Micro-Computed Tomographical Imaging of Soft Biological Materials Using Contrast Techniques

A Beginner’s Guide to the Characterization of Hydrogel Microarchitecture for Cellular Applications

The extracellular matrix (ECM) is a three-dimensional, acellular scaffold of living tissues. Incorporating the ECM into cell culture models is a goal of cell biology studies and requires biocompatible materials that can mimic the ECM. Among such materials are hydrogels: polymeric networks that derive most of their mass from water. With the tuning of their properties, these polymer networks can resemble living tissues. The microarchitectural properties of hydrogels, such as porosity, pore size, fiber length, and surface topology can determine cell plasticity. The adequate characterization of these parameters requires reliable and reproducible methods. However, most methods were historically standardized using other biological specimens, such as 2D cell cultures, biopsies, or even animal models. Therefore, their translation comes with technical limitations when applied to hydrogel-based cell culture systems. In our current work, we have reviewed the most common techniques employed in the characterization of hydrogel microarchitectures. Our review provides a concise description of the underlying principles of each method and summarizes the collective data obtained from cell-free and cell-loaded hydrogels. The advantages and limitations of each technique are discussed, and comparisons are made. The information presented in our current work will be of interest to researchers who employ hydrogels as platforms for cell culture, 3D bioprinting, and other fields within hydrogel-based research.

Computed Tomography as a Characterization Tool for Engineered Scaffolds with Biomedical Applications

The ever-growing field of materials with applications in the biomedical field holds great promise regarding the design and fabrication of devices with specific characteristics, especially scaffolds with personalized geometry and architecture. The continuous technological development pushes the limits of innovation in obtaining adequate scaffolds and establishing their characteristics and performance. To this end, computed tomography (CT) proved to be a reliable, nondestructive, high-performance machine, enabling visualization and structure analysis at submicronic resolutions. CT allows both qualitative and quantitative data of the 3D model, offering an overall image of its specific architectural features and reliable numerical data for rigorous analyses. The precise engineering of scaffolds consists in the fabrication of objects with well-defined morphometric parameters (e.g., shape, porosity, wall thickness) and in their performance validation through thorough control over their behavior (in situ visualization, degradation, new tissue formation, wear, etc.). This review is focused on the use of CT in biomaterial science with the aim of qualitatively and quantitatively assessing the scaffolds’ features and monitoring their behavior following in vivo or in vitro experiments. Furthermore, the paper presents the benefits and limitations regarding the employment of this technique when engineering materials with applications in the biomedical field.

Quality control methods in musculoskeletal tissue engineering: from imaging to biosensors

Tissue engineering is rapidly progressing toward clinical application. In the musculoskeletal field, there has been an increasing necessity for bone and cartilage replacement. Despite the promising translational potential of tissue engineering approaches, careful attention should be given to the quality of developed constructs to increase the real applicability to patients. After a general introduction to musculoskeletal tissue engineering, this narrative review aims to offer an overview of methods, starting from classical techniques, such as gene expression analysis and histology, to less common methods, such as Raman spectroscopy, microcomputed tomography, and biosensors, that can be employed to assess the quality of constructs in terms of viability, morphology, or matrix deposition. A particular emphasis is given to standards and good practices (GXP), which can be applicable in different sectors. Moreover, a classification of the methods into destructive, noninvasive, or conservative based on the possible further development of a preimplant quality monitoring system is proposed. Biosensors in musculoskeletal tissue engineering have not yet been used but have been proposed as a novel technology that can be exploited with numerous advantages, including minimal invasiveness, making them suitable for the development of preimplant quality control systems.

A versatile technique for high-resolution three-dimensional imaging of human arterial segments using microcomputed tomography

Background

Quantitative methods for evaluating microstructure of arterial specimens typically rely on histologic techniques that involve random sampling, which cannot account for the unique spatial distribution of features in three dimensions.

Methods

To overcome this limitation, we demonstrate a nondestructive method for three-dimensional imaging of intact human blood vessels using microcomputed tomography (microCT). Human artery segments were dehydrated and stained in an iodine solution then imaged with a standard laboratory microCT scanner. Image visualization and segmentation was performed using commercially available and open source software.

Results

Staining of cadaveric vessels with iodine enabled clear visualization of the arterial wall with microCT, preserved tissue morphology, and generated high-resolution images with a voxel size of 5.4 μm. Various components of the arterial wall were segmented using a combination of manual and automatic thresholding algorithms.

Conclusions

Our approach allows for spatial mapping of human artery tissue samples that can guide targeted histologic analysis of smaller tissue segments, provide geometric data to inform finite element models, quantify degree of atherosclerosis, and help to evaluate the foreign body response to intravascular medical implants. (JVS-Vascular Science 2020;2:13-19.).

Clinical Relevance

In this article, we describe a powerful technique for whole artery analysis of pathologic human tissue specimens that provides high-resolution spatial detail regarding composition of the blood vessel wall. The protocol described here is a valuable adjunct that can be used as a research tool to inform finite element modeling of arteries, quantify pathologic response (ie, neointimal hyperplasia and vascular calcification), and evaluate the tissue/device interface of implanted medical devices.

[Application of iodine staining technique for tumor identification in Micro-CT of mouse model with skull base-infratemporal fossa tumor]

OBJECTIVE

To establish an animal model with malignant tumor in the skull base-infratemporal region, and to explore the role of iodine staining technique in identifying tumor tissues with Micro-CT data.

