Refraction and scattering of X-rays in analyzer-based imaging

Absorption, refraction and scattering retrieval in X-ray analyzer-based imaging

A three-image algorithm is proposed in order to retrieve the absorption, refraction and ultra-small-angle X-ray scattering (USAXS) properties of the object in X-ray analyzer-based imaging. Based on the Gaussian fitting to the rocking curve, the novel algorithm is theoretically derived and presented, and validated by synchrotron radiation experiments. Compared with multiple-image radiography, this algorithm only requires a minimum of three intensity measurements, and is therefore advantageous in terms of simplified acquisition procedure and reduced data collection times, which are especially important for specific applications such as in vivo imaging and phase tomography. Moreover, the retrieval algorithm can be specialized to particular cases where some degree of a priori knowledge on the object is available, potentially reducing the minimum number of intensity measurements to two. Furthermore, the effect of angular mis-alignment on the accuracy of the retrieved images was theoretically investigated, which can be of use in image interpretation and optimization of the data acquisition procedure.

Choosing sensitivity to reduce X-ray dose in medical phase contrast imaging

In medical X-ray imaging, phase contrast imaging is to measure refraction angles caused by the patient. The X-ray dose for a given image quality depends on the sensitivity of the setup, i.e. on the angular measurement range. Measurement ranges of existing phase contrast setups are either too high or too low for perfectly imaging a human finger in air: There is a gap in available measurement ranges, which prevents a reduction of X-ray dose. To fill the gap, this work proposes a novel variant of a Talbot-Lau interferometer. Instead of a single phase grating, it uses two phase gratings, each consisting of tiny prisms. The height of the prisms is an additional factor in the measurement range, which allows to fill the gap. The potential is a dose-reduction by a factor of 5.4 compared to Talbot-Lau setups of same post-patient length. Simulation results indicate a polychromatic visibility of up to 20%.

Interpretation and Utility of the Moments of Small-Angle X-Ray Scattering Distributions

Small angle x-ray scattering has been proven to be a valuable method for accessing structural information below the spatial resolution limit implied by direct imaging. Here, we theoretically derive the relation that links the subpixel differential phase signal provided by the sample to the moments of scattering distributions accessible by refraction sensitive x-ray imaging techniques. As an important special case we explain the scatter or dark-field contrast in terms of the sample's phase signal. Further, we establish that, for binary phase objects, the nth moment scales with the difference of the refractive index decrement to the power of n. Finally, we experimentally demonstrate the utility of the moments by quantitatively determining the particle sizes of a range of powders with a laboratory-based setup.

Small angle x-ray scattering with edge-illumination

Sensitivity to sub-pixel sample features has been demonstrated as a valuable capability of phase contrast x-ray imaging. Here, we report on a method to obtain angular-resolved small angle x-ray scattering distributions with edge-illumination- based imaging utilizing incoherent illumination from an x-ray tube. Our approach provides both the three established image modalities (absorption, differential phase and scatter strength), plus a number of additional contrasts related to unresolved sample features. The complementarity of these contrasts is experimentally validated by using different materials in powder form. As a significant application example we show that the extended complementary contrasts could allow the diagnosis of pulmonary emphysema in a murine model. In support of this, we demonstrate that the properties of the retrieved scattering distributions are consistent with the expectation of increased feature sizes related to pulmonary emphysema. Combined with the simplicity of implementation of edge-illumination, these findings suggest a high potential for exploiting extended sub-pixel contrasts in the diagnosis of lung diseases and beyond.

Multiple scattering tomography

Multiple scattering represents a challenge for numerous modern tomographic imaging techniques. In this Letter, we derive an appropriate line integral that allows for the tomographic reconstruction of angular resolved scattering distributions, even in the presence of multiple scattering. The line integral is applicable to a wide range of imaging techniques utilizing various kinds of probes. Here, we use x-ray grating interferometry to experimentally validate the framework and to demonstrate additional structural sensitivity, which exemplifies the impact of multiple scattering tomography.

