Simon Rutishauser

High-resolution tomographic imaging of a human cerebellum: comparison of absorption and grating-based phase contrast

Human brain tissue belongs to the most impressive and delicate three-dimensional structures in nature. Its outstanding functional importance in the organism implies a strong need for brain imaging modalities. Although magnetic resonance imaging provides deep insights, its spatial resolution is insufficient to study the structure on the level of individual cells. Therefore, our knowledge of brain microstructure currently relies on two-dimensional techniques, optical and electron microscopy, which generally require severe preparation procedures including sectioning and staining. X-ray absorption microtomography yields the necessary spatial resolution, but since the composition of the different types of brain tissue is similar, the images show only marginal contrast. An alternative to absorption could be X-ray phase contrast, which is known for much better discrimination of soft tissues but requires more intricate machinery. In the present communication, we report an evaluation of the recently developed X-ray grating interferometry technique, applied to obtain phase-contrast as well as absorption-contrast synchrotron radiation-based microtomography of human cerebellum. The results are quantitatively compared with synchrotron radiation-based microtomography in optimized absorption-contrast mode. It is demonstrated that grating interferometry allows identifying besides the blood vessels, the stratum moleculare, the stratum granulosum and the white matter. Along the periphery of the stratum granulosum, we have detected microstructures about 40 µm in diameter, which we associate with the Purkinje cells because of their location, size, shape and density. The detection of individual Purkinje cells without the application of any stain or contrast agent is unique in the field of computed tomography and sets new standards in non-destructive three-dimensional imaging.

Exploring the wavefront of hard X-ray free-electron laser radiation

The high photon flux and femtosecond pulse duration of hard X-ray free-electron lasers have spurred a large variety of novel and fascinating experiments in physical, chemical and biological sciences. Many of these experiments depend fundamentally on a clean, well-defined wavefront. Here we explore the wavefront properties of hard X-ray free-electron laser radiation by means of a grating interferometer, from which we obtain shot-to-shot wavefront information with an excellent angular sensitivity on the order of ten nanoradian. The wavefront distortions introduced by optical elements are observed in-situ and under operational conditions. The source-point position and fluctuations are measured with unprecedented accuracy in longitudinal and lateral direction, both during nominal operation and as the X-ray free-electron laser is driven into saturation.

Recent developments in X-ray Talbot interferometry at ESRF-ID19

In this paper we describe the design of different X-ray Talbot interferometers that have been built at the tomography beamline ID19 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and give a short review of performance characteristics, of current developments, and of the results obtained with these instruments so far. Among the applications so far, soft-tissue imaging has been a particular focus, as demonstrated in a recent paper by Schulz et al. (J. Roy. Soc. Interface, in press).

Sensitivity of X-ray grating interferometry

It is known that the sensitivity of X-ray phase-contrast grating interferometry with regard to electron density variations present in the sample is related to the minimum detectable refraction angle. In this article a numerical framework is developed that allows for a realistic and quantitative determination of the sensitivity. The framework is validated by comparisons with experimental results and then used for the quantification of several influences on the sensitivity, such as spatial coherence or the number of phase step images. In particular, we identify the ideal inter-grating distance with respect to the highest sensitivity for parallel beam geometry. This knowledge will help to optimize existing synchrotron-based grating interferometry setups.

X-ray wavefront characterization using a rotating shearing interferometer technique

A fast and accurate method to characterize the X-ray wavefront by rotating one of the two gratings of an X-ray shearing interferometer is described and investigated step by step. Such a shearing interferometer consists of a phase grating mounted on a rotation stage, and an absorption grating used as a transmission mask. The mathematical relations for X-ray Moiré fringe analysis when using this device are derived and discussed in the context of the previous literature assumptions. X-ray beam wavefronts without and after X-ray reflective optical elements have been characterized at beamline B16 at Diamond Light Source (DLS) using the presented X-ray rotating shearing interferometer (RSI) technique. It has been demonstrated that this improved method allows accurate calculation of the wavefront radius of curvature and the wavefront distortion, even when one has no previous information on the grating projection pattern period, magnification ratio and the initial grating orientation. As the RSI technique does not require any a priori knowledge of the beam features, it is suitable for routine characterization of wavefronts of a wide range of radii of curvature.

