Diffraction-enhanced X-ray imaging of articular cartilage

High-Resolution Phase-Contrast Tomography on Human Collagenous Tissues: A Comprehensive Review

Phase-contrast X-ray imaging is becoming increasingly considered since its first applications, which occurred almost 30 years ago. Particular emphasis was placed on studies that use this technique to investigate soft tissues, which cannot otherwise be investigated at a high resolution and in a three-dimensional manner, using conventional absorption-based settings. Indeed, its consistency and discrimination power in low absorbing samples, unified to being a not destructive analysis, are pushing interests on its utilization from researchers of different specializations, from botany, through zoology, to human physio-pathology research. In this regard, a challenging method for 3D imaging and quantitative analysis of collagenous tissues has spread in recent years: it is based on the unique characteristics of synchrotron radiation phase-contrast microTomography (PhC-microCT). In this review, the focus has been placed on the research based on the exploitation of synchrotron PhC-microCT for the investigation of collagenous tissue physio-pathologies from solely human samples. Collagen tissues' elasto-mechanic role bonds it to the morphology of the site it is extracted from, which could weaken the results coming from animal experimentations. Encouraging outcomes proved this technique to be suitable to access and quantify human collagenous tissues and persuaded different researchers to approach it. A brief mention was also dedicated to the results obtained on collagenous tissues using new and promising high-resolution phase-contrast tomographic laboratory-based setups, which will certainly represent the real step forward in the diffusion of this relatively young imaging technique.

Imaging of Cartilage and Chondral Defects: An Overview

A healthy articular cartilage is paramount to joint function. Cartilage defects, whether acute or chronic, are a significant source of morbidity. This review summarizes various imaging modalities used for cartilage assessment. While radiographs are insensitive, they are still widely used to indirectly assess cartilage. Ultrasound has shown promise in the detection of cartilage defects, but its efficacy is limited in many joints due to inadequate visualization. CT arthrography has the potential to assess internal derangements of joints along with cartilage, especially in patients with contraindications to MRI. MRI remains the favored imaging modality to assess cartilage. The conventional imaging techniques are able to assess cartilage abnormalities when cartilage is already damaged. The newer imaging techniques are thus targeted at detecting biochemical and structural changes in cartilage before an actual visible irreversible loss. These include, but are not limited to, T2 and T2* mapping, dGEMRI, T1ρ imaging, gagCEST imaging, sodium MRI and integrated PET with MRI. A brief discussion of the advances in the surgical management of cartilage defects and post-operative imaging assessment is also included.

Cartilage Tissue Engineering Approaches Need to Assess Fibrocartilage When Hydrogel Constructs Are Mechanically Loaded

Chondrocytes that are impregnated within hydrogel constructs sense applied mechanical force and can respond by expressing collagens, which are deposited into the extracellular matrix (ECM). The intention of most cartilage tissue engineering is to form hyaline cartilage, but if mechanical stimulation pushes the ratio of collagen type I (Col1) to collagen type II (Col2) in the ECM too high, then fibrocartilage can form instead. With a focus on Col1 and Col2 expression, the first part of this article reviews the latest studies on hyaline cartilage regeneration within hydrogel constructs that are subjected to compression forces (one of the major types of the forces within joints) in vitro. Since the mechanical loading conditions involving compression and other forces in joints are difficult to reproduce in vitro, implantation of hydrogel constructs in vivo is also reviewed, again with a focus on Col1 and Col2 production within the newly formed cartilage. Furthermore, mechanotransduction pathways that may be related to the expression of Col1 and Col2 within chondrocytes are reviewed and examined. Also, two recently-emerged, novel approaches of load-shielding and synchrotron radiation (SR)–based imaging techniques are discussed and highlighted for future applications to the regeneration of hyaline cartilage. Going forward, all cartilage tissue engineering experiments should assess thoroughly whether fibrocartilage or hyaline cartilage is formed.

Edge-illumination x-ray phase-contrast imaging

Although early demonstration dates back to the mid-sixties, x-ray phase-contrast imaging (XPCI) became hugely popular in the mid-90s, thanks to the advent of 3rd generation synchrotron facilities. Its ability to reveal object features that had so far been considered invisible to x-rays immediately suggested great potential for applications across the life and the physical sciences, and an increasing number of groups worldwide started experimenting with it. At that time, it looked like a synchrotron facility was strictly necessary to perform XPCI with some degree of efficiency-the only alternative being micro-focal sources, the limited flux of which imposed excessively long exposure times. However, new approaches emerged in the mid-00s that overcame this limitation, and allowed XPCI implementations with conventional, non-micro-focal x-ray sources. One of these approaches showing particular promise for 'real-world' applications is edge-illumination XPCI: this article describes the key steps in its evolution in the context of contemporary developments in XPCI research, and presents its current state-of-the-art, especially in terms of transition towards practical applications.

Multiscale X-ray phase contrast imaging of human cartilage for investigating osteoarthritis formation

Background

The evolution of cartilage degeneration is still not fully understood, partly due to its thinness, low radio-opacity and therefore lack of adequately resolving imaging techniques. X-ray phase-contrast imaging (X-PCI) offers increased sensitivity with respect to standard radiography and CT allowing an enhanced visibility of adjoining, low density structures with an almost histological image resolution. This study examined the feasibility of X-PCI for high-resolution (sub-) micrometer analysis of different stages in tissue degeneration of human cartilage samples and compare it to histology and transmission electron microscopy.

