Maarten Depypere

Development of micro-CT protocols for in vivo follow-up of mouse bone architecture without major radiation side effects

In vivo micro-computed tomography (micro-CT) will offer unique information on the time-related changes in bone mass and structure of living mice, provided that radiation-induced side effects are prevented. Lowering the radiation dose, however, inevitably decreases the image quality. In this study we developed and validated a protocol for in vivo micro-CT imaging of mouse bone architecture that retains high quality images but avoids radiation-induced side effects on bone structure and hematological parameters. The left hindlimb of male C57Bl/6 mice was scanned in vivo at 3 consecutive time points, separated each time by a 2-week interval. Two protocols for in vivo micro-CT imaging were evaluated, with pixel sizes of 9 and 18 μm and administered radiation doses of 434 mGy and 166 mGy per scan, respectively. These radiation doses were found not to influence trabecular or cortical bone architecture in pre-pubertal or adult mice. In addition, there was no evidence for hematological side effects as peripheral blood cell counts and the colony-forming capacity of hematopoietic progenitor cells from bone marrow and spleen were not altered. Although the images obtained with these in vivo micro-CT protocols were more blurred than those obtained with high resolution (5 μm) ex vivo CT imaging, longitudinal follow-up of trabecular bone architecture in an orchidectomy model proved to be feasible using the 9 μm pixel size protocol in combination with a suitable bone segmentation technique (i.e. local thresholding). The image quality of the 18 μm pixel size protocol was too degraded for accurate bone segmentation and the use of this protocol is therefore restricted to monitor marked changes in bone structure such as bone metastatic lesions or fracture healing. In conclusion, we developed two micro-CT protocols which are appropriate for detailed as well as global longitudinal studies of mouse bone architecture and lack noticeable radiation-induced side effects.

A comparative evaluation of cone beam CT and micro-CT on trabecular bone structures in the human mandible.

OBJECTIVES The main purpose of this study was to determine the accuracy of cone beam CT (CBCT) in measuring the trabecular bone microstructure, in comparison with micro-CT. The subobjective was to examine to what extent bone quality assessment is influenced by X-ray tube current and voltage settings as well as soft tissue surrounding the bone. METHODS Eight human mandibular bone samples were scanned using three different clinical exposure protocol within water (W1-3) and without water (NW1-3) by a high-resolution (80 µm) CBCT machine (3D Accuitomo 170(®); Morita, Kyoto, Japan). Subsequently, the samples underwent micro-CT scanning (SkyScan 1174®; SkyScan, Antwerp, Belgium). After image acquisition, similar volumes of interest of the trabecular structures captured with CBCT and micro-CT were aligned with each other. Segmentation was then performed, and the morphometric parameters were quantified within the volumes of interest by CTAn software (CTAnalyser(®); SkyScan, Antwerp, Belgium). Descriptive statistical analyses and multiple comparisons between all protocols were applied in R software. RESULTS High positive Pearsons correlation coefficients were observed between CBCT and micro-CT protocols for all tested morphometric indices except for trabecular thickness. No significant differences were observed between all exposure protocols except for trabecular separation. When examining the soft-tissue effect on trabecular bone structures, no significant differences between NW (1-3) and W (1-3) protocols were observed for all variables. CONCLUSIONS The present study demonstrated the potential of high-resolution CBCT imaging for in vivo applications of quantitative bone morphometry and bone quality assessment. However, the overestimation of morphometric parameters and acquisition settings in CBCT must be taken into account.

3D volumetric displacement and strain analysis of composite polymerization

OBJECTIVE The present study aimed at a better understanding of the internal shrinkage patterns within different cavity sizes. METHODS Ten cylindrical cavities in two sizes were filled with a flowable composite and scanned using X-ray micro-computed tomography (μ-CT) before filling, before and after polymerization. Three-dimensional (3D) non-rigid image registration was applied to sets of two subsequent μ-CT images, before and after polymerization in order to calculate the displacements and strains caused by polymerization shrinkage. RESULTS 3D volumetric displacement analysis disclosed a main vertical component for both the small and large cavities, however in the latter the downward direction reversed to an upward direction from a depth of approximately 2mm due to debonding at the bottom. Air bubbles and voids in the restorations increased upon polymerization, causing a reverse in strain in the surrounding areas. SIGNIFICANCE Polymerization-induced shrinkage stress in composite restorations cannot be measured directly. This exploratory study revealed more information on cavity-size dependent shrinkage patterns and opens the way to more extensive studies using different composite materials and varying geometric cavity configurations.

