Grating-based Talbot–Lau x-ray interferometry is a popular method for measuring absorption, phase shift, and small-angle scattering. The standard acquisition method for this modality is phase stepping, where the Talbot pattern is reconstructed from multiple images acquired at different grating positions. We review the implicit assumptions in phase-stepping reconstruction, and find that the assumptions of perfectly known grating positions and homoscedastic noise variance are violated in some scenarios. Additionally, we investigate a recently reported estimation bias in the visibility and dark-field signal. To adapt the phase-stepping reconstruction to these findings, we propose three improvements to the reconstruction. These improvements are (a) to use prior knowledge to compute more accurate grating positions to reduce moiré artifacts, (b) to utilize noise variance information to reduce dark-field and phase noise in high-visibility acquisitions, and (c) to perform correction of an estimation bias in the interferometer visibility, leading to more quantitative dark-field imaging in acquisitions with a low signal-to-noise ratio. We demonstrate the benefit of our methods on simulated data, as well as on images acquired with a Talbot–Lau interferometer.
X-ray grating-based phase-contrast imaging might open up entirely new opportunities in medical imaging. However, transferring the interferometer technique from laboratory setups to conventional imaging systems the necessary rigidity of the system is difficult to achieve. Therefore, vibrations or distortions of the system lead to inaccuracies within the phase-stepping procedure. Given insufficient stability of the phase-step positions, up to now, artifacts in phase-contrast images occur, which lower the image quality. This is a problem with regard to the intended use of phase-contrast imaging in clinical routine as for example tiny structures of the human anatomy cannot be observed. In this contribution we evaluate an algorithm proposed by Vargas et.al.1 and applied to X-ray imaging by Pelzer et.al. that enables us to reconstruct a differential phase-contrast image without the knowledge of the specific phase-step positions. This method was tested in comparison to the standard reconstruction by Fourier analysis. The quality of phase-contrast images remains stable, even if the phase-step positions are completely unknown and not uniformly distributed. To also achieve attenuation and dark-field images the proposed algorithm has been combined with a further algorithm of Vargas et al.3 Using this algorithm, the phase-step positions can be reconstructed. With the help of the proper phase-step positions it is possible to get information about the phase, the amplitude and the offset of the measured data. We evaluated this algorithm concerning the measurement of thick objects which show a high absorbency.
Interferometric x-ray imaging becomes more and more attractive for applications such as medical imaging or non-destructive testing, because it provides the opportunity to obtain additional information on the internal structure of radiographed objects.<sup>12</sup> Therefore, three types of images are acquired: An attenuation image like in conventional x-ray imaging, an image of the differential phase-shift generated by the object and the so called dark-field image, which contains information about the object’s granularity even on sub-pixel scale.<sup>3</sup> However, most experiments addressing grating-based x-ray phase-contrast imaging with polychromatic sources are restricted to energies up to about 40 keV. For the application of this imaging method to thicker objects like human specimens or dense components, higher tube voltages are required. This is why we designed and constructed a laboratory setup for high energies, which is able to image larger objects.<sup>4</sup> To evaluate the performance of the setup, the mean visibility of the field of view was measured for several tube voltages. The result shows that the mean visibility has a peak value of 23% at a tube voltage of 60 kV and is constantly greater than 16% up to a tube voltage of 120 kV. Thus, good image quality is provided even for high energies. To further substantiate the performance of the setup at high energies, a human ex-vivo foot was examined at a tube voltage of 75 kV. The interferometric x-ray images show a good image quality and a promising diagnostic power.
X-ray grating-based phase-contrast Talbot-Lau interferometry is a promising imaging technology that has the potential to raise soft tissue contrast in comparison to conventional attenuation-based imaging. Additionally, it is sensitive to attenuation, refraction and scattering of the radiation and thus provides complementary and otherwise inaccessible information due to the dark-field image, which shows the sub-pixel size granularity of the measured object.
Until recent progress the method has been mainly limited to photon energies below 40 keV. Scaling the method to photon energies that are sufficient to pass large and spacious objects represents a challenging task. This is caused by increasing demands regarding the fabrication process of the gratings and the broad spectra that come along with the use of polychromatic X-ray sources operated at high acceleration voltages.
We designed a setup that is capable to reach high visibilities in the range from 50 to 120 kV. Therefore, spacious and dense parts of the human body with high attenuation can be measured, such as a human knee.