Methods

Ten 10%-formalin preserved healthy and moderately degenerated osteochondral samples, post-mortem extracted from human knee joints, were examined using four different X-PCI tomographic set-ups using synchrotron radiation the European Synchrotron Radiation Facility (France) and the Swiss Light Source (Switzerland). Volumetric datasets were acquired with voxel sizes between 0.7 × 0.7 × 0.7 and 0.1 × 0.1 × 0.1 µm³. Data were reconstructed by a filtered back-projection algorithm, post-processed by ImageJ, the WEKA machine learning pixel classification tool and VGStudio max. For correlation, osteochondral samples were processed for histology and transmission electron microscopy.

Results

X-PCI provides a three-dimensional visualization of healthy and moderately degenerated cartilage samples down to a (sub-)cellular level with good correlation to histologic and transmission electron microscopy images. X-PCI is able to resolve the three layers and the architectural organization of cartilage including changes in chondrocyte cell morphology, chondrocyte subgroup distribution and (re-)organization as well as its subtle matrix structures.

Conclusions

X-PCI captures comprehensive cartilage tissue transformation in its environment and might serve as a tissue-preserving, staining-free and volumetric virtual histology tool for examining and chronicling cartilage behavior in basic research/laboratory experiments of cartilage disease evolution.

High-Frequency Acoustic Imaging Using Adhesive-Free Polymer Transducer

The piezoelectric polymer PVDF and its copolymers have a long history as transducer materials for medical and biological applications. An efficient use of these polymers can potentially both lower the production cost and offer an environment-friendly alternative for medical transducers which today is dominated by piezoelectric ceramics containing lead. The main goal of the current work has been to compare the image quality of a low-cost in-house transducers made from the copolymer P(VDF-TrFE) to a commercial PVDF transducer. Several test objects were explored with the transducers used in a scanning acoustic microscope, including a human articular cartilage sample, a coin surface, and an etched metal film with fine line structures. To evaluate the image quality, C- and B-scan images were obtained from the recorded time series, and compared in terms of resolution, SNR, point-spread function, and depth imaging capability. The investigation is believed to provide useful information about both the strengths and limitations of low-cost polymer transducers.

Principles of Different X-ray Phase-Contrast Imaging: A Review

Numerous advances have been made in X-ray technology in recent years. X-ray imaging plays an important role in the nondestructive exploration of the internal structures of objects. However, the contrast of X-ray absorption images remains low, especially for materials with low atomic numbers, such as biological samples. X-ray phase-contrast images have an intrinsically higher contrast than absorption images. In this review, the principles, milestones, and recent progress of X-ray phase-contrast imaging methods are demonstrated. In addition, prospective applications are presented.

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

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

Fast diffraction-enhanced imaging using continuous sample rotation and analyzer crystal scanning

Diffraction-enhanced imaging (DEI) has high sensitivity and a wide dynamic range of density and thus can be used for fine imaging of biological and organic samples that include large differences in density. A fast DEI method composed of continuous fast sample rotations and slow analyzer crystal scanning was developed to shorten the measurement period. Fine sectional images of a biological sample were successfully obtained within a half measurement period of the conventional step-scanning method while keeping the same exposure time. In addition, a fine three-dimensional image of a rat tail was obtained with a 375 s measurement period.

Quantitative Assessment of Degenerative Cartilage and Subchondral Bony Lesions in a Preserved Cadaveric Knee: Propagation-Based Phase-Contrast CT Versus Conventional MRI and CT

OBJECTIVE

The aim of this study was to quantitatively assess hyaline cartilage and subchondral bone conditions in a fully preserved cadaveric human knee joint using high-resolution x-ray propagation-based phase-contrast imaging (PBI) CT and to compare the performance of the new technique with conventional CT and MRI.

MATERIALS AND METHODS

A cadaveric human knee was examined using an x-ray beam of 60 keV, a detector with a 90-mm² FOV, and a pixel size of 46 × 46 μm². PBI CT images were reconstructed with both the filtered back projection algorithm and the equally sloped tomography method. Conventional 3-T MRI and CT were also performed. Measurements of cartilage thickness, cartilage lesions, International Cartilage Repair Society scoring, and detection of subchondral bone changes were evaluated. Visual inspection of the specimen akin to arthroscopy was conducted and served as a standard of reference for lesion detection.

RESULTS

Loss of cartilage height was visible on PBI CT and MRI. Quantification of cartilage thickness showed a strong correlation between the two modalities. Cartilage lesions appeared darker than the adjacent cartilage on PBI CT. PBI CT showed similar agreement to MRI for depicting cartilage substance defects or lesions compared with the visual inspection. The assessment of subchondral bone cysts showed moderate to strong agreement between PBI CT and CT.

CONCLUSION

In contrast to the standard clinical methods of MRI and CT, PBI CT is able to simultaneously depict cartilage and bony changes at high resolution. Though still an experimental technique, PBI CT is a promising high-resolution imaging method to evaluate comprehensive changes of osteoarthritic disease in a clinical setting.

Micro-imaging of Brain Cancer Radiation Therapy Using Phase-contrast Computed Tomography

PURPOSE

Experimental neuroimaging provides a wide range of methods for the visualization of brain anatomic morphology down to subcellular detail. Still, each technique-specific detection mechanism presents compromises among the achievable field-of-view size, spatial resolution, and nervous tissue sensitivity, leading to partial sample coverage, unresolved morphologic structures, or sparse labeling of neuronal populations and often also to obligatory sample dissection or other sample invasive manipulations. X-ray phase-contrast imaging computed tomography (PCI-CT) is an experimental imaging method that simultaneously provides micrometric spatial resolution, high soft-tissue sensitivity, and ex vivo full organ rodent brain coverage without any need for sample dissection, staining or labeling, or contrast agent injection. In the present study, we explored the benefits and limitations of PCI-CT use for in vitro imaging of normal and cancerous brain neuromorphology after in vivo treatment with synchrotron-generated x-ray microbeam radiation therapy (MRT), a spatially fractionated experimental high-dose radiosurgery. The goals were visualization of the MRT effects on nervous tissue and a qualitative comparison of the results to the histologic and high-field magnetic resonance imaging findings.