METHODS

Sedation anesthesia was carried out on 12 BABL/c nude mice using inhaled isoflurane, and then WSU-HN6 cells that cultured and immortalized from human tongue squamous cell carcinoma were injected into the right infratemporal fossa via the submandibular area. The procedure was carried out under ultrasonographic guidance. The nude mice were sacrificed after 3 weeks observation. The head specimens were fixed and scanned by Micro-CT, and repeated scans were performed after staining with 3.75% compound iodine solution. Following decalcification in 20% EDTA for 2-4 weeks, the head specimens were embedded and sectioned. Hematoxylin and eosin staining and Pan-Keratin immunohistochemical staining were carried out. Bright-field microscopy and stereomicroscopy were used to visualize. The Micro-CT data were analyzed using iPlan software (Brainlab).

RESULTS

Non-traumatic ultrasonography was used to guide HN-6 cells injection and confirm skull-base tumor formation in all the animals. Ultrasonographic guidance reduced the risk of cervical vessel injury when transferring tumor cells into the skull base space. An obvious asymmetrical appearance was detected via ultrasonography 3 weeks after tumor cell injection. The Micro-CT analysis showed that the bone was obviously damaged on the right side of the skull base, but the soft tissue image was unrecognizable. After four days staining with compound iodine solution, the morphology of the tumor and surrounding soft tissue could be clearly identified. Hematoxylin and eosin staining showed the tumor formation of the right infratemporal fossa region accompanied by bone destruction. Human keratin immunohistochemical staining showed that the tumor tissue originated from human squamous cell carcinoma, and the polynuclear osteoclasts could be seen at the margin of the skull base bone resorption.

CONCLUSION

The animal model with malignant tumor in the skull base-infratemporal region could be successfully established via submandibular injection under ultrasound-guidance. Bone changes of the skull were easily observed on Micro-CT, but the tumor counter was not able to be distinguished from surrounding soft tissue. The 3.75% compound iodine staining of the head specimen could help discern the tumor and surrounding soft tissue in more details.

Iodine-enhanced micro-computed tomography of atherosclerotic plaque morphology complements conventional histology

BACKGROUND AND AIMS

Visualization of arterial lesions in situ can enhance understanding of atherosclerosis progression and potentially improve experimental therapies. Conventional histology methods for assessing atherosclerotic lesions are robust but are destructive and may prevent further tissue analysis. The objective of the current study was to evaluate a novel, nondestructive method for visualization and characterization of atherosclerotic lesions as an alternative or complementary to routine histology. Thus, we tested the hypothesis that micro-computed tomography (micro-CT) paired with an iodine-based radiopaque stain would effectively characterize atherosclerotic plaques in a manner comparable to routine histology while maintaining sample integrity and providing whole-volume data.

METHODS

We examined porcine coronary arteries with varying degrees of atherosclerosis, using micro-CT in the absence and presence of iohexol (240 mgI/ml). Following iohexol washout, routine histological assessment of the samples was performed with hematoxylin and eosin and Masson's trichrome.

RESULTS

Iohexol staining generated soft tissue delineation and subsequent atherosclerotic plaque assessment via augmented radiopacity, permitting three-dimensional (3D) reconstruction of these lesions, maintaining in situ architecture. Although plaque distribution and arterial wall tissue layers were discernible, micro-CT was incapable of discriminating cell types comprising the plaque. Calcium phosphate deposition was readily located and visualized in 3D space, independent of iohexol.

CONCLUSIONS

The results of this study establish micro-CT, combined with a diffusible radiopaque contrast agent, as a powerful imaging modality for visualizing in situ architecture of atherosclerotic plaques. Our findings demonstrate that micro-CT can be used to identify plaque distribution and calcium deposition complementary to routine histological analysis.

Challenges of Post-measurement Histology for the Dielectric Characterisation of Heterogeneous Biological Tissues

The dielectric properties of biological tissues are typically measured using the open-ended coaxial probe technique, which is based on the assumption that the tissue sample is homogeneous. Therefore, for heterogeneous tissue samples, additional post-measurement sample processing is conducted. Specifically, post-measurement histological analysis may be performed in order to associate the measured dielectric properties with the tissue types present in a heterogeneous sample. Accurate post-measurement histological analysis enables identification of the constituent tissue types that contributed to the measured dielectric properties, and their relative distributions. There is no standard protocol for conducting post-measurement histological analysis, which leads to high numbers of excluded tissue samples and inconsistencies in the resulting reported data for heterogeneous tissues. To this extent, this study examines the post-measurement histological process and the challenges in associating the acquired dielectric properties with the different tissue types present in heterogeneous samples. The results demonstrate that the histological process inevitably alters the morphology of samples, thus introducing errors in the interpretation of the dielectric properties acquired from heterogeneous biological samples. Notably, sample size was seen to shrink by up to 90% through the histological process, meaning that sensing volume determined from fresh tissues is not directly applicable to histology images.

New Insights Into 3-Dimensional Anatomy of the Facial Mimetic Muscles Related to the Nasolabial Fold: An Iodine Staining Technique Based on Micro-Computed Tomography

BACKGROUND

This study aimed to introduce a novel approach to study the facial mimetic muscles (FMMs) related to the nasolabial fold (NLF) and realize the visualization of complex 3-dimensional (3D) structures of the FMM.

