360 results on '"MENDEZ, MARTIN"'
Search Results
152. Mito-communication
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Mendez, Martin, primary
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- 2009
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153. Detection of obstructive sleep apnea in children using decreases in the amplitude fluctuations of PPG signal and HRV
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Gil, Eduardo, primary, Mendez, Martin, additional, Vergara, Jose Maria, additional, Cerutti, Sergio, additional, Bianchi, Anna Maria, additional, and Laguna, Pablo, additional
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- 2008
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154. Detection of Sleep Apnea from surface ECG based on features extracted by an Autoregressive Model
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Mendez, Martin O., primary, Ruini, Davide D., additional, Villantieri, Omar P., additional, Matteucci, Matteo, additional, Penzel, Thomas, additional, Cerutti, Sergio, additional, and Bianchi, Anna M., additional
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- 2007
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155. Sleep Monitoring Through a Textile Recording System
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Devot, Sandrine, primary, Bianchi, Anna M., additional, Naujoka, Elke, additional, Mendez, Martin O., additional, Braurs, Andreas, additional, and Cerutti, Sergio, additional
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- 2007
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156. Conservation genetics of the franciscana dolphin in Northern Argentina: population structure, by-catch impacts, and management implications
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Mendez, Martin, primary, Rosenbaum, H. C., additional, and Bordino, P., additional
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- 2007
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157. Signal Processing and Feature Extraction for Sleep Evaluation in Wearable Devices
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Bianchi, Anna M., primary, Villantieri, Omar P., additional, Mendez, Martin O., additional, and Cerutti, Sergio, additional
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- 2006
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158. Mesh-based approach for the 3D analysis of anatomical structures of interest in Radiotherapy.
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Mejia-Rodriguez, Aldo R., Scalco, Elisa, Tresoldi, Daniele, Bianchi, Anna M., Arce-Santana, Edgar R., Mendez, Martin O., and Rizzo, Giovanna
- Abstract
In this paper a method based on mesh surfaces approximations for the 3D analysis of anatomical structures in Radiotherapy (RT) is presented. Parotid glands meshes constructed from Megavoltage CT (MVCT) images were studied in terms of volume, distance between center of mass (distCOM) of the right and left parotids, dice similarity coefficient (DICE), maximum distance between meshes (DMax) and the average symmetric distance (ASD). A comparison with the standard binary images approach was performed. While absence of significant differences in terms of volume, DistCOM and DICE indices suggests that both approaches are comparable, the fact that the ASD showed significant difference (p=0.002) and the DMax was almost significant (p=0.053) suggests that the mesh approach should be adopted to provide accurate comparison between 3D anatomical structures of interest in RT. [ABSTRACT FROM PUBLISHER]
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- 2012
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159. Approximate string matching using phase correlation.
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Alba, Alfonso, Rodriguez-Kessler, Margarita, Arce-Santana, Edgar R., and Mendez, Martin O.
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A novel method for approximate string matching with applications to bioinformatics is presented in this paper. Unlike most methods in the literature, the proposed method does not depend on the computation of the edit distance between two sequences, but uses instead a similarity index obtained by applying the phase correlation method. The resulting algorithm provides a finer control over the false positive rate, allowing users to pick out relevant matchings in less time, and can be applied for both offline and online processing. [ABSTRACT FROM PUBLISHER]
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- 2012
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160. Assessment of the Heart Rate Variability during Arousal from Sleep by Cohen's Class Time-Frequency Distributions.
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Jarm, Tomaz, Kramar, Peter, Zupanic, Anze, Mendez, Martin, Bianchi, A.M., Villantieri, O.P., and Cerutti, S.
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Arousal from sleep is a normal physiologic event which produces well defined changes in the sympatho-vagal balance. Arousal from sleep is related to sleep fragmentation and some sleep disorders as obstructive sleep apnea. However when repetitive arousals are found during sleep time, bad sleep quality and as consequence sleepiness during the day are associated. We studied the dynamic of the HRV during arousal accompanied by muscular activity. Ten isolated arousals free from any pathologic event where studied. Three Time- Frequency distributions (TFDs), Born-Jordan, Choi-Williams and Smooth Pseudo Wigner-Ville Distributions, were analyzed in order to evaluate their performance during arousal episodes. The three TFDs showed the same performance when analytic HRV signal is used. LF component suggests a major participation of the sympathetic activity at the beginning of the arousal episode while HF component suggests a major role of the parasympathetic drive at after arousal episode. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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161. A Fully Constrained Optimization Method for Time-Resolved Multispectral Fluorescence Lifetime Imaging Microscopy Data Unmixing.
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Gutierrez-Navarro, Omar, Campos-Delgado, Daniel U., Arce-Santana, Edgar, Mendez, Martin O., and Jo, Javier A.
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FLUORESCENCE microscopy ,TIME-domain analysis ,CONSTRAINED optimization ,TIME-resolved measurements ,LEAST squares ,SIMULATION methods & models ,FLUORESCENT dyes ,HYPERSPECTRAL imaging systems - Abstract
This paper presents a new unmixing methodology of multispectral fluorescence lifetime imaging microscopy (m-FLIM) data, in which the spectrum is defined as the combination of time-domain fluorescence decays at multiple emission wavelengths. The method is based on a quadratic constrained optimization (CO) algorithm that provides a closed-form solution under equality and inequality restrictions. In this paper, it is assumed that the time-resolved fluorescence spectrum profiles of the constituent components are linearly independent and known a priori. For comparison purposes, the standard least squares (LS) solution and two constrained versions nonnegativity constrained least squares (NCLS) and fully constrained least squares (FCLS) (Heinz and Chang, 2001) are also tested. Their performance was evaluated by using synthetic simulations, as well as imaged samples from fluorescent dyes and ex vivo tissue. In all the synthetic evaluations, the CO obtained the best accuracy in the estimations of the proportional contributions. CO could achieve an improvement ranging between 41% and 59% in the relative error compared to LS, NCLS, and FCLS at different signal-to-noise ratios. A liquid mixture of fluorescent dyes was also prepared and imaged in order to provide a controlled scenario with real data, where CO and FCLS obtained the best performance. The CO and FCLS were also tested with 20 ex vivo samples of human coronary arteries, where the expected concentrations are qualitatively known. A certainty measure was employed to assess the confidence in the estimations made by each algorithm. The experiments confirmed a better performance of CO, since this method is optimal with respect to equality and inequality restrictions in the linear unmixing formulation. Thus, the evaluation showed that CO achieves an accurate characterization of the samples. Furthermore, CO is a computational efficient alternative to estimate the abundance of components in m-FLIM data, since a global optimal solution is always guaranteed in a closed form. [ABSTRACT FROM PUBLISHER]
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- 2013
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162. Conservation genetics of harvested river turtles, Podocnemis expansa and Podocnemis unifilis, in the Peruvian Amazon: All roads lead to Iquitos.