METHODS AND MATERIALS

MRT was administered in vivo to the brain of both healthy and cancer-bearing rats. At 45 days after treatment, the brain was dissected out and imaged ex vivo using propagation-based PCI-CT.

RESULTS

PCI-CT visualizes the brain anatomy and microvasculature in 3 dimensions and distinguishes cancerous tissue morphology, necrosis, and intratumor accumulation of iron and calcium deposits. Moreover, PCI-CT detects the effects of MRT throughout the treatment target areas (eg, the formation of micrometer-thick radiation-induced tissue ablation). The observed neurostructures were confirmed by histologic and immunohistochemistry examination and related to the micro-magnetic resonance imaging data.

CONCLUSIONS

PCI-CT enabled a unique 3D neuroimaging approach for ex vivo studies on small animal models in that it concurrently delivers high-resolution insight of local brain tissue morphology in both normal and cancerous micro-milieu, localizes radiosurgical damage, and highlights the deep microvasculature. This method could assist experimental small animal neurology studies in the postmortem evaluation of neuropathology or treatment effects.

Imaging of Hydrogel Microsphere Structure and Foreign Body Response Based on Endogenous X-Ray Phase Contrast

Transplantation of functional islets encapsulated in stable biomaterials has the potential to cure Type I diabetes. However, the success of these materials requires the ability to quantitatively evaluate their stability. Imaging techniques that enable monitoring of biomaterial performance are critical to further development in the field. X-ray phase-contrast (XPC) imaging is an emerging class of X-ray techniques that have shown significant promise for imaging biomaterial and soft tissue structures. In this study, XPC imaging techniques are shown to enable three dimensional (3D) imaging and evaluation of islet volume, alginate hydrogel structure, and local soft tissue features ex vivo. Rat islets were encapsulated in sterile ultrapurified alginate systems produced using a high-throughput microfluidic system. The encapsulated islets were implanted in omentum pouches created in a rodent model of type 1 diabetes. Microbeads were imaged with XPC imaging before implantation and as whole tissue samples after explantation from the animals. XPC microcomputed tomography (μCT) was performed with systems using tube-based and synchrotron X-ray sources. Islets could be identified within alginate beads and the islet volume was quantified in the synchrotron-based μCT volumes. Omental adipose tissue could be distinguished from inflammatory regions resulting from implanted beads in harvested samples with both XPC imaging techniques. Individual beads and the local encapsulation response were observed and quantified using quantitative measurements, which showed good agreement with histology. The 3D structure of the microbeads could be characterized with XPC imaging and failed beads could also be identified. These results point to the substantial potential of XPC imaging as a tool for imaging biomaterials in small animal models and deliver a critical step toward in vivo imaging.

X-ray phase contrast tomography; proof of principle for post-mortem imaging

OBJECTIVE

To demonstrate the feasibility of using X-ray phase-contrast tomography to assess internal organs in a post-mortem piglet model, as a possible non-invasive imaging autopsy technique.

METHODS

Tomographic images of a new-born piglet were obtained using a free-space propagation X-ray phase-contrast imaging setup at a synchrotron (European Synchrotron Radiation Facility, Grenoble, France). A monochromatic X-ray beam (52 keV) was used in combination with a detector pixel size of 46 × 46 µm(2). A phase-retrieval algorithm was applied to all projections, which were then reconstructed into tomograms using the filtered-back projection algorithm. Images were assessed for diagnostic quality.

RESULTS

Images obtained with the free-space propagation setup presented high soft-tissue contrast and sufficient resolution for resolving organ structure. All of the main body organs (heart, lungs, kidneys, liver and intestines) were easily identified and adequately visualized. In addition, grey/white matter differentiation in the cerebellum while still contained within the skull was shown.

CONCLUSION

The feasibility of using X-ray phase-contrast tomography as a post-mortem imaging technique in an animal model has been demonstrated. Future studies will focus on translating this experiment to a laboratory-based setup.

ADVANCES IN KNOWLEDGE

Appropriate image processing and analysis enable the simultaneous visualization of both soft- and hard-tissue structures in X-ray phase-contrast images of a complex, thick sample.

Volumetric quantitative characterization of human patellar cartilage with topological and geometrical features on phase-contrast X-ray computed tomography

Phase-contrast X-ray computed tomography (PCI-CT) has attracted significant interest in recent years for its ability to provide significantly improved image contrast in low absorbing materials such as soft biological tissue. In the research context of cartilage imaging, previous studies have demonstrated the ability of PCI-CT to visualize structural details of human patellar cartilage matrix and capture changes to chondrocyte organization induced by osteoarthritis. This study evaluates the use of geometrical and topological features for volumetric characterization of such chondrocyte patterns in the presence (or absence) of osteoarthritic damage. Geometrical features derived from the scaling index method (SIM) and topological features derived from Minkowski Functionals were extracted from 1392 volumes of interest (VOI) annotated on PCI-CT images of ex vivo human patellar cartilage specimens. These features were subsequently used in a machine learning task with support vector regression to classify VOIs as healthy or osteoarthritic; classification performance was evaluated using the area under the receiver operating characteristic curve (AUC). Our results show that the classification performance of SIM-derived geometrical features (AUC: 0.90 ± 0.09) is significantly better than Minkowski Functionals volume (AUC: 0.54 ± 0.02), surface (AUC: 0.72 ± 0.06), mean breadth (AUC: 0.74 ± 0.06) and Euler characteristic (AUC: 0.78 ± 0.04) (p < 10(-4)). These results suggest that such geometrical features can provide a detailed characterization of the chondrocyte organization in the cartilage matrix in an automated manner, while also enabling classification of cartilage as healthy or osteoarthritic with high accuracy. Such features could potentially serve as diagnostic imaging markers for evaluating osteoarthritis progression and its response to different therapeutic intervention strategies.

