Multi-modality CT imaging of human bone for improved validation of subject-specific finite element analysis

INTRODUCTION Finite element analysis (FE) is a promising alternative to dual-energy x-ray absorptiometry for improved prediction of fracture risk as FE can incorporate bone density, geometry, and microarchitecture [1]. However, the accuracy of FE models is heavily influenced by the spatial resolution of the CT scan [2].  In this study we quantify the architectural differences in trabecular bone between different modalities, specifically high-resolution peripheral quantitative computed tomography (HR-pQCT) and micro-computed tomography (μCT), and measure their effect on the resulting FE models. METHODS Two cadaveric, human distal tibiae were imaged using HR-pQCT (XtremeCT, Scanco Medical, Switzerland), with a voxel size of 82μm. After imaging, four cubes, 10mm edge length, were cut and scanned using μCT (μCT-35, Scanco Medical, Switzerland.), with a voxel size of 20μm. During a sensitivity analysis, the μCT image data was segmented using a threshold based technique and resampled to voxel sizes of 40μm, 60μm, and 80μm to assess the effect of voxel size on the FE results. The FE results were statistical compared with a one-way repeated measures ANOVA and a Tukey’s post-hoc test. Based on the known location of each cube recorded during the cutting process, the μCT data was manually aligned to the larger HR-pQCT image, where mutual information registration was applied for accurate alignment.  Virtual cubes were extracted from the registered HR-pQCT data. All cube image data was rescaled to preserve the bone volume/total volume ratio. Subsequently, all image data was converted to hexahedron elements for FE analysis and subjected to 1% uniaxial compression (FAIM v6.0, Numerics88) in the x-, y-, and z-directions. The resulting FE data was compared with a two-way ANOVA, with Bonferroni multiple comparisons test. RESULTS The sensitivity analysis found no statistically significant differences in reaction force between any cubes with different resolutions when compressed in the z-direction. The mean percent error, when compared to the 20μm cube, for the 40μm, 60μm, and 80μm cubes was 0.06%, 0.45%, and 0.76%, respectively. In the y-direction, there were significant differences between the 20μm and the 80μm (p < 0.01; 62.51% error). When loaded in the x-direction, there were significant differences between the 20μm FE cube and the 60μm and 80μm FE cubes (p<0.05; respective mean percent errors of 121.27% and 182.83%). With increasing voxel size, the reaction force was overestimated. As shown in figure 1, the registration between μCT and HR-pQCT was successful. However, there were statistically significant differences found between the μCT versus HR-pQCT FE data in the y- and x-direction (p<0.01; 23.95% and 21.04% error, respectively) but none were found for the z-direction (p>0.05, 0.14% error.) Figure 1. Registered distal tibia (HR-pQCT - red) to the μCT acquired cubes (yellow). The registered image appears green in overlap. DISCUSSION AND CONCLUSIONS It was shown there is little difference in the FE results loaded in the z-direction upon rescaling μCT data (mean percent errors less than 0.8%). However, there are large changes in the non-axial (i.e. x and y) directions. It is recommended that μCT data be rescaled up to 40μm. In addition, it was shown that μCT data could be successfully registered to HR-pQCT (see figure 1). For a given resolution, μCT data is only comparable to HR-pQCT data in the z-direction. In conclusion, HR-pQCT may not be an ideal substitute for μCT micro-architectural data.  This study improves our understanding on the use of HR-pQCT imaging for quantification of bone micro-architecture