The authors will show investigations on the resulting attenuation, differential phase-contrast and dark-field images. The images experimentally show that X-ray grating-based phase-contrast radiography is feasible with highly absorbing parts of the human body containing massive bones.
With energy-resolving photon-counting detectors in grating-based x-ray phase-contrast imaging it is possible to reduce the dose needed and optimize the imaging chain towards better performance. The advantage of photon- counting detector’s linear energy response and absence of electronic noise in attenuation based imaging is known. The access to the energy information of the photons counted provides even further potential for optimization by applying energy weighting factors. We have evaluated energy weighting for grating-based phase-contrast imaging. Measurements with the hybrid photon-counting detector Dosepix were performed. The concept of energy binning implemented in the pixel electronics allows individual storing of the energy information of the incoming photons in 16 energy bins for each pixel. With this technique the full spectral information can be obtained pixel wise from one single acquisition. On the differential phase-contrast data taken, we applied different types of energy weighting factors. The results presented in this contribution demonstrate the advantages of energy-resolved photon-counting in differential phase-contrast imaging. Using a x-ray spectrum centred significantly above the interferometers design energy leads to poor image quality. But with the proposed method and detector the quality was enhanced by 2.8 times in signal-to-noise ratio squared. As this is proportional to dose, energy- resolved photon-counting might be valuable especially for medical applications.
Grating-based X-ray phase-contrast imaging is a promising imaging modality to increase soft tissue contrast in comparison to conventional attenuation-based radiography. Complementary and otherwise inaccessible information is provided by the dark-field image, which shows the sub-pixel size granularity of the measured object. This could especially turn out to be useful in mammography, where tumourous tissue is connected with the presence of supertiny microcalcifications. In addition to the well-established image reconstruction process, an analysis method was introduced by Modregger, <sup>1</sup> which is based on deconvolution of the underlying scattering distribution within a single pixel revealing information about the sample. Subsequently, the different contrast modalities can be calculated with the scattering distribution. The method already proved to deliver additional information in the higher moments of the scattering distribution and possibly reaches better image quality with respect to an increased contrast-to-noise ratio. Several measurements were carried out using melamine foams as phantoms. We analysed the dependency of the deconvolution-based method with respect to the dark-field image on different parameters such as dose, number of iterations of the iterative deconvolution-algorithm and dark-field signal. A disagreement was found in the reconstructed dark-field values between the FFT method and the iterative method. Usage of the resulting characteristics might be helpful in future applications.
Photon-counting detectors in medical x-ray imaging provide a higher dose efficiency than integrating detectors. Even further possibilities for imaging applications arise, if the energy of each photon counted is measured, as for example K-edge-imaging or optimizing image quality by applying energy weighting factors.
In this contribution, we show results of the characterization of the Dosepix detector. This hybrid photon- counting pixel detector allows energy resolved measurements with a novel concept of energy binning included in the pixel electronics. Based on ideas of the Medipix detector family, it provides three different modes of operation: An integration mode, a photon-counting mode, and an energy-binning mode. In energy-binning mode, it is possible to set 16 energy thresholds in each pixel individually to derive a binned energy spectrum in every pixel in one acquisition. The hybrid setup allows using different sensor materials. For the measurements 300 μm Si and 1 mm CdTe were used. The detector matrix consists of 16 x 16 square pixels for CdTe (16 x 12 for Si) with a pixel pitch of 220 μm. The Dosepix was originally intended for applications in the field of radiation measurement. Therefore it is not optimized towards medical imaging. The detector concept itself still promises potential as an imaging detector.
We present spectra measured in one single pixel as well as in the whole pixel matrix in energy-binning mode with a conventional x-ray tube. In addition, results concerning the count rate linearity for the different sensor materials are shown as well as measurements regarding energy resolution.
Interferometric X-ray imaging becomes more and more attractive for applications such as medical imaging or non-destructive testing, where a compact setup is needed. Therefore a so-called Talbot-Lau interferometer in combination with a conventional X-ray tube is used.
Thereby, three different kinds of images can be obtained. An attenuation image like in conventional X-ray
imaging, an image of the differential phase-shifts caused by the object and the so-called dark-field image. The dark-field image shows information about the object's granularity even in sub-pixel dimensions what especially seems very promising for applications like mammography.