X-ray Phase Contrast Allows Three Dimensional, Quantitative Imaging of Hydrogel Implants

Three dimensional imaging techniques are needed for the evaluation and assessment of biomaterials used for tissue engineering and drug delivery applications. Hydrogels are a particularly popular class of materials for medical applications but are difficult to image in tissue using most available imaging modalities. Imaging techniques based on X-ray Phase Contrast (XPC) have shown promise for tissue engineering applications due to their ability to provide image contrast based on multiple X-ray properties. In this manuscript, we investigate the use of XPC for imaging a model hydrogel and soft tissue structure. Porous fibrin loaded poly(ethylene glycol) hydrogels were synthesized and implanted in a rodent subcutaneous model. Samples were explanted and imaged with an analyzer-based XPC technique and processed and stained for histology for comparison. Both hydrogel and soft tissues structures could be identified in XPC images. Structure in skeletal muscle adjacent could be visualized and invading fibrovascular tissue could be quantified. There were no differences between invading tissue measurements from XPC and the gold-standard histology. These results provide evidence of the significant potential of techniques based on XPC for 3D imaging of hydrogel structure and local tissue response.

Computer-aided diagnosis for phase-contrast X-ray computed tomography: quantitative characterization of human patellar cartilage with high-dimensional geometric features

Phase-contrast computed tomography (PCI-CT) has shown tremendous potential as an imaging modality for visualizing human cartilage with high spatial resolution. Previous studies have demonstrated the ability of PCI-CT to visualize (1) structural details of the human patellar cartilage matrix and (2) changes to chondrocyte organization induced by osteoarthritis. This study investigates the use of high-dimensional geometric features in characterizing such chondrocyte patterns in the presence or absence of osteoarthritic damage. Geometrical features derived from the scaling index method (SIM) and statistical features derived from gray-level co-occurrence matrices were extracted from 842 regions of interest (ROI) annotated on PCI-CT images of ex vivo human patellar cartilage specimens. These features were subsequently used in a machine learning task with support vector regression to classify ROIs as healthy or osteoarthritic; classification performance was evaluated using the area under the receiver-operating characteristic curve (AUC). SIM-derived geometrical features exhibited the best classification performance (AUC, 0.95 ± 0.06) and were most robust to changes in ROI size. These results suggest that such geometrical features can provide a detailed characterization of the chondrocyte organization in the cartilage matrix in an automated and non-subjective manner, while also enabling classification of cartilage as healthy or osteoarthritic with high accuracy. Such features could potentially serve as imaging markers for evaluating osteoarthritis progression and its response to different therapeutic intervention strategies.

Diffraction-enhanced Synchrotron Imaging of Bovine Ovaries Ex Vivo

OBJECTIVES

The objective of this study was to test the hypothesis that diffraction-enhanced imaging (DEI), a synchrotron x-ray imaging technique, would provide greater contrast for evaluating bovine ovaries compared with conventional diagnostic ultrasonography.

MATERIALS AND METHODS

Bovine ovaries were evaluated ex vivo as follows: fresh without radiographic arterial contrast (n = 2), fresh with contrast (n = 1), preserved in 10% formalin without contrast (n = 2), and preserved with contrast (n = 1). Each ovary was imaged with DEI and subsequently with ultrasonography and histology. The ability to visualize and differentiate preantral and antral follicles, corpora lutea (CL), and cumulus oocyte complexes (COCs) were compared using DEI, ultrasonography, and histology. The diameter of follicles and CL were measured and compared using ultrasonography, DEI, and histology. The diameter of the smallest follicle detected was reported using each of the three imaging methods. The number of antral follicles (antral follicle count ≥2 mm) was compared between ultrasonography and DEI.

RESULTS

DEI enabled the detection of 71% of follicles and 67% of CL that were detected ultrasonographically. However, DEI did not allow the detection of COCs and cell layers of the follicle wall that were visualized histologically. Luteal tissues were not easily distinguished using DEI, and DEI was inferior for differentiating follicles and CL compared with ultrasonography. The mean follicle diameter was similar between DEI (4.00 ± 0.35 mm, fresh with contrast; 9.62 ± 2.43 mm, fresh without contrast) and ultrasonography (3.85 ± 0.28 mm, fresh with contrast; 8.97 ± 2.60 mm, fresh without contrast) (P > .05). However, the mean follicle diameter was greater using both DEI (4.00 ± 0.35 mm) and ultrasonography (3.85 ± 0.28 mm) compared with histology (2.21 ± 0.38 mm; P = .01, fresh ovaries with contrast). The mean CL diameter was similar between DEI (11.64 ± 1.67 mm), ultrasonography (9.34 ± 0.35 mm), and histology (9.59 ± 0.36 mm) (P > .05). The mean diameter of the smallest follicle detected was similar between DEI (3.06 ± 0.45 mm) and ultrasonography (2.95 ± 0.74 mm); both DEI and ultrasonographic measurements were greater than histology (0.39 ± 0.04 mm, P < .0001). The mean antral follicle count was similar between ultrasonography (6.50 ± 0.71 mm, fresh with no contrast; 6.50 ± 2.50 mm, preserved with no contrast) and DEI (4.50 ± 0.50 mm, fresh with no contrast; 6.50 ± 0.50 mm, preserved with no contrast) (P > .05).

