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Background Insufficient physical activity (PA) is a risk factor for obesity and non-communicable diseases and seems more prevalent among socioeconomically disadvantaged (SED) adults. Recreational walking (RW) is an important type of leisure time PA which can be done for free and without specific equipment or facilities. Environmental factors influencing PA may be particularly important for SED adults who are more reliant on their direct environment. The objectively measured environment is not always congruent with the perception of residents, and this may differ by socioeconomic group. This study aims to identify the local environmental factors important for RW as perceived by SED adults. Methods This study is part of the CIVISANO project. Purposeful convenience sampling was used to recruit 38 SED adults (25-65 y/o) in two Flemish semi-urban municipalities. Individual walk-along interviews have been performed in the participants’ neighborhood. 20 participants joined the focus group discussion (n = 4) that were intended for member checking, to categorize identified environmental factors and to identify local actions to promote recreational walking. MaxQDA was used for content analysis. Results The way the environment is perceived by SED adults plays an important role in their RW behavior. Results demonstrate the interrelation of different environmental types (physical, sociocultural, economic, political and information) and sizes (micro, meso, macro). Improving communication and knowledge transfer, stimulation of physical, sociocultural, and economic accessibility, and promotion of physical and social safety are identified most important action points for the local government to facilitate RW. Conclusions Our findings indicate that the perceived local environment can play an important role in promoting RW among SED adults. Future studies should investigate if changes in these environments and in residents’ perception lead to changes in RW behavior of SED adults. Key messages

We report a study of scattering dynamics in crystals employing momentum-resolved scanning transmission electron microscopy under varying illumination conditions. As we perform successive changes of the probe focus, multiple real-space signals are obtained in dependence of the shape of the incident electron wave. With support from extensive simulations, each signal is shown to be characterised by an optimum focus for which the contrast is maximum and which differs among different signals. For instance, a systematic focus mismatch is found between images formed by high-angle scattering, being sensitive to thickness and chemical composition, and the first moment in diffraction space, being sensitive to electric fields. It follows that a single recording at one specific probe focus is usually insufficient to characterise materials comprehensively. Most importantly, we demonstrate in experiment and simulation that the second moment (mu(20) + mu(02)) = < p(2)> of the diffracted intensity exhibits a contrast maximum when the electron probe is focused at the top and bottom faces of the specimen, making the presented concept attractive for measuring local topography. Given the versatility of < p(2)>, we furthermore present a detailed study of its large-angle convergence both analytically using the Mott scattering approach, and by dynamical simulations using the multislice algorithm including thermal diffuse scattering. Both approaches are in very good agreement and yield logarithmic divergence with increasing scattering angle.

Background The prevalence of obesity worldwide has been steadily increasing. This has generated an uptake in research determining the cause of obesity. One of the primary drivers that have been identified is the “obesogenic environment', which is an environment that contains features that increase excess weight in individuals or populations. Obesogenic environments are unevenly distributed with socio-economically disadvantaged neighbourhoods being more affected. Methods This survey is part of the CIVISANO (‘Community-based participatory research to tackle the disparities in health-enhancing lifestyle within the municipality environment') project, which will be conducted in two municipalities. Respondents will be socio-economically disadvantaged adults between 25- and 65-years old. To gain information on individual characteristics and perceptions of the obesogenic environment, residents will be invited to complete a survey. Since it is known that socio-economically disadvantaged respondents are difficult to reach a non-random active recruitment approach will be applied. The following variables are included in the survey: sociodemographic information, subjective health and anthropometrics, dietary habits, food security and physical activity. Items from the SPOTLIGHT project survey and NEMS-P survey were added to assess respondent's perceptions of features of the obesogenic environment. Additional variables on individual characteristics were based on items from the Belgian National Health Interview Survey of 2019. Results Through the survey, we expect to gain insight into the perceptions of socio-economically disadvantaged adults on obesogenic features of their local environment. Initial results from the survey will be available in the upcoming months. Conclusions The construction of instruments to assess the context-specific features, which can be compared with objective measurements, can increase our knowledge of contextual determinants of the obesogenic environment. Key messages The first Flemish survey that focuses on perceptions of socio-economically disadvantaged adults on obesogenic features of their local environment in the municipal context. Gaining a better understanding of the local obesogenic environment creates an opportunity to promote and strengthen the development of effective local interventions to build healthier environments.