At-wavelength characterization of refractive x-ray lenses using a two-dimensional grating interferometer

We report on the application of a two-dimensional hard x-ray grating interferometer to x-ray optics metrology. The interferometer is sensitive to refraction angles in two perpendicular directions with a precision of 10 nrad. It is used to observe the wavefront changes induced by a single parabolic beryllium focusing lens of large radius of curvature. The lens shape is reconstructed and its residual aberrations are analyzed. Its profile differs from an ideal parabolic shape by less than 2 μm or λ/50 at λ = 0.54 A wavelength.

Comparison of two x-ray phase-contrast imaging methods with a microfocus source

We present a comparison for high-resolution imaging with a laboratory source between grating-based (GBI) and propagation-based (PBI) x-ray phase-contrast imaging. The comparison is done through simulations and experiments using a liquid-metal-jet x-ray microfocus source. Radiation doses required for detection in projection images are simulated as a function of the diameter of a cylindrical sample. Using monochromatic radiation, simulations show a lower dose requirement for PBI for small object features and a lower dose for GBI for larger object features. Using polychromatic radiation, such as that from a laboratory microfocus source, experiments and simulations show a lower dose requirement for PBI for a large range of feature sizes. Tested on a biological sample, GBI shows higher noise levels than PBI, but its advantage of quantitative refractive index reconstruction for multi-material samples becomes apparent.

A sensitive x-ray phase contrast technique for rapid imaging using a single phase grid analyzer

Phase contrast x-ray imaging (PCXI) is a promising imaging modality, capable of sensitively differentiating soft tissue structures at high spatial resolution. However, high sensitivity often comes at the cost of a long exposure time or multiple exposures per image, limiting the imaging speed and possibly increasing the radiation dose. Here, we demonstrate a PCXI method that uses a single short exposure to sensitively capture sample phase information, permitting high speed x-ray movies and live animal imaging. The method illuminates a checkerboard phase grid to produce a fine grid-like intensity reference pattern at the detector, then spatially maps sample-induced distortions of this pattern to recover differential phase images of the sample. The use of a phase grid is an improvement on our previous absorption grid work in two ways. There is minimal loss in x-ray flux, permitting faster imaging, and, a very fine pattern is produced for homogenous high spatial resolution. We describe how this pattern permits retrieval of five images from a single exposure; the sample phase gradient images in the horizontal and vertical directions, a projected phase depth image, an edge-enhanced image, and a type of scattering image. Finally, we describe how the reconstruction technique can achieve subpixel distortion retrieval and study the behavior of the technique in regard to analysis technique, Talbot distance, and exposure time.

Single-shot analysis of hard x-ray laser radiation using a noninvasive grating spectrometer

We present a spectrometer setup based on grating dispersion for hard x-ray free-electron lasers. This setup consists of a focusing spectrometer grating and a charge-integrating microstrip detector. Measurement results acquired at Linac Coherent Light Source are presented, demonstrating noninvasive monitoring of single-shot spectra with a resolution of 2.0×10(-4) ±0.3×10(-4) at photon energy of 6 keV with more than 95% transmission of the main beam.

X-ray submicrometer phase contrast imaging with a Fresnel zone plate and a two dimensional grating interferometer

The application of a two dimensional (2D) grating interferometer-Fresnel zone plate combination for quantitative submicron phase contrast imaging is reported. The combination of the two optical elements allows quick recovery of the phase shift introduced by a sample in a hard X-ray beam, avoiding artifacts observed when using the one dimensional (1D) interferometer for a sample with features oriented in the unsensitive direction of the interferometer. The setup provides submicron resolution due to the optics magnification ratio and a fine sensitivity in both transverse orientations due to the 2D analysis gratings. The method opens up possibilities for sub-micro phase contrast tomography of microscopic objects made of light and/or homogeneous materials with randomly oriented features.