CONCLUSIONS

The contrast resolution of antral follicles, CL, and COCs in bovine ovaries was inferior using DEI compared with ultrasonography and histology. Alternative synchrotron techniques, such as phase-contrast computed tomography and DEI computed tomography, may prove more effective than DEI for imaging ovaries ex vivo.

Limited-angle tomography for analyzer-based phase-contrast x-ray imaging

Multiple-image radiography (MIR) is an analyzer-based phase-contrast x-ray imaging method, which is emerging as a potential alternative to conventional radiography. MIR simultaneously generates three planar parametric images containing information about scattering, refraction and attenuation properties of the object. The MIR planar images are linear tomographic projections of the corresponding object properties, which allows reconstruction of volumetric images using computed tomography (CT) methods. However, when acquiring a full range of linear projections around the tissue of interest is not feasible or the scanning time is limited, limited-angle tomography techniques can be used to reconstruct these volumetric images near the central plane, which is the plane that contains the pivot point of the tomographic movement. In this work, we use computer simulations to explore the applicability of limited-angle tomography to MIR. We also investigate the accuracy of reconstructions as a function of number of tomographic angles for a fixed total radiation exposure. We use this function to find an optimal range of angles over which data should be acquired for limited-angle tomography MIR (LAT-MIR). Next, we apply the LAT-MIR technique to experimentally acquired MIR projections obtained in a cadaveric human thumb study. We compare the reconstructed slices near the central plane to the same slices reconstructed by CT-MIR using the full angular view around the object. Finally, we perform a task-based evaluation of LAT-MIR performance for different numbers of angular views, and use template matching to detect cartilage in the refraction image near the central plane. We use the signal-to-noise ratio of this test as the detectability metric to investigate an optimum range of tomographic angles for detecting soft tissues in LAT-MIR. Both results show that there is an optimum range of angular view for data acquisition where LAT-MIR yields the best performance, comparable to CT-MIR only if one considers volumetric images near the central plane and not the whole volume.

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

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

Characterization of speckle in lung images acquired with a benchtop in-line x-ray phase-contrast system

We investigate the manifestation of speckle in propagation-based x-ray phase-contrast imaging of mouse lungs in situ by use of a benchtop imager. The key contributions of the work are the demonstration that lung speckle can be observed by use of a benchtop imaging system employing a polychromatic tube-source and a systematic experimental investigation of how the texture of the speckle pattern depends on the parameters of the imaging system. Our analyses consists of image texture characterization based on the statistical properties of pixel intensity values.

An initial evaluation of analyser-based phase-contrast X-ray imaging of carotid plaque microstructure

Carotid artery plaque instability can result in rupture and lead to ischaemic stroke. Stability of plaques appears to be a function of composition. Current non-invasive imaging techniques are limited in their ability to classify distinct histological regions within plaques. Phase-contrast (PC) X-ray imaging methods are an emerging class of techniques that have shown promise for identifying soft-tissue features without use of exogenous contrast agents. This is the first study to apply analyser-based X-ray PC imaging in CT mode to provide three-dimensional (3D) images of excised atherosclerotic plaques. The results provide proof of principle for this technique as a promising method for analysis of carotid plaque microstructure. Multiple image radiography CT (MIR-CT), a tomographic implementation of X-ray PC imaging that employs crystal optics, was employed to image excised carotid plaques. MIR-CT imaging yields three complementary images of the plaque's 3D X-ray absorption, refraction and scatter properties. These images were compared with histological sections of the tissue. X-ray PC images were able to identify the interface between the plaque and the medial wall. In addition, lipid-rich and highly vascularized regions were visible in the images as well as features depicting inflammation. This preliminary research shows MIR-CT imaging can reveal details about plaque structure not provided by traditional absorption-based X-ray imaging and appears to identify specific histological regions within plaques. This is the first study to apply analyser-based X-ray PC imaging to human carotid artery plaques to identify distinct soft-tissue regions.

Analyzer-based phase-contrast x-ray imaging of carotid plaque microstructure

BACKGROUND

Plaque vulnerability depends, in part, on composition. Imaging techniques are needed that can aid the prediction of plaque stability. High-contrast images of soft-tissue structure have been obtained with x-ray phase-contrast (PC) imaging. This research investigates multiple image radiography (MIR), an x-ray PC imaging technique, for evaluation of human carotid artery plaques.

METHODS

Carotid plaques were imaged with ultrasound and subsequently excised and formalin fixed. MIR imaging was performed. By using synchrotron radiation, conventional radiographs were acquired for comparison. Image texture measures were computed for soft-tissue regions of the plaques.

RESULTS

Ultrasound evaluation identified plaques as homogeneous without calcifications. MIR images revealed complex heterogeneous structure with multiple microcalcifications consistent with histology, and possessed more image texture in specific regions than conventional radiographs (P < .05). MIR refraction images allowed imaging of the geometric structure of tissue interfaces within the plaques, while scatter images contained more texture in soft-tissue regions than absorption or refraction images.

CONCLUSIONS

X-ray PC imaging better depicts plaque soft-tissue heterogeneity than ultrasound or conventional radiographs. MIR imaging technique should be investigated further as a viable imaging technique to identify high-risk plaques.

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

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

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

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

A proposed model of naturally occurring osteoarthritis in the domestic rabbit

Osteoarthritis affects one in eight American adults over the age of 25 y and is a leading cause of chronic disability in the US. Translational research to investigate treatments for this naturally occurring joint disease requires an appropriate animal model. The authors conducted a retrospective study to assess the potential of naturally occurring osteoarthritis in the domestic rabbit as a model of the human disease. Analysis of radiographic images showed that the presence and severity of osteoarthritis were significantly influenced by both age and body weight. The most commonly affected joints were the knee and the hip. The findings reported here suggest that the rabbit is an excellent model of spontaneously arising osteoarthritis that may be useful in translational research pertaining to the human disease.