METHODS

Micro-computed tomography (micro-CT) and iodine staining technique were used to obtain the 2-dimensional radiographs of the FMM. Materialise Mimics software was then used to reconstruct the 3D model of the FMM.

RESULTS

The zygomaticus major muscle was not directly connected to the orbicularis oris muscle (OOM). The terminal fibers of the zygomaticus major muscle were located in the levator anguli oris. The surface of the FMM was covered with some unknown muscle fibers in the NLF. The terminal fibers of the zygolabialis ended in the middle of the ipsilateral OOM rather than the edge of the OOM. At the same anatomical level, the FMM fibers were so closely connected to each other that they were not separated by adipose tissue. A detailed 3D sequence of the muscle fibers in the modiolus at the corner of the mouth was determined. The muscle fibers in the modiolus were organized rather than disorganized. The terminal fibers of the levator anguli oris were located at the terminal fibers of the musculus depressor anguli oris at the corner of mouth.

CONCLUSIONS

Although the micro-CT had many disadvantages, the micro-CT combined with iodine staining technique enabled the 3D anatomical study of the FMM related to NLF. We imported the 2-dimensional images obtained by micro-CT scanning into Mimics software, successfully reconstructed the FMM related to the NLF, and finally obtained images of complex 3D structures of the FMM related to the NLF. The shapes, positions, and 3D spatial relationships between the FMM related to the NLF were clearly visualized. The novel insights into the 3D anatomy of the FMM related to NLF may help understand the formation of the NLF. Finally, the results of this study may help improve rejuvenation surgery of the NLF in the near future.

Advances in bioinks and in vivo imaging of biomaterials for CNS applications

Due to increasing life expectancy incidence of neurological disorders is rapidly rising, thus adding urgency to develop effective strategies for treatment. Stem cell-based therapies were considered highly promising and while progress in this field is evident, outcomes of clinical trials are rather disappointing. Suboptimal engraftment, poor cell survival and uncontrolled differentiation may be the reasons behind dismal results. Clearly, new direction is needed and we postulate that with recent progress in biomaterials and bioprinting, regenerative approaches for neurological applications may be finally successful. The use of biomaterials aids engraftment of stem cells, protects them from harmful microenvironment and importantly, it facilitates the incorporation of cell-supporting molecules. The biomaterials used in bioprinting (the bioinks) form a scaffold for embedding the cells/biomolecules of interest, but also could be exploited as a source of endogenous contrast or supplemented with contrast agents for imaging. Additionally, bioprinting enables patient-specific customization with shape/size tailored for actual needs. In stroke or traumatic brain injury for example lesions are localized and focal, and usually progress with significant loss of tissue volume creating space that could be filled with artificial tissue using bioprinting modalities. The value of imaging for bioprinting technology is advantageous on many levels including design of custom shapes scaffolds based on anatomical 3D scans, assessment of performance and integration after scaffold implantation, or to learn about the degradation over time. In this review, we focus on bioprinting technology describing different printing techniques and properties of biomaterials in the context of requirements for neurological applications. We also discuss the need for in vivo imaging of implanted materials and tissue constructs reviewing applicable imaging modalities and type of information they can provide. STATEMENT OF SIGNIFICANCE: Current stem cell-based regenerative strategies for neurological diseases are ineffective due to inaccurate engraftment, low cell viability and suboptimal differentiation. Bioprinting and embedding stem cells within biomaterials at high precision, including building complex multi-material and multi-cell type composites may bring a breakthrough in this field. We provide here comprehensive review of bioinks, bioprinting techniques applicable to application for neurological disorders. Appreciating importance of longitudinal monitoring of implanted scaffolds, we discuss advantages of various imaging modalities available and suitable for imaging biomaterials in the central nervous system. Our goal is to inspire new experimental approaches combining imaging, biomaterials/bioinks, advanced manufacturing and tissue engineering approaches, and stimulate interest in image-guided therapies based on bioprinting.

A review of methods to measure tendon dimensions

Tendons are soft tissues of the musculoskeletal system that are designed to facilitate joint movement. Tendons exhibit a wide range of mechanical properties matched to their functions and, as a result, have been of interest to researchers for many decades. Dimensions are an important aspect of tendon properties.

Change in the dimensions of tissues is often seen as a sign of injury and degeneration, as it may suggest inflammation or general disorder of the tissue. Dimensions are also important for determining the mechanical properties and behaviours of materials, particularly the stress, strain, and elastic modulus. This makes the dimensions significant in the context of a mechanical study of degenerated tendons. Additionally, tendon dimensions are useful in planning harvesting for tendon transfer and joint reconstruction purposes.

Historically, many methods have been used in an attempt to accurately measure the dimensions of soft tissue, since improper measurement can lead to large errors in the calculated properties. These methods can be categorised as destructive (by approximation), contact, and non-contact and can be considered in terms of in vivo and ex vivo.

