Permanent implants made of titanium or its alloys are the gold standard in many orthopedic and traumatological
applications due to their good biocompatibility and mechanical properties. However, a second surgical intervention is
required for this kind of implants as they have to be removed in the case of children that are still growing or on patient’s
demand. Therefore, magnesium-based implants are considered for medical applications as they are degraded under
physiological conditions. The major challenge is tailoring the degradation in a manner that is suitable for a biological
environment and such that stabilization of the bone is provided for a controlled period. In order to understand failure
mechanisms of magnesium-based implants in orthopedic applications and, further, to better understand the
osseointegration, screw implants in bone are studied under mechanical load by means of a push-out device installed at
the imaging beamline P05 of PETRA III at DESY. Conventional absorption contrast microtomography and phasecontrast
techniques are applied in order to monitor the bone-to-implant interface under increasing load conditions. In this
proof-of-concept study, first results from an in situ push-out experiment are presented.
Chronic obstructive pulmonary disease (COPD) is one of the leading causes of morbidity and mortality worldwide and
emphysema is one of its main components. The disorder is characterized by irreversible destruction of the alveolar walls
and enlargement of distal airspaces. Despite the severe changes in the lung tissue morphology, conventional chest
radiographs have only a limited sensitivity for the detection of mild to moderate emphysema. X-ray dark-field is an
imaging modality that can significantly increase the visibility of lung tissue on radiographic images. The dark-field
signal is generated by coherent, small-angle scattering of x-rays on the air-tissue interfaces in the lung. Therefore,
morphological changes in the lung can be clearly visualized on dark-field images. This is demonstrated by a preclinical
study with a small-animal emphysema model. To generate a murine model of pulmonary emphysema, a female
C57BL/6N mouse was treated with a single orotracheal application of porcine pancreatic elastase (80 U/kg body weight)
dissolved in phosphate-buffered saline (PBS). Control mouse received PBS. The mice were imaged using a small-animal
dark-field scanner. While conventional x-ray transmission radiography images revealed only subtle indirect signs of the
pulmonary disorder, the difference between healthy and emphysematous lungs could be clearly directly visualized on the
dark-field images. The dose applied to the animals is compatible with longitudinal studies. The imaging results correlate
well with histology. The results of this study reveal the high potential of dark-field radiography for clinical lung imaging.
The main shortcoming of conventional biomedical x-ray imaging is the weak soft-tissue contrast caused by the small differences in the absorption coefficients between different materials. This issue can be addressed by x-ray phasesensitive imaging approaches, e.g. x-ray Talbot-Lau grating interferometry. The advantage of the three-grating Talbot-Lau approach is that it allows to acquire x-ray phase-contrast and dark-field images with a conventional lab source. However, through the introduction of the grating interferometer some constraints are imposed on the setup geometry. In general, the grating pitch and the mean x-ray energy determine the setup dimensions. The minimal length of the setup increases linearly with energy and is proportional to p<sup>2</sup>, where p is the grating pitch. Thus, a high-energy (100 keV) compact grating-based setup for x-ray imaging can be realized only if gratings with aspect-ratio of approximately 300 and a pitch of 1-2 μm were available. However, production challenges limit the availability of such gratings. In this study we consider the use of non-binary phase-gratings as means of designing a more compact grating interferometer for phase-contrast imaging. We present simulation and experimental data for both monochromatic and polychromatic case. The results reveal that phase-gratings with triangular-shaped structures yield visibilities that can be used for imaging purposes at significantly shorter distances than binary gratings. This opens the possibility to design a high-energy compact setup for x-ray phase-contrast imaging. Furthermore, we discuss different techniques to achieve triangular-shaped phase-shifting structures.
We have developed a compact grating-based in-vivo phase-contrast micro-CT system with a rotating gantry. The 50 W microfocus x-ray source operates with 20 to 50 kV peak energy. The length of the rotating interferometer is around 47 cm. Pixel size in the object is 30 micron; the field of view is approx. 35 mm in diameter, suited to image a mouse. The interferometer consists of three gratings: an absorption grating close to the x-ray source, a phase grating to introduce a π/2 phase shift and an absorption analyzer grating positioned at the first fractional Talbot distance. Numerous drives and actuators are used to provide angular and linear grating alignment, phase stepping and object/gantry precision positioning. Phantom studies were conducted to investigate performance, accuracy and stability of the scanner. In particular, the influences of gantry rotation and of temperature fluctuations on the interferometric image acquisition were characterized. Also dose measurements were performed. The first imaging results obtained with the system show the complementary nature of phase-contrast micro-CT images with respect to absorption-based micro-CT. Future improvements, necessary to optimize the scanner for in-vivo small-animal CT scanning on a regular and easy-to-use basis, are also discussed.
Modern X-ray techniques opened the possibility to reconstruct phase contrast (PC) information. This provides
significantly improved soft-tissue contrast when compared to conventional computed tomography (CT). While PCCT
significantly ameliorates contrast information, radiation dose continues to be an issue when translated to the clinic.