Segmentation of Trabecular Jaw Bone on Cone Beam CT Datasets.

BACKGROUND The term bone quality is often used in a dentomaxillofacial context, for example in implant planning, as bone density and bone structure have been linked to primary implant success. PURPOSE This research aimed to investigate the performance of adaptive thresholding of trabecular bone in cone beam CT (CBCT) images. The segmentation quality was assessed for different imaging devices and upper and lower jaws. MATERIALS AND METHODS Four jaws were scanned with eight CBCT scanners and one micro-CT device. Images of the jaws were spatially aligned with the micro-CT images. Two volumes of interest for each jaw were manually delineated. Trabecular bone in the volumes of interest in the micro-CT images was segmented so that the micro-CT images could serve as high-resolution ground truth images. The volumes of interest in the CBCT images were segmented using both global and adaptive thresholding. RESULTS Segmentation was significantly better for the lower jaw than for the upper jaw. Differences in performance between the scanners were significant for both jaws. Adaptive thresholding performed significantly better in segmenting the bone structure out of CBCT images. CONCLUSIONS When assessing jaw bone structure, the observer should always choose adaptive thresholding. It remains a challenge to identify the optimal threshold selection for the structural assessment of jaw bone.

Effects of Immediate and Delayed Loading on Peri‐Implant Trabecular Structures: A Cone Beam CT Evaluation

PURPOSE To develop a method for characterizing trabecular bone microarchitecture using cone beam computed tomography (CBCT) and to evaluate trabecular bone changes after rehabilitation using immediate versus delayed implant protocols. MATERIALS AND METHODS Six mongrel dogs randomly received 27 titanium implants in the maxillary incisor or mandibular premolar areas, following one of four protocols: (1) normal extraction socket healing; (2) immediate implant placement and immediate loading; (3) delayed implant placement and delayed loading; (4) delayed implant placement and immediate loading. The animals were euthanized at 8 weeks, and block biopsies were scanned using high resolution CBCT. Standard bone structural variables were assessed in coronal, middle, and apical levels. RESULTS Coronal and middle regions had more compact, more platelike, and thicker trabeculae. Protocols (2), (3), and (4) had significantly higher values (p < 0.001) than protocol (1) for bone surface density, bone surface volume ratio, and connectivity density, while significantly lower values (p < 0.001) were found for trabecular separation and fractal dimension. However, protocols (2), (3), and (4) did not show significantly different bone remodeling. CONCLUSIONS Compared with normal extraction healing, the implant protocols have an improved bone structural integration. Results do not suggest a different bone remodeling pattern when a delayed versus an immediate implant protocol is used.

An iterative dual energy CT reconstruction method for a K-edge contrast material

We present and evaluate an iterative dual energy CT reconstruction algorithm for a K-edge contrast material in microCT imaging. This allows improved discrimination of contrast enhanced structures such as vasculature from surrounding bony structures. The energy dependence of the attenuation is modeled by decomposing the linear attenuation coefficient into three basis functions. Any material without a K-edge in the imaging energy range can be modeled by two basis functions describing the Compton scatter and the photoelectric effect respectively. A K-edge material is described by using its mass attenuation coefficient as third basis function. During reconstruction the basis function coefficients are determined for each voxel of the image by maximizing the likelihood of the data. The relative weights of the Compton and photoelectric components are constrained to those of expected materials to reduce the number of unknowns to two per voxel. The proposed method is validated on simulated and real microCT projections. The presented method was found to perform better than a typical post-reconstruction approach with respect to beam-hardening and noise at the expense of increased computation time.