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Pineda-Catalan, Oscar, Mendez, Martin, Gleizer, Anya, García-Dávila, Carmen, Aguirre, A. Alonso, Pinedo-Vasquez, Miguel, and Amato, George
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TURTLES , *PODOCNEMIS expansa , *PODOCNEMIS unifilis , *MITOCHONDRIAL DNA analysis , *HAPLOTYPES , *PROTECTED areas - Abstract
We present a mtDNA analysis of Podocnemis expansa ( n = 81) and Podocnemis unifilis ( n = 228) turtles traded in Peru to evaluate the potential origin of these animals. In particular, we were interested in the relationship between samples reported in the Iquitos markets (IMs) and a Pacaya Samiria Natural Reserve (PSNR) where illegal hunting is presumed. Our mtDNA data showed that, for both species, all haplotypes found within the PSNR were observed in the IM, and that these markets also displayed haplotypes not documented in the reserve. This suggests that the IMs are recipients of Podocnemis turtles from within and outside the PSNR. The fact that most of the haplotype diversity observed in the markets was not found within the PSNR strongly suggests that Podocnemis genetic diversity is exploited in areas where conservation actions are limited. Hence, we recommend expanding Podocnemis conservation efforts outside of protected areas. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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163. Pneumonia without the need for admission, the experience in an emergency department
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Arévalo-Velasco, Agustín, Alario-García, María J., Méndez-Martín, Violante, and García-García, Ángel
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- 2012
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164. Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation
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Schuh, B., Mendez-Martin, F., Völker, B., George, E.P., Clemens, H., Pippan, R., and Hohenwarter, A.
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Polymers and Plastics ,High-entropy alloys ,Severe plastic deformation ,Metals and Alloys ,Ceramics and Composites ,3 dimensional atom probe tomography ,Compositionally complex alloys ,Microstructure ,Electronic, Optical and Magnetic Materials - Abstract
An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50nm. As a result, strength increased significantly to 1950MPa, and hardness to ∼520HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630HV at about 450°C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910HV after 100h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5min at 450°C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites.
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165. Fibrosis of the Gluteal Muscles
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H. Duran Sacristan, L. Ferrandez, A. Sanchez-Barba, J. Mendez Martin, C. Linan, and L. Lopez-Duran Stern
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medicine.anatomical_structure ,Text mining ,Fibrosis ,business.industry ,medicine ,Orthopedics and Sports Medicine ,Surgery ,General Medicine ,Anatomy ,Gluteal muscles ,medicine.disease ,business - Published
- 1974
166. [Nutritional effect of essential amino acid supplement during hemodialysis]
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A, Ortiz Gonzalez, M P, Sanchez-Robles, J R, Gutierrez Gonzalez, and A, Mendez Martin
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Adult ,Nitrogen ,Renal Dialysis ,Humans ,Histidine ,Nutritional Physiological Phenomena ,Amino Acids, Essential ,Middle Aged ,Nutrition Disorders - Published
- 1983
167. High-resolution characterization of intermetallic TiAl-powder for additive manufacturing
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David Wimler, Gammer, C., Eckert, J., Mendez-Martin, F., Lindemann, J., Clemens, H., and Mayer, S.
168. Creep strength and microstructural evolution of 9-12%Cr heat resistant steels during creep exposure at 600°C and 650°C
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Francisca Mendez Martin, Clara Panait, Walter Bendick, Mihaela Albu, Bernhard Sonderegger, Gerald Kothleitner, Christof Sommitsch, Anne-Françoise Gourgues-Lorenzon, Jacques Besson, Fuchsmann, A., Centre des Matériaux (MAT), MINES ParisTech - École nationale supérieure des mines de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS), V & M France CEV, Salzgitter Mannesmann Forschung GmbH, J. Lecomte-Beckers, Q. Contrepois, T. Beck, and B. Kuhn
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9% Cr tempered martensitic steels ,laves phases ,creep strength ,12% Cr tempered martensitic steels ,ComputingMilieux_MISCELLANEOUS ,Z phases ,[SPI.MAT]Engineering Sciences [physics]/Materials - Abstract
International audience
169. A-phase classification using convolutional neural networks.
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Arce-Santana, Edgar R., Alba, Alfonso, Mendez, Martin O., and Arce-Guevara, Valdemar
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CONVOLUTIONAL neural networks , *ELECTROENCEPHALOGRAPHY , *RAPID eye movement sleep , *SLEEP stages , *ALGORITHMS - Abstract
A series of short events, called A-phases, can be observed in the human electroencephalogram (EEG) during Non-Rapid Eye Movement (NREM) sleep. These events can be classified in three groups (A1, A2, and A3) according to their spectral contents, and are thought to play a role in the transitions between the different sleep stages. A-phase detection and classification is usually performed manually by a trained expert, but it is a tedious and time-consuming task. In the past two decades, various researchers have designed algorithms to automatically detect and classify the A-phases with varying degrees of success, but the problem remains open. In this paper, a different approach is proposed: instead of attempting to design a general classifier for all subjects, we propose to train ad-hoc classifiers for each subject using as little data as possible, in order to drastically reduce the amount of time required from the expert. The proposed classifiers are based on deep convolutional neural networks using the log-spectrogram of the EEG signal as input data. Results are encouraging, achieving average accuracies of 80.31% when discriminating between A-phases and non A-phases, and 71.87% when classifying among A-phase sub-types, with only 25% of the total A-phases used for training. When additional expert-validated data is considered, the sub-type classification accuracy increases to 78.92%. These results show that a semi-automatic annotation system with assistance from an expert could provide a better alternative to fully automatic classifiers. Graphical abstract A/N Deep Learning Classifier. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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170. From microscopic to atomistic scale: Temperature effect on yttria distribution in mechanically alloyed FeCrMnNiCo powder particles.
