Optimization of phase contrast imaging using hard x rays

Abstract

X ray radiography and tomography are important tools in medicine as well as in life science and materials science. Not long ago an approach called in-line holography based on simple propagation became possible using partially coherent synchrotron beams like the ones available at the European Synchrotron Radiation Facility (ESRF). Theoretical and experimental work by Cloetens et al. [Appl. Phys. Lett 75, 2912 (1999)] have shown that quantitative retrieval of the optical phase, from a set of radiographs taken at different sample-to-detector distances, is feasible. Mathematically speaking we are dealing with a direct method based on linearization in order to solve an inverse nonlinear problem. The phase retrieval can be combined with classical tomography in order to obtain a three-dimensional representation of the object’s electron density (holotomography). In order to optimize the image contrast for the numerical phase retrieval process, we have carried out calculations resulting in an optimized choice of value and number of the sample-to-detector distances as well as of the photon energy. These results were then confirmed by experiments on the ESRF long beamline ID19.

Phase retrieval for object and probe in the optical near-field

“…The flat field of the illumination does not need to be known as near-field ptychography does not rely on the faulty flat field correction. However, in the case of a clean probe, the flat field correction is approximately correct and high quality reconstructions of the object (without the probe) can also be obtained by quicker single-step techniques that are for example based on the contrast transfer function (CTF) [11,[95][96][97]. A flat field corrected and Fourier transformed hologram F [I ∆ ] (q x , q y ) which was recorded at some distance ∆ with respect to the sample can be approximately described by [95] F [I ∆ ] (q x , q y ) ≈ 2πδ D (q x , q y ) + 2F [φ] (q x , q y ) sin(X ) , (5.361)…”

Three-dimensional single-cell imaging with X-ray waveguides in the holographic regime

“…in the holographic regime which is favoured in view of its stronger phase effects (contrast) in the measured image, another phase-retrieval approach is commonly used, which also relies on Fourier filtering (Zabler et al, 2005;Cloetens et al, 2006;Langer et al, 2008). It is based on the contrast transfer function (CTF) (Guigay, 1977), which provides a linearization of the image formation also for smaller Fresnel numbers as long as the objects are 'weak enough', i.e.…”

Combining Coherent Hard X-Ray Tomographies with Phase Retrieval to Generate Three-Dimensional Models of Forming Bone

“…Samples were imaged with a high flux monoenergetic beam of 26 keV using an effective pixel size of 0.647 µm. Multiple scans of 3999 images were recorded for each sample, with an exposure time of 100 ms. Five sampleto-detector distances (d1 = 10 mm, d2 = 16 mm, d3 = 33 mm, d4 = 84 mm, and d5 = 101 mm) were used to maximize phase contrast in preparation for computation phase retrieval into 3D reconstructed holotomography datasets (Zabler et al, 2005). The energy chosen was sufficiently high to circumvent visible radiation degradation of the sample (e.g., bubble and soft tissue motion), due to exceptionally low absorption.…”

“…While phase contrast imaging is long known to be very sensitive to small density objects in the beam (Cloetens et al, 1999b), quantification of the X-ray interaction with the material is not straightforward, requiring integration of results from multiple different measurements to resolve ambiguities. This is needed because not all structural features are recorded in a single tomographic dataset due to missing spatial frequencies in the projected radiographic images (known as missing frequencies in the Fresnel propagation transfer function) (Zabler et al, 2005). Consequently, several (at least two) projection images have to be captured per tomographic rotation angle, using measurements obtained at different sample-to-detector distances (Langer et al, 2010a;Langer and Peyrin, 2016).…”