Dirk Van Dyck

Advanced Electron Microscopy for Advanced Materials

The idea of this Review is to introduce newly developed possibilities of advanced electron microscopy to the materials science community. Over the last decade, electron microscopy has evolved into a full analytical tool, able to provide atomic scale information on the position, nature, and even the valency atoms. This information is classically obtained in two dimensions (2D), but can now also be obtained in 3D. We show examples of applications in the field of nanoparticles and interfaces.

Clinical comparison of 6 aberrometers. Part 1: Technical specifications

Purpose: To provide a detailed assessment of the techniques, technical features, and practical use of 6 aberrometers made available to our institution from September 2002 to January 2004. Setting: Department of Ophthalmology, University Hospital Antwerp, Antwerp, Belgium. Methods: A number of technical and practical parameters are listed for the Visual Function Analyzer (Tracey), the OPD‐scan (ARK 10000; Nidek), the Zywave (Bausch & Lomb), the WASCA (Carl Zeiss Meditec), the MultiSpot Hartmann‐Shack device, and the Allegretto Wave Analyzer including working principles, data acquisition, aberrometer alignment, wavefront calculation, and data analysis. Operator and patient comfort as well as practical advantages and disadvantages are discussed. Conclusion: All devices met at least half the following parameters: alignment, correction for source wavelength, data averaging, measurement quality check, and inhibition of accommodation.

Automated breast tumor diagnosis and grading based on wavelet chromatin texture description

In this paper, wavelets were employed for multi-scale image analysis to extract parameters for the description of chromatin texture in the cytological diagnosis and grading of invasive breast cancer. Their value was estimated by comparing the performance of co-occurrence, densitometric, and morphometric parameters in an automated K-nearest neighbor (Knn) classification scheme based on light microscopic images of isolated nuclei of paraffin-embedded tissue. This design allowed a multifaceted cytological retrospective study of which the practical value can be judged easily. Results show that wavelets perform excellently with classification scores comparable with densitometric and co-occurrence features. Moreover, because wavelets showed a high additive value with the other textural groups, this panel allowed a very profound description with higher recognition scores than previously reported (76% for individual nuclei, 100% for cases). Morphometric parameters performed less well and only slightly increased correct classification. The major drawback, besides image segmentation errors demanding operator supervision, emanated to be the few false-negative cases, which restrict the immediate practical use. However, an enlargement of the parameter set may avoid this misclassification, resulting in an applicable expert system of practical use.

‘Big Bang’ tomography as a new route to atomic-resolution electron tomography

Until now it has not been possible to image at atomic resolution using classical electron tomographic methods, except when the target is a perfectly crystalline nano-object imaged along a few zone axes. The main reasons are that mechanical tilting in an electron microscope with sub-ångström precision over a very large angular range is difficult, that many real-life objects such as dielectric layers in microelectronic devices impose geometrical constraints and that many radiation-sensitive objects such as proteins limit the total electron dose. Hence, there is a need for a new tomographic scheme that is able to deduce three-dimensional information from only one or a few projections. Here we present an electron tomographic method that can be used to determine, from only one viewing direction and with sub-ångström precision, both the position of individual atoms in the plane of observation and their vertical position. The concept is based on the fact that an experimentally reconstructed exit wave consists of the superposition of the spherical waves that have been scattered by the individual atoms of the object. Furthermore, the phase of a Fourier component of a spherical wave increases with the distance of propagation at a known ‘phase speed’. If we assume that an atom is a point-like object, the relationship between the phase and the phase speed of each Fourier component is linear, and the distance between the atom and the plane of observation can therefore be determined by linear fitting. This picture has similarities with Big Bang cosmology, in which the Universe expands from a point-like origin such that the distance of any galaxy from the origin is linearly proportional to the speed at which it moves away from the origin (Hubble expansion). The proof of concept of the method has been demonstrated experimentally for graphene with a two-layer structure and it will work optimally for similar layered materials, such as boron nitride and molybdenum disulphide.