Diffraction enhanced imaging of a rat model of gastric acid aspiration pneumonitis

RATIONALE AND OBJECTIVES

Diffraction-enhanced imaging (DEI) is a type of phase contrast x-ray imaging that has improved image contrast at a lower dose than conventional radiography for many imaging applications, but no studies have been done to determine if DEI might be useful for diagnosing lung injury. The goals of this study were to determine if DEI could differentiate between healthy and injured lungs for a rat model of gastric aspiration and to compare diffraction-enhanced images with chest radiographs.

MATERIALS AND METHODS

Radiographs and diffraction-enhanced chest images of adult Sprague Dawley rats were obtained before and 4 hours after the aspiration of 0.4 mL/kg of 0.1 mol/L hydrochloric acid. Lung damage was confirmed with histopathology.

RESULTS

The radiographs and diffraction-enhanced peak images revealed regions of atelectasis in the injured rat lung. The diffraction-enhanced peak images revealed the full extent of the lung with improved clarity relative to the chest radiographs, especially in the portion of the lower lobe that extended behind the diaphragm on the anteroposterior projection.

CONCLUSIONS

For a rat model of gastric acid aspiration, DEI is capable of distinguishing between a healthy and an injured lung and more clearly than radiography reveals the full extent of the lung and the lung damage.

Experimental study on phase-contrast imaging with synchrotron hard X-ray for repairing osteonecrosis of the femoral head

Synchrotron radiation light is 1 of 4 artificial light sources, the others being electric light, X-ray, and laser. Phase-contrast imaging with hard X-ray has achieved wide application in many scientific fields, such as biomedicine and material science. This article compares the effectiveness of nanohydroxyapatite/collagen (nHAC) and autologous mesenchymal stem cell for the repair of defects in a rabbit model with osteonecrosis of the femoral head under the monitoring of phase-contrast imaging with synchrotron hard X-ray. We established models of bilateral osteonecrosis of the femoral head defect using New Zealand rabbits and divided them into 3 groups. Imaging techniques such as phase-contrast imaging and diffraction enhanced imaging with synchrotron hard X-ray were applied to assess the degradation and repair process of nHAC and mesenchymal stem cell at 4, 8, and 12 weeks postoperatively. We found phase-contrast imaging with synchrotron hard X-ray displayed the reparative process of the bone defect, degradation of nHAC, and osteocyte substitution. There were significant differences in the repair of the bone defect and osteogenesis in groups B and C compared with group A (control). Osteogenesis was more significant in group C. We provided experimental data for the development and application of synchrotron hard X-ray imaging techniques and concluded that phase-contrast microimaging with synchrotron hard X-ray displays the reparative process of bone tissue at a micro-level and plays an important role in the development of tissue engineering.

Correlation of dynamic impact testing, histopathology and visual macroscopic assessment in human osteoarthritic cartilage

OBJECTIVE

Improved staging of cartilage degeneration is required, particularly during the early stages. We correlated mechanical properties with histological and macroscopic findings.

METHODS

One hundred and twenty cartilage samples were obtained during total knee arthroplasty. Two adjacent plugs were harvested--one for histological classification and one for macroscopic and biomechanical purposes. Dynamic impact testing was performed; normal stress, dissipated energy (∆E), tangent modulus and stiffness were evaluated.

RESULTS

Samples were classified according to six categories of the ICRS histological scale. Mechanical characteristics revealing significant differences between the groups (p < 0.01) were specific damping and related absolute ∆E. A significant correlation was found between the macroscopic score and specific damping, as well as absolute and relative ∆E (p < 0.01). A strong relation was revealed between relative ∆E and cartilage thickness (p < 0.001; R (2) = 0.69).

CONCLUSIONS

Only ∆E correlated with the condition of the cartilage--the value increased with decreasing quality-and is the most suitable characteristic. This change appears substantial in initial stages of cartilage deterioration.

In vivo x-ray phase contrast analyzer-based imaging for longitudinal osteoarthritis studies in guinea pigs

Over the last two decades phase contrast x-ray imaging techniques have been extensively studied for applications in the biomedical field. Published results demonstrate the high capability of these imaging modalities of improving the image contrast of biological samples with respect to standard absorption-based radiography and routinely used clinical imaging techniques. A clear depiction of the anatomic structures and a more accurate disease diagnosis may be provided by using radiation doses comparable to or lower than those used in current clinical methods. In the literature many works show images of phantoms and excised biological samples proving the high sensitivity of the phase contrast imaging methods for in vitro investigations. In this scenario, the applications of the so-called analyzer-based x-ray imaging (ABI) phase contrast technique are particularly noteworthy. The objective of this work is to demonstrate the feasibility of in vivo x-ray ABI phase contrast imaging for biomedical applications and in particular with respect to joint anatomic depiction and osteoarthritis detection. ABI in planar and tomographic modes was performed in vivo on articular joints of guinea pigs in order to investigate the animals with respect to osteoarthritis by using highly monochromatic x-rays of 52 keV and a low noise detector with a pixel size of 47 × 47 µm(2). Images give strong evidence of the ability of ABI in depicting both anatomic structures in complex systems as living organisms and all known signs of osteoarthritis with high contrast, high spatial resolution and with an acceptable radiation dose. This paper presents the first proof of principle study of in vivo application of ABI. The technical challenges encountered when imaging an animal in vivo are discussed. This experimental study is an important step toward the study of clinical applications of phase contrast x-ray imaging techniques.