Introduction Parental barriers are a key factor to determine physical activity behaviors, such as active commuting to school in young people. The purpose of this study was to test the reliability and validity of a questionnaire which addresses parental barriers towards active commuting to school. Methods This paper describes the development, following the Delphi method, and the validation of a Likert-scale questionnaire of 23 items (Parental Perception of Barriers towards Active Commuting to School (PABACS)) designed to determine the main parental barriers. These barriers are categorized into 3 scales (general, walking, and cycling barriers) and the overall questionnaire. A total of 207 parents of children and adolescents in Granada (Spain) in 2016 completed the survey on two occasions. The internal consistency was examined using Cronbach's alpha tests and Intra-Class Correlation (ICC) and Kappa index were used to assess the reliability of the different questions. To examine the predictive validity, spearman's rho tests were conducted. Results The questionnaire showed a good internal consistency for the overall questionnaire and for the three scales. The Intra-Class Correlation values were moderate. The overall questionnaire and the general and walking barriers scales showed a moderate validity to predict active modes of commuting, as well as the items ‘distance’ and ‘walking convenience’. Conclusions This study provides a reliable tool to assess parental barriers towards active commuting to school, which will help future studies to identify the most important barriers on which institutions have to focus to increase active commuting to school rates.

Background e- and mHealth interventions using self-regulation techniques like action and coping planning have the potential to tackle the worldwide problem of physical inactivity. However, they often use one-week self-regulation cycles, providing support toward an active lifestyle on a weekly basis. This may be too long to anticipate on certain contextual factors that may fluctuate from day to day and may influence physical activity. Consequently, the formulated action and coping plans often lack specificity and instrumentality, which may decrease effectiveness of the intervention. The aim of this study was to evaluate effectiveness of a self-regulation, app-based intervention called ‘MyDayPlan’. “MyDayPlan’ provides an innovative daily cycle in which users are guided towards more physical activity via self-regulation techniques such as goal setting, action planning, coping planning and self-monitoring of behaviour. Methods An ABAB single-case design was conducted in 35 inactive adults between 18 and 58 years (M = 40 years). The A phases (A1 and A2) were the control phases in which the ‘MyDayPlan’ intervention was not provided. The B phases (B1 and B2) were the intervention phases in which ‘MyDayPlan’ was used on a daily basis. The length of the four phases varied within and between the participants. Each phase lasted a minimum of 5 days and the total study lasted 32 days for each participant. Participants wore a Fitbit activity tracker during waking hours to assess number of daily steps as an outcome. Single cases were aggregated and data were analysed using multilevel models to test intervention effects and possible carry-over effects. Results Results showed an average intervention effect with a significant increase in number of daily steps from the control to intervention phases for each AB combination. From A1 to B1, an increase of 1424 steps (95% CI [775.42, 2072.32], t (1082) = 4.31,p p = .003) were found. Furthermore, the number of daily steps decreased significantly (1134 steps) when going from the first intervention phase (B1) to the second control phase (A2) (95% CI [− 1755.60, − 512.38], t (1082) = − 3.58, p p = .60) and intervention phases (95% CI [− 128.79,284.22], t (1078) = .74, p = .46). This reveals, in contrast to what was hypothesized, no evidence for a carry-over effect after removing the ‘MyDayPlan’ app after the first intervention phase (B1). Conclusion This study adds evidence that the self-regulation mHealth intervention, ‘MyDayPlan’ has the capacity to positively influence physical activity levels in an inactive adult population. Furthermore, this study provides evidence for the potential of interventions adopting a daily self-regulation cycle in general.