Noise and analyzer-crystal angular position analysis for analyzer-based phase-contrast imaging

The analyzer-based phase-contrast x-ray imaging (ABI) method is emerging as a potential alternative to conventional radiography. Like many of the modern imaging techniques, ABI is a computed imaging method (meaning that images are calculated from raw data). ABI can simultaneously generate a number of planar parametric images containing information about absorption, refraction, and scattering properties of an object. These images are estimated from raw data acquired by measuring (sampling) the angular intensity profile of the x-ray beam passed through the object at different angular positions of the analyzer crystal. The noise in the estimated ABI parametric images depends upon imaging conditions like the source intensity (flux), measurements angular positions, object properties, and the estimation method. In this paper, we use the Cramér-Rao lower bound (CRLB) to quantify the noise properties in parametric images and to investigate the effect of source intensity, different analyzer-crystal angular positions and object properties on this bound, assuming a fixed radiation dose delivered to an object. The CRLB is the minimum bound for the variance of an unbiased estimator and defines the best noise performance that one can obtain regardless of which estimation method is used to estimate ABI parametric images. The main result of this paper is that the variance (hence the noise) in parametric images is directly proportional to the source intensity and only a limited number of analyzer-crystal angular measurements (eleven for uniform and three for optimal non-uniform) are required to get the best parametric images. The following angular measurements only spread the total dose to the measurements without improving or worsening CRLB, but the added measurements may improve parametric images by reducing estimation bias. Next, using CRLB we evaluate the multiple-image radiography, diffraction enhanced imaging and scatter diffraction enhanced imaging estimation techniques, though the proposed methodology can be used to evaluate any other ABI parametric image estimation technique.

On the possibility of quantitative refractive-index tomography of large biomedical samples with hard X-rays

One of the most promising applications of the X-ray phase-contrast imaging is the three dimensional tomographic reconstruction of the index of refraction. However, results reported so far are limited to relatively small samples. We present here the tomographic reconstruction of the index of refraction distribution of a large biomedical sample (> 10 cm diameter). A quantitative study comparing the absorption and phase contrast (analyzer-based) tomography images shows that the distribution of the index of refraction obtained with the phase contrast method provides a more accurate depiction (3-10 times larger signal to noise ratio values) of the sample internal structure. Thanks to the higher sensitivity of this method, the improved precision was obtained using an incoming photon fluence on the sample several times smaller than in the case of absorption imaging.

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.

Determining particle size distributions from a single projection image

Imaging techniques employed to measure the structure of granular, particulate and porous materials are limited by scale, temporal resolution and, for biological samples, radiation exposure. This paper describes a technique for determining the distribution of particle sizes in opaque samples, for particle volume fractions less than ten percent, using a single projection radiograph. The method is based on the derived property of the additivity of the particles' spatial autocorrelation function in projection images. Simulations and experiments demonstrate the ability to use this property to determine the distribution of particle sizes in a material.

Talbot-Lau x-ray interferometry for high energy density plasma diagnostic

High resolution density diagnostics are difficult in high energy density laboratory plasmas (HEDLP) experiments due to the scarcity of probes that can penetrate above solid density plasmas. Hard x-rays are one possible probe for such dense plasmas. We study the possibility of applying an x-ray method recently developed for medical imaging, differential phase-contrast with Talbot-Lau interferometers, for the diagnostic of electron density and small-scale hydrodynamic instabilities in HEDLP experiments. The Talbot method uses micro-periodic gratings to measure the refraction and ultra-small angle scatter of x-rays through an object and is attractive for HEDLP diagnostic due to its capability to work with incoherent and polychromatic x-ray sources such as the laser driven backlighters used for HEDLP radiography. Our paper studies the potential of the Talbot method for HEDLP diagnostic, its adaptation to the HEDLP environment, and its extension of high x-ray energy using micro-periodic mirrors. The analysis is illustrated with experimental results obtained using a laboratory Talbot interferometer.

Synchrotron radiation in cancer treatments and diagnostics: an overview

During the last 30 years many groups have carried out experiments and trials to develop new imaging and radiotherapy techniques in oncology, based on the use of synchrotron X-rays. There are several synchrotron biomedical stations around the world, which offer an excellent platform to improve either the imaging diagnosis or radiotherapy treatment for different tumour types. In the coming months the first radiotherapy clinical trials will be seen at the Biomedical Beamline at the ESRF synchrotron in Grenoble (France). In this article we highlight the results of some of the techniques and strategies that have been developed at different biomedical synchrotron stations.

