Damage detection by using FBGs and strain field pattern recognition techniques

A novel methodology for damage detection and location in structures is proposed. The methodology is based on strain measurements and consists in the development of strain field pattern recognition techniques. The aforementioned are based on PCA (principal component analysis) and damage indices (T 2 and Q). We propose the use of fiber Bragg gratings (FBGs) as strain sensors

Reconstruction of an acoustically sound-soft obstacle from one incident field and the far field pattern

The problem considered is that of determining the shape of a plane acoustically sound-soft obstacle from the knowledge of the far-field pattern for one time-harmonic incident field. An iterative procedure is proposed based on two boundary integrals representing the incident field and the far-field pattern, respectively. Numerical examples are included which show that the procedure gives accurate numerical approximations in relatively few iterations.

On the induced electric field gradients in the human body for magnetic stimulation by gradient coils in MRI

Prior theoretical studies indicate that the negative spatial derivative of the electric field induced by magnetic stimulation may he one of the main factors contributing to depolarization of the nerve fiber. This paper studies this parameter for peripheral nerve stimulation (PNS) induced by time.-varying gradient fields during MRI scans. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic human, full-body model. Whole-body cylindrical and planar gradient sets in MRI systems and various input signals have been explored. The spatial distributions of the induced electric field and their gradients are calculated and attempts are made to correlate these areas with reported experimental stimulation data. The induced electrical field pattern is similar for both the planar coils and cylindrical coils. This study provides some insight into the spatial characteristics of the induced field gradients for PNS in MRI, which may be used to further evaluate the sites where magnetic stimulation is likely to occur and to optimize gradient coil design.

Novel method to improve the signal-to-noise ratio in the far-field results obtained from planar near-field measurements

A method to reduce the noise power in far-field pattern without modifying the desired signal is proposed. Therefore, an important signal-to-noise ratio improvement may be achieved. The method is used when the antenna measurement is performed in planar near-field, where the recorded data are assumed to be corrupted with white Gaussian and space-stationary noise, because of the receiver additive noise. Back-propagating the measured field from the scan plane to the antenna under test (AUT) plane, the noise remains white Gaussian and space-stationary, whereas the desired field is theoretically concentrated in the aperture antenna. Thanks to this fact, a spatial filtering may be applied, cancelling the field which is located out of the AUT dimensions and which is only composed by noise. Next, a planar field to far-field transformation is carried out, achieving a great improvement compared to the pattern obtained directly from the measurement. To verify the effectiveness of the method, two examples will be presented using both simulated and measured near-field data.

Reduction of truncation errors in planar, cylindrical and partial spherical near-field antenna measurements

A method to reduce truncation errors in near-field antenna measurements is presented. The method is based on the Gerchberg-Papoulis iterative algorithm used to extrapolate band-limited functions and it is able to extend the valid region of the calculated far-field pattern up to the whole forward hemisphere. The extension of the valid region is achieved by the iterative application of a transformation between two different domains. After each transformation, a filtering process that is based on known information at each domain is applied. The first domain is the spectral domain in which the plane wave spectrum (PWS) is reliable only within a known region. The second domain is the field distribution over the antenna under test (AUT) plane in which the desired field is assumed to be concentrated on the antenna aperture. The method can be applied to any scanning geometry, but in this paper, only the planar, cylindrical, and partial spherical near-field measurements are considered. Several simulation and measurement examples are presented to verify the effectiveness of the method.

A New Method to Reduce Truncation Errors in Partial Spherical Near-Field Measurements

A new and effective method for reduction of truncation errors in partial spherical near-field (SNF) measurements is proposed. The method is useful when measuring electrically large antennas, where the measurement time with the classical SNF technique is prohibitively long and an acquisition over the whole spherical surface is not practical. Therefore, to reduce the data acquisition time, partial sphere measurement is usually made, taking samples over a portion of the spherical surface in the direction of the main beam. But in this case, the radiation pattern is not known outside the measured angular sector as well as a truncation error is present in the calculated far-field pattern within this sector. The method is based on the Gerchberg-Papoulis algorithm used to extrapolate functions and it is able to extend the valid region of the calculated far-field pattern up to the whole forward hemisphere. To verify the effectiveness of the method, several examples are presented using both simulated and measured truncated near-field data.