With respect to optimizing the output of interferometric X-ray imaging in any application, it is inevitable to
know the energy response of the interferometer as well as the energy dependence of the interactions of X- rays with matter.
In this contribution, simulations and measurements using a Medipix 2 and a Timepix detector are presented.
Grating-based X-ray phase-contrast imaging (XPCI) is a promising modality to increase soft-tissue contrast in medical imaging and especially in the case of mammography. Several groups worldwide have performed investigations on grating-based Talbot-Lau X-ray imaging of breast tissue, but in most cases focussed on the soft tissue contrast enhancement of the differential phase image.
In this contribution, we present promising measurements with a Talbot-Lau interferometer of several mastectomy breast tissue samples especially focussed on the sensitivity of the dark-field signal of microcalcifications and with a comparable dose value to conventional mammography. We can present a contrast improvement for calcifications in surrounding breast tissue for the dark-field image by a factor of 10 related to the attenuation
Grating-based X-ray phase-contrast imaging with a Talbot-Lau interferometer is a promising method which
might be able to increase soft tissue contrast and to gain additional information in comparison to attenuationbased
imaging. The method provides an attenuation image, a differential phase image and a dark-field image.
A conventional polychromatic X-ray tube can be used together with a Talbot-Lau interferometer consisting of
a source grating, a phase grating and an absorption grating. The dark-field image shows information about the
sub-pixel-size granularity of the measured object. This supplemental information is supposed to be suitable in
applications, such as mammography or nondestructive testing.
In this contribution we present results of measurements investigating the thickness-dependent behavior of
dark-field imaging. The measurements are performed with a wedge-shaped, granular object with our X-ray
phase-contrast imaging set-up and calculating the dark-field image. Measurements with this special phantom
show a resurgence of visibility contrast with increasing thickness of the object after passing a minimum. The
reason of this artifact is not completely clear up to now, but might be found in attenuation effects in the object
in combination with the polychromatic X-ray spectrum or in residual amplitudes in our fitting algorithm for low
visibilities and low intensities at large thicknesses. Understandig the thickness-dependent behavior of the X-ray
dark-field advances the understanding of the formation of the dark-field image.
X-ray phase-contrast imaging (XPCI) using a Talbot-Lau interferometer is a promising modality to increase
soft-tissue contrast in medical imaging and has drawn the attention of many groups worldwide. Nevertheless,
due to the many free parameters to be optimized in a XPCI setup, there's a long way to go to find the optimal
geometry, design energy and spectrum for a future clinical application.
In this contribution, we present a fast procedure to optimize the visibility response of a Talbot-Lau Interferometer
with respect to an introduced filter material. This is done by performing a virtual filtering of the
known reference tube spectrum, followed by calculating the visibility using the simulated detector and interferometer
responses. Additionally, our procedure can also be used to optimize the general setup lengths, the grating
properties or the design energy.
We present recent results of the optimization process of our lab setup, where the simulations predict a visibility
increase of approximately 72% compared to the non-optimized state.
In the future, we are going to extend the functionality of our optimization algorithm to perform simulations
that allow the prediction of best suitable spectrum for a given application at a certain noise level tolerated and
at the lowest dose possible.
In recent years X-ray phase-contrast imaging became applicable in the hard X-ray regime through the use of a
grating-based Talbot-Lau interferometer and was demonstrated to be a promising technique to gain contrast in
different fields of medical imaging. In addition to absorption imaging, phase-contrast and dark-field imaging is
capable to yield completely new information and is able to provide structural information about a specimen at a
scale much less than imaging system-based resolution. Therefore an effective implementation of this information
in medical imaging applications benefits substantially from a detailed look onto interferometer setup based effects
on the phase signal.
For the calculation of the dark-field signal, the loss in intensity modulation, represented by the contrast ratio
V / V<sub>0</sub>, due to local scattering effects within the specimen structure is exploited. By using an energy-resolving
detector, spectral effects of a X-ray tube spectrum on the interferometer image quality can be determined. In
this contribution we will show first results on spectroscopic
dark-field imaging, with a focus to the potential
utilization on porous bone structure. Measurements were carried out using a Talbot-Lau interferometer in
connection with a hybrid photon-counting semiconductor Timepix detector, which provides an adjustable lower
threshold for photon detection.
