X-ray phase imaging-From static observation to dynamic observation-

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.

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.

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.

Conventional X-ray absorption contrast imaging does not depict soft tissues, such as cartilage, in sufficient detail. For visualization of the soft tissues, X-ray phase-contrast imaging is more sensitive than absorption-contrast imaging. The basic concept of the X-ray phase-contrast imaging used in this study is similar to that of differential interference contrast (Nomarski) microscopy. We applied Talbot-Lau X-ray interferometry to visualize the joint structures in the right hand and knee of a donated cadaver. This imaging system simultaneously produced three different types of images: an absorption image, a differential phase image, and a visibility image. The interface between the articular cartilage of the metacarpo-phalangeal joint and fluid or the bony cortex was clearly demonstrated on the differential phase image, whereas this interface was unclear on the absorption image. Within the knee joint, the surface of the articular cartilage was demonstrated both on the differential phase and visibility images; the medial collateral ligament and medial meniscus were also visualized successfully. These results are clinically significant for the diagnosis and therapeutic estimation of rheumatoid arthritis and related joint diseases. This feasibility study on the clinical application of this imaging tool was a collaborative effort of researchers in the fields of physics, radiology, and gross anatomy.

X-ray phase imaging reaching clinical uses.

Conventional x-ray imaging relies on differences in attenuation in a material to produce image contrast. While useful for differentiating structures with large density variations, the subtle differences in attenuation for low-density materials can be difficult to detect. X-ray phase imaging, on the other hand, relies on differences in phase delay which is typically several orders of magnitude larger than attenuation. However, most methods of producing x-ray phase images rely on specialized synchrotron sources, small and low power microfocus sources or the careful alignment of several precision gratings. We demonstrate that focusing polycapillary optics can produce small focal spots from conventional x-ray sources to enable phase imaging. Moreover, in conjunction with focusing optics, the use of a simple, low cost wire mesh to structure the beam can significantly improve phase reconstructions.

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.

The X-ray radiographic imaging technique is very important in medical, biological, inspection, material science, and other fields. However, it is not enough to obtain a clear X-ray image of samples with low absorbance materials, such as biological soft tissues. Then, we have used an X-ray phase-imaging method of an X-ray Talbot interferometer. In this method, X-ray gratings were required to have a narrow pitch and high aspect ratio structure. Therefore, we have developed and fabricated high-aspect-ratio X-ray gratings with a pitch of , a height of , and a large effective area of using X-ray lithography and narrow electroforming techniques. In this paper, we discuss the fabrication process of X-ray gratings with a narrow pitch and high-aspect-ratio structure, and results of X-ray phase tomography using an X-ray Talbot interferometer with these X-ray gratings.

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.

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.

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.

Interferometric temperature measurement around an isolated boiling bubble was conducted for studying heat transfer from liquid phase to the bubble. Finite fringe interferogram was captured with Mach-Zehnder type interferometer and a high-speed camera. Brightness distribution in the interferogram was converted to series wrapped phase image and axisymmetric differential temperature image through wrapped phase image, differential phase image, unwrapped phase image, and differential refractive index image by the Hilbert transform and Abel inversion. The resulted differential temperature image shows that the superheated liquid layer on the heating surface is lifted up and stretched by the growing bubble, and that hot liquid flow is accompanied by departed bubble.

Grating-based x-ray phase-contrast imaging is a fast developing new modality not only for medical imaging, but as well for other fields such as material sciences. While these many possible applications arise, the knowledge of the noise behavior is essential. In this work, the authors used a least squares fitting algorithm to calculate the noise behavior of the three quantities absorption, differential phase, and dark-field image. Further, the calculated error formula of the differential phase image was verified by measurements. Therefore, a Talbot interferometer was setup, using a microfocus x-ray tube as source and a Timepix detector for photon counting. Additionally, simulations regarding this topic were performed. It turned out that the variance of the reconstructed phase is only dependent of the total number of photons used to generate the phase image and the visibility of the experimental setup. These results could be evaluated in measurements as well as in simulations. Furthermore, the correlation between absorption and dark-field image was calculated. These results provide the understanding of the noise characteristics of grating-based phase-contrast imaging and will help to improve image quality.

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

Recent Developments in X-ray Phase Imaging