ESTIMATING DIELECTRIC PROPERTIES OF BIOLOGICAL TISSUE

INTRODUCTION Microwave imaging has been of interest in recent decades, offering the potential of an affordable and non-ionizing medical diagnostic modality. This technique is sensitive to changes in dielectric properties such as permittivity and conductivity. One approach is microwave radar imaging, which creates images by focusing signals caused by reflections at material interfaces. In order to improve the images from radar approaches, patient-specific dielectric property estimations have been used to determine the speed of wave travel within the tissue [1]. Estimating dielectric properties of biological tissue can also be useful in emerging quantitative applications including bone health assessment. Methods such as local rod probes and antenna measurements of planar samples have been developed to estimate dielectric properties, but are of limited use for in vivo measurements. We have previously developed methods of permittivity estimation with a custom antenna, however this approach requires two measurements at different separation distances and is unable to estimate conductivity. This study aims to improve on methods of estimating permittivity and to add an estimate of conductivity of in vivo biological tissue by incorporating an antenna calibration method. METHODS In order to remove the influence of the antennas on measurements, a previously developed calibration method [2] was adapted to be used with a custom ultra-wideband antenna system [3], allowing permittivity and conductivity to be estimated over a range of frequencies. The two antennas are characterized as 2x2 matrices at each frequency, determined from two calibration measurements: the first is performed with the antennas separated by an electrical conductor, and the second measurement is done with the antennas in direct contact with one another. Measurements were performed using a vector network analyzer (Agilent, PNA-L, N5230A), and take less than 15 seconds. Measurement samples were placed between the two antennas, with their surfaces in contact with the entire antenna aperture. Dielectric properties were then estimated using the magnitude and phase of the calibrated transmission data. To validate this method, dielectric properties of several liquids were estimated and compared to literature values. RESULTS A general agreement was seen between the estimated and literature dielectric properties of several liquids, particularly for high permittivity materials. The estimated and literature permittivity of distilled water is shown in Figure 1. Several biological tissues were then measured such as human calf and heel, and porcine bone excisions. Literature values for these properties are limited as they are often done using local probes which only measure the properties at the surface of a sample.DISCUSSION AND CONCLUSIONS A calibration method has been adapted to enable an ultra-wideband antenna system to assess dielectric properties of in vivo tissue at microwave frequencies. The estimated properties of the tested liquids align closely to literature, providing confidence in estimates of biological tissues which have limited literature values. This technique can be used towards microwave radar signal speed estimates, and for quantitative property measurements. Future improvements could include a skin subtraction method to isolate the properties of bone or other tissue under the skin, and development towards microwave bone health assessment

A PRINCIPLE INVESTIGATION INTO THE FEASIBILITY OF USING MICROWAVE IMAGING TO MONITOR BONE HEALTH

INTRODUCTION Assessing bone health is of particular interest in age-associated disease and traumas such as osteoporosis, and fractures from extreme sports. Having tools that can safely and accurately assess bone health allows for the screening, diagnosis, and monitoring of disease or injury. The current gold standard for assessing bone health is high-resolution peripheral quantitative computed tomography (HR-pQCT) allowing direct three-dimensional (3D) visualization of bone. Recent evidence suggests microwave imaging can be a complementary medical imaging tool to HR-pQCT for dynamic assessment of full bone health [1]. Specifically, it was shown that microwave properties of cancellous bone are sensitive to physical changes in bone. However, this study was purely exploratory and provided no direct evidence for changes in dielectric properties with varying bone health. In this study, we aim to understand the interaction of electromagnetic waves with bone as a composite material, specifically the material anisotropy. Such information would be crucial to understanding how microwave measurements relate to the physical characteristics of the bone. METHODS Image data for the right and left tibia and radius of one female and two male subjects was acquired from HR-pQCT (XtremeCTII, Scanco Medical). The 3D image data was smoothed with a Gaussian filter (σ = 1.6) and segmented using histogram based segmentation. Cubes of edge length 82 voxels (5.002 mm) were extracted from the segmented images based on the bone center of geometry. The extracted cubes were imported into electromagnetic simulation software (SEMCAD X, Schmid & Partner Engineering AG). A parallel plate waveguide filled with air was excited with a Gaussian pulse polarized in the z-axis (f0 = 6.5 GHz, BW = 11 GHz). The bone and marrow were assigned material properties from literature [2]. Resulting data was exported and processed using custom MATLAB scripts (R2013a, MathWorks). Three simulations were performed per image such that the electromagnetic wave was polarized in each of the three anatomical directions: anterior-posterior, medial-lateral, and proximal-distal. RESULTS The effective permittivity, ε’r, was calculated for each of the anatomical directions and plotted across the frequency range of the input signal. A representative plot for all images is shown in Figure 1. The effective permittivity for each orientation tend to vary around a common permittivity.DISCUSSION AND CONCLUSIONS The results presented here provide a rudimentary but novel insight into the anisotropic behaviour of bone at microwave frequencies. Furthermore, it presents a technique for 3D model acquisition and simulation of bone not yet present in literature. This technique will allow further exploration of the electromagnetic properties of bone such as a deeper insight into the anisotropic behaviour and development of a model for the effective medium of bone as a composite material. With such information, the microwave measurements of bone could be directly related to the bone’s physical properties. This would prove the potential of microwaves to assess bone health for disease or trauma and allow the development of in vivo imaging tools for assessing disease and trauma

