Frieder J. Koch

Experimental Realisation of High-sensitivity Laboratory X-ray Grating-based Phase-contrast Computed Tomography

The possibility to perform high-sensitivity X-ray phase-contrast imaging with laboratory grating-based phase-contrast computed tomography (gbPC-CT) setups is of great interest for a broad range of high-resolution biomedical applications. However, achieving high sensitivity with laboratory gbPC-CT setups still poses a challenge because several factors such as the reduced flux, the polychromaticity of the spectrum, and the limited coherence of the X-ray source reduce the performance of laboratory gbPC-CT in comparison to gbPC-CT at synchrotron facilities. In this work, we present our laboratory X-ray Talbot-Lau interferometry setup operating at 40 kVp and describe how we achieve the high sensitivity yet unrivalled by any other laboratory X-ray phase-contrast technique. We provide the angular sensitivity expressed via the minimum resolvable refraction angle both in theory and experiment, and compare our data with other differential phase-contrast setups. Furthermore, we show that the good stability of our high-sensitivity setup allows for tomographic scans, by which even the electron density can be retrieved quantitatively as has been demonstrated in several preclinical studies.

In-vivo X-ray Dark-Field Chest Radiography of a Pig

X-ray chest radiography is an inexpensive and broadly available tool for initial assessment of the lung in clinical routine, but typically lacks diagnostic sensitivity for detection of pulmonary diseases in their early stages. Recent X-ray dark-field (XDF) imaging studies on mice have shown significant improvements in imaging-based lung diagnostics. Especially in the case of early diagnosis of chronic obstructive pulmonary disease (COPD), XDF imaging clearly outperforms conventional radiography. However, a translation of this technique towards the investigation of larger mammals and finally humans has not yet been achieved. In this letter, we present the first in-vivo XDF full-field chest radiographs (32 × 35 cm2) of a living pig, acquired with clinically compatible parameters (40 s scan time, approx. 80 µSv dose). For imaging, we developed a novel high-energy XDF system that overcomes the limitations of currently established setups. Our XDF radiographs yield sufficiently high image quality to enable radiographic evaluation of the lungs. We consider this a milestone in the bench-to-bedside translation of XDF imaging and expect XDF imaging to become an invaluable tool in clinical practice, both as a general chest X-ray modality and as a dedicated tool for high-risk patients affected by smoking, industrial work and indoor cooking.

Large field-of-view tiled grating structures for X-ray phase-contrast imaging

X-ray grating-based interferometry promises unique new diagnostic possibilities in medical imaging and materials analysis. To transfer this method from scientific laboratories or small-animal applications to clinical radiography applications, compact setups with a large field of view (FoV) are required. Currently the FoV is limited by the grating area, which is restricted due to the complex manufacturing process. One possibility to increase the FoV is tiling individual grating tiles to create one large area grating mounted on a carrier substrate. We investigate theoretically the accuracy needed for a tiling process in all degrees of freedom by applying a simulation approach. We show how the resulting precision requirements can be met using a custom-built frame for exact positioning. Precise alignment is achieved by comparing the fringe patterns of two neighboring grating tiles in a grating interferometer. With this method, the FoV can be extended to practically any desired length in one dimension. First results of a phase-contrast scanning setup with a full FoV of 384 mm × 24 mm show the suitability of this method.

X-ray Phase-Contrast Imaging and Metrology through Unified Modulated Pattern Analysis

We present a method for x-ray phase-contrast imaging and metrology applications based on the sample-induced modulation and subsequent computational demodulation of a random or periodic reference interference pattern. The proposed unified modulated pattern analysis (UMPA) technique is a versatile approach and allows tuning of signal sensitivity, spatial resolution, and scan time. We characterize the method and demonstrate its potential for high-sensitivity, quantitative phase imaging, and metrology to overcome the limitations of existing methods.

X-ray grating interferometry at photon energies over 180 keV

We report on the implementation and characterization of grating interferometry operating at an x-ray energy of 183 keV. With the possibility to use this technique at high x-ray energies, bigger specimens could be studied in a quantitative way. Also, imaging strongly absorbing specimens will benefit from the advantages of the phase and dark-field signals provided by grating interferometry. However, especially at these high photon energies the performance of the absorption grating becomes a key point on the quality of the system, because the grating lines need to keep their small width of a couple of micrometers and exhibit a greater height of hundreds of micrometers. The performance of high aspect ratio absorption gratings fabricated with different techniques is discussed. Further, a dark-field image of an alkaline multicell battery highlights the potential of high energy x-ray grating based imaging.

