Analyzer-based Imaging Research Articles

Multiple imaging radiography at LNLS

An analyzer-based X-ray phase-contrast imaging (ABI) setup has been mounted at the Brazilian Synchrotron Light Laboratory (LNLS) for multiple imaging radiography (MIR) purposes. The algorithm employed for treating the MIR data collected at LNLS is described, and its reliability in extracting the distinct types of contrast that can be obtained with MIR is demonstrated by analyzing a test sample (thin polyamide wire). As a practical application, the possibility of studying ophthalmic tissues, corneal sequestra in this case, via MIR is investigated.

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

A new algorithm is introduced for separation of the scattered and non-scattered parts of a monochromatic and well collimated synchrotron radiation beam transmitted through a sample and analyzed by reflection from a perfect crystal in the non-dispersive setting. The observed rocking curve is described by the Voigt function, which is a convolution of Lorentzian and Gaussian functions. For the actual fitting, pseudo-Voigtians are used. The fit yields the scaled integrated intensity (the effect of absorption), the center of the rocking curve (the effect of refraction), and the intensity of the transmitted beam is divided into the scattered and non-scattered parts. The algorithm is tested using samples that exhibit various degrees of refraction and scattering. Very close fits are achieved in an angular range that is 15 times the full width at half-maximum of the intrinsic rocking curve of the analyzer. The scattering part has long tails of Lorentzian shape owing to the ;long-slit geometry' of the set-up. Quantitative images of absorption, refraction and scattering are constructed and compared with results of earlier treatments. The portion of scattering and the second moment of the observed rocking curve both increase linearly with the sample thickness and yield identical maps of the effects of scattering. The effects of refraction are calculated using the geometrical optics approximation, and a good agreement with experiment is found. The fits with reduced number of data points (minimum number is five) yield closely the same results as fits to the full data set.

Visrock: a program for digital topography and X-ray microdiffraction imaging

Visrockis a program for interactive analysis of sequences of digital X-ray images.Visrockwas developed in the context of the rocking-curve imaging method of full-field X-ray microdiffraction imaging. Its functionality is based on parallel profile analysis of millions of local diffraction profiles. Options for subsequent visualization of the spatial distribution of extracted parameters include automatic contrast enhancement, noise reduction and multi-peak analysis. In addition to microdiffraction imaging, further useful applications of the program lie particularly in computed tomography, sequential radiography and analyser-based imaging.

Two dimensional diffraction enhanced imaging algorithm

Analyzer-based imaging provides refraction (phase) contrast due to angular filtering during reflection. By using two analyzer crystals with perpendicular diffraction planes (the so-called Bragg magnifier) the observable contrast is extended to two refraction directions. However, a nonlinear contribution of the two refraction directions to the images occurs and thus the standard diffraction-enhanced imaging algorithms are not suitable for image analysis in the case of the Bragg magnifier. Therefore, in this letter the authors introduce an algorithm (two dimensional diffraction enhanced imaging), which takes this nonlinear contribution into account and allows them to reconstruct the absorption, the horizontal, and the vertical refraction images.

Nonlinear extension of the x-ray diffraction enhanced imaging

Diffraction enhanced imaging is the analyzer-based x-ray imaging technique which allows extraction of the refraction and absorption contrasts from two images taken on the opposite sides of the rocking curve of the analyzer. It is widely used in different fields of science. However, the information provided by the method is qualitative rather than quantitative. This happens because either side of the rocking curve is approximated as a line. One can overcome this problem of considering the rocking curve instead of the approximation. This letter is dedicated to the implementation of this idea and includes theoretical background and experimental validation.

A numerical wave-optical approach for the simulation of analyzer-based x-ray imaging

An advanced wave-optical approach for simulating a monochromator-analyzer set-up in Bragg geometry with high accuracy is presented. The polychromaticity of the incident wave on the monochromator is accounted for by using a distribution of incoherent point sources along the surface of the crystal. The resulting diffracted amplitude is modified by the sample and can be well represented by a scalar representation of the optical field where the limitations of the usual 'weak object' approximation are removed. The subsequent diffraction mechanism on the analyzer is described by the convolution of the incoming wave with the Green-Riemann function of the analyzer. The free space propagation up to the detector position is well reproduced by a classical Fresnel-Kirchhoff integral. The preliminary results of this innovative approach show an excellent agreement with experimental data.

General reconstruction formulas for analyzer-based computed tomography

General formulas are derived for computed tomography reconstruction in analyzer-based imaging. Two image acquisition geometries are investigated. In the first geometry, which is widely used, the rotation axis of the object is perpendicular to that of the analyzer crystal. In the second geometry, in which the rotation axes are parallel, the reconstruction formulas are shown to be significantly simpler and more stable with respect to noise in experimental data.

Fourier optics approach to X-ray analyser-based imaging

The transfer functions of the analyser-based imaging system are expressed and discussed in terms of the analyser point-spread function and of an offset function which takes into account the partial coherence characteristics of the X-ray beam, particularly its wavelength spread. A description of the imaging process in terms of ambiguity functions is presented and is used to discuss the validity conditions of the transfer functions.