New methods to reduce leakage errors in planar near-field measurements

This paper describes two methods to cancel the effect of two kinds of leakage signals which may be presented when an antenna is measured in a planar near-field range. One method tries to reduce leakage bias errors from the receiver¿s quadrature detector and it is based on estimating the bias constant added to every near-field data sample. Then, that constant is subtracted from the data, removing its undesired effect on the far-field pattern. The estimation is performed by back-propagating the field from the scan plane to the antenna under test plane (AUT) and averaging all the data located outside the AUT aperture. The second method is able to cancel the effect of the leakage from faulty transmission lines, connectors or rotary joints. The basis of this method is also a reconstruction process to determine the field distribution on the AUT plane. Once this distribution is known, a spatial filtering is applied to cancel the contribution due to those faulty elements. After that, a near-field-to-far-field transformation is applied, obtaining a new radiation pattern where the leakage effects have disappeared. To verify the effectiveness of both methods, several examples are presented.

Methods for noise reduction in far-field patterns obtained from cylindrical near-field antenna measurements

Two different methods to reduce the noise power in the far-field pattern of an antenna as measured in cylindrical near-field (CNF) are proposed. Both methods are based on the same principle: the data recorded in the CNF measurement, assumed to be corrupted by white Gaussian and space-stationary noise, are transformed into a new domain where it is possible to filter out a portion of noise. Those filtered data are then used to calculate a far-field pattern with less noise power than that one obtained from the measured data without applying any filtering. Statistical analyses are carried out to deduce the expressions of the signal-to-noise ratio improvement achieved with each method. Although the idea of the two alternatives is the same, there are important differences between them. The first one applies a modal filtering, requires an oversampling and improves the far-field pattern in all directions. The second method employs a spatial filtering on the antenna plane, does not require oversampling and the far-field pattern is only improved in the forward hemisphere. Several examples are presented using both simulated and measured near-field data to verify the effectiveness of the methods.

Application of postprocessing techniques methods for noise reduction in cylindrical near field antenna measurements

Three different methods to reduce the noise power in the far-field pattern of an antenna when it is measured in a cylindrical near field system are presented and compared. The first one is based on a modal filtering while the other two are based on spatial filtering, either on an antenna plane or either on a cylinder of smaller radius. Simulated and measured results will be presented.

The visual evoked magnetic response to a pattern onset stimulus

The topography of the visual evoked magnetic response to a pattern onset stimulus was studied in four normal subjects. The topography of th CIIm component was consistent when measured on the same subject nine months apart. Full field responses were more variable than half field responses. With decreasing check size, the field pattern changes from a simple distribution with one outgoing and one ingoing area of field to a more complex pattern with in and outgoing fields over each hemisphere of the brain. The source may originate at the pole or from within the calcarine fissure.

Distributed source analysis of the pattern onset visual evoked magnetic response

Distributed source analyses of half-field pattern onset visual evoked magnetic responses (VEMR) were carried out by the authors with a view to locating the source of the largest of the components, the CIIm. The analyses were performed using a series of realistic source spaces taking into account the anatomy of the visual cortex. Accuracy was enhanced by constraining the source distributions to lie within the visual cortex only. Further constraints on the source space yielded reliable, but possibly less meaningful, solutions.

How to Assess Histology in Eosinophilic Esophagitis: Standardization by the EEsAI Histopathology Questionnaire

Background and Aims: The international EEsAI study group iscurrently developing the first activity index specific forEosinophilic Esophagitis (EoE). So far histologic assessment inEoE i s not standardized but urgently needed to harmonizehistologic endpoints i n clinical trials. G oal: To develop andevaluate the EEsAI histopathology questionnaire.Methods: B ased o n multiple Delphi r ounds w ith national a ndinternational ( Europe and North America) E oE e xperthistopathologists and gastroenterologists, w e have d evelopedthe EEsAI histopathology questionnaire.Results: T he EEsAI histopathology questionnaire t akes i ntoaccount the following items: number of eosinphils per highpower field, pattern of i nflammation, eosinophil abscesses,basal l ayer e nlargement, and lamina propria f ibrosis. D istinctdefinitions a nd categories f or every item were established byagreement among experts. EoE expert pathologists had amedian of 22 minutes to complete the questionnaire and judgedit as well feasible. The histopathology questionnaire was pilotedin 20 patients and is currently evaluated in a cohort of 150 adultEoE patients.Conclusions: T he EEsAI histopathology questionnaire is thefirst tool that assesses EoE severity according to clearly definedhistologic criteria. These definitions, as well as their categories,established by i nternational E oE e xpert pathologists, w illstandardize h istologic EoE s everity assessment and therebyfacilitate the comparison of histologic outcome measures inclinical trials.