Background Electronic health (eHealth) and mobile health (mHealth) interventions have the potential to tackle the worldwide problem of physical inactivity. However, they often suffer from large attrition rates. Consequently, feasibility and acceptability of interventions have become important matters in the creation of e- and mHealth interventions. The aim of this study was to evaluate participants’ opinions regarding acceptability and feasibility of a self-regulation, app-based intervention called ‘MyDayPlan’. ‘MyDayPlan’ provides an innovative daily cycle providing several self-regulation techniques throughout the day that guide users towards an active lifestyle via various self-regulation techniques. Methods Semi-structured interviews were conducted with 20 adults after using the app for 2 weeks. A directed content analysis was performed using NVivo Software. Results ‘MyDayPlan’ was well-received and seems to be feasible and acceptable with inactive adults. The straightforward lay out and ease of use of the app were appreciated. Furthermore, the incorporation of the techniques ‘action planning’, and ‘prompting review of behavioral goals’ was positively evaluated. However, the users gave some recommendations: implementation of activity trackers to self-monitor physical activity could be of added value. Furthermore, increasing intuitiveness by minimizing text input and providing more preprogrammed options could further increase the ease of use. Finally, users indicated that they would benefit from more guidance during the “coping planning” component (barrier identification/problem solving), for example by receiving more tailored examples. Conclusions Based on these findings, adaptations will be made to the ‘MyDayPlan’ app before evaluating its effectiveness. Furthermore, involving potential end users and evaluating acceptability and feasibility during the development of an e- and mHealth intervention is key. Also, creating interventions with a large ease of use and straightforward layout that provides tailored support during action and coping planning is key.

Background Ecological momentary assessment (EMA) is a method of collecting real-time data based on repeated measures and observations that take place in participant’s daily environment. EMA has many advantages over more traditional, retrospective questionnaires. However, EMA faces some challenges to reach its full potential. The aims of this systematic review are to (1) investigate whether and how content validity of the items (i.e. the specific questions that are part of a larger EMA questionnaire) used in EMA studies on physical activity and sedentary behaviour was assessed, and (2) provide an overview of important methodological considerations of EMA in measuring physical activity and sedentary behaviour. Methods Thirty papers (twenty unique studies) were systematically reviewed and variables were coded and analysed within the following 4 domains: (1) Content validity, (2) Sampling approach, (3) Data input modalities and (4) Degree of EMA completion. Results Only about half of the studies reported the specific items (n = 12) and the source of the items (n = 11). None of the studies specifically assessed the content validity of the items used. Only a minority (n = 5) of the studies reported any training, and one tested the comprehensibility of the EMA items. A wide variability was found in the design and methodology of the EMA. A minority of the studies (n = 7) reported a rationale for the used prompt frequency, time selection, and monitoring period. Retrospective assessment periods varied from ‘now’ to ‘in the last 3.5 hours’. In some studies there was a possibility to delay (n = 6) or deactivate (n = 10) the prompt, and some provided reminders after the first prompt (n = 9). Conclusions Almost no EMA studies reported the content validation of the items used. We recommend using the COSMIN checklist (COnsensus-based Standards for the selection of health Measurement INstruments) to report on the content validity of EMA items. Furthermore, as often no rationale was provided for several methodological decisions, the following three recommendations are made. First, provide a rationale for choosing the sampling modalities. Second, to ensure assessment ‘in the moment’, think carefully about the retrospective assessment period, reminders, and deactivation of the prompt. Third, as high completion rates are important for representativeness of the data and generalizability of the findings, report completion rates. Trial registration This review is registered in PROSPERO, the International prospective register of systematic reviews (registration number: CRD42017077996).

Additional file 2. Coding Scheme.

Additional file 1. Interview guide.

Additional file 1: Table with coded information from the included studies.

Additional file 3. Screenshots of ‘MyDayPlan’.

Background Physical inactivity is a global pandemic associated with a high burden of disease and premature mortality. There is also a trend in growing economic inequalities which impacts population health. There is no global analysis of the relationship between income inequality and population levels of physical inactivity. Methods Two thousand sixteen World Health Organisation’s country level data about compliance with the 2010 global physical activity guidelines were analysed against country level income interquantile ratio data obtained from the World Bank, OECD and World Income Inequality Database. The analysis was stratified by country income (Low, Middle and High) according to the World Bank classification and gender. Multiple regression was used to quantify the association between physical activity and income inequality. Models were adjusted for GDP and percentage of GDP spent on health care for each country and out of pocket health care spent. Results Significantly higher levels of inactivity and a wider gap between the percentage of women and men meeting global physical activity guidelines were found in countries with higher income inequality in high and middle income countries irrespective of a country wealth and spend on health care. For example, in higher income countries, for each point increase in the interquantile ratio data, levels of inactivity in women were 3.73% (CI 0.89 6.57) higher, levels of inactivity in men were 2.04% (CI 0.08 4.15) higher and the gap in inactivity levels between women and men was 1.50% larger (CI 0.16 2.83). Similar relationships were found in middle income countries with lower effect sizes. These relationships were, however, not demonstrated in the low-income countries. Conclusions Economic inequalities, particularly in high- and middle- income countries might contribute to physical inactivity and might be an important factor to consider and address in order to combat the global inactivity pandemic and to achieve the World Health Organisation target for inactivity reduction.