Four-Dimensional Imaging of Soft Tissue and Implanted Biomaterial Mechanics: A Barbed Suture Case Study for Tendon Repair

Timely, recent developments in X-ray microcomputed tomography (XμCT) imaging such as increased resolution and improved sample preparation enable nondestructive time-lapse imaging of polymeric biomaterials when implanted in soft tissue, which we demonstrate herein. Imaging the full three-dimensional (3D) structure of an implanted biomaterial provides new opportunities to assess the micromechanics of the interface between the implant and tissues and how this changes over time as force is applied in load-bearing musculoskeletal applications. In this paper, we present a case study demonstrating in situ XμCT and finite element analysis, using a dynamically loaded barbed suture repair for its novel use in tendon tissue. The aim of this study was to identify the distribution of stress in the suture and tendon as load is applied. The data gained demonstrate a clear 3D visualization of microscale features in both the tissue and implant in wet conditions. XμCT imaging has revealed, for the first time, pores around the suture, preventing full engagement of all the barbs with the tendon tissue. Subsequent finite element analysis reveals the localized stress and strain, which are not evenly distributed along the suture, or throughout the tissue. This case study demonstrates for the first time a powerful in situ mechanical imaging tool, which could be readily adapted by other laboratories to interrogate and optimize the interface between the implanted biomaterials and the soft tissue.

Correlative Detection of Isolated Single and Multi-Cellular Calcifications in the Internal Elastic Lamina of Human Coronary Artery Samples

Histopathology protocols often require sectioning and processing of numerous microscopy slides to survey a sample. Trade-offs between workload and sampling density means that small features can be missed. Aiming to reduce the workload of routine histology protocols and the concern over missed pathology in skipped sections, we developed a prototype x-ray tomographic scanner dedicated to rapid scouting and identification of regions of interest in pathology specimens, thereby allowing targeted histopathology analysis to replace blanket searches. In coronary artery samples of a deceased HIV patient, the scanner, called Tomopath, obtained depth-resolved cross-sectional images at 15 µm resolution in a 15-minute scan, which guided the subsequent histological sectioning and microscopy. When compared to a commercial tabletop micro-CT scanner, the prototype provided several-fold contrast-to-noise ratio in 1/11^th the scan time. Correlated tomographic and histological images revealed two types of micro calcifications: scattered loose calcifications typically found in atherosclerotic lesions; isolated focal calcifications in one or several cells in the internal elastic lamina and occasionally in the tunica media, which we speculate were the initiation of medial calcification linked to kidney disease, but rarely detected at this early stage due to their similarity to particle contaminants introduced during histological processing, if not for the evidence from the tomography scan prior to sectioning. Thus, in addition to its utility as a scouting tool, in this study it provided complementary information to histological microscopy. Overall, the prototype scanner represents a step toward a dedicated scouting and complementary imaging tool for routine use in pathology labs.

Hydrogel-based scaffolds to support intrathecal stem cell transplantation as a gateway to the spinal cord: clinical needs, biomaterials, and imaging technologies

The prospects for cell replacement in spinal cord diseases are impeded by inefficient stem cell delivery. The deep location of the spinal cord and complex surgical access, as well as densely packed vital structures, question the feasibility of the widespread use of multiple spinal cord punctures to inject stem cells. Disorders characterized by disseminated pathology are particularly appealing for the distribution of cells globally throughout the spinal cord in a minimally invasive fashion. The intrathecal space, with access to a relatively large surface area along the spinal cord, is an attractive route for global stem cell delivery, and, indeed, is highly promising, but the success of this approach relies on the ability of cells (1) to survive in the cerebrospinal fluid (CSF), (2) to adhere to the spinal cord surface, and (3) to migrate, ultimately, into the parenchyma. Intrathecal infusion of cell suspension, however, has been insufficient and we postulate that embedding transplanted cells within hydrogel scaffolds will facilitate reaching these goals. In this review, we focus on practical considerations that render the intrathecal approach clinically viable, and then discuss the characteristics of various biomaterials that are suitable to serve as scaffolds. We also propose strategies to modulate the local microenvironment with nanoparticle carriers to improve the functionality of cellular grafts. Finally, we provide an overview of imaging modalities for in vivo monitoring and characterization of biomaterials and stem cells. This comprehensive review should serve as a guide for those planning preclinical and clinical studies on intrathecal stem cell transplantation.

Synthesis and Preclinical Characterization of a Cationic Iodinated Imaging Contrast Agent (CA4+) and Its Use for Quantitative Computed Tomography of Ex Vivo Human Hip Cartilage

Contrast agents that go beyond qualitative visualization and enable quantitative assessments of functional tissue performance represent the next generation of clinically useful imaging tools. An optimized and efficient large-scale synthesis of a cationic iodinated contrast agent (CA4+) is described for imaging articular cartilage. Contrast-enhanced CT (CECT) using CA4+ reveals significantly greater agent uptake of CA4+ in articular cartilage compared to that of similar anionic or nonionic agents, and CA4+ uptake follows Donnan equilibrium theory. The CA4+ CECT attenuation obtained from imaging ex vivo human hip cartilage correlates with the glycosaminoglycan content, equilibrium modulus, and coefficient of friction, which are key indicators of cartilage functional performance and osteoarthritis stage. Finally, preliminary toxicity studies in a rat model show no adverse events, and a pharmacokinetics study documents a peak plasma concentration 30 min after dosing, with the agent no longer present in vivo at 96 h via excretion in the urine.

Iodine and freeze-drying enhanced high-resolution MicroCT imaging for reconstructing 3D intraneural topography of human peripheral nerve fascicles

BACKGROUND

The precise annotation and accurate identification of the topography of fascicles to the end organs are prerequisites for studying human peripheral nerves.

