Propagation-based x-ray phase imaging using focusing polycapillary optics

Focusing polycapillary optics yield high gains in intensity and increased spatial resolution for a variety of clinical, lab-based, synchrotron, or in situ analysis applications. In this dissertation we investigate the extension of two applications of focusing polycapillary optics. The first is the application of polycapillary optics in radiation therapy. This discussion includes measurements and calculation of dose for focused beam orthovoltage therapy. A system has been designed to investigate whether the polycapillary optics can produce an X-ray beam which can give more accurate dose painting due to the higher dose concentration at the focal spot. X-ray exposures were measured with radio sensitive film. The measured spectrum of the X-ray beam was used to calculate the dose and compared with the measured dose on the films. The second application is medical imaging and quantitative phase retrieval. X-ray phase imaging can display subtle differences in low-density materials more readily than conventional X-ray imaging. However, due to spatial coherence requirements on the X-ray source, producing X-ray phase images has traditionally required either highly specialized sources, such as synchrotrons or small and low power microfocus sources, or multiple exposures with several carefully stepped precision gratings. To find appropriate approaches for enabling X-ray phase imaging in a clinically meaningful way, two techniques were tested. First, focusing polycapillary optics were applied in this work with conventional X-ray sources to enable propagation-based X-ray phase imaging. Quantitative phase retrieval was compared for two simple computational algorithms. Second, mesh-based X-ray phase imaging was demonstrated. In this technique, a simple, low-cost, coarse wire mesh and simple processing relaxes the spatial coherence constraint and allows quantitative phase retrieval. The mesh-based system combined with polycapillary optics significantly improved the accuracy of quantitative phase retrieval.

X-ray phase imaging reaching clinical uses.

X-ray phase imaging based on the Lau effect is demonstrated with an incoherent laboratory X-ray source. Its optical configuration resembles the inverse geometry of the X-ray Talbot–Lau interferometer, which employs a large-area amplitude grating. However, the proposed approach avoids the use and hence fabrication difficulty of such a grating, which is advantageous when constructing an X-ray phase imaging apparatus. With a Mo-target source, differential phase images of polymer spheres were successfully obtained and a design concept for downsizing the system is also described.

Under the JST-ERATO project in progress to develop X-ray and neutron phase-imaging methods together, recent achievements have been selected and reviewed after describing the merit and the principle of the phase imaging method. For X-ray phase imaging, recent developments of four-dimensional phase tomography and phase microscopy at SPring-8, Japan are mainly presented. For neutron phase imaging, an approach in combination with the time-of-flight method developed at J-PARC, Japan is described with the description of new Gd grating fabrication.

Taking advantage of the fact that an X-ray Talbot interferometer functions with X-rays of a broad energy bandwidth, high-speed X-ray phase tomography has been demonstrated by using white synchrotron light. Time resolution in addition to three-dimensional spatial resolution has been attained, and we report this achievement as the first four-dimensional (4D) X-ray phase tomography. Moire image movies of samples rotating at a speed of 1 or 2 rps generated by a Talbot interferometer were recorded at a frame rate of up to 1 kf/s, and differential phase image movies of the same frame rate were created by the Fourier-transform method. Consequently, a sub-second time resolution was achieved in the 4D phase tomography, while the spatial resolution was below 0.1 mm and 0.05 mm in axial and in-plane directions, respectively. An X-ray Talbot interferometer generates visibility images in addition to differential phase images, showing the distribution of microstructures, which cause ultra-small angle scattering but cannot be resolved individually with system spatial resolution. Tomographic image reconstruction from the visibility images was also demonstrated.

Phase contrast X-ray Imaging has the potential to enhance soft material image contrast and improve target detectability in heterogeneous and complex materials. Applications could be in areas such as separating cancers from normal dense tissue in medical imaging and in separating plastics from explosives in baggage and security imaging. We are investigating the benefits of single photon processing detectors in yielding information such as the energy and time of arrival of photons in enabling novel phase imaging and phase retrieval techniques. We will present the recent advances in simulations, system design and computational advances with the use of quantum processing detectors for x-ray phase imaging.

We are attempting to expand the technology of X-ray grating phase imaging/tomography to enable dynamic observation. X-ray phase imaging has been performed mainly for static cases, and this challenge is significant since properties of materials (and hopefully their functions) would be understood by observing their dynamics in addition to their structure, which is an inherent advantage of X-ray imaging. Our recent activities in combination with white synchrotron radiation for this purpose are described. Taking advantage of the fact that an X-ray grating interferometer functions with X-rays of a broad energy bandwidth (and therefore high flux), movies of differential phase images and visibility images are obtained with a time resolution of a millisecond. The time resolution of X-ray phase tomography can therefore be a second. This study is performed as a part of a project to explore X-ray grating interferometry, and our other current activities are also briefly outlined.

When only data transmission signals with a bandwidth of 1 MHz exist in the rover, the position can be obtained using the differential group delay data of the same-beam very long baseline interferometry (VLBI). The relative position between a lunar rover and a lander can be determined with an error of several hundreds of meters. When the guidance information of the rover is used to determine relative position, the rover’s wheel skid behavior and integral movement may influence the accuracy of the determined position. This paper proposes a new method for accurately determining relative position. The differential group delay and biased differential phase delay are obtained from the same-beam VLBI observation, while the modified biased differential phase delay is obtained using the statistic mean value of the differential group delay and the biased phase delay as basis. The small bias in the modified biased phase delay is estimated together with other parameters when the relative position of the rover is calculated. The effectiveness of the proposed method is confirmed using the same-beam VLBI observation data of SELENE. The radio sources onboard the rover and the lander are designed for same-beam VLBI observations. The results of the simulations of the differential delay of the same-beam VLBI observation between the rover and the lander show that the differential delay is sensitive to relative position. An approach to solving the relative position and a strategy for tracking are also introduced. When the lunar topography data near the rover are used and the observations are scheduled properly, the determined relative position of the rover may be nearly as accurate as that solved using differential phase delay data.