Additional file 2: Adapted CREMAS checklist.

Additional file 1: Fig. S1: Relationship in low income countries between within country income inequality and percentage of people inactive within a country. Country code are presented and can be cross-referenced with real names in Table S1. The linear raw association is presented with 95% CI ribbon. Fig. S2: Relationship in middle income countries between within country income inequality and percentage of people inactive within a country. Country code are presented and can be cross-referenced with real names in Table S1. The linear raw association is presented with 95% CI ribbon. Fig. 3: Relationship in high income countries between within country income inequality and percentage of people inactive within a country. Country code are presented and can be cross-referenced with real names in Table S1. The linear raw association is presented with 95% CI ribbon. Table S1: Country code and name reference table. Table S2: Results of sensitivity analysis.

Additional file 3: PRISMA Checklist.

To investigate how healthcare buildings, especially hospitals, need to be designed to take up an active role in patient mobilisation and as such contribute to patient recovery, we are in need of a research approach to map patients’ physical activity in relation to the (indoor) built environment. Tracking participants’ physical activity is an important part of this as it allows to collect objective data on the kind, duration and intensity of movement which can then be discussed in relation to the built environment. The use of activity trackers can thus be considered a relevant method as part of a larger research approach. We illustrate how self-reflection can add to set up activity tracking through registering with and experiencing of wearing an activity tracker (Axivity). To conclude advantages and limitations of the self-reflection process are discussed in relation to the complex context of healthcare settings.

The main features and the GPU implementation of the MULTEM program are presented and described. This new program performs accurate and fast multislice simulations by including higher order expansion of the multislice solution of the high energy Schrodinger equation, the correct subslicing of the three-dimensional potential and top-bottom surfaces. The program implements different kinds of simulation for CTEM, STEM, ED, PED, CBED, ADF-TEM and ABF-HC with proper treatment of the spatial and temporal incoherences. The multislice approach described here treats the specimen as amorphous material which allows a straightforward implementation of the frozen phonon approximation. The generalized transmission function for each slice is calculated when is needed and then discarded. This allows us to perform large simulations that can include millions of atoms and keep the computer memory requirements to a reasonable level. (C) 2015 Elsevier B.V. All rights reserved.

This paper describes an approach to retrieve the three-dimensional atomic structure of a nanocrystalline particle from the reconstructed electron exit wave function in a single projection direction. The method employs wave propagation to determine the local exit surface of each atomic column together with its mass. The exit wave in between colums is used as internal calibration so as to remove the background noise and improve the precision to the level of single atom sensitivity. The validity of the approach is tested with exit wave functions of a gold wedge reconstructed from simulated images containing different levels of noise. (C) 2014 Elsevier Ltd. All rights reserved.

We describe the state of the art of the research towards ultrasonic measurement methods for use in lead-bismuth cooled liquid metal reactors. Our current research activities are highly focused on specific tasks in the MYRRHA system, which is a fast spectrum research reactor cooled with the eutectic mixture of lead and bismuth (LBE) and is conceived as an accelerator driven system capable of operating in both sub-critical and critical mode. As liquid metal is opaque to light, normal visual feedback during fuel manipulations in the reactor vessel is not available and must therefore be replaced by a system that is not hindered by the opacity of the coolant. In this respect ultrasonic measurement techniques have been proposed and even developed in the past for operation in sodium cooled reactors. To our knowledge, no such systems have ever been deployed in lead based reactors and we are the first to have a research program in this direction as will be detailed in this paper. We give an overview of the acoustic properties of LBE and compare them with the properties of sodium and water to theoretically show the feasibility of ultrasonic systems operating in LBE. In the second part of the paper we discuss the results of the validation experiments in water and LBE. A typical scene is ultrasonically probed by a mechanical scanning system while the signals are processed to render a 3D visualization on a computer screen. It will become clear that mechanical scanning is capable of producing acceptable images but that it is a time consuming process that is not fit to solve the initial task to providing feedback during manipulations in the reactor vessel. That is why we propose to use several dedicated ultrasonic systems each adapted to a specific task and capable to provide real-time feedback of the ongoing manipulations, as is detailed in the third and final part of the paper.