Predicting Bone Adaptation in Astronauts during and after Spaceflight

A method was previously developed to identify participant-specific parameters in a model of trabecular bone adaptation from longitudinal computed tomography (CT) imaging. In this study, we use these numerical methods to estimate changes in astronaut bone health during the distinct phases of spaceflight and recovery on Earth. Astronauts (N = 16) received high-resolution peripheral quantitative CT (HR-pQCT) scans of their distal tibia prior to launch (L), upon their return from an approximately six-month stay on the international space station (R+0), and after six (R+6) and 12 (R+12) months of recovery. To model trabecular bone adaptation, we determined participant-specific parameters at each time interval and estimated their bone structure at R+0, R+6, and R+12. To assess the fit of our model to this population, we compared static and dynamic bone morphometry as well as the Dice coefficient and symmetric distance at each measurement. In general, modeled and observed static morphometry were highly correlated (R2> 0.94) and statistically different (p p < 0.0001). The Dice coefficient and symmetric distance indicated a reasonable local fit between observed and predicted bone volumes. This work applies a general and versatile computational framework to test bone adaptation models. Future work can explore and test increasingly sophisticated models (e.g., those including load or physiological factors) on a participant-specific basis

Exogenously delivered iPSCs disrupt the natural repair response of endogenous MPCs after bone injury

Abstract Promoting bone healing including fracture non-unions are promising targets for bone tissue engineering due to the limited success of current clinical treatment methods. There has been significant research on the use of stem cells with and without biomaterial scaffolds to treat bone fractures due to their promising regenerative capabilities. However, the relative roles of exogenous vs. endogenous stem cells and their overall contribution to in vivo fracture repair is not well understood. The purpose of this study was to determine the interaction between exogenous and endogenous stem cells during bone healing. This study was conducted using a standardized burr-hole bone injury model in a mesenchymal progenitor cell (MPC) lineage-tracing mouse under normal homeostatic and osteoporotic conditions. Burr-hole injuries were treated with a collagen-I biomaterial loaded with and without labelled induced pluripotent stem cells (iPSCs). Using lineage-tracing, the roles of exogenous and endogenous stem cells during bone healing were examined. It was observed that treatment with iPSCs resulted in muted healing compared to untreated controls in intact mice post-injury. When the cell populations were examined histologically, iPSC-treated burr-hole defects presented with a dramatic reduction in endogenous MPCs and cell proliferation throughout the injury site. However, when the ovaries were removed and an osteoporotic-like phenotype induced in the mice, iPSCs treatment resulted in increased bone formation relative to untreated controls. In the absence of iPSCs, endogenous MPCs demonstrated robust proliferative and osteogenic capacity to undertake repair and this behaviour was disrupted in the presence of iPSCs which instead took on an osteoblast fate but with little proliferation. This study clearly demonstrates that exogenously delivered cell populations can impact the normal function of endogenous stem/progenitor populations during the normal healing cascade. These interactions need to be better understood to inform cell and biomaterial therapies to treat fractures

Three-Dimensional Quantitative Morphometric Analysis (QMA) for In Situ Joint and Tissue Assessment of Osteoarthritis in a Preclinical Rabbit Disease Model.