Vulnerability of the superficial zone of immature articular cartilage to compressive injury

OBJECTIVE

The zonal composition and functioning of adult articular cartilage causes depth-dependent responses to compressive injury. In immature cartilage, shear and compressive moduli as well as collagen and sulfated glycosaminoglycan (sGAG) content also vary with depth. However, there is little understanding of the depth-dependent damage caused by injury. Since injury to immature knee joints most often causes articular cartilage lesions, this study was undertaken to characterize the zonal dependence of biomechanical, biochemical, and matrix-associated changes caused by compressive injury.

METHODS

Disks from the superficial and deeper zones of bovine calves were biomechanically characterized. Injury to the disks was achieved by applying a final strain of 50% compression at 100%/second, followed by biomechanical recharacterization. Tissue compaction upon injury as well as sGAG density, sGAG loss, and biosynthesis were measured. Collagen fiber orientation and matrix damage were assessed using histology, diffraction-enhanced x-ray imaging, and texture analysis.

RESULTS

Injured superficial zone disks showed surface disruption, tissue compaction by 20.3 ± 4.3% (mean ± SEM), and immediate biomechanical impairment that was revealed by a mean ± SEM decrease in dynamic stiffness to 7.1 ± 3.3% of the value before injury and equilibrium moduli that were below the level of detection. Tissue areas that appeared intact on histology showed clear textural alterations. Injured deeper zone disks showed collagen crimping but remained undamaged and biomechanically intact. Superficial zone disks did not lose sGAG immediately after injury, but lost 17.8 ± 1.4% of sGAG after 48 hours; deeper zone disks lost only 2.8 ± 0.3% of sGAG content. Biomechanical impairment was associated primarily with structural damage.

CONCLUSION

The soft superficial zone of immature cartilage is vulnerable to compressive injury, causing superficial matrix disruption, extensive compaction, and textural alteration, which results in immediate loss of biomechanical function. In conjunction with delayed superficial sGAG loss, these changes may predispose the articular surface to further softening and tissue damage, thus increasing the risk of development of secondary osteoarthritis.

Diffraction-enhanced radiography of various mouse organs

OBJECTIVE

The purposes of this study were to evaluate whether a novel radiographic technique, diffraction-enhanced radiographic imaging, would render high-contrast images of mouse livers, hearts, and kidneys and to determine whether blood vessels and bile ducts can be differentiated on images of mouse livers.

MATERIALS AND METHODS

For imaging of the bile ducts, mouse livers were excised 20 or 35 days after ligation of the common bile duct. Livers, hearts, and kidneys of control mice also were excised for imaging. The diffraction-enhanced imaging experiments were performed with a silicon 333 crystal diffraction plane and an 18-keV x-ray beam. The beam incident to the sample measured 20 mm (horizontal) x 11 mm (vertical). Images were acquired with the analyzer crystal set at different positions of the rocking curve.

RESULTS

Only dilated bile ducts, no normal bile ducts, were found. With diffraction-enhanced imaging without a contrast agent, the blood vessels of the liver, heart, and kidney were visualized to a scale of tens of micrometers.

CONCLUSION

Diffraction-enhanced imaging with a silicon 333 crystal plane had excellent contrast in the detection of blood vessels and pathologically dilated bile ducts and may be a promising radiographic technique for basic medical research.

In-laboratory diffraction-enhanced X-ray imaging for articular cartilage

The loss of articular cartilage characteristic of osteoarthritis can only be diagnosed by joint space narrowing when conventional radiography is used. This is due to the lack of X-ray contrast of soft tissues. Whereas conventional radiography harnesses the X-ray attenuation properties of tissues, Diffraction Enhanced Imaging (DEI), a novel radiographic technique, allows the visualization of soft tissues simultaneous with calcified tissues by virtue of its ability to not only harness X-ray attenuation but also the X-ray refraction from tissue boundaries. Previously, DEI was dependent upon synchrotron X-rays, but more recently, the development of nonsynchrotron DEI units has been explored. These developments serve to elaborate the full potential of radiography. Here, we tested the potential of an in-laboratory DEI system, called Diffraction-Enhanced X-ray Imaging (DEXI), to render images of articular cartilage displaying varying degrees of degradation, ex vivo. DEXI allowed visualization of even early stages of cartilage degeneration such as surface fibrillation. This may be of eventual clinical significance for the diagnosis of early stages of degeneration, or at the very least, to visualize soft tissue degeneration simultaneous with bone changes.

Toward the x-ray microdiffraction imaging of bone and tissue-engineered bone

The hierarchical structure of bone makes the X-ray microdiffraction scanning techniques one of the most effective tool to investigate the structural features of this tissue at different length scales: the atomic/nanometer scale of the X-ray scattering signals and the macroscopic scale of the scanned sample area. The potentiality of the microdiffraction approach has been verified also by investigations on tissue-engineered bone substitutes used to repair large hard bone defects. The aim of this review is to present the most representative and recent results obtained through high-resolution scanning microdiffraction techniques studying both natural and tissue-engineered bone. The rapid evolution of the instrumental set-ups and the advanced methods of data analysis are described. Recent examples in which X-ray microbeams were used for imaging quantitative features of natural bone tissue and engineered bone substitutes are presented along with the qualitative and quantitative information extracted from the two-dimensional patterns collected on bone samples and on ex vivo cell seeded bioceramic implants. Thanks to the microdiffraction approach, several aspects of the mechanisms leading to the generation of the new bone, coupled to the scaffold resorption in the tissue-engineered constructs, have been tentatively interpreted. The potential of X-ray microdiffraction as an imaging tool in the field of bone tissue engineering is discussed and the key role of high-spatial resolution, availability of automatic tools (for dealing with the huge amount of experimental data) and advanced analysis techniques is elucidated. Finally, future perspectives in the field are presented.