In this paper, we investigate how precise atoms of a small nanocluster can ultimately be located in three dimensions (3D) from a tilt series of images acquired using annular dark field (ADF) scanning transmission electron microscopy (STEM). Therefore, we derive an expression for the statistical precision with which the 3D atomic position coordinates can be estimated in a quantitative analysis. Evaluating this statistical precision as a function of the microscope settings also allows us to derive the optimal experimental design. In this manner, the optimal angular tilt range, required electron dose, optimal detector angles, and number of projection images can be determined.

It is now established that the 3D structure of homogeneous nanocrystals can be recovered from in-line hologram of single projections. The method builds on a quantitative contrast interpretation of electron exit wave functions. Since simulated exit wave functions of single and bilayers of graphene reveal the atomic structure of carbon-based materials with sufficient resolution, we explore theoretically how the approach can be expanded beyond periodic carbon-based materials to include non-periodic molecular structures. We show here theoretically that the 3D atomic structure of randomly oriented oleic acid molecules can be recovered from a single projection.

Resolution and sensitivity of the latest generation aberration-corrected transmission electron microscopes allow the vast majority of single atoms to be imaged with sub-Ångstrom resolution and their locations determined in an image plane with a precision that exceeds the 1.9-pm wavelength of 300 kV electrons. Such unprecedented performance allows expansion of electron microscopic investigations with atomic resolution into the third dimension. Here we report a general tomographic method to recover the three-dimensional shape of a crystalline particle from high-resolution images of a single projection without the need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality, while minimizing electron beam-induced structure modifications even for small particles or surfaces. We apply it to germanium, gold and magnesium oxide particles, and achieve a depth resolution of 1–2 Å, which is smaller than inter-atomic distances., The resolution of transmission electron microscopes allows the imaging of single atoms and determination of their locations in a plane. Here, the authors present a tomographic method to recover the three-dimensional shape of a crystalline particle without the need for sample rotation.

A standard textbook derivation for the scattering of electrons by a weak potential under the first Born approximation suggests that the far-field scattered wave should be in phase with the incident wave. However, it is well known that waves scattered from a weak phase object should be phase-shifted by pi/2 relative to the incident wave. A disturbing consequence of this missing phase is that, according to the Optical Theorem, the total scattering cross section would be zero in the first Born approximation. We resolve this mystery pedagogically by showing that the first Born approximation fails to conserve electrons even to first order. Modifying the derivation to conserve electrons introduces the correct phase without changing the scattering amplitude. We also show that the far-field expansion for the scattered waves used in many texts is inappropriate for computing an exit wave from a sample, and that the near-field expansion also give the appropriately phase-shifted result. (C) 2011 Elsevier B.V. All rights reserved.

This paper is the second part of a two-part paper on direct structure inversion from exit waves. In the first part, a method has been proposed to quantitatively determine structure parameters with atomic resolution such as atom column positions, surface profile and the number of atoms in the atom columns. In this part, the theory will be demonstrated by means of a Au[110] exit wave reconstructed from a set of focal-series images. The procedures to analyze the experimentally reconstructed exit wave in terms of quantitative structure information are described in detail.

A widely used performance criterion in high-resolution transmission electron microscopy (HRTEM) is the information limit. It corresponds to the inverse of the maximum spatial object frequency that is linearly transmitted with sufficient intensity from the exit plane of the object to the image plane and is limited due to partial temporal coherence. In practice, the information limit is often measured from a diffractogram or from Young's fringes assuming a weak phase object scattering beyond the inverse of the information limit. However, for an aberration corrected electron microscope, with an information limit in the sub-angstrom range, weak phase objects are no longer applicable since they do not scatter sufficiently in this range. Therefore, one relies on more strongly scattering objects such as crystals of heavy atoms observed along a low index zone axis. In that case, dynamical scattering becomes important such that the non-linear and linear interaction may be equally important. The non-linear interaction may then set the experimental cut-off frequency observed in a diffractogram. The goal of this paper is to quantify both the linear and the non-linear information transfer in terms of closed form analytical expressions. Whereas the cut-off frequency set by the linear transfer can be directly related with the attainable resolution, information from the non-linear transfer can only be extracted using quantitative, model-based methods. In contrast to the historic definition of the information limit depending on microscope parameters only, the expressions derived in this paper explicitly incorporate their dependence on the structure parameters as well. In order to emphasize this dependence and to distinguish from the usual information limit, the expressions derived for the inverse cut-off frequencies will be referred to as the linear and non-linear structural information limit. The present findings confirm the well-known result that partial temporal coherence has different effects on the transfer of the linear and non-linear terms, such that the non-linear imaging contributions are damped less than the linear imaging contributions at high spatial frequencies. This will be important when coherent aberrations such as spherical aberration and defocus are reduced.

