence on the stiffness and failure load of the tibia. This study demonstrated the application of a novel spatiotemporal approach in the comprehensive analysis of bone adaptations in the spatiotemporal space.

Longitudinal effects of Parathyroid Hormone treatment on morphological, densitometric and mechanical properties of mouse tibia

The use of Parathyroid Hormone (PTH) as bone anabolic is limited due to cost-benefit assessments. Preclinical studies evaluating the effects of PTH on bone have reported variable and often contradictory results. Here, we have applied a new approach using a combination of in-vivo longitudinal µCT, image processing techniques and finite element models to monitor early local changes in the whole tibia (divided in 40 compartments) and mechanical properties of female C57BL/6J mice treated with PTH 1-34, compared to controls. Compared with standard 3D bone morphometric analysis, our new approach allowed detection of much smaller and localised changes in bone mineral content (BMC) at very early time points (1 week vs 3 weeks with standard methods) and showed that changes do not occur uniformly over time and across the anatomical space. Indeed, in the PTH treated mice, significant changes in BMC were observed in the medial and posterior sectors of the proximal tibia, a week after treatment, and in the medial sector of the tibia midshaft region a week later (p < 0.05). By the third week, two thirds of the regions showed significantly higher values of BMC (p < 0.05). The effect of PTH on bone regional volume is similar to that on BMC, but there is almost no effect of PTH on bone tissue mineral density. The differences in estimated mechanical properties became significant after three weeks of treatment (p < 0.05). These results provide the first evidence of an early and localised PTH effect on murine bone, and show that our novel partitioning approach, compared to the standard evaluation protocol, allows a more precise quantification of bone changes following treatment, which would facilitate preclinical testing of novel mono- and/or combination therapies throughout the bone.

Longitudinal imaging of the ageing mouse

Several non-invasive imaging techniques are used to investigate the effect of pathologies and treatments over time in mouse models. Each preclinical in vivo technique provides longitudinal and quantitative measurements of changes in tissues and organs, which are fundamental for the evaluation of alterations in phenotype due to pathologies, interventions and treatments. However, it is still unclear how these imaging modalities can be used to study ageing with mice models. Almost all age related pathologies in mice such as osteoporosis, arthritis, diabetes, cancer, thrombi, dementia, to name a few, can be imaged in vivo by at least one longitudinal imaging modality. These measurements are the basis for quantification of treatment effects in the development phase of a novel treatment prior to its clinical testing. Furthermore, the non-invasive nature of such investigations allows the assessment of different tissue and organ phenotypes in the same animal and over time, providing the opportunity to study the dysfunction of multiple tissues associated with the ageing process. This review paper aims to provide an overview of the applications of the most commonly used in vivo imaging modalities used in mouse studies: micro-computed-tomography, preclinical magnetic-resonance-imaging, preclinical positron-emission-tomography, preclinical single photon emission computed tomography, ultrasound, intravital microscopy, and whole body optical imaging.