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Mayer, Michael, Svoboda, Jiri, Mendez-Martin, Francisca, Fellner, Simon, Gammer, Christoph, Razumovskiy, Vsevolod, Resch, Laura, Sprengel, Wolfgang, Stark, Andreas, Zeisl, Stefan, and Ressel, Gerald
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TEMPERATURE effect , *ATOM-probe tomography , *POWDERS , *POSITRON annihilation , *MECHANICAL alloying , *FACE centered cubic structure , *DISPERSION strengthening , *ALLOY powders - Abstract
Mechanical alloying (MA), the state-of-the-art processing step to produce oxide dispersion strengthened materials, shows a deficiency regarding time and costs hindering a broader applicability. Therefore, in order to investigate the effect of cryogenic MA temperatures and to understand the mechanism behind the refinement and dissolution of yttria, face-centered cubic FeCrMnNiCo powders are mechanically alloyed with yttria at room and cryogenic temperatures using a novel cryogenic attritor. Mechanically alloyed powders are thus analyzed using a comprehensive set of experimental methods. Transmission electron microscopy reveals a stronger decrease of the oxide particle size upon cryogenic MA while at both temperatures the hereby observed particles in a size over 10 nm still show yttria crystal structure. Nevertheless, a substantial amount of yttria is refined below 10 nm forming nanoclusters without detectable crystal structure. Positron annihilation spectroscopy suggests a vacancy assisted dissolution of yttria into these nanoclusters while detailed investigation of these nanoclusters by atom probe tomography suggests smaller clusters in the cryoalloyed sample. The results imply that this vacancy assisted dissolution seems to be enhanced at cryogenic temperatures as first principle calculations and a change of the chemical composition of the nanoclusters imply higher vacancy densities at cryogenic MA temperatures stabilizing smaller nanoclusters. [Display omitted] • Two milling temperature independent size regimes for the refinement and dissolution of yttria are proposed. • Yttria at microscale is present as particles whereas yttria at nanoscale is proposed to be dissolved. • Vacancies are suggested to play a crucial role in the dissolution of yttria during mechanical alloying. • During cryomilling, the nucleation and stabilization of smaller nanoclusters of dissolved yttria is promoted. • Undissolved yttria appeared in smaller sizes within the cryomilled specimen indicating a beneficial effect of cryomilling. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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171. Giant hardening response in AlMgZn(Cu) alloys
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Stefan Pogatscher, Francisca Mendez-Martin, Matheus A. Tunes, Peter J. Uggowitzer, Ramona Tosone, Daniel Marchand, Phillip Dumitraschkewitz, William A. Curtin, and Lukas Stemper
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Materials science ,Polymers and Plastics ,Alloy ,Nucleation ,02 engineering and technology ,engineering.material ,01 natural sciences ,thermomechanical treatment ,Precipitation hardening ,0502 economics and business ,0103 physical sciences ,Formability ,microstructure evolution ,050207 economics ,Composite material ,Tensile testing ,010302 applied physics ,aluminum alloys ,050208 finance ,Precipitation (chemistry) ,05 social sciences ,Metals and Alloys ,021001 nanoscience & nanotechnology ,Electronic, Optical and Magnetic Materials ,mechanical testing ,Ceramics and Composites ,engineering ,Hardening (metallurgy) ,Thermomechanical processing ,Deformation (engineering) ,0210 nano-technology ,precipitation hardening - Abstract
This study presents a thermomechanical processing concept which is capable of exploiting the full industrial application potential of recently introduced AlMgZn(Cu) alloys. The beneficial linkage of alloy design and processing allows not only to satisfy the long-standing trade-off between high mechanical strength in use and good formability during processing but also addresses the need for economically feasible processing times. After an only 3-hour short pre-aging treatment at 100 degrees C, the two investigated alloys, based on commercial EN AW-5182 and modified with additions of Zn and Zn+Cu respectively, show high formability due to increased work-hardening. Then, these alloys exhibit a giant hardening response of up to 184 MPa to reach a yield strength of 410 MPa after a 20-minute short final heat treatment at 185 degrees C, i.e. paint-baking. This rapid hardening response strongly depends on the number density, size distribution and constitution of precursors acting as preferential nucleation sites for T-phase precursor precipitation during the final high-temperature aging treatment and is significantly increased by the addition of Cu. Minor deformation (2%) after pre-aging and before final heat treatment further enhances the development of hardening precipitates additionally by activating dislocation-supported nucleation and growth. Tensile testing, quantitative and analytical electron-microscopy methods, atom probe analysis and DFT calculations were used to characterize the alloys investigated in this work over the thermomechanical processing route. The influence of pre-strain on the hardening response and the role of Cu additions in early-stage cluster nucleation are discussed in detail and supported by in-situ STEM experiments and first-principles calculations. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd.
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172. On the existence of orthorhombic martensite in a near-α titanium base alloy used for additive manufacturing
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Christian Fleißner-Rieger, Matheus Araujo Tunes, Christoph Gammer, Tanja Jörg, Tanja Pfeifer, Michael Musi, Francisca Mendez-Martin, and Helmut Clemens
- Subjects
010302 applied physics ,Mechanics of Materials ,Mechanical Engineering ,0103 physical sciences ,Materials Chemistry ,Metals and Alloys ,02 engineering and technology ,021001 nanoscience & nanotechnology ,0210 nano-technology ,01 natural sciences - Full Text
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173. SEM and FIB-SEM investigations on potential gas shales in the Dniepr-Donets Basin (Ukraine): pore space evolution in organic matter during thermal maturation.
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D Misch, F Mendez-Martin, G Hawranek, P Onuk, D Gross, and R F Sachsenhofer
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- 2016
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174. Carbon distribution in multi-phase γ-TiAl based alloys and its influence on mechanical properties and phase formation.