Depth sectioning in high angular annular dark field scanning transmission electron microscopy is considered a candidate for three-dimensional characterization on the atomic scale. However at present the depth resolution is still far from the atomic level, due to strong limitations in the opening angle of the beam. In this paper we introduce a new, parameter based tomographic reconstruction algorithm that allows to make maximal use of the prior knowledge about the constituent atom types and the microscope settings, so as to retrieve the atomic positions and push the resolution to the atomic level in all three dimensions.

In order to interpret the amplitude and phase of the exit wave in terms of mass and position of the atoms, one has to “invert” the dynamic scattering of the electrons in the object so as to obtain a starting structure which can then be used as a “seed” for further quantitative structure refinement. This is especially challenging in case of a zone axis condition when the interaction of the electrons with the atom column is very strong. Based on the channelling theory we will show that the channelling map not only yields a circle on the Argand plot but also a circular “defocus curve” for every column. The former gives the number of atoms in each column, while the latter provides the defocus value for each column, which reveals the surface roughness at the exit plane with single atom sensitivity.

To determine if perceptions of the social and physical environment are associated with active transport and leisure-time sports among Belgian youth and to investigate if this relationship is moderated by self-efficacy.In February-May 2008, 1445 adolescents (17.4+/-0.6 yrs) were recruited in 20 randomly selected Belgian schools. Physical activity, psychosocial, and environmental factors were assessed using validated questionnaires. Moderated multilevel regression analysis was used to examine the association between physical activity and possible correlates.Social environmental variables (modeling and social support) were positively associated with active transportation and leisure-time sports (p0.05). Higher land use mix diversity, higher street connectivity, more attractive environments, better access to recreational facilities, and higher emotional satisfaction with the neighborhood were associated with more active transportation (p0.05). Higher perceived safety from traffic, better access to recreational facilities, more physical activity equipment at home, and fewer electronic devices in the bedroom were associated with more leisure-time sports (p0.05). Lower perceived safety and poorer access to (recreational) facilities were only associated with lower active transport among youth with lower self-efficacy (p0.05).Creating more supportive environments could have the potential to affect the physical activity levels of Belgian adolescents with both high and low self-efficacy.

Tomography with high angular annular dark field scanning transmission electron microscopy at atomic resolution can be greatly improved if one is able to take advantage of prior knowledge. In this paper we present a reconstruction technique that explicitly takes into account the microscope parameters and the atomic nature of the projected object. This results in a more accurate estimate of the atomic positions and in a good resistance to noise. The reconstruction is a maximum likelihood estimator of the object. Moreover, the limits to the precision have been explored, allowing for a prediction of the amount of expected noise in the reconstruction for a certain experimental setup. We believe that the proposed reconstruction technique can be generalized to other tomographic experiments.

Using a combination of electron holography and energy filtering, interference fringes produced after inelastic interaction of electrons with hydrogen molecules are examined. Surprisingly, the coherence of inelastic scattering increases when moving from the surface of a hydrogen-containing bubble to the vacuum. This phenomenon can be understood in terms of the Feynman two-slit thought experiment with a variable ambiguity of the which-way registration. (C) 2006 Elsevier B.V. All rights reserved.

Electron channelling occurs when the incident electron beam is parallel to the atom columns of an object, such as a crystal or a particular crystal defect. Then, the electrons are trapped in the electrostatic potential of an atom column in which they scatter dynamically. This picture provides physical insight and explains why a one-to-one correspondence is maintained between the exit wave and the projected structure, even in case of strong dynamical scattering. Moreover, the theory is very useful to invert the dynamical scattering, that is, to derive the projected structure from the exit wave. Finally, it can be used to determine the composition of an atom column with single atom sensitivity or to explain dynamical electron diffraction effects. In this paper, an overview of the channelling theory will be given together with some recent applications.