This work utilises advances in multi-tissue imaging, and incorporates new metrics which define in situ joint changes and individual tissue changes in osteoarthritis (OA). The aims are to (1) demonstrate a protocol for processing intact animal joints for microCT to visualise relevant joint, bone and cartilage structures for understanding OA in a preclinical rabbit model, and (2) introduce a comprehensive three-dimensional (3D) quantitative morphometric analysis (QMA), including an assessment of reproducibility. Sixteen rabbit joints with and without transection of the anterior cruciate ligament were scanned with microCT and contrast agents, and processed for histology. Semi-quantitative evaluation was performed on matching two-dimensional (2D) histology and microCT images. Subsequently, 3D QMA was performed; including measures of cartilage, subchondral cortical and epiphyseal bone, and novel tibio-femoral joint metrics. Reproducibility of the QMA was tested on seven additional joints. A significant correlation was observed in cartilage thickness from matching histology-microCT pairs. The lateral compartment of operated joints had larger joint space width, thicker femoral cartilage and reduced bone volume, while osteophytes could be detected quantitatively. Measures between the in situ tibia and femur indicated an altered loading scenario. High measurement reproducibility was observed for all new parameters; with ICC ranging from 0.754 to 0.998. In conclusion, this study provides a novel 3D QMA to quantify macro and micro tissue measures in the joint of a rabbit OA model. New metrics were established consisting of: an angle to quantitatively measure osteophytes (σ), an angle to indicate erosion between the lateral and medial femoral condyles (ρ), a vector defining altered angulation (λ, α, β, γ) and a twist angle (τ) measuring instability and tissue degeneration between the femur and tibia, a length measure of joint space width (JSW), and a slope and intercept (m, Χ) of joint contact to demonstrate altered loading with disease progression, as well as traditional bone and cartilage and histo-morphometry measures. We demonstrate correlation of microCT and histology, sensitive discrimination of OA change and robust reproducibility

User reproducibility for cartilage segmentation.

<p>Plot of 2D cartilage thickness (Cg.Th) measured by three users segmenting a random selection of matching microCT and histology images; 10 femur and 10 tibia image pairs from both NO and OP joints, demonstrating user independence of the segmentation procedure. ICC (microCT) = 0.996 (0.992, 0.998); ICC (histology) = 0.993 (0.984, 0.997), p < 0.001.</p

Bivariate Regressions of QMA measures.

<p>Relationships between QMA measures indicate significant associations between and within tissue measures (Cg.Th, Ct.Th) and whole joint measures (JSW, χ, β, σ). Correlations were observed between Cg.Th and JSW both (a) laterally (R = 0.64, p < 0.01) and (b) medially (R = 0.66, p < 0.01), where Cg.Th is the addition of mean tibial and femoral compartmental values. This was also seen laterally (c) but not medially (d) for JSW and χ (R = 0.88, p < 0.01). The β angle is strongly negatively correlated with both (e) lateral and (f) medial tibial Cg.Th (R = -0.69 and -0.66, respectively, p < 0.01), and Ct.Th is negatively correlated with σ in the (g) lateral and (h) medial tibia (R = -0.52 and -0.55, respectively, p < 0.05).</p

Measurement reproducibility.

<p>Typical HEX1/HEX2/HEX3 scans show excellent measurement reproducibility (ICC > 0.74) for cartilage, bone and <i>in situ</i> joint measures. (a) Bone morphometric ICC values were low for tibial BS/BV (0.077) and Tb.Th (0.403), due to penetration of Hexabrix^® into bone tissue. Scale bar = 5 mm. (b) Femoral and (c) tibial cartilage thickness maps demonstrate good reproducibility in cartilage compartments.</p