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Klein, Thomas, Schachermayer, Michael, Mendez-Martin, Francisca, Schöberl, Thomas, Rashkova, Boryana, Clemens, Helmut, and Mayer, Svea
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TITANIUM-aluminum alloys , *MECHANICAL behavior of materials , *HIGH temperatures , *CREEP (Materials) , *STRENGTH of materials , *CARBON - Abstract
Advanced intermetallic γ -TiAl based alloys are attractive light-weight materials for high-temperature application. In order to extend their service temperature limits, alloying with low-density elements, such as C, is of particular interest and has been shown to effectively increase high-temperature strength as well as creep resistance. In the present study the local chemical composition of the constituent phases of the so-called TNM alloy and a C-containing derivative thereof is characterized by atom probe tomography. In both alloys Mo is found to preferentially locate in the β o phase, in contrast to Nb, which is dispersed in similar levels in all phases. In the C-containing alloy, C is enriched in the α 2 phase, dissolved in the γ phase, but depleted in the β o phase. Furthermore, the investigation of interfaces through site-specific sample preparation reveals segregation of C at phase interfaces and their close vicinity. Finally, a correlation of the mechanical properties with the C distribution is established by nanoindentation technique. Both the γ and the α 2 phase significantly harden through the addition of C, which is in good agreement with the C concentration present within these phases as observed by atom probe tomography. However, the β o phase softens through the addition of C, which is not a direct consequence of the C distribution, but follows from the absence of finely dispersed ω o particles in the β o phase of the C-containing alloy. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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175. Multi-Scale Evaluation of Sleep Quality Based on Motion Signal from Unobtrusive Device.
- Author
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Coluzzi, Davide, Baselli, Giuseppe, Bianchi, Anna Maria, Guerrero-Mora, Guillermina, Kortelainen, Juha M., Tenhunen, Mirja L., and Mendez, Martin O.
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SLEEP quality , *PRESSURE sensors , *SLEEP disorders , *HOME environment , *METABOLIC disorders , *MOSQUITO nets - Abstract
Sleep disorders are a growing threat nowadays as they are linked to neurological, cardiovascular and metabolic diseases. The gold standard methodology for sleep study is polysomnography (PSG), an intrusive and onerous technique that can disrupt normal routines. In this perspective, m-Health technologies offer an unobtrusive and rapid solution for home monitoring. We developed a multi-scale method based on motion signal extracted from an unobtrusive device to evaluate sleep behavior. Data used in this study were collected during two different acquisition campaigns by using a Pressure Bed Sensor (PBS). The first one was carried out with 22 subjects for sleep problems, and the second one comprises 11 healthy shift workers. All underwent full PSG and PBS recordings. The algorithm consists of extracting sleep quality and fragmentation indexes correlating to clinical metrics. In particular, the method classifies sleep windows of 1-s of the motion signal into: displacement (DI), quiet sleep (QS), disrupted sleep (DS) and absence from the bed (ABS). QS proved to be positively correlated ( 0.72 ± 0.014 ) to Sleep Efficiency (SE) and DS/DI positively correlated ( 0.85 ± 0.007 ) to the Apnea-Hypopnea Index (AHI). The work proved to be potentially helpful in the early investigation of sleep in the home environment. The minimized intrusiveness of the device together with a low complexity and good performance might provide valuable indications for the home monitoring of sleep disorders and for subjects' awareness. [ABSTRACT FROM AUTHOR]
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- 2022
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176. EEG segmentation for improving automatic CAP detection.
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Mariani, Sara, Grassi, Andrea, Mendez, Martin O., Milioli, Giulia, Parrino, Liborio, Terzano, Mario G., and Bianchi, Anna M.
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ELECTROENCEPHALOGRAPHY , *IMAGE segmentation , *MEDICAL practice , *COMPUTERS in medicine , *SLEEP , *NEUROSCIENCES - Abstract
Highlights: [•] A completely automatic method for the classification of the Cyclic Alternating Pattern of Sleep. [•] A novel approach to microstructure analysis based on EEG segmentation. [•] A computationally efficient tool that puts the basis for a smooth and accurate evaluation of CAP in routine clinical practice. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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177. Strain aging characterization and physical modelling of over-aging in dual phase steel.
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Soliman, Mohamed, Shan, Yao V, Mendez-Martin, Francisca, Kozeschnik, Ernst, and Palkowski, Heinz
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DUAL-phase steel , *SCANNING transmission electron microscopy , *ATOM-probe tomography , *TENSILE tests , *CHEMICAL potential - Abstract
This study presents an integrated work of experimental investigations and physical modeling of bake hardening (BH) response in dual-phase steel (DP). A DP steel with a martensite volume percentage of 22% was produced by intercritical annealing followed by quenching in brine. Aging experiments with up to 5% pre-straining were carried out in the temperature range of 100 to 220 °C, for 2 to 2·104 min at temperature. The DP steel was characterized using light optical, scanning electron and transmission electron microscopy. The pinning effect of the dislocations was revealed by atom probe tomographic analysis. The increase in the yield strength accompanying the aging phenomenon, measured using tensile tests, showed a two-step increase, followed by an over-aging stage. The dependence of the time-interval of each stage on the pre-strain value and aging temperature was analyzed. A physical-based model for interpreting the over-aging stage in DP steel was developed. A new concept for over-aging, correlating it to carbon-diffusion from ferrite to martensite due to the gradient in the chemical potential at the interface between the two phases, was introduced. This diffusion causes a partial dissolution of the already formed Cottrell atmosphere/carbide precipitates. Finally, the time required for the onset of over-aging, calculated using physical simulations, was compared with experimental results showing a good matching between experiment and simulation. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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178. In situ analysis of the effect of high heating rates and initial microstructure on the formation and homogeneity of austenite.
- Author
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Eggbauer, Annika, Lukas, Marina, Ressel, Gerald, Prevedel, Petri, Mendez-Martin, Francisca, Keckes, Jozef, Stark, Andreas, and Ebner, Reinhold
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CARBON steel , *MICROSTRUCTURE , *HEAT treatment , *COST control , *GRAIN size - Abstract
Decreasing processing time of a quench and temper heat treatment is of high interest for industry due to the possibility of cost reduction. One option to reduce processing time is to shorten the austenitizing cycle by applying high heating rates and minimum holding times. However, due to the high heating rates, the analysis of their influences on the formation kinetics of austenite and its crystallographic parameters is challenging. Thus, this work concentrates on the in situ analysis of the austenitization process by means of high-energy X-ray diffraction to study a range of heating rates applied to ferritic–pearlitic and soft annealed initial microstructures. The transformation kinetics from ferrite/pearlite and soft annealed state to austenite, the cementite dissolution behavior and the homogeneity of the freshly formed austenite were analyzed. The results indicate three distinct steps of austenite formation independent of initial microstructure and heating rate: (1) nucleation of carbon-rich austenite at cementite–ferrite interfaces, (2) growth of austenite phase fraction accompanied by a reduction of the carbon content, until reaching the mean carbon content of the steel, followed by growth of the austenite grain size, (3) regarding austenite homogeneity, the combination of austenitization temperature and initial microstructure are the main influencing factors. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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179. Solid bitumen in shales: Petrographic characteristics and implications for reservoir characterization.