X-ray phase imaging has found limited clinical use due to requirements on x-ray coherence that may not be easily translated to clinical practice. Instead, this work employs a conventional source to create structured illumination with a simple wire mesh. The system has been employed to produce high contrast absorption images with simultaneous differential phase contrast images. In previous work we have demonstrated accurate quantitative phase extraction. In this work, we have incorporated the dark field information to successfully reveal additional structure in low contrast objects.

X-ray attenuation, phase, and dark-field images (so-called multimodal images) provide complementary information to one another. Different experimental techniques can capture these contrast mechanisms, and the corresponding images can be retrieved using various theoretical algorithms. Our previous works developed the ‘Multimodal Intrinsic Speckle-Tracking’ (MIST) algorithm, which is suitable for multimodal image retrieval from intensity data acquired using speckle-based x-ray imaging (SBXI). MIST is derived from the x-ray Fokker–Planck equation, requiring the inversion of derivative operators that are often numerically unstable. These numerical instabilities can be addressed by employing appropriate regularization techniques, such as Tikhonov regularization. However, the output from such regularization is highly sensitive to the choice of the Tikhonov regularization parameter, making it crucial to select this value carefully and optimally. In this work, we present an automated iterative algorithm to optimize the regularization of the inverse Laplacian operator in our most recently published and most general MIST variant, addressing the operator’s instability near the Fourier-space origin. Our algorithm leverages the inherent stability of the phase solution obtained from the transport-of-intensity equation for SBXI, using it as a reliable optimization reference image for the complex but better-performing x-ray Fokker–Planck-based algorithm that incorporates the dark-field signal. We applied the algorithm to experimental SBXI data collected using synchrotron light of a sample comprised of four different rods. The phase and dark-field images of the four-rod sample were optimally retrieved using our developed algorithm, eliminating the tedious and subjective task of selecting a suitable Tikhonov regularization parameter. The developed regularization-optimization algorithm makes MIST a more user-friendly multimodal-retrieval algorithm by eliminating the need for manual parameter selection. We anticipate that our optimization algorithm can also be applied to other image retrieval approaches derived from the Fokker–Planck equation, such as those designed for propagation-based x-ray imaging, as they also involve the unstable inverse Laplacian operator.

Pioneering study on phase measurement and X-ray phase imaging by Talbot interferometry

Recent Developments in X-ray Phase Imaging

X-ray Phase Imaging

ObjectivesPhase-contrast imaging is a novel X-ray based technique that provides enhanced soft tissue contrast. The aim of this study was to evaluate the feasibility of visualizing human carotid arteries by grating-based phase-contrast tomography (PC-CT) at two different experimental set-ups: (i) applying synchrotron radiation and (ii) using a conventional X-ray tube.Materials and MethodsFive ex-vivo carotid artery specimens were examined with PC-CT either at the European Synchrotron Radiation Facility using a monochromatic X-ray beam (2 specimens; 23 keV; pixel size 5.4 µm), or at a laboratory set-up on a conventional X-ray tube (3 specimens; 35-40 kVp; 70 mA; pixel size 100 µm). Tomographic images were reconstructed and compared to histopathology. Two independent readers determined vessel dimensions and one reader determined signal-to-noise ratios (SNR) between PC-CT and absorption images.ResultsIn total, 51 sections were included in the analysis. Images from both set-ups provided sufficient contrast to differentiate individual vessel layers. All PCI-based measurements strongly predicted but significantly overestimated lumen, intima and vessel wall area for both the synchrotron and the laboratory-based measurements as compared with histology (all p<0.001 with slope >0.53 per mm2, 95%-CI: 0.35 to 0.70). Although synchrotron-based images were characterized by higher SNRs than laboratory-based images; both PC-CT set-ups had superior SNRs compared to corresponding conventional absorption-based images (p<0.001). Inter-reader reproducibility was excellent (ICCs >0.98 and >0.84 for synchrotron and for laboratory-based measurements; respectively).ConclusionExperimental PC-CT of carotid specimens is feasible with both synchrotron and conventional X-ray sources, producing high-resolution images suitable for vessel characterization and atherosclerosis research.

Mammographic screening has played a pivotal role in reducing mortality rates associated with breast cancer. Despite an overall decline of breast cancer death rates of 43% from 1989 through 2020, breast cancer remains a predominant form of invasive cancer among women. Conventional X-ray imaging used in mammography can yield limited contrast when it comes to substances with similar X-ray attenuation, such as that between benign and cancerous tissue. Mesh-based X-ray phase imaging allows for higher contrast and resolution in similar-density materials. This thesis aims to explore characteristics of the mesh used in this kind of system.To investigate the significance of mesh orthogonality in mesh-based phase imaging, simulations were conducted. Three types of meshes were compared: an orthogonal mesh and oblique meshes with deviations from the horizontal or vertical axes. The use of mesh shifting demonstrated a significant enhancement in phase retrieval within the imaging systems that employed non-orthogonal meshes when compared to the trials conducted without mesh shifting. Next, two meshes of different compositions, stainless steel and nickel copper alloy meshes, were used in a biological phase imaging set-up. Theoretically, a higher-atomic-number mesh would produce a stronger absorption gradient, resulting in better phase images. However, noticeable, periodic artifacts were observed within the alloy mesh phase images that appeared rotated to the same degree as the mesh. Further investigation into other processing techniques might be advantageous to reduce artifacts.