Cartilage lesions and the development of osteoarthritis after internal fixation of ankle fractures: a prospective study

BACKGROUND

The role of the location and severity of the initial cartilage lesions associated with an ankle fracture in the development of posttraumatic osteoarthritis has not been established, to our knowledge.

METHODS

We performed a long-term follow-up study of a consecutive, prospectively included cohort of 288 ankle fractures that were treated operatively between June 1993 and November 1997. Arthroscopy had been performed in all cases in order to classify the extent and location of cartilage damage. One hundred and nine patients (47%) were available for follow-up after a mean of 12.9 years. The main outcome parameters were the American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot score for clinical evaluation and a modified Kannus osteoarthritis score for radiographic assessment of the development of posttraumatic osteoarthritis.

RESULTS

Cartilage damage anywhere in the ankle joint was associated with a suboptimal clinical outcome (odds ratio, 5.0 [95% confidence interval = 1.3 to 20.1]; p = 0.02) and with a suboptimal radiographic outcome (odds ratio = 3.4 [95% confidence interval = 1.0 to 11.2]; p = 0.04). An association was also found between the development of clinical signs of osteoarthritis and a deep lesion (>50% of the cartilage thickness) on the anterior aspect of the talus (odds ratio = 12.3 [95% confidence interval = 1.4 to 108.0]; p = 0.02) and a deep lesion on the lateral aspect of the talus (odds ratio = 5.4 [95% confidence interval = 1.2 to 23.5]; p = 0.02). A deep lesion on the medial malleolus was associated with the development of clinical signs of osteoarthritis (odds ratio = 5.2 [95% confidence interval = 1.9 to 14.6]; p < 0.01) and radiographic signs of osteoarthritis (odds ratio = 2.9 [95% confidence interval = 1.1 to 7.9]; p = 0.03) of osteoarthritis. There was no significant correlation between cartilage lesions on the fibula and the long-term outcome.

CONCLUSIONS

Our findings show that initial cartilage damage seen arthroscopically following an ankle fracture is an independent predictor of the development of posttraumatic osteoarthritis. Specifically, lesions on the anterior and lateral aspects of the talus and on the medial malleolus correlate with an unfavorable clinical outcome.

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

RATIONALE AND OBJECTIVES

This study was designed to demonstrate the feasibility of the use of phase-contrast computed tomographic (CT) imaging for the identification of articular cartilage abnormalities of the knees in a mouse model of collagen-induced arthritis.

MATERIALS AND METHODS

Arthritis was induced in nine male DBA/1 J mice by the intradermal injection of collagen. After 50 days, the nine mice were sacrificed, along with four mice that did not receive intradermal injections of collagen. Phase-contrast CT imaging using a microfocus x-ray source of the entire knee was performed. The images were evaluated by two blinded readers, and histopathologic grades were considered the reference standard. The phase-contrast CT images of cartilage were graded 0, I, or II. Evaluation of the grading agreement between the phase-contrast CT images and histopathologic findings was performed using correlation analysis.

RESULTS

Phase-contrast CT images highly reflected the subchondral bone status in the assessment of articular cartilage abnormalities in the mouse model of collagen-induced arthritis. Three-dimensional reformed images showed the articular surface and subchondral bony status of the knee joints. On the basis of the histopathology of the 26 knee joints, 12 joints were grade 0, six joints were grade I, and eight joints were grade II. Grading agreement between the use of the phase-contrast CT images and histopathologic results was high (r = 0.76).

CONCLUSIONS

Phase-contrast CT imaging using a microfocus x-ray source offers a promising tool for the assessment of articular cartilage abnormalities of the knees in a mouse model.

Comparison of diffraction-enhanced computed tomography and monochromatic synchrotron radiation computed tomography of human trabecular bone

Diffraction-enhanced imaging (DEI) is an x-ray-based medical imaging modality that, when used in tomography mode (DECT), can generate a three-dimensional map of both the apparent absorption coefficient and the out-of-plane gradient of the index of refraction of the sample. DECT is known to have contrast gains over monochromatic synchrotron radiation CT (SRCT) for soft tissue structures. The goal of this experiment was to compare contrast-to-noise ratio (CNR) and resolution in images of human trabecular bone acquired using SRCT with images acquired using DECT. All images were acquired at the National Synchrotron Light Source (Upton, NY, USA) at beamline X15 A at an x-ray energy of 40 keV and the silicon [3 3 3] reflection. SRCT, apparent absorption DECT and refraction DECT slice images of the trabecular bone were created. The apparent absorption DECT images have significantly higher spatial resolution and CNR than the corresponding SRCT images. Thus, DECT will prove to be a useful tool for imaging applications in which high contrast and high spatial resolution are required for both soft tissue features and bone.

The design and application of an in-laboratory diffraction-enhanced x-ray imaging instrument

We describe the design and application of a new in-laboratory diffraction-enhanced x-ray imaging (DEXI) instrument that uses a nonsynchrotron, conventional x-ray source to image the internal structure of an object. In the work presented here, a human cadaveric thumb is used as a test-sample to demonstrate the imaging capability of our instrument. A 22 keV monochromatic x-ray beam is prepared using a mismatched, two-crystal monochromator; a silicon analyzer crystal is placed in a parallel crystal geometry with the monochromator allowing both diffraction-enhanced imaging and multiple-imaging radiography to be performed. The DEXI instrument was found to have an experimentally determined spatial resolution of 160+/-7 mum in the horizontal direction and 153+/-7 mum in the vertical direction. As applied to biomedical imaging, the DEXI instrument can detect soft tissues, such as tendons and other connective tissues, that are normally difficult or impossible to image via conventional x-ray techniques.