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Misch, D., Gross, D., Hawranek, G., Horsfield, B., Klaver, J., Mendez-Martin, F., Urai, J.L., Vranjes-Wessely, S., Sachsenhofer, R.F., Schmatz, J., Li, J., and Zou, C.
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QUARTZ , *BITUMEN , *SHALE , *SCANNING electron microscopy , *RESERVOIRS , *MICROSCOPY - Abstract
Abstract The presence of solid bitumen strongly affects hydrocarbon storage and expulsion from a source rock as it might either cause blockage of pore throats leading to lower effective gas permeability, or contribute to hydrocarbon storage and provide migration pathways when a continuous network of hydrocarbon-wet organic matter (OM) pores is formed. Furthermore, organic matter transformation reactions are suggested to influence mineral diagenesis as well. In an attempt to characterize different solid bitumen types and transformation stages over a broad maturity interval (0.5–2.7%Ro) and for varying primary kerogen compositions, we reviewed optical and scanning electron microscopy (SEM) data of 35 solid bitumen-rich shale samples with a Cambrian to Triassic age. We were able to identify in-situ pre-oil solid bitumen, as well as remobilized post-oil solid bitumen at various maturity stages from the early oil window onwards. Solid bitumen is the main host for SEM-visible organic matter porosity; onset of porosity development in solid bitumen differs considerably between predominantly oil-prone (e.g., alginites, amorphous OM from algal and bacterial precursors) and gas-prone (vitrinite-rich) kerogen compositions. Furthermore, solid bitumen (pyrobitumen) in rocks with a terrestrially dominated OM composition seems to be considerably less mobile within the source rock compared to pre- and post-oil solid bitumen in oil-prone rocks, and less reactive in terms of porosity generation. In most samples, several solid bitumen populations with varying fluorescence properties and bitumen reflectance were observed, complicating the use of these petrographic maturity indicators. The apparently different solid bitumen populations often form continuous networks at the SEM-scale. Microstructural features such as irregularly distributed sponge-like porosity or detrital and authigenic mineral inclusions in the sub-micrometer scale were found to have a great influence on texture and reflectance under reflected light microscopy. The formation of authigenic minerals (quartz, various carbonate phases with different Ca/Mg/Fe proportions, magnetite in Cambrian samples) was observed frequently in post-oil solid bitumen of oil-prone rocks, indicating a close genetic relationship between transformation products formed during hydrocarbon generation (e.g., acetate, carbon dioxide and methane) and the dissolution and precipitation of minerals during diagenesis. In some cases, stylolite-like features in the sub-micrometer scale were found, showing that processes well-known from reservoir characterization at core-scale also play a role at the micrometer-scale. Furthermore, the observed strong interaction between organic matter transformation and mineral authigenesis indicates a substantial aqueous component even in pores filled apparently exclusively with solid bitumen. Highlights • Combined optical and scanning electron microscopy was applied to 35 Cambrian to Triassic solid bitumen-rich shales. • Solid bitumen shows complex textural features at microscale. • A pore evolution model was established for oil- and gas-prone organofacies. • Solid bitumen reflectance varies considerably within one sample. • Mineral authigenesis is strongly influenced by solid bitumen transformation. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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180. A new Probabilistic Active Contour region-based method for multiclass medical image segmentation.
- Author
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Arce-Santana, Edgar R., Mejia-Rodriguez, Aldo R., Martinez-Peña, Enrique, Alba, Alfonso, Mendez, Martin, Scalco, Elisa, Mastropietro, Alfonso, and Rizzo, Giovanna
- Subjects
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IMAGE segmentation , *MAGNETIC resonance imaging of the brain , *DIAGNOSTIC imaging , *GAUSSIAN distribution , *OPTICAL coherence tomography - Abstract
In medical imaging, the availability of robust and accurate automatic segmentation methods is very important for a user-independent and time-saving delineation of regions of interest. In this work, we present a new variational formulation for multiclass image segmentation based on active contours and probability density functions demonstrating that the method is fast, accurate, and effective for MRI brain image segmentation. We define an energy function assuming that the regions to segment are independent. The first term of this function measures how much the pixels belong to each class and forces the regions to be disjoint. In order for this term to be outlier-resistant, probability density functions were used allowing to define the structures to be segmented. The second one is the classical regularization term which constrains the border length of each region removing inhomogeneities and noise. Experiments with synthetic and real images showed that this approach is robust to noise and presents an accuracy comparable to other classical segmentation approaches (in average DICE coefficient over 90% and ASD below one pixel), with further advantages related to segmentation speed. Graphical Abstract. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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181. Phase evolution and carbon redistribution during continuous tempering of martensite studied with high resolution techniques.
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Vieweg, Annika, Povoden-Karadeniz, Erwin, Ressel, Gerald, Prevedel, Petri, Wojcik, Tomasz, Mendez-Martin, Francisca, Stark, Andreas, Keckes, Jozef, and Kozeschnik, Ernst
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MARTENSITE , *PHASE transitions , *TEMPERING , *LOW temperatures , *STOICHIOMETRY , *PRECIPITATION (Chemistry) kinetics - Abstract
Using high resolution techniques, such as, 3D atom probe tomography, transmission electron microscopy and synchrotron radiation, the continuous tempering process of a 50CrMo4 steel is analyzed. Differences of continuous heating compared to isothermal holding at certain tempering temperatures are discussed. Focus is laid on in-depth characterization of phase development and carbon redistribution effects, such as, segregation and clustering. The results of this work suggest that segregation is the dominant effect at low temperatures (< 150 °C), while cluster formation within these segregated areas is prominent at intermediate temperatures (150–250 °C). Epsilon carbide is formed during continuous heating with off-stoichiometric chemical composition (270–350 °C). It transforms finally into cementite ( T > 350 °C). [ABSTRACT FROM AUTHOR]
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- 2017
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182. Atomistic insights into milling mechanisms in an Fe-YO model alloy.