NEW METHOD

In this study, we present a feasible imaging method that acquires 3D high-resolution (HR) topography of peripheral nerve fascicles using an iodine and freeze-drying (IFD) micro-computed tomography (microCT) method to greatly increase the contrast of fascicle images.

RESULTS

The enhanced microCT imaging method can facilitate the reconstruction of high-contrast HR fascicle images, fascicle segmentation and extraction, feature analysis, and the tracing of fascicle topography to end organs, which define fascicle functions.

COMPARISON WITH EXISTING METHODS

The complex intraneural aggregation and distribution of fascicles is typically assessed using histological techniques or MR imaging to acquire coarse axial three-dimensional (3D) maps. However, the disadvantages of histological techniques (static, axial manual registration, and data instability) and MR imaging (low-resolution) limit these applications in reconstructing the topography of nerve fascicles.

CONCLUSIONS

Thus, enhanced microCT is a new technique for acquiring 3D intraneural topography of the human peripheral nerve fascicles both to improve our understanding of neurobiological principles and to guide accurate repair in the clinic. Additionally, 3D microstructure data can be used as a biofabrication model, which in turn can be used to fabricate scaffolds to repair long nerve gaps.

Scaffolds for whole organ tissue engineering: Construction and in vitro evaluation of a seamless, spherical and hollow collagen bladder construct with appendices

The field of regenerative medicine has developed promising techniques to improve current neobladder strategies used for radical cystectomies or congenital anomalies. Scaffolds made from molecularly defined biomaterials are instrumental in the regeneration of tissues, but are generally confined to small flat patches and do not comprise the whole organ. We have developed a simple, one-step casting method to produce a seamless large hollow collagen-based scaffold, mimicking the shape of the whole bladder, and with integrated anastomotic sites for ureters and urethra. The hollow bladder scaffold is highly standardized, with uniform wall thickness and a unidirectional pore structure to facilitate cell infiltration in vivo. Human and porcine bladder urothelial and smooth muscle cells were able to attach to the scaffold and maintained their phenotype in vitro. The closed luminal side and the porous outside of the scaffold facilitated the formation of an urothelial lining and infiltration of smooth muscle cells, respectively. The cells aligned according to the provided scaffold template. The technology used is highly adjustable (shape, size, materials) and may be used as a starting point for research to an off-the-shelf medical device suitable for neobladders.

STATEMENT OF SIGNIFICANCE

In this study, we describe the development of a simple, one-step casting method to produce a seamless large hollow collagen-based scaffold mimicking the shape of the whole bladder with integrated anastomotic sites for ureters and urethra. The hollow bladder scaffold is highly standardized with uniform wall thickness and a unidirectional pore structure to facilitate cell infiltration in vivo. The closed luminal surface and the porous exterior of the scaffold facilitated the formation of a urothelial lining and infiltration of smooth muscle cells, respectively. The applied technology is highly adjustable (shape, size, materials) and can be the starting point for research to an off-the-shelf medical device suitable for neobladders.

Computational Aerodynamic Analysis of a Micro-CT Based Bio-Realistic Fruit Fly Wing

The aerodynamic features of a bio-realistic 3D fruit fly wing in steady state (snapshot) flight conditions were analyzed numerically. The wing geometry was created from high resolution micro-computed tomography (micro-CT) of the fruit fly Drosophila virilis. Computational fluid dynamics (CFD) analyses of the wing were conducted at ultra-low Reynolds numbers ranging from 71 to 200, and at angles of attack ranging from -10° to +30°. It was found that in the 3D bio-realistic model, the corrugations of the wing created localized circulation regions in the flow field, most notably at higher angles of attack near the wing tip. Analyses of a simplified flat wing geometry showed higher lift to drag performance values for any given angle of attack at these Reynolds numbers, though very similar performance is noted at -10°. Results have indicated that the simplified flat wing can successfully be used to approximate high-level properties such as aerodynamic coefficients and overall performance trends as well as large flow-field structures. However, local pressure peaks and near-wing flow features induced by the corrugations are unable to be replicated by the simple wing. We therefore recommend that accurate 3D bio-realistic geometries be used when modelling insect wings where such information is useful.

Imaging Approaches in Functional Assessment of Implantable Myogenic Biomaterials and Engineered Muscle Tissue

The fields of tissue engineering and regenerative medicine utilize implantable biomaterials and engineered tissues to regenerate damaged cells or replace lost tissues. There are distinct challenges in all facets of this research, but functional assessments and monitoring of such complex environments as muscle tissues present the current strategic priority. Many extant methods for addressing these questions result in the destruction or alteration of tissues or cell populations under investigation. Modern advances in non-invasive imaging modalities present opportunities to rethink some of the anachronistic methods, however, their standard employment may not be optimal when considering advancements in myology. New image analysis protocols and/or combinations of established modalities need to be addressed. This review focuses on efficacies and limitations of available imaging modalities to the functional assessment of implantable myogenic biomaterials and engineered muscle tissues.