Neovascularization Potential of Blood Outgrowth Endothelial Cells From Patients With Stable Ischemic Heart Failure Is Preserved

Background Blood outgrowth endothelial cells (BOECs) mediate therapeutic neovascularization in experimental models, but outgrowth characteristics and functionality of BOECs from patients with ischemic cardiomyopathy (ICMP) are unknown. We compared outgrowth efficiency and in vitro and in vivo functionality of BOECs derived from ICMP with BOECs from age‐matched (ACON) and healthy young (CON) controls. Methods and Results We isolated 3.6±0.6 BOEC colonies/100×106 mononuclear cells (MNCs) from 60‐mL blood samples of ICMP patients (n=45; age: 66±1 years; LVEF: 31±2%) versus 3.5±0.9 colonies/100×106 MNCs in ACON (n=32; age: 60±1 years) and 2.6±0.4 colonies/100×106 MNCs in CON (n=55; age: 34±1 years), P=0.29. Endothelial lineage (VEGFR2+/CD31+/CD146+) and progenitor (CD34+/CD133−) marker expression was comparable in ICMP and CON. Growth kinetics were similar between groups (P=0.38) and not affected by left ventricular systolic dysfunction, maladaptive remodeling, or presence of cardiovascular risk factors in ICMP patients. In vitro neovascularization potential, assessed by network remodeling on Matrigel and three‐dimensional spheroid sprouting, did not differ in ICMP from (A)CON. Secretome analysis showed a marked proangiogenic profile, with highest release of angiopoietin‐2 (1.4±0.3×105 pg/106 ICMP‐BOECs) and placental growth factor (5.8±1.5×103 pg/106 ICMP BOECs), independent of age or ischemic disease. Senescence‐associated β‐galactosidase staining showed comparable senescence in BOECs from ICMP (5.8±2.1%; n=17), ACON (3.9±1.1%; n=7), and CON (9.0±2.8%; n=13), P=0.19. High‐resolution microcomputed tomography analysis in the ischemic hindlimb of nude mice confirmed increased arteriogenesis in the thigh region after intramuscular injections of BOECs from ICMP (P=0.025; n=8) and CON (P=0.048; n=5) over vehicle control (n=8), both to a similar extent (P=0.831). Conclusions BOECs can be successfully culture‐expanded from patients with ICMP. In contrast to impaired functionality of ICMP‐derived bone marrow MNCs, BOECs retain a robust proangiogenic profile, both in vitro and in vivo, with therapeutic potential for targeting ischemic disease.

Endothelial Msx1 transduces hemodynamic changes into an arteriogenic remodeling response

During peripheral arterial disease, MSX1 acts downstream of BMP–SMAD signaling to transduce the arterial shear stimulus into an arteriogenic remodeling response. MSX1 activates collateral endothelium into a proinflammatory state through ICAM1/VCAM1 up-regulation, resulting in increased leukocyte infiltration and collateral remodeling.

Validating cone-beam computed tomography for peri-implant bone morphometric analysis

Cone-beam computed tomography (CBCT) has been recently used to analyse trabecular bone structure around dental implants. To validate the use of CBCT for three-dimensional (3D) peri-implant trabecular bone morphometry by comparing it to two-dimensional (2D) histology, 36 alveolar bone samples (with implants n=27 vs. without implants n=9) from six mongrel dogs, were scanned ex vivo using a high-resolution (80 µm) CBCT. After scanning, all samples were decalcified and then sectioned into thin histological sections (∼6 μm) to obtain high contrast 2D images. By using CTAn imaging software, bone morphometric parameters including trabecular number (Tb.N), thickness (Tb.Th), separation (Tb.Sp) and bone volume fraction (BV/TV) were examined on both CBCT and corresponding histological images. Higher Tb.Th and Tb.Sp, lower BV/TV and Tb.N were found on CBCT images (P<0.001). Both measurements on the peri-implant trabecular bone structure showed moderate to high correlation (r=0.65–0.85). The Bland–Altman plots showed strongest agreement for Tb.Th followed by Tb.Sp, Tb.N and BV/TV, regardless of the presence of implants. The current findings support the assumption that peri-implant trabecular bone structures based on high-resolution CBCT measurements are representative for the underlying histological bone characteristics, indicating a potential clinical diagnostic use of CBCT-based peri-implant bone morphometric characterisation.

The minimal entropy prior for simultaneous reconstruction and segmentation of in vivo microct trabecular bone images

We present the minimal entropy prior for the iterative reconstruction of µCT projection data using the maximum a posteriori (MAP) framework. The minimal entropy prior restricts the number of intensity values in the reconstruction without requiring a priori information about the number of image tissues or tissue intensity estimates. We utilise the presented prior in the MAP framework to simultaneously reconstruct and segment trabecular bone from low quality µCT images. The MAP approach allows the straightforward incorporation of the in vivo scanner point spread function, while the entropy prior yields a segmentation that obeys to projection data.