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Ressel, G., Holec, D., Fian, A., Mendez-Martin, F., and Leitner, H.
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YTTRIUM oxides , *IRON powder , *MATHEMATICAL models , *X-ray diffraction , *ATOM-probe tomography , *X-ray photoelectron spectroscopy , *MECHANICAL alloying , *IRON alloys , *MILLING (Metalwork) - Abstract
This experimental study combined with first principles modeling focuses on the distribution and behavior of yttria in pure iron powder particles prepared by mechanical alloying. A profound verification of the mechanism during milling is still missing in literature. Atom probe tomography and X-ray photoelectron spectroscopy measurements directly after mechanical alloying revealed yttria dissolved in the iron matrix, which later rearranged in clusters. These findings are corroborated by ab initio calculations demonstrating that the formation energy for Y substitutional defect in bcc-Fe is significantly lower in the close neighborhood of vacancies. X-ray diffraction measurements revealed that mechanical alloying for at least 12 hours caused a dramatic decrease in domain size and an extraordinary increase of defect density. [ABSTRACT FROM AUTHOR]
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- 2014
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183. Quench rate sensitivity of age-hardenable Al-Zn-Mg-Cu alloys with respect to the Zn/Mg ratio: An in situ SAXS and HEXRD study.
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Graf, Gloria, Spoerk-Erdely, Petra, Staron, Peter, Stark, Andreas, Mendez Martin, Francisca, Clemens, Helmut, and Klein, Thomas
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PRECIPITATION hardening , *ATOM-probe tomography , *SMALL-angle X-ray scattering , *ALLOYS , *MANUFACTURING processes , *HEAT treatment , *COPPER-zinc alloys - Abstract
Al-Zn-Mg-Cu alloys with a low Zn/Mg ratio have attracted considerable attention in recent years as a result of an attractive property portfolio based on their ability of age hardening via the precipitation of the T-Mg 32 (Al,Zn) 49 phase and its precursors. In this study, the quench rate sensitivity of an Al-Mg-Zn-Cu alloy with low Zn/Mg ratio is investigated and compared to a commercial reference Al-Zn-Mg-Cu alloy (AA7075) showing a high Zn/Mg ratio. The impact of five different cooling rates in the range of 0.25–100 K/s on the precipitation of quench-induced particles was studied by means of in situ small-angle X-ray scattering and high-energy X-ray diffraction. Subsequent continuous annealing experiments showed the influence of the cooling rate on the precipitation of fine hardening phases. Selected heat treatment conditions were further studied via scanning electron microscopy and atom probe tomography in order to reveal the microstructure and the chemical composition of the T-Mg 32 (Al,Zn) 49 precipitates. The results showed that the Al-Mg-Zn-Cu alloy with low Zn/Mg ratio exhibits a lower quench rate sensitivity than the commercial AA7075 alloy. The lowest cooling rates, at which no quench-induced precipitation occurs, are in the range of 1 K/s for the investigated Al-Mg-Zn-Cu alloy and 100 K/s for the AA7075 alloy. The robust precipitation behavior of the Al-Mg-Zn-Cu alloy with a low Zn/Mg ratio is expected to be beneficial for advanced manufacturing processes. [Display omitted] [ABSTRACT FROM AUTHOR]
- Published
- 2022
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184. On the existence of orthorhombic martensite in a near-α titanium base alloy used for additive manufacturing.
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Fleißner-Rieger, Christian, Tunes, Matheus Araujo, Gammer, Christoph, Jörg, Tanja, Pfeifer, Tanja, Musi, Michael, Mendez-Martin, Francisca, and Clemens, Helmut
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TITANIUM alloys , *MARTENSITE , *ATOM-probe tomography , *TITANIUM powder , *CRYSTAL defects , *RESIDUAL stresses , *MANUFACTURING processes - Abstract
• A second type of martensite was found in a Ti6242S alloy used for additive manufacturing. • Appearance of the martensitic variant can be controlled via processing parameters. • Orthorhombic martensite is softer; Softening indicates a gain in ductility. • Nanometer-sized lattice defects affect precipitate morphology. Inspired by a novel orthorhombic phase transformation in the powder material, the adaption of cooling rates during the additive manufacturing process originates a new martensite variant with beneficial material specifics for this cutting-edge production technique. [Display omitted] Additive manufacturing is a state-of-the-art production technology to produce tailor-made and highly complex parts. Among various other alloys, Ti base alloys are frequently used in this manufacturing technique due to their well-balanced properties and their wide range of applications. Allotropic phases and the occurrence of athermal phase transformations are the main reasons why these alloys hold a great development potential and are the basis of extensive use. High cooling rates during manufacturing lead to martensitic phases and the formation of nanometer-sized microstructures resulting in extraordinary strength. Simultaneously, such high cooling rates cause a high amount of lattice defects and the occurrence of residual stresses, which finally may result in delamination effects and cracks. Usually, a common approach to reduce residual stresses during additive manufacturing is to decrease thermal gradients by increasing the heat input or preheating the building platform. Instead of applying the typical approaches to lower thermal gradients, this study deals with the origin of the 'softer' orthorhombic martensite by accelerating the solidification process. The implementation of the orthorhombic phase in bulk components was inspired by a new phase transformation herein reported for the first time in the powder material, which also validates the possible occurrence of two martensitic phases in the same alloy. Various sophisticated characterization techniques like high energy and high-temperature X-ray diffraction, high-resolution transmission electron microscopy as well as atom probe tomography were applied to characterize this softer orthorhombic martensitic phase in detail aiming to highlight the opportunities accompanied by this new approach for additive manufacturing of titanium alloys. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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185. Efficient automatic classifiers for the detection of A phases of the cyclic alternating pattern in sleep.