General angular momentum recoupling coefficients can be expressed as a summation formula over products of 6-j coefficients. Yutsis, Levinson and Vanagas developed graphical techniques for representing the general recoupling coefficient as a cubic graph and they describe a set of reduction rules allowing a stepwise generation of the corresponding summation formula. This paper gives an overview of the state-of-the-art heuristic algorithms, used in the latest version of our GYutsis program, for calculating general recoupling coefficients. By means of an experimental setup we show that, in particular for problems of higher order, this approach yields summation formulae which are at least as good, but are often more concise than those obtained by previous algorithms. We also give a counter-example showing that the widespread convention of reducing girth cycles first does not always lead to a shortest reduction.

The Belgian Nuclear Research Centre SCK-CEN is in the process of developing MYRRHA a fast flux research reactor, cooled by lead bismuth eutectic mixture (LBE). As LBE is opaque to visual light, ultrasonic techniques will be employed to provide feedback to submerged operations. One of the tasks tackled using ultrasound is locating a potentially lost fuel assembly. Unfortunately, standard machined stainless steel objects, such as a fuel assembly, reflect an ultrasonic pulse in a specular way which induces stringent requirements on the alignment of the ultrasonic sensor with respect to the fuel assembly. An alternative approach consists in relying on reflections from the known surroundings: a lost fuel assembly will block the line-of-sight to the diaphragm resulting in an anomaly in the reflection. In that way, it suffices to align the sensor with the surroundings. The alignment requirement in the latter approach can be further relaxed by using a tiling of retroreflectors on the lower surface of the diaphragm. In this paper, we present the first results in developing a retroreflecting surface for ultrasound in LBE. We also demonstrate how retroreflective surfaces can be used for localizing lost objects.

We propose an improved image simulation procedure for atomic-resolution annular dark-field scanning transmission electron microscopy (STEM) based on the multislice formulation, which takes thermal diffuse scattering fully into account. The improvement with regard to the classical frozen phonon approach is realized by separating the lattice configuration statistics from the dynamical scattering so as to avoid repetitive calculations. As an example, the influence of phonon scattering on the image contrast is calculated and investigated. STEM image simulation of crystals can be applied with reasonable computing times to problems involving a large number of atoms and thick or large supercells.

Generalized recoupling coe! cients or 3nj-coe! cients can be expressed as multiple sums over products of Racah or 6j-coe! cients [1]. The problem of finding an optimal summation formula (i.e. with a minimal number of Racah coe! cients) for a given 3nj-coe! cient is equivalent to finding an optimal reduction of a so-called Yutsis graph [2]. In terms of graph theory Yutsis graphs are connected simple cubic graphs which can be partitioned into two vertex induced trees. The two parts are necessarily of the same size. In this area Yutsis graphs are also studied under the name of cubic dual Hamiltonian graphs [3]. We present algorithms for determining whether a cubic graph is a Yutsis graph. This is interesting for generating large test cases for programs (as in [4], [5] or [6]) that determine a summation formula for a 3njcoe! cient. Moreover, we give the numbers of Yutsis and non-Yutsis cubic graphs with up to 30 vertices and cubic polyhedra with up to 38 vertices. All these numbers have been computed by two independent programs in order to reduce the probability of error. Since the decision problem whether a given cubic graph is Yutsis or not is NP-complete, we couldn’t hope for a polynomial time worst case performance of our programs. Nevertheless the programs described in this article are very fast on average.

White matter (WM) fiber tractography (i.e., the reconstruction of the 3D architecture of WM fiber pathways) is known to be an important application of diffusion tensor magnetic resonance imaging (DT-MRI). For the quantitative evaluation of several fiber-tracking properties, such as accuracy, noise sensitivity, and robustness, synthetic ground-truth DT-MRI data are required. Moreover, an accurate simulated phantom is also required for optimization of the user-defined tractography parameters, and objective comparisons between fiber-tracking algorithms. Therefore, in this study a mathematical framework for simulating DT-MRI data, based on the physical properties of WM fiber bundles, is presented. We obtained a model of a WM fiber bundle by parameterizing the various features that characterize this bundle. We then evaluated three different synthetic DT-MRI models using experimental data in order to test the proposed methodology, and to determine the optimum model and parameter settings for constructing a realistic simulated DT-MRI phantom. Several examples of how the mathematical framework can be applied to compare fiber-tracking algorithms are presented.