Clinical comparison of 6 aberrometers Part 2: Statistical comparison in a test group

PURPOSE: To compare and mutually validate the measurements of 6 aberrometers: the Visual Function Analyzer (Tracey), the OPD‐Scan (ARK‐10000, Nidek), the Zywave (Bausch & Lomb), the WASCA (Carl Zeiss Meditec), the MultiSpot Hartmann‐Shack device, and the Allegretto Wave Analyzer. SETTING: University Hospital Antwerp, Antwerp, Belgium. METHODS: This prospective study was conducted on a group of 44 healthy eyes with refractions ranging from −5.25 diopters (D) to +5.25 D (cylinder 0 to −2 D). For each aberrometer and each eye, the averaged Zernike data were used to calculate various kinds of root‐mean‐square (RMS). These parameters, together with the refractive parameters, were then analyzed with a repeated‐measures analysis of variance (ANOVA) test, complemented by paired t tests. A similar analysis was done for the comparison of the variances of these parameters. RESULTS: The aberrometers gave comparable values for all studied parameters with the following exceptions: The OPD‐Scan underestimated the polynomials describing 4‐ and 5‐fold symmetries, and the Visual Function Analyzer slightly overestimated the astigmatism terms. The 3rd‐order radial RMS value was different for each device, as well as the RMS in the central 2.0 mm zone. The WASCA presented the lowest variance. CONCLUSION: These results suggest that in healthy eyes, all aberrometers produced globally similar results but they may vary in some details.

Direct structure reconstruction in HRTEM

Abstract In this paper we propose a new method for the interpretation of exit wave functions in electron microscopy. These wave functions can experimentally be obtained by an holographic wave function reconstruction technique such as focal series reconstruction, off-axis holography or ptychography. We aim at the development of an approximate method that allows to visualise the different atomic columns of the structure in terms of the 1S channelling eigenfunctions which are highly localised at the cores of the separate columns. It will be shown that this method yields very accurate lateral information, but that the technique is intrinsically rather insensitive to the chemical composition in the different columns. Nevertheless, this procedure may provide an indispensable tool for the correct interpretation of reconstructed high-resolution electron wave functions.

Machine learning study of several classifiers trained with texture analysis features to differentiate benign from malignant soft-tissue tumors in T1-MRI images

To study, from a machine learning perspective, the performance of several machine learning classifiers that use texture analysis features extracted from soft‐tissue tumors in nonenhanced T1‐MRI images to discriminate between malignant and benign tumors.

Color Texture Classification by Wavelet Energy Correlation Signatures

In the last decade, multiscale techniques for gray-level texture analysis have been intensively studied. In this paper, we aim on extending these techniques to color images. We introduce wavelet energy-correlation signatures and we derive the transformation of these signatures upon linear color space transformations. Classification experiments demonstrate that the wavelet correlation features contain more information than the intensity or the energy features of each color plane separately. The influence of image representation in color space is evaluated.

Exit wave reconstructions using through focus series of HREM images

The through focus exit wave reconstruction technique uses a series of high resolution electron microscopy images to reconstruct the complex electron wavefunction at the exit plane of the specimen. The feasibility of the through focus exit wave reconstruction method and its most important limitations are discussed. It is shown that—provided all aberrations of the microscope are well corrected for—a large improvement in the interpretability of the information can be obtained. Microsc. Res. Tech. 49:301–323, 2000.

Accurate multislice theory for elastic electron scattering in transmission electron microscopy

Abstract Combining the concept of the slice-transmission-operator matrix with the transfer matrix technique, the complete Schrodinger equation for elastic electron scattering in solids is rigorously solved within the framework of the multislice scheme. This new approach extends the range in which the multislice theory is accurate. It is shown that the multislice theory can in principle be as accurate as the Bloch wave theory and the Green function theory in all cases of elastic electron scattering. It is pointed out that in some cases such as the calculation of high-order Laue zone (HOLZ) reflections, more accurate corrections should be made for the conventional high-energy approximation and therefore the conventional multislice method (CMS). A new multislice formula is developed for calculating the transmitted wave. This formula takes back-scattering effects into account and may therefore be accurate for large beam tilt. It is also shown that the new multislice approach keeps all the advantages of the CMS with respect to the Bloch wave method.