On qualitative and quantitative analysis in analyser-based imaging

Using rigorous wave-optical formalism, a general expression is obtained for the image intensity distribution in combined analyser-based/propagation-based phase-contrast imaging. This expression takes into account partial coherence of the wave incident on the object as well as the finite resolution of the detector system. Using this general expression, two approaches based on the geometrical optics and weak-object approximations are applied to derive simple solutions to the inverse problem of reconstruction of the phase and amplitude of the object wave. With the help of numerical experiments, the two approaches are compared in terms of their validity conditions and are shown to impose certain restrictions on the properties of the object wave. In particular, it is shown that violation of the validity conditions of the geometrical optics or weak-object approximations results in the appearance of strong reconstruction artefacts in the transmitted intensity near the edges of the objects. The effect of the incident wavefront non-uniformity due to imperfections of the imaging set-up on image formation and phase/amplitude reconstruction is also discussed. A solution to this problem is proposed in the form of a multi-image phase/amplitude reconstruction algorithm based on the geometrical optics approximation. This algorithm and an algorithm based on the weak-object approximation are applied to simulated and experimental images of fibres.

A computed tomography implementation of multiple‐image radiography

Conventional x-ray computed tomography (CT) produces a single volumetric image that represents the spatially variant linear x-ray attenuation coefficient of an object. However, in many situations, differences in the x-ray attenuation properties of soft tissues are very small and difficult to measure in conventional x-ray imaging. In this work, we investigate an analyzer-based imaging method, called computed tomography multiple-image radiography (CT-MIR), which is a tomographic implementation of the recently proposed multiple-image radiography method. The CT-MIR method reconstructs concurrently three physical properties of the object. In addition to x-ray attenuation, CT-MIR produces volumetric images that represent the refraction and ultrasmall-angle scattering properties of the object. These three images can provide a rich description of the object's physical properties that are revealed by the probing x-ray beam. An imaging model for CT-MIR that is based on the x-ray transform of the object properties is established. The CT-MIR method is demonstrated by use of experimental data acquired at a synchroton radiation imaging beamline, and is compared to the pre-existing diffraction-enhanced imaging CT method. We also investigate the merit of an iterative reconstruction method for use with future clinical implementations of CT-MIR, which we anticipate would be photon limited.

Phase contrast X-ray imaging of mice and rabbit lungs: a comparative study

The significant degree of X-ray phase contrast created by air-tissue interfaces, coupled with the poor radiographic contrast of conventional chest radiographs, makes the inflated lung an ideal candidate for investigating the potential diagnostic improvement afforded by phase contrast X-ray imaging. In small animals these methods highlight the lung airways and lobe boundaries and reveal the lung tissue as a speckled intensity pattern not seen in other soft tissues. We have compared analyser-based and propagation-based phase contrast imaging modalities, together with conventional radiographic imaging, to ascertain which technique shows the greatest image enhancement for various lung sizes. The conventional radiographic image of a mouse was obtained on a Siemens Nova 3000 mammography system, whilst phase contrast images of mice and rabbit chests were acquired at the medical imaging beamline (20B2) at the SPring-8 synchrotron radiation research facility in Japan. For mice aged 1 day, 1 week and 1 month old it was determined that analyser-based imaging showed the greatest overall image contrast, however, for an adult rabbit both techniques yielded excellent contrast. The success of these methods in creating high quality images for rabbit lungs raises the possibility of improving human lung imaging using phase contrast techniques.

A method to extract quantitative information in analyzer-based x-ray phase contrast imaging

Analyzer-based imaging is a powerful phase-sensitive technique that generates improved contrast compared to standard absorption radiography. Combining numerically two images taken on either side at ±1/2 of the full width at half-maximum (FWHM) of the rocking curve provides images of “pure refraction” and of “apparent absorption.” In this study, a similar approach is made by combining symmetrical images with respect to the peak of the analyzer rocking curve but at general positions, ±α⋅FWHM. These two approaches do not consider the ultrasmall angle scattering produced by the object independently, which can lead to inconsistent results. An accurate way to separately retrieve the quantitative information intrinsic to the object is proposed. It is based on a statistical analysis of the local rocking curve, and allows one to overcome the problems encountered using the previous approaches.

Exploiting the x-ray refraction contrast with an analyser: the state of the art

Besides attenuation, x-ray refraction occurs in an x-ray beam passing through an object containing details of different refractive indices or of non-uniform thicknesses. The emerging x-rays present characteristic angular deflections, which are on the microradian scale. Different refraction directions can be resolved by using an angular analyser, for instance a perfect crystal, which can then be revealed by a detector placed behind it. By varying the alignment of the analyser with regard to the incoming x-ray beam, the unrefracted x-rays can either be recorded on a detector or rejected, thus, either contributing or not contributing to the image contrast.The x-ray refraction contrast can greatly exceed the absorption contrast, for instance in imaging low Z materials, where the potentialities of the technique have been exploited from the beginning. An analyser-based imaging technique has been presented in various versions also assuming different names.An important variation, based on an algorithm that differentiates the separation of the refraction contrast from the absorption contrast in an image, has been introduced with the name `diffraction enhanced imaging' (DEI).The interest in the potential medical application of the diffraction imaging technique has given origin to many projects utilizing analyser-based refraction imaging. Initiated with conventional x-ray generators, it has been rapidly developed in synchrotron radiation sites thanks to the highly intense collimated beams available that permit fast projection and tomographic imaging. Mammographic in vitro protocols and cartilage characterization are the most exploited applications of DEI. The potential medical application has refocused the interest towards the utilization of presently available conventional high-power x-ray generators that can open the doors to large-scale utilization. The aim of this paper is to present a review of the technique in its theoretical and practical aspects; an abundant bibliography, highlights the most significant and recent results.