This paper aims to give an overview of the technique of energy-filtered transmission electron microscopy (EFTEM). It explains the basic principles of the technique and points to the relevant literature for more detailed issues. Experimental examples are given to show the power of EFTEM to study the chemical composition of nanoscale samples in materials science. Advanced EFTEM applications like imaging spectroscopy and EFTEM tomography are briefly discussed.

In recent years, the successful implementation of a spherical-aberration corrector in a Philips CM 200 FEG ST microscope achieved by Haider et al. has attracted a great deal of attention. However, thus far extensive applications of this novel high-resolution transmission electron microscope (HRTEM) to materials research have been hampered by the problems concerning optimum imaging conditions and image interpretation. In this paper, we present our points of view concerning atomic imaging in an aberration-corrected HRTEM. Since atomic resolution images can also be obtained with other techniques such as through-focus exit-wave function reconstruction (TF-EWR), we have to emphasis that the strength of the aberration-corrected HRTEM particularly lies on its ability to resolve the atomic structure in real time. However, for this purpose it is mandatory that the image contrast be related in a one-to-one function with the projected structure of the object. We analyzed the atomic imaging conditions in much detail and we come to the following conclusion: this novel facility is no doubt a powerful and advanced HRTEM instrument in achieving atomic images with its highest resolution (information limit). We furthermore demonstrate that the combination of the new microscope and TF-EWR will yield optimal results. (C) 2003 Elsevier B.V. All rights reserved.

Atomic resolution transmission electron microscopy, even with an aberration free microscope, is only able to resolve and refine amorphous structures at the atomic level for very small foil thicknesses. Then, a precision of the order of 0.01 A is possible, but this may require long recording times, especially for light atoms. For larger thicknesses, amorphous structures can in principle only be resolved and refined using electron tomography.

General angular momentum recoupling coefficients can be expressed as a summation formula over products of 6- j coefficients. Yutsis, Levinson and Vanagas developed graphical techniques for representing the general recoupling coefficient as a cubic graph and they describe a set of reduction rules allowing a stepwise generation of the corresponding summation formula. This paper is a follow up to [Van Dyck and Fack, Comput. Phys. Comm. 151 (2003) 353–368] where we described a heuristic algorithm based on these techniques. In this article we separate the heuristic from the algorithm and describe some new heuristic approaches which can be plugged into the generic algorithm. We show that these new heuristics lead to good results: in many cases we get a more efficient summation formula than our previous approach, in particular for problems of higher order. In addition the new features and the use of our program GYutsis , which implements these techniques, is described both for end users and application programmers. Program summary Title of program: CycleCostAlgorithm, GYutsis Catalogue number: ADSA Program Summary URL: http://cpc.cs.qub.ac.uk/summaries/ADSA Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland. Users may obtain the program also by downloading either the compressed tar file gyutsis.tgz (for Unix and Linux) or the zip file gyutsis.zip (for Windows) from our website ( http://caagt.rug.ac.be/yutsis/ ). An applet version of the program is also available on our website and can be run in a web browser from the URL http://caagt.rug.ac.be/yutsis/GYutsisApplet.html . Licensing provisions: none Computers for which the program is designed: any computer with Sun's Java Runtime Environment 1.4 or higher installed. Programming language used: Java 1.2 (Compiler: Sun's SDK 1.4.0) No. of lines in program: approximately 9400 No. of bytes in distributed program, including test data, etc.: 544 117 Distribution format: tar gzip file Nature of physical problem: A general recoupling coefficient for an arbitrary number of (integer or half-integer) angular momenta can be expressed as a formula consisting of products of 6- j coefficients summed over a certain number of variables. Such a formula can be generated using the program GYutsis (with a graphical user front end) or CycleCostAlgorithm (with a text-mode user front end). Method of solution: Using the graphical techniques of Yutsis, Levinson and Vanagas (1962) a summation formula for a general recoupling coefficient is obtained by representing the coefficient as a Yutsis graph and by performing a selection of reduction rules valid for such graphs. Each reduction rule contributes to the final summation formula by a numerical factor or by an additional summation variable. Whereas an optimal summation formula (i.e. with a minimum number of summation variables) is hard to obtain, we present here some new heuristic approaches for selecting an edge from a k -cycle in order to transform it into an ( k −1)-cycle ( k >3) in such a way that a ‘good’ summation formula is obtained. Typical running time: From instantaneously for the typical problems to 30 s for the heaviest problems on a Pentium II-350 Linux-system with 256 MB RAM.