Sine systemate chaos? A versatile tool for earthworm taxonomy: non-destructive imaging of freshly fixed and museum specimens using micro-computed tomography

In spite of the high relevance of lumbricid earthworms ('Oligochaeta': Lumbricidae) for soil structure and functioning, the taxonomy of this group of terrestrial invertebrates remains in a quasi-chaotic state. Earthworm taxonomy traditionally relies on the interpretation of external and internal morphological characters, but the acquisition of these data is often hampered by tedious dissections or restricted access to valuable and rare museum specimens. The present state of affairs, in conjunction with the difficulty of establishing primary homologies for multiple morphological features, has led to an almost unrivaled instability in the taxonomy and systematics of certain earthworm groups, including Lumbricidae. As a potential remedy, we apply for the first time a non-destructive imaging technique to lumbricids and explore the future application of this approach to earthworm taxonomy. High-resolution micro-computed tomography (μCT) scanning of freshly fixed and museum specimens was carried out using two cosmopolitan species, Aporrectodea caliginosa and A. trapezoides. By combining two-dimensional and three-dimensional dataset visualization techniques, we demonstrate that the morphological features commonly used in earthworm taxonomy can now be analyzed without the need for dissection, whether freshly fixed or museum specimens collected more than 60 years ago are studied. Our analyses show that μCT in combination with soft tissue staining can be successfully applied to lumbricid earthworms. An extension of the approach to other families is poised to strengthen earthworm taxonomy by providing a versatile tool to resolve the taxonomic chaos currently present in this ecologically important, but taxonomically neglected group of terrestrial invertebrates.

Three-dimensional imaging of the mouse heart and vasculature using micro-CT and whole-body perfusion of iodine or phosphotungstic acid

Recent studies have investigated histological staining compounds as micro-computed tomography (micro-CT) contrast agents, delivered by soaking tissue specimens in stain and relying on passive diffusion for agent uptake. This study describes a perfusion approach using iodine or phosphotungstic acid (PTA) stains, delivered to an intact mouse, to capitalize on the microvasculature as a delivery conduit for parenchymal staining and direct contact for staining artery walls. Twelve C57BL/6 mice, arterially perfused with either 25% Lugol's solution or 5% PTA solution were scanned intact and reconstructed with 26 µm isotropic voxels. The animals were fixed and the heart and surrounding vessels were excised, embedded and scanned; isolated heart images were reconstructed with 13 µm isotropic voxels. Myocardial enhancement and artery diameters were measured. Both stains successfully enhanced the myocardium and vessel walls. Interestingly, Lugol's solution provided a significantly higher enhancement of the myocardium than PTA [2502 ± 437 vs 656 ± 178 Hounsfield units (HU); p < 0.0001], delineating myofiber architecture and orientation. There was no significant difference in vessel wall enhancement (Lugol's, 1036 ± 635 HU; PTA, 738 ± 124 HU; p = 0.29), but coronary arteries were more effectively segmented from the PTA-stained hearts, enabling segmented imaging of fifth- order coronary artery branches. The combination of whole mouse perfusion delivery and use of heavy metal-containing stains affords high-resolution imaging of the mouse heart and vasculature by micro-CT. The differential imaging patterns of Lugol's- and PTA-stained tissues reveals new opportunities for micro-analyses of cardiac and vascular tissues.

Imaging challenges in biomaterials and tissue engineering

Biomaterials are employed in the fields of tissue engineering and regenerative medicine (TERM) in order to enhance the regeneration or replacement of tissue function and/or structure. The unique environments resulting from the presence of biomaterials, cells, and tissues result in distinct challenges in regards to monitoring and assessing the results of these interventions. Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are invasive, and do not provide information on functional status. Imaging techniques are needed that enable non-destructive, longitudinal, quantitative, and three-dimensional analysis of TERM strategies. This review focuses on evaluating the application of available imaging modalities for assessment of biomaterials and tissue in TERM applications. Included is a discussion of limitations of these techniques and identification of areas for further development.

Translational paradigms in scientific and clinical imaging of cardiac development

Congenital heart defects (CHD) are the most prevalent congenital disease, with 45% of deaths resulting from a congenital defect due to a cardiac malformation. Clinically significant CHD permit survival upon birth, but may become immediately life threatening. Advances in surgical intervention have significantly reduced perinatal mortality, but the outcome for many malformations is bleak. Furthermore, patients living while tolerating a CHD often acquire additional complications due to the long-term systemic blood flow changes caused by even subtle anatomical abnormalities. Accurate diagnosis of defects during fetal development is critical for interventional planning and improving patient outcomes. Advances in quantitative, multidimensional imaging are necessary to uncover the basic scientific and clinically relevant morphogenetic changes and associated hemodynamic consequences influencing normal and abnormal heart development. Ultrasound is the most widely used clinical imaging technology for assessing fetal cardiac development. Ultrasound-based fetal assessment modalities include motion mode (M-mode), two dimensional (2D), and 3D/4D imaging. These datasets can be combined with computational fluid dynamics analysis to yield quantitative, volumetric, and physiological data. Additional imaging modalities, however, are available to study basic mechanisms of cardiogenesis, including optical coherence tomography, microcomputed tomography, and magnetic resonance imaging. Each imaging technology has its advantages and disadvantages regarding resolution, depth of penetration, soft tissue contrast considerations, and cost. In this review, we analyze the current clinical and scientific imaging technologies, research studies utilizing them, and appropriate animal models reflecting clinically relevant cardiogenesis and cardiac malformations. We conclude with discussing the translational impact and future opportunities for cardiovascular development imaging 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.