Talbot phase-contrast x-ray imaging for the small joints of the hand

A high-resolution radiographic method for soft tissues in the small joints of the hand would aid in the study and treatment of rheumatoid arthritis (RA) and osteoarthritis (OA), which often attacks these joints. Of particular interest would be imaging with <100 µm resolution the joint cartilage, whose integrity is a main indicator of disease. Differential phase-contrast (DPC) or refraction-based x-ray imaging with Talbot grating interferometers could provide such a method, since it enhances soft tissue contrast and can be implemented with conventional x-ray tubes. A numerical joint phantom was first developed to assess the angular sensitivity and spectrum needed for a hand DPC system. The model predicts that, due to quite similar refraction indexes for joint soft tissues, the refraction effects are very small, requiring high angular resolution. To compare our model to experiment we built a high-resolution bench-top interferometer using 10 µm period gratings, a W anode tube and a CCD-based detector. Imaging experiments on animal cartilage and on a human finger support the model predictions. For instance, the estimated difference between the index of refraction of cartilage and water is of only several percent at ∼25 keV mean energy, comparable to that between the linear attenuation coefficients. The potential advantage of DPC imaging thus comes mainly from the edge enhancement at the soft tissue interfaces. Experiments using a cadaveric human finger are also qualitatively consistent with the joint model, showing that refraction contrast is dominated by tendon embedded in muscle, with the cartilage layer difficult to observe in our conditions. Nevertheless, the model predicts that a DPC radiographic system for the small hand joints of the hand could be feasible using a low energy quasi-monochromatic source, such as a K-edge filtered Rh or Mo tube, in conjunction with a ∼2 m long 'symmetric' interferometer operated in a high Talbot order.

Comparison of in vitro breast cancer visibility in analyser-based computed tomography with histopathology, mammography, computed tomography and magnetic resonance imaging

High-resolution analyser-based X-ray imaging computed tomography (HR ABI-CT) findings on in vitro human breast cancer are compared with histopathology, mammography, computed tomography (CT) and magnetic resonance imaging. The HR ABI-CT images provided significantly better low-contrast visibility compared with the standard radiological images. Fine cancer structures indistinguishable and superimposed in mammograms were seen, and could be matched with the histopathological results. The mean glandular dose was less than 1 mGy in mammography and 12-13 mGy in CT and ABI-CT. The excellent visibility of in vitro breast cancer suggests that HR ABI-CT may have a valuable role in the future as an adjunct or even alternative to current breast diagnostics, when radiation dose is further decreased, and compact synchrotron radiation sources become available.

Development of optics for x-ray phase-contrast imaging of high energy density plasmas

Phase-contrast or refraction-enhanced x-ray radiography can be useful for the diagnostic of low-Z high energy density plasmas, such as imploding inertial confinement fusion (ICF) pellets, due to its sensitivity to density gradients. To separate and quantify the absorption and refraction contributions to x-ray images, methods based on microperiodic optics, such as shearing interferometry, can be used. To enable applying such methods with the energetic x rays needed for ICF radiography, we investigate a new type of optics consisting of grazing incidence microperiodic mirrors. Using such mirrors, efficient phase-contrast imaging systems could be built for energies up to ∼100 keV. In addition, a simple lithographic method is proposed for the production of the microperiodic x-ray mirrors based on the difference in the total reflection between a low-Z substrate and a high-Z film. Prototype mirrors fabricated with this method show promising characteristics in laboratory tests.

X-ray phase, absorption and scatter retrieval using two or more phase contrast images

We have developed two phase-retrieval techniques for analyser-based phase contrast imaging that provide information about an object's X-ray absorption, refraction and scattering properties. The first requires rocking curves to be measured with and without the sample and improves upon existing techniques by accurately fitting the curves with Pearson type VII functions. The second employs an iterative approach using two simultaneously recorded images by exploiting the Laue crystal geometry. This technique provides a substantial reduction in X-ray dose and enables quantitative phase retrieval to be performed on images of moving objects.

Development of microperiodic mirrors for hard x-ray phase-contrast imaging

Differential phase-contrast imaging with hard x rays can have important applications in medicine, material sciences, and energy research. Phase-contrast methods based on microperiodic optics, such as shearing interferometry, are particularly attractive because they allow the use of conventional x-ray tubes. To enable shearing interferometry with x rays up to 100?keV, we propose using grazing-incidence microperiodic mirrors. In addition, a simple lithographic method is proposed for the production of the microperiodic x-ray mirrors, based on the difference in grazing-incidence reflectivity between a low-Z substrate and a high-Z film. Using this method, we produced prototype mirrors with 5-100?mum periods and 90?mm active length. Experimental tests with x rays up to 60?keV indicate good microperiodic mirror reflectivity and high-contrast fringe patterns, encouraging further development of the proposed imaging concept.