Vaiheohjatut antenniryhmät GSM-tukiasemissa

Diplomityön aiheena oli selvittää onko Suomessa GSM-tukiasemissa käytössä vaiheohjattuja antenniryhmiäja olisiko tällaisten antennien käyttöön kiinnostusta tai mitään es-teitä. Lähtökohtana tälle työlle oli ajatus GSM-tukiasema-antennista, jota voitaisiin kääntää tarpeen mukaan haluttuun suuntaan. Vaiheohjatut antenniryhmät mahdollistavat juuri tällaisen antennin keilan kääntämisen ja muokkaamisen elektronisesti, ilman kuluvia osia. Keilan muitakin ominaisuuksia voidaan säätää, kuten muotoa ja keilojen määrää. Nämä ominaisuudet mahdollistaisivat esimerkiksi ruuhkaisilla alueilla keilojen lisäämisen, jolloin alueen puhelujen välityskapasiteetti kasvaisi.Tarvittaessa voitaisiin myös keilan muotoa muuttaa. Esimerkiksi hätätilanteessasaadaan haluttu keila suunnattua tarkasti tietylle alueelle tai toiselle tukiasemalle ja näin varmistettua kuuluvuus. Myös huoltotoimenpiteet joissakin tapauksissa helpottuisivat. Etenkin vaikeakulkuisissa paikoissa sijaitsevien tukiasemien ensiapu, esimerkiksi antennin fyysisesti kääntyessä, voitaisiin hoitaa etänä kääntämällä pelkkää keilaa ja tässä tapauksessa kääntää antenni tukiaseman normaalin huollon yhteydessä. Suurimpia ongelmakohtia kyseisen tekniikan käyttöönotossa on ollut hinta, mutta muun muassa uusien valmistustekniikoiden avulla vaiheohjattujen antenniryhmien hintoja ollaan saatu pudotettua.

A multiwave range test for obstacle reconstructions with unknown physical properties

We develop a new multiwave version of the range test for shape reconstruction in inverse scattering theory. The range test [R. Potthast, et al., A ‘range test’ for determining scatterers with unknown physical properties, Inverse Problems 19(3) (2003) 533–547] has originally been proposed to obtain knowledge about an unknown scatterer when the far field pattern for only one plane wave is given. Here, we extend the method to the case of multiple waves and show that the full shape of the unknown scatterer can be reconstructed. We further will clarify the relation between the range test methods, the potential method [A. Kirsch, R. Kress, On an integral equation of the first kind in inverse acoustic scattering, in: Inverse Problems (Oberwolfach, 1986), Internationale Schriftenreihe zur Numerischen Mathematik, vol. 77, Birkhäuser, Basel, 1986, pp. 93–102] and the singular sources method [R. Potthast, Point sources and multipoles in inverse scattering theory, Habilitation Thesis, Göttingen, 1999]. In particular, we propose a new version of the Kirsch–Kress method using the range test and a new approach to the singular sources method based on the range test and potential method. Numerical examples of reconstructions for all four methods are provided.

The point-source method for 3D reconstructions for the Helmholtz and Maxwell equations

We use the point-source method (PSM) to reconstruct a scattered field from its associated far field pattern. The reconstruction scheme is described and numerical results are presented for three-dimensional acoustic and electromagnetic scattering problems. We give new proofs of the algorithms, based on the Green and Stratton-Chu formulae, which are more general than with the former use of the reciprocity relation. This allows us to handle the case of limited aperture data and arbitrary incident fields. Both for 3D acoustics and electromagnetics, numerical reconstructions of the field for different settings and with noisy data are shown. For shape reconstruction in acoustics, we develop an appropriate strategy to identify areas with good reconstruction quality and combine different such regions into one joint function. Then, we show how shapes of unknown sound-soft scatterers are found as level curves of the total reconstructed field.