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Mariani, Sara, Manfredini, Elena, Rosso, Valentina, Grassi, Andrea, Mendez, Martin, Alba, Alfonso, Matteucci, Matteo, Parrino, Liborio, Terzano, Mario, Cerutti, Sergio, and Bianchi, Anna
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SLEEP stages , *POLYSOMNOGRAPHY , *ALGORITHMS , *CLINICAL trials , *MACHINE learning , *ARTIFICIAL neural networks , *SUPPORT vector machines - Abstract
This study aims to develop an automatic detector of the A phases of the cyclic alternating pattern, periodic activity that generally occurs during non-REM (NREM) sleep. Eight polysomnographic recordings from healthy subjects were examined. From EEG recordings, five band descriptors, an activity descriptor and a variance descriptor were extracted and used to train different machine-learning algorithms. A visual scoring provided by an expert clinician was used as golden standard. Four alternative mathematical machine-learning techniques were implemented: (1) discriminant classifier, (2) support vector machines, (3) adaptive boosting, and (4) supervised artificial neural network. The results of the classification, compared with the visual analysis, showed average accuracies equal to 84.9 and 81.5% for the linear discriminant and the neural network, respectively, while AdaBoost had a slightly lower accuracy, equal to 79.4%. The SVM leads to accuracy of 81.9%. The performance achieved by the automatic classification is encouraging, since an efficient automatic classifier would benefit the practice in everyday clinics, preventing the physician from the time-consuming activity of the visually scoring of the sleep microstructure over whole 8-h sleep recordings. Finally, the classification based on learning algorithms would provide an objective criterion, overcoming the problems of inter-scorer disagreement. [ABSTRACT FROM AUTHOR]
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- 2012
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186. Characterization of A phases during the Cyclic Alternating Pattern of sleep
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Mariani, Sara, Manfredini, Elena, Rosso, Valentina, Mendez, Martin O., Bianchi, Anna M., Matteucci, Matteo, Terzano, Mario G., Cerutti, Sergio, and Parrino, Liborio
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ELECTROENCEPHALOGRAPHY , *SIGNAL processing , *MICROSTRUCTURE , *SLEEP disorders , *RECEIVER operating characteristic curves , *NEUROLOGY - Abstract
Abstract: Objective: This study aims to identify, starting from a single EEG trace, quantitative distinctive features characterizing the A phases of the Cyclic Alternating Pattern (CAP). Methods: The C3-A2 or C4-A1 EEG leads of the night recording of eight healthy adult subjects were used for this analysis. CAP was scored by an expert and the portions relative to NREM were selected. Nine descriptors were computed: band descriptors (low delta, high delta, theta, alpha, sigma and beta); Hjorth activity in the low delta and high delta bands; differential variance of the EEG signal. The information content of each descriptor in recognizing the A phases was evaluated through the computation of the ROC curves and the statistics sensitivity, specificity and accuracy. Results: The ROC curves show that all the descriptors have a certain significance in characterizing A phases. The average accuracy obtained by thresholding the descriptors ranges from 59.89 (sigma descriptor) to 72.44 (differential EEG variance). Conclusions: The results show that it is possible to attribute a significant quantitative value to the information content of the descriptors. Significance: This study gives a mathematical confirm to the features of CAP generally described qualitatively, and puts the bases for the creation of automatic detection methods. [Copyright &y& Elsevier]
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- 2011
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187. Precipitation behavior of a Co-free Fe-Ni-Cr-Mo-Ti-Al maraging steel after severe plastic deformation.
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Zeisl, S., Lassnig, A., Hohenwarter, A., and Mendez-Martin, F.
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MARAGING steel , *MATERIAL plasticity , *ATOM-probe tomography , *DEFORMATIONS (Mechanics) , *HEAT treatment - Abstract
Maraging steels are martensitic steels that are strengthened by the precipitation of nano-sized intermetallic phases. Severe plastic deformation is known to influence diffusion, stability and properties of metallic materials. The goal of this study is to reveal the influence of severe plastic deformation on the hardness and the precipitation behavior of a Co-free maraging steel. For this study the as-quenched material was deformed with high-pressure torsion. Afterwards, a series of heat treatments were conducted. The hardness increase was measured and correlated with micro-to-nanoscale microstructural characteristics, focused on the precipitate characterization with atom probe tomography. Severe plastic deformation transformed the microstructure to a globular microstructure, delayed and minimized austenite reversion as well as accelerated and influenced the precipitation reaction, affecting the size, morphology and type of intermetallic phases. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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188. Giant hardening response in AlMgZn(Cu) alloys.
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Stemper, Lukas, Tunes, Matheus A., Dumitraschkewitz, Phillip, Mendez-Martin, Francisca, Tosone, Ramona, Marchand, Daniel, Curtin, William A., Uggowitzer, Peter J., and Pogatscher, Stefan
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ALLOYS , *HEAT treatment , *DISCONTINUOUS precipitation , *INDUSTRIAL capacity , *TENSILE tests - Abstract
This study presents a thermomechanical processing concept which is capable of exploiting the full industrial application potential of recently introduced AlMgZn(Cu) alloys. The beneficial linkage of alloy design and processing allows not only to satisfy the long-standing trade-off between high mechanical strength in use and good formability during processing but also addresses the need for economically feasible processing times. After an only 3-hour short pre-aging treatment at 100°C, the two investigated alloys, based on commercial EN AW-5182 and modified with additions of Zn and Zn+Cu respectively, show high formability due to increased work-hardening. Then, these alloys exhibit a giant hardening response of up to 184 MPa to reach a yield strength of 410 MPa after a 20-minute short final heat treatment at 185°C, i.e. paint-baking. This rapid hardening response strongly depends on the number density, size distribution and constitution of precursors acting as preferential nucleation sites for T-phase precursor precipitation during the final high-temperature aging treatment and is significantly increased by the addition of Cu. Minor deformation (2%) after pre-aging and before final heat treatment further enhances the development of hardening precipitates additionally by activating dislocation-supported nucleation and growth. Tensile testing, quantitative and analytical electron-microscopy methods, atom probe analysis and DFT calculations were used to characterize the alloys investigated in this work over the thermomechanical processing route. The influence of pre-strain on the hardening response and the role of Cu additions in early-stage cluster nucleation are discussed in detail and supported by in-situ STEM experiments and first-principles calculations. Image, graphical abstract [ABSTRACT FROM AUTHOR]
- Published
- 2021
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189. Petrographic and sorption-based characterization of bituminous organic matter in the Mandal Formation, Central Graben (Norway).
- Author
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Misch, D., Riedl, F., Liu, B., Horsfield, B., Ziegs, V., Mendez-Martin, F., Vranjes-Wessely, S., and Sachsenhofer, R.F.