Quantitative in vivo imaging of embryonic development: opportunities and challenges

Animal models are critically important for a mechanistic understanding of embryonic morphogenesis. For decades, visualizing these rapid and complex multidimensional events has relied on projection images and thin section reconstructions. While much insight has been gained, fixed tissue specimens offer limited information on dynamic processes that are essential for tissue assembly and organ patterning. Quantitative imaging is required to unlock the important basic science and clinically relevant secrets that remain hidden. Recent advances in live imaging technology have enabled quantitative longitudinal analysis of embryonic morphogenesis at multiple length and time scales. Four different imaging modalities are currently being used to monitor embryonic morphogenesis: optical, ultrasound, magnetic resonance imaging (MRI), and micro-computed tomography (micro-CT). Each has its advantages and limitations with respect to spatial resolution, depth of field, scanning speed, and tissue contrast. In addition, new processing tools have been developed to enhance live imaging capabilities. In this review, we analyze each type of imaging source and its use in quantitative study of embryonic morphogenesis in small animal models. We describe the physics behind their function, identify some examples in which the modality has revealed new quantitative insights, and then conclude with a discussion of new research directions with live imaging.

Longitudinal in vivo imaging to assess blood flow and oxygenation in implantable engineered tissues

The functionality of vascular networks within implanted prevascularized tissues is difficult to assess using traditional analysis techniques, such as histology. This is largely due to the inability to visualize hemodynamics in vivo longitudinally. Therefore, we have developed dynamic imaging methods to measure blood flow and hemoglobin oxygen saturation in implanted prevascularized tissues noninvasively and longitudinally. Using laser speckle imaging, multispectral imaging, and intravital microscopy, we demonstrate that fibrin-based tissue implants anastomose with the host (severe combined immunodeficient mice) in as short as 20 h. Anastomosis results in initial perfusion with highly oxygenated blood, and an increase in average hemoglobin oxygenation of 53%. However, shear rates in the preformed vessels were low (20.8±12.8 s(-1)), and flow did not persist in the vast majority of preformed vessels due to thrombus formation. These findings suggest that designing an appropriate vascular network structure in prevascularized tissues to maintain shear rates above the threshold for thrombosis may be necessary to maintain flow following implantation. We conclude that wide-field and microscopic functional imaging can dynamically assess blood flow and oxygenation in vivo in prevascularized tissues, and can be used to rapidly evaluate and improve prevascularization strategies.

Coronary artery wall imaging in mice using osmium tetroxide and micro-computed tomography (micro-CT)

The high spatial resolution of micro-computed tomography (micro-CT) is ideal for 3D imaging of coronary arteries in intact mouse heart specimens. Previously, micro-CT of mouse heart specimens utilized intravascular contrast agents that hardened within the vessel lumen and allowed a vascular cast to be made. However, for mouse coronary artery disease models, it is highly desirable to image coronary artery walls and highlight plaques. For this purpose, we describe an ex vivo contrast-enhanced micro-CT imaging technique based on tissue staining with osmium tetroxide (OsO(4) ) solution. As a tissue-staining contrast agent, OsO(4) is retained in the vessel wall and surrounding tissue during the fixation process and cleared from the vessel lumens. Its high X-ray attenuation makes the artery wall visible in CT. Additionally, since OsO(4) preferentially binds to lipids, it highlights lipid deposition in the artery wall. We performed micro-CT of heart specimens of 5- to 25-week-old C57BL/6 wild-type mice and 5- to 13-week-old apolipoprotein E knockout (apoE(-/-) ) mice at 10 μm resolution. The results show that walls of coronary arteries as small as 45 μm in diameter are visible using a table-top micro-CT scanner. Similar image clarity was achieved with 1/2000th the scan time using a synchrotron CT scanner. In 13-week-old apoE mice, lipid-rich plaques are visible in the aorta. Our study shows that the combination of OsO(4) and micro-CT permits the visualization of the coronary artery wall in intact mouse hearts.

X-ray microtomography in biology

Progress in high-resolution X-ray microtomography has provided us with a practical approach to determining three-dimensional (3D) structures of opaque samples at micrometer to submicrometer resolution. In this review, we give an introduction to hard X-ray microtomography and its application to the visualization of 3D structures of biological soft tissues. Practical aspects of sample preparation, handling, data collection, 3D reconstruction, and structure analysis are described. Furthermore, different sample contrasting methods are approached in detail. Examples of microtomographic studies are overviewed to present an outline of biological applications of X-ray microtomography. We also provide perspectives of biological microtomography as the convergence of sciences in X-ray optics, biology, and structural analysis.

Potential for imaging engineered tissues with X-ray phase contrast

As the field of tissue engineering advances, it is crucial to develop imaging methods capable of providing detailed three-dimensional information on tissue structure. X-ray imaging techniques based on phase-contrast (PC) have great potential for a number of biomedical applications due to their ability to provide information about soft tissue structure without exogenous contrast agents. X-ray PC techniques retain the excellent spatial resolution, tissue penetration, and calcified tissue contrast of conventional X-ray techniques while providing drastically improved imaging of soft tissue and biomaterials. This suggests that X-ray PC techniques are very promising for evaluation of engineered tissues. In this review, four different implementations of X-ray PC imaging are described and applications to tissues of relevance to tissue engineering reviewed. In addition, recent applications of X-ray PC to the evaluation of biomaterial scaffolds and engineered tissues are presented and areas for further development and application of these techniques are discussed. Imaging techniques based on X-ray PC have significant potential for improving our ability to image and characterize engineered tissues, and their continued development and optimization could have significant impact on the field of tissue engineering.

