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Concept

Phase-contrast imaging

Résumé

Phase-contrast imaging is a method of that has a range of different applications. It measures differences in the refractive index of different materials to differentiate between structures under analysis. In conventional light microscopy, phase contrast can be employed to distinguish between structures of similar transparency, and to examine crystals on the basis of their double refraction. This has uses in biological, medical and geological science. In X-ray tomography, the same physical principles can be used to increase image contrast by highlighting small details of differing refractive index within structures that are otherwise uniform. In transmission electron microscopy (TEM), phase contrast enables very high resolution (HR) imaging, making it possible to distinguish features a few Angstrom apart (at this point highest resolution is 40 pm). Atomic Physics Phase Contrast imaging is commonly used in atomic physics to describe a range of techniques for dispersively imag

Source officielle

https://en.wikipedia.org/wiki/Phase-contrast_imaging

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The course deals with the concept of measuring in different domains, particularly in the electrical, optical, and microscale domains. The course will end with a perspective on quantum measurements, which could trigger the ultimate revolution in metrology.

Sample preparation and direct observation techniques (optical microscopy, AFM, electron microscopy) and their practical application to the study of morphology and microdeformation in polymers.

Introduction and application of photon based tools for chemical sciences: from basic concepts to optical and x-ray lasers

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High resolution differential laser interferometry for the VLTI (Very Large Telescope Interferometer)

Olivier Boris Scherler

One method for locating extrasolar planets is to observe the lateral movement of a star in the sky caused by a planet in orbit around it. In order to detect this displacement, the angular position of the star has to be measured with high accuracy. This technique is called astrometry. The Very Large Telescope Interferometer (VLTI) is operated by the European Southern Observatory and located at the Paranal Observatory in Chile. The purpose of the PRIMA instrument (Phase Referenced Imaging and Micro-arcsecond Astrometry) of the VLTI is to perform high-resolution astrometric measurements and high-resolution imaging of faint stars using white light interferometry, by combining the light collected by two telescopes. In order to allow the detection of extrasolar planets, the astrometric measurement has to be performed with micro-arcsecond accuracy. In astrometric mode the PRIMA instrument observes two targets at the same time: the object of scientific interest, and a bright reference star. The angular position of the science object relative to the reference star is obtained by monitoring the differential optical path travelled by the light of each object in two separate white-light interferometers. The aim of this work was to develop a high-resolution laser metrology based on superheterodyne interferometry, with an accuracy of 5 nm over a differential optical path of 100 mm. Moreover the laser source had to be stabilised on an absolute frequency reference, in order to ensure the long-term stability and calibration required to achieve the target performance. Superheterodyne interferometry allowed the direct measurement of the differential optical path using two heterodyne interferometers working with two different frequency shifts. The differential phase measurement between the two interferometers was obtained by electronic mixing of the two heterodyne signals, leading to the differential optical path needed for the astrometric measurement

Institut de Microtechnique, Université de Neuchâtel2006

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Structure determination techniques are an essential tool to investigate and understand molecules, especially in biology, where functionality crucially depends on structure. The dominant high-resolution methods, such as transmission electron microscopy and X-ray diffraction, rely on ensemble averaging, making them unsuitable for evidencing structural variations of flexible molecules. Common single-molecule imaging techniques like scanning probe microscopy achieve high resolution but lack access to 3D objects. A possible way to overcome these limitations is offered by low-energy electron holography (LEEH). So far, LEEH achieved molecular imaging at resolutions in the range of 7-8 Å on the single-molecule level. A resolution in the range of a few Å, which would give access to atomic details of molecular structures, is in theory attainable. Obtaining a resolution near the theoretical limit requires a fully optimized LEEH setup and workflow, for which in particular sample and emitter preparation are crucial. Here, I present a novel LEEH microscope with a unique experimental workflow, which is capable of resolving molecular substructures with an unprecedented resolution for LEEH experiments. The newly constructed LEEH microscope takes advantage of an efficient vibration isolation system and is remote-controlled to minimize external perturbations. The setup is capable of low-temperature (50 K) measurements, which enhances the stability of the system. A reliable way for preparing highly coherent electron emitters constituted by sharp tungsten tips is presented along with different methods for improving their performance via functionalization and UHV treatments. I show an optimized and reliable substrate preparation protocol resulting in ultra-clean free-standing single-layer graphene (SLG) samples. Native electrospray ion beam deposition (ES-IBD) allows for the transfer of non-volatile molecules from solution into gas phase and onto the substrate. By employing a quadrupole mass selection, highly pure molecular ion beams are obtained, which are soft-landed onto the SLG substrate in a controlled and reproducible manner while retaining an intact structure. This versatile preparation method allows us to study a great variety of molecules by LEEH. Data on proteins with masses ranging from 12-820 kDa reveal molecular shapes consistent with model structures. Locally, structural details as small as 5 Å were identified, thus we are approaching our goal of a resolution in the range of a few Å. The application of our methodology to flexible proteins, exemplified by antibodies, allowed for the imaging of their conformational variability and for determining the influence of the sample preparation on their structure. This gave us the possibility to directly image gas-phase related conformations. Data of compact molecular systems with masses below 3 kDa reveal that our LEEH microscope is able to investigate small molecules, such as porphyrines, at high imaging contrast. The presented results indicate that an optimized LEEH setup and workflow in combination with ES-IBD is a versatile microscopy method, which is filling the gap between high-resolution and single-molecule techniques by elucidating structural details of various molecular species.

EPFL2022

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An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

Diana Khabipova, José Pedro Rebelo Ferreira Marques

Phase imaging benefits from strong susceptibility effects at very high field and the high signal-to-noise ratio (SNR) afforded by multi-channel coils. Combining the information from coils is not trivial, however, as the phase that originates in local field effects (the source of interesting contrast) is modified by the inhomogeneous sensitivity of each coil. This has historically been addressed by referencing individual coil sensitivities to that of a volume coil, but alternative approaches are required for ultra-high field systems in which no such coil is available. An additional challenge in phase imaging is that the phase that develops up to the echo time is " wrapped" into a range of 2p radians. Phase wraps need to be removed in order to reveal the underlying phase distribution of interest. Beginning with a coil combination using a homogeneous reference volume coil -the Roemer approach -which can be applied at 3 T and lower field strengths, we review alternative methods for combining single-echo and multi-echo phase images where no such reference coil is available. These are applied to high-resolution data acquired at 7 T and their effectiveness assessed via an index of agreement between phase values over channels and the contrast-to-noise ratio in combined images. The virtual receiver coil and COMPOSER approaches were both found to be computationally efficient and effective. The main features of spatial and temporal phase unwrapping methods are reviewed, placing particular emphasis on recent developments in temporal phase unwrapping and Laplacian approaches. The features and performance of these are illustrated in application to simulated and high-resolution in vivo data. Temporal unwrapping was the fastest of the methods tested and the Laplacian the most robust in images with low SNR. (C) 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Wiley2017

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High resolution differential laser interferometry for the VLTI (Very Large Telescope Interferometer)

Olivier Boris Scherler

Institut de Microtechnique, Université de Neuchâtel2006

An illustrated comparison of processing methods for MR phase imaging and QSM: combining array coil signals and phase unwrapping

Diana Khabipova, José Pedro Rebelo Ferreira Marques

Wiley2017

EPFL2022