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- *
ORGANIC compounds , *GAS absorption & adsorption , *SURFACE area , *SOLVENT extraction , *SCANNING electron microscopy - Abstract
The Upper Jurassic Mandal Fm. of the Central Graben, Norway represents an important source rock that charged major petroleum accumulations in the North Sea, including the giant Ekofisk field. Nevertheless, exploration to date has been less successful than expected in marginal basin position such as the Cod Terrace, the Mandal High or the Søgne Basin, probably due to higher proportions of thermally stable (type III) kerogen. In an attempt to delineate changes in initial kerogen composition from later effects such as delayed expulsion of hydrocarbons, traditional organic petrography and scanning electron microscopy were combined with organic geochemical proxies and gas adsorption tests. The kerogen composition of the Mandal Fm. shows considerable variation. Samples hosting autochthonous coaly layers were found in wells from the Søgne Basin and the Cod Terrace, for which less generative potential was previously postulated. Nevertheless, samples hosting mainly vitrodetrinite were also found in basinal wells. A correlation of total organic carbon contents with liptinite percentages highlights enhanced bioproductivity or preservation efficiency for samples with abundant algal organic matter, that were likely deposited under deeper water and possibly oxygen-depleted conditions. By combining organic geochemical proxies with nitrogen sorption data, it could be proven that in case of the Mandal Fm., the (bituminous) organic matter fraction represents the controlling factor on abundance of micro- and mesopores and hence adsorptive gas retention. The amount of bitumen extractable from the Rock-Eval S2 peak (S2 bitumen) shows a strong correlation with the total inner surface area, suggesting that small mesopores (<10–15 nm) are mainly associated with the high-molecular bituminous fraction represented by the S2 bitumen , which appears non-porous at SEM-scale. Furthermore, the total inner surface area decreases strongly with thermal maturity, documenting a change in pore characteristics of the organic matter fraction (growth of mesopores and occurrence of macropores) by advancing hydrocarbon generation. Pyrobitumen-rich Upper Visean reference samples at peak oil and early wet gas window maturity show intense sponge-like pyrobitumen-hosted porosity coinciding with a low relative proportion of S2 bitumen (high petroleum quality). Pyrobitumen is not affected by solvent extraction, thus not contributing high-molecular weight compounds to the extracted fraction. Such inert meso- to macroporous residues might contribute only relatively little to gas sorption capacity, but might represent important storage space for free gas, as well as flow pathways during expulsion. • Samples rich in vitrodetrinite are common both in marginal and basinal positions • Coaly layers were exclusively found in the Søgne and Cod areas • A trend of TOC vs. liptinite indicates enhanced bioproductivity or preservation • High-molecular weight bitumen controls micro- and mesopores • The total inner surface area decreases with thermal maturity [ABSTRACT FROM AUTHOR]
- Published
- 2019
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190. Editorial: Stress and the brain: advances in neurophysiological measures for mental stress detection and reduction.
- Author
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Coelli S, Maggioni E, and Mendez MO
- Abstract
Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
- Published
- 2024
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191. Multifractal analysis of heart rate variability in pregnancy during sleep.
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Mendez MO, Bianchi AM, Recker F, Strizek B, Murguía JS, Reali P, and Jimenez-Cruz J
- Abstract
Understanding the complex dynamics of heart rate variability (HRV) during pregnancy is crucial for monitoring both maternal well-being and fetal health. In this study, we use the Multifractal Detrended Fluctuations Analysis approach to investigate HRV patterns in pregnant individuals during sleep based on RR interval maxima (MM fluctuations). In addition, we study the type of multifractality within MM fluctuations, that is, if it arises from a broad probability density function or from varying long-range correlations. Furthermore, to provide a comprehensive view of HRV changes during sleep in pregnancy, classical temporal and spectral HRV indices were calculated at quarterly intervals during sleep. Our study population consists of 21 recordings from nonpregnant women, 18 from the first trimester (early-pregnancy) and 18 from the second trimester (middle-pregnancy) of pregnancy. Results. There are statistically significant differences ( p -value < 0.05) in mean heart rate, rms heart rate, mean MM fluctuations, and standard deviation of MM fluctuations, particularly in the third and fourth quarter of sleep between pregnant and non-pregnant states. In addition, the early-pregnancy group shows significant differences ( p -value < 0.05) in spectral indices during the first and fourth quarter of sleep compared to the non-pregnancy group. Furthermore, the results of our research show striking similarities in the average multifractal structure of MM fluctuations between pregnant and non-pregnant states during normal sleep. These results highlight the influence of different long-range correlations within the MM fluctuations, which could be primarily associated with the emergence of sleep cycles on multifractality during sleep. Finally, we performed a separability analysis between groups using temporal and spectral HRV indices as features per sleep quarter. Employing only three features after Principal Component Analysis (PCA) to the original feature set, achieving complete separability among all groups appears feasible. Using multifractal analysis, our study provides a comprehensive understanding of the complex HRV patterns during pregnancy, which holds promise for maternal and fetal health monitoring. The separability analysis also provides valuable insights into the potential for group differentiation using simple measures such as mean heart rate, rms heart rate, and mean MM fluctuations or in the transformed feature space based on PCA., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2024 Mendez, Bianchi, Recker, Strizek, Murguía, Reali and Jimenez-Cruz.)
- Published
- 2024
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192. Microstructure-Property Correlation and Its Evolution during Aging in an Al 4.4 Co 26 Cr 19 Fe 18 Ni 27 Ti 5.6 High-Entropy Alloy.
- Author
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Biermair F, Mendez-Martin F, Razumovskiy VI, Moitzi F, and Ressel G
- Abstract
The efficient energy use in multiple sectors of modern industry is partly based on the efficient use of high-strength, high-performance alloys that retain remarkable mechanical properties at elevated and high temperatures. High-entropy alloys (HEAs) represent the most recent class of these materials with a high potential for high-temperature high-strength applications. Aside from their chemical composition and microstructure-property relationship, limited information on the effect of heat treatment as a decisive factor for alloy design is available in the literature. This work intends to contribute to this research topic by investigating the effect of heat treatment on the microstructure and mechanical performance of an Al
4.4 Co26 Cr19 Fe18 Ni27 Ti5.6 HEