Cristina Cozzini

A Fourier-domain algorithm for total-variation regularized phase retrieval in differential X-ray phase contrast imaging

Phase retrieval in differential X-ray phase contrast imaging involves a one dimensional integration step. In the presence of noise, standard integration methods result in image blurring and streak artifacts. This work proposes a regularized integration method which takes the availability of two dimensional data as well as the integration-specific frequency-dependent noise amplification into account. In more detail, a Fourier-domain algorithm is developed comprising a frequency-dependent minimization of the total variation orthogonal to the direction of integration. For both simulated and experimental data, the novel method yielded strong artefact reduction without increased blurring superior to the results obtained by standard integration methods or regularization techniques in the image domain.

Energy dispersive X-ray diffraction spectral resolution considerations for security screening applications

Energy dispersive X-ray diffraction (EDXRD) is a very effective method for explosive and narcotic threat detection in baggage screening. The XRD profiles arise from the molecular interference when X-rays are coherently scattered by a substance. The accurate identification of the target material depends on the ability to detect and resolve the peaks present in the coherent scatter profiles. A high-energy resolution High Purity Germanium (HPGe) detector is therefore generally used in such type of systems. To evaluate the suitability of cost-effective room-temperature semiconductor detectors for next-generation baggage screening systems, an assessment of the minimal requirements for the system resolution is required. In this study a hybrid Monte Carlo code has been modified to account for the molecular interference function that gives rise to the coherent scatter signature. A model for a realistic response function for Cadmium Zinc Telluride (CZT) detectors is then used to convolve the spectral output. This simulation tool is then used to assess the system design features and their influence on spectral resolution.

Low-dose, phase-contrast mammography with high signal-to-noise ratio

Differential phase-contrast X-ray imaging using a Talbot-Lau interferometer has recently shown promising results for applications in medical imaging. However, reducing the applied radiation dose remains a major challenge. In this study, we consider the realization of a Talbot-Lau interferometer in a high Talbot order to increase the signal-to-noise ratio for low-dose applications. The quantitative performance of π and π/2 systems at high Talbot orders is analyzed through simulations, and the design energy and X-ray spectrum are optimized for mammography. It is found that operation even at very high Talbot orders is feasible and beneficial for image quality. As long as the X-ray spectrum is matched to the visibility spectrum, the SNR continuously increases with the Talbot order for π-systems. We find that the optimal X-ray spectra and design energies are almost independent of the Talbot order and that the overall imaging performance is robust against small variations in these parameters. Discontinuous spectra, such as that from molybdenum, are less robust because the characteristic lines may coincide with minima in the visibility spectra; however, they may offer slightly better performance. We verify this hypothesis by realizing a prototype system with a mean fringe visibility of above 40% at the seventh Talbot order. With this prototype, a proof-of-principle measurement of a freshly dissected breast at reasonable compression to 4 cm is conducted with a mean glandular dose of only 3 mGy but with a high SNR.

Fast one-dimensional wave-front propagation for x-ray differential phase-contrast imaging

Numerical wave-optical simulations of X-ray differential phase-contrast imaging using grating interferometry require the oversampling of gratings and object structures in the range of few micrometers. Consequently, fields of view of few millimeters already use large amounts of a computers main memory to store the propagating wave front, limiting the scope of the investigations to only small-scale problems. In this study, we apply an approximation to the Fresnel-Kirchhoff diffraction theory to overcome these restrictions by dividing the two-dimensional wave front up into 1D lines, which are processed separately. The approach enables simulations with samples of clinically relevant dimensions by significantly reducing the memory footprint and the execution time and, thus, allows the qualitative comparison of different setup configurations. We analyze advantages as well as limitations and present the simulation of a virtual mammography phantom of several centimeters of size.

Modeling scattering for security applications: A multiple beam X-Ray Diffraction Imaging system

In the past decades several authors have demonstrated the ability of X-Ray Diffraction Imaging (XDI) in providing spatial and material specific information about the object under investigation [1]. Energy Dispersive X-Ray Diffraction (EDXRD) systems detect diffraction patterns by using a polychromatic spectrum and measuring the coherently scattered beam at a specific angle. This type of molecular-specific information plays a key role in security screening modalities providing high detection as well as low false alarm rates [2]. In the past, one of the main limitations to the widespread use of this technology in airport screening was the slow scanning rate compared to conventional systems. With the advent of novel system topologies such as the 3rd Generation energy-dispersive XDI configurations [3], this issue can be overcome [4]. In the present work a simulation tool accounting for molecular interference and providing multiple collimation configuration options has been validated with a single source dual-channel geometry. The primary-scatter and cross-scatter signals generated by two neighboring voxels have been studied here, analyzing both coherent and incoherent scatter contributions; and a method for a first order material independent cross-scatter correction has been evaluated. The tool has then been applied for first investigations of a Multiple Inverse Fan Beam (MIFB) concept.

Fourier domain image fusion for differential X-ray phase-contrast breast imaging

X-Ray Phase-Contrast (XPC) imaging is a novel technology with a great potential for applications in clinical practice, with breast imaging being of special interest. This work introduces an intuitive methodology to combine and visualize relevant diagnostic features, present in the X-ray attenuation, phase shift and scattering information retrieved in XPC imaging, using a Fourier domain fusion algorithm. The method allows to present complementary information from the three acquired signals in one single image, minimizing the noise component and maintaining visual similarity to a conventional X-ray image, but with noticeable enhancement in diagnostic features, details and resolution. Radiologists experienced in mammography applied the image fusion method to XPC measurements of mastectomy samples and evaluated the feature content of each input and the fused image. This assessment validated that the combination of all the relevant diagnostic features, contained in the XPC images, was present in the fused image as well.

Effect of object size, position, and detector pixel size on X-ray absorption, differential phase-contrast and dark-field signal

X-ray phase-contrast and dark-field imaging are two new modalities that have great potential for applications in different fields like medical diagnostics or materials science. The use of grating interferometers allows the detection of both differential phase shift and dark-field signal together with the absorption signal in a single acquisition. We present wave-optical simulations to quantitatively analyze the response of a grating-based X-ray phase-contrast and dark-field imaging setup to variations of the sample relative to the system. Specifically, we investigated changes in the size and the position of the object. Furthermore, we examined the influence of different detector pixel sizes while sample and interferometer remained unchanged. The results of this study contribute to a better understanding of the signal formation and represent a step towards the full characterization of the response of grating interferometry setups to specific sample geometries.

A visibility optimization study for grating-based X-ray Phase Contrast Imaging

An X-ray phase contrast grating interferometer consists of three gratings in the beam line to generate interference patterns and acquire phase shift information if an object is placed into the beam. These gratings have a pitch of a few micrometers and therefore very accurate alignment is needed to acquire phase information reliably. In this work we have investigated different setup parameters and environmental influences on the detected signal quality by Six Sigma methods. We have used this information to optimize our setup and established a semi-automatic alignment procedure to reproduce a stable signal quality.