Note: Gratings on low absorbing substrates for x-ray phase contrast imaging

Grating based X-ray phase contrast imaging is on the verge of being applied in clinical settings. To achieve this goal, compact setups with high sensitivity and dose efficiency are necessary. Both can be increased by eliminating unwanted absorption in the beam path, which is mainly due to the grating substrates. Fabrication of gratings via deep X-ray lithography can address this issue by replacing the commonly used silicon substrate with materials with lower X-ray absorption that fulfill certain boundary conditions. Gratings were produced on both graphite and polymer substrates without compromising on structure quality. These gratings were tested in a three-grating setup with a source operated at 40 kVp and lead to an increase in the detector photon count rate of almost a factor of 4 compared to a set of gratings on silicon substrates. As the visibility was hardly affected, this corresponds to a significant increase in sensitivity and therefore dose efficiency.

Increasing the aperture of x-ray mosaic lenses by freeze drying

Point focus x-ray mosaic lenses are limited in aperture by the aspect ratio that can be reached in the micro fabrication process. In lithography based micro fabrication processes, which are used to fabricate the lens pillar structures, the achievable aspect ratio is restricted by structure collapse due to capillary forces which occur during drying after development. Capillary forces can be avoided by freeze drying, hence avoiding the direct phase change from liquid to gas. Substituting conventional drying by freeze drying using cyclohexane at a temperature of  −10 °C, we could increase the achievable aspect ratio for the triangular pillar structures with edge length of 10 to 45 µm of the x-ray mosaic lenses by up to a factor of 2.2 with no further changes in process, material or structural geometry. A maximum aspect ratio of 30 was achieved for pillars with 10 µm edge length. The process can readily be employed to other structures or lithography techniques.

Quantitative characterization of X-ray lenses from two fabrication techniques with grating interferometry

Refractive X-ray lenses are in use at a large number of synchrotron experiments. Several materials and fabrication techniques are available for their production, each having their own strengths and drawbacks. We present a grating interferometer for the quantitative analysis of single refractive X-ray lenses and employ it for the study of a beryllium point focus lens and a polymer line focus lens, highlighting the differences in the outcome of the fabrication methods. The residuals of a line fit to the phase gradient are used to quantify local lens defects, while shape aberrations are quantified by the decomposition of the retrieved wavefront phase profile into either Zernike or Legendre polynomials, depending on the focus and aperture shape. While the polymer lens shows better material homogeneity, the beryllium lens shows higher shape accuracy.

Increasing the field of view of x-ray phase contrast imaging using stitched gratings on low absorbent carriers

X-ray phase contrast imaging has become a promising biomedical imaging technique for enhancing soft-tissue contrast. In addition to an absorption contrast image it provides two more types of image, a phase contrast and a small-angle scattering contrast image recorded at the same time. In biomedical imaging their combination allows for the conventional investigation of e.g. bone fractures on the one hand and for soft-tissue investigation like cancer detection on the other hand. Among the different methods of X-ray phase contrast imaging the grating based approach, the Talbot-Lau interferometry, has the highest potential for commercial use in biomedical imaging at the moment, because commercially available X-ray sources can be used in a compact setup. In Talbot-Lau interferometers, core elements are phase and absorption gratings with challenging specifications because of their high aspect ratios (structure height over width). For the long grating lamellas structural heights of more than 100 μm together with structural width in the micron range are requested. We are developing a fabrication process based on deep x-ray lithography and electroforming (LIGA) to fabricate these challenging structures. In case of LIGA gratings the structural area is currently limited to several centimeters by several centimeters which limit the field of view in grating based X-ray phase contrast imaging. In order to increase the grating area significantly we are developing a stitching method for gratings using a 625 μm thick silicon wafer as a carrier substrate. In this work we compare the silicon carrier with an alternative one, polyimide, for patient dose reduction and for the use at lower energies in terms of transmission and image reconstruction problems.

Large area gratings by x-ray LIGA dynamic exposure for x-ray phase-contrast imaging

Abstract. X-ray differential phase-contrast imaging (DPCI) using a Talbot–Lau interferometer at a conventional tube source has continuously found applications since its first demonstration. It requires high aspect ratio grating structures with a feature size in the micrometer range that are fabricated using lithographie, galvanik und abformung technology. To overcome the current limitation in grating area, an exposure strategy—continuous exposure—has been developed. In this case, the mask is fixed in respect to the synchrotron beam and only the substrate is scanned. Thus, the grating area is given by the scanning length which is much larger than the actual mask size. The design, needs, and tolerances to adopt this process of dynamic exposure will be described. Furthermore, the first tests using this method will be presented. Gratings with a metal aspect ratio of 11 and a period of 10  μm were fabricated on an area of 165  mm×65  mm. First imaging results demonstrate the suitability of this method. No differences in the visibility or in x-ray image compared to gratings fabricated by the standard method could be found.