Possible dose reduction can be achieved by using more efficient reconstruction algorithms. In this work, dose reduction
is achieved by applying a compressed sensing (CS) reconstruction to a highly sparse set of PCCT projections. The
applied reconstruction algorithm is based on a non-uniform fast Fourier transform (NUFFT), where sparse sets of
projections are reconstructed with a CS algorithm, employing wavelet domain sparsity and finite differences
minimization. We evaluated this approach with both phantom and real data. Measured data from a conventional X-ray
source were acquired using grating-based interferometry. The resulting reconstructions are compared visually, and
quantitatively on the basis of standard deviation within different regions-of-interest. The assessment of phantom and
measured data demonstrated the possibility to reconstruct from drastically fewer projections than the Nyquist-theorem
demands. The measured standard deviations were comparable or even lower compared to full dose reconstructions. In
this initial evaluation of CS-based methods in PCCT, we presented a considerable reduction of necessary projections.
Thus, radiation dose can be reduced while maintaining the superior soft-tissue contrast and image quality of PCCT. In
the future, approaches such as the presented, will enable 4D PCCT, for instance in cardiac applications.
Phase-contrast imaging approaches suffer from a severe problem which is known in Magnetic Resonance Imaging
(MRI) and Synthetic Aperture Radar (SAR) as phase-wrapping. This work focuses on an unwrapping solution for
the grating based phase-contrast interferometer with X-rays. The approach delivers three types of information
about the x-rayed object - the absorption, differential phase-contrast and dark-field information whereas the
observed differential phase values are physically limited to the interval (-π, π]; values higher or lower than the
interval borders are mapped (wrapped) back into it. In contrast to existing phase-unwrapping algorithms for MRI
and SAR the presented algorithm uses the absorption image as additional information to identify and correct
phase-wrapped values. The idea of the unwrapping algorithm is based on the observation that at locations with
phase-wrapped values the contrast in the absorption image is high and the behavior of the gradient is similar
to the real (unwrapped) phase values. This can be expressed as a cost function which has to be minimized by
an integer optimizer. Applied on simulated and real datasets showed that 95.6% of phase-wraps were correctly
unwrapped. Based on the results we conclude that it is possible to use the absorption information in order to
identify and correct phase-wrapped values.
In this paper we describe the design of different X-ray Talbot interferometers that have been built at the tomography
beamline ID19 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and give a short review of
performance characteristics, of current developments, and of the results obtained with these instruments so far. Among the
applications so far, soft-tissue imaging has been a particular focus, as demonstrated in a recent paper by Schulz et al. <i>(J.
Roy. Soc. Interface</i>, in press).
Phase contrast imaging with conventional X-ray tubes as e.g. in computer tomography scanners (CTscanners)
requires a setup of three different types of optical gratings. One grating is used to obtain
a spatially coherent radiation, the second grating defines a periodic phase shift and the third is used
as a periodic absorption grating. In order to absorb high energy radiation, absorption gratings with
periods of a few microns only and extreme aspect ratios (>80) are fabricated, employing a modified
LIGA process. However, above a critical structural height, structures collapse due to e.g. capillary
effects. To overcome this limitation a new variant of the LIGA process has been developed. It is
characterized by structuring of a resist on both sides of a membrane, resulting in a moderate aspect
ratio on both sides of the membrane instead of an extreme aspect ratio on one side. To get a perfect
overlay of both structures the grating structure on the front side of a membrane patterned by the
standard LIGA-process is used as the mask for structuring the second resist layer on the backside of
the membrane. A second electroforming step fills the gaps on the backside.
The basic principles of x-ray image formation in radiography have remained essentially unchanged since R¨ontgen
first discovered x-rays over a hundred years ago. The conventional approach relies on x-ray absorption as
the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image
formation. This approach ignores another, potentially more useful source of contrast, namely phase and scattering
information. Phase-contrast imaging techniques, which can be understood using wave optics rather than ray
optics, offer ways to augment or complement standard absorption contrast by incorporating phase information.
The recent development of grating based phase- and darkfield-contrast imaging with x-rays1 pawed the way for many potential applications to medical imaging and structure determination in material science.
Here we present our recent contributions to the field of interferometric phase-contrast and dark-field x-ray imaging. We introduce a new material dependent scattering parameter, the <i>Linear Diffusion Coefficient</i>, and a quantitative mathematical formalism to extend the dark-field x-ray images into three dimensions by tomographic reconstruction. Further, the results of two experiments that illustrate the potential of dark-field imaging for computed tomography are shown.
New developments in X-ray instrumentation and analysis have facilitated the development and improvement
of various scanning X-ray microscopy techniques. In this contribution, we offer an overview of recent scanning
hard X-ray microscopy measurements performed at the Swiss Light Source. We discuss scanning transmission
X-ray microscopy in its transmission, phase contrast, and dark-field imaging modalities. We demonstrate how
small-angle X-ray scattering analysis techniques can be used to yield additional information. If the illumination
is coherent, coherent diffraction imaging techniques can be brought to bear. We discuss how, from scanning
microscopy measurements, detailed measurements of the X-ray scattering distributions can be used to extract
high-resolution images. These microscopy techniques with their respective imaging power can easily be combined
to multimodal X-ray microscopy.
We report advances and complementary results concerning a recently developed method for high-sensitivity grating-based hard x-ray phase tomography. We demonstrate how the soft tissue sensitivity of the technique can be used to obtain in-vitro tomographic images of a tumor bearing rat brain specimen, without use of contrast agents. In particular, we demonstrate that brain tumors and the white and gray brain matter structure in a rat's cerebellum can be resolved by this approach. The findings are potentially interesting from a clinical point of view, since a similar approach using three transmission gratings can be implemented with more readily available x-ray sources, such as standard x-ray tubes. Moreover, open the results the way to in-vivo experiments in the near future.