Micro-computed tomography with iodine staining resolves the arrangement of muscle fibres

We illustrate here microCT images in which contrast between muscle and connective tissue has been achieved by means of staining with iodine. Enhancement is shown to be dependent on the concentration of iodine solution (I(2)KI), time in solution and specimen size. Histological examination confirms that the arrangement of individual muscle fibres can be visualised on the enhanced microCT images, and that the iodine accumulates in the muscle fibres in preference to the surrounding connective tissues. We explore the application of this technique to describe the fibrous structure of skeletal muscle, and conclude that it has the potential to become a non-destructive and cost-effective method for investigating muscle fascicle architecture, particularly in comparative morphological studies.

A comparison of bioreactors for culture of fetal mesenchymal stem cells for bone tissue engineering

Bioreactors provide a dynamic culture system for efficient exchange of nutrients and mechanical stimulus necessary for the generation of effective tissue engineered bone grafts (TEBG). We have shown that biaxial rotating (BXR) bioreactor-matured human fetal mesenchymal stem cell (hfMSC) mediated-TEBG can heal a rat critical sized femoral defect. However, it is not known whether optimal bioreactors exist for bone TE (BTE) applications. We systematically compared this BXR bioreactor with three most commonly used systems: Spinner Flask (SF), Perfusion and Rotating Wall Vessel (RWV) bioreactors, for their application in BTE. The BXR bioreactor achieved higher levels of cellularity and confluence (1.4-2.5x, p < 0.05) in large 785 mm(3) macroporous scaffolds not achieved in the other bioreactors operating in optimal settings. BXR bioreactor-treated scaffolds experienced earlier and more robust osteogenic differentiation on von Kossa staining, ALP induction (1.2-1.6×, p < 0.01) and calcium deposition (1.3-2.3×, p < 0.01). We developed a Micro CT quantification method which demonstrated homogenous distribution of hfMSC in BXR bioreactor-treated grafts, but not with the other three. BXR bioreactor enabled superior cellular proliferation, spatial distribution and osteogenic induction of hfMSC over other commonly used bioreactors. In addition, we developed and validated a non-invasive quantitative micro CT-based technique for analyzing neo-tissue formation and its spatial distribution within scaffolds.

X-ray imaging of poly(ethylene glycol) hydrogels without contrast agents

Hydrogels have shown promise for a number of tissue engineering applications. However, their high water content results in little or no image contrast when using conventional X-ray imaging techniques. X-ray imaging techniques based on phase-contrast have shown promise for biomedical application due to their ability to provide information about the X-ray refraction properties of samples. Nonporous and porous poly(ethylene glycol) hydrogels were synthesized and imaged using a synchrotron light source employing a silicon analyzer crystal and an X-ray energy of 40-keV. Data were acquired at 21 angular analyzer positions spanning the range of -5 to 5 μrad. Images that depict the projected X-ray absorption, refraction, and ultra-small-angle scatter (USAXS) properties of the hydrogels were reconstructed from the measurement data. The poly(ethylene glycol) hydrogels could be discerned from surrounding water and soft tissue in the refraction image but not the absorption or USAXS images. In addition, the refraction images of the porous hydrogels have a speckle pattern resulting in increased image texture in comparison to nonporous hydrogels. To our knowledge, this is the first study to show that X-ray phase-contrast imaging techniques can identify and provide detail on hydrogel structure without the addition of contrast agents.

Rapid 3D phenotyping of cardiovascular development in mouse embryos by micro-CT with iodine staining

BACKGROUND

Microcomputed tomography (micro-CT) has been used extensively in research to generate high-resolution 3D images of calcified tissues in small animals nondestructively. It has been especially useful for the characterization of skeletal mutations but limited in its utility for the analysis of soft tissue such as the cardiovascular system. Visualization of the cardiovascular system has been largely restricted to structures that can be filled with radiopaque intravascular contrast agents in adult animals. Recent ex vivo studies using osmium tetroxide, iodinated contrast agents, inorganic iodine, and phosphotungstic acid have demonstrated the ability to stain soft tissues differentially, allowing for high intertissue contrast in micro-CT images. In the present study, we demonstrate the application of this technology for visualization of cardiovascular structures in developing mouse embryos using Lugol solution (aqueous potassium iodide plus iodine).

METHODS AND RESULTS

We show the optimization of this method to obtain ex vivo micro-CT images of embryonic and neonatal mice with excellent soft-tissue contrast. We demonstrate the utility of this method to visualize key structures during cardiovascular development at various stages of embryogenesis. Our method benefits from the ease of sample preparation, low toxicity, and low cost. Furthermore, we show how multiple cardiac defects can be demonstrated by micro-CT in a single specimen with a known genetic lesion. Indeed, a previously undescribed cardiac venous abnormality is revealed in a PlexinD1 mutant mouse.

CONCLUSIONS

Micro-CT of iodine-stained tissue is a valuable technique for the characterization of cardiovascular development and defects in mouse models of congenital heart disease.