Phase-contrast imaging is a novel modality in the field of medical X-ray imaging. The pioneer method is the
grating-based interferometry which has no special requirements to the X-ray source and object size. Furthermore,
it provides three different types of information of an investigated object simultaneously - absorption, differential
phase-contrast and dark-field images. Differential phase-contrast and dark-field images represent a completely
new information which has not yet been investigated and studied in context of medical imaging. In order to
introduce phase-contrast imaging as a new modality into medical environment the resulting information about
the object has to be correctly interpreted. The three output images reflect different properties of the same object
the main challenge is to combine and visualize these data in such a way that it diminish the information explosion
and reduce the complexity of its interpretation.
This paper presents an intuitive image fusion approach which allows to operate with grating-based phase-contrast
images. It combines information of the three different images and provides a single image. The approach
is implemented in a fusion framework which is aimed to support physicians in study and analysis. The framework
provides the user with an intuitive graphical user interface allowing to control the fusion process. The example
given in this work shows the functionality of the proposed method and the great potential of phase-contrast
imaging in medical practice.
We present energy-dependent measurement results, regarding
grating-based X-ray phase-contrast imaging. These
were done using a Talbot grating interferometer according to the proposal made by Weitkamp et al.<sup>1</sup> using a
commercial microfocus X-ray tube producing a standard tungsten spectrum and a Talbot-Lau grating interferometer
with a medical X-ray tube. The spectroscopic, pixelated, photon-counting Timepix detector was used
for photon detection. Using these setups, we measured the visibilities for different energies at different source
to phase-grating distances. These results showed a constant maximum visibility, shifting to higher energies increasing
the distance. This behaviour can be explained by changing the distance leads to a change in the setup's
effective design energy. The results also showed that the expected drop of visibility for a misaligned setup does
not take place when a polychromatic X-ray source is used. Additionally, images obtained with phase-contrast
imaging at different energy thresholds are presented and evaluated. This knowledge about the energy-dependency
of the setup's parameters will help to optimise X-ray phase-contrast imaging towards the clinical application.
Differential phase-contrast imaging with X-ray tubes based on Talbot Interferometry is influenced by conventional
X-ray imaging setups. Parameters, which are optimized for conventional setups, may not be optimal for
differential phase-contrast imaging. Therefore, there is a high potential for optimization of differential phase-contrast
imaging. Quantities like visibility, contrast to noise ratio, and dose can be combined to form an objective
function. For differential phase-contrast imaging, those possible objective functions are generally not known analytically
and are expected to be non-linear. The optimization of differential phase-contrast is still possible as
the quantities, which are necessary to form an objective function, can be obtained by a simulation. Additionally,
setup parameters can be varied more purposefully within the simulation than it would be possible in an experimental
setup. To take particle as well as wave contributions into account, a Monte-Carlo simulation framework
and a wave field simulation framework are used. Numerical optimization procedures are an adequate approach to
find optimal setups for differential phase-contrast imaging. The objective function can be obtained by numerical
simulations. Hence, different optimization procedures will be evaluated and compared. Results for an optimized
phase grating and an optimized analyzer grating are presented. The appropriate optimization procedure and the
optimal setup depend on the intended application of the setup and the constraints which the setup parameters
have to obey.
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.
We present a simulation framework for X-ray phase-contrast computed tomography imaging (PCTI) inheriting the wave- as well as the particle-behavior of photons. The developed tool includes the modeling of a partially
coherent X-ray source, the propagation of the X-ray photons through samples, and the interfering properties of
photons. Hence, the simulation is capable of physically modeling a grating-based interferometric imaging system
reported in e.g. Pfeiffer et al.<sup>5</sup> The information gained comprises the three potentially measurable images,
which are the absorption image, the phase image, and the darkfield image. Results on such a setup concerning
spatial and temporal coherence will be shown. Samples consisting of elements and structures similar to biological tissue were implemented to demonstrate the applicability on medical imaging. For the purpose of CT-imaging a head-like phantom was simulated and the results show the advantage of PCTI for thick biological objects.
The simulation was developed with a modular concept so that the influences of each imaging component can be
considered seperately. Thus the grating based interferometry for
X-ray phase-contrast imaging can be optimized towards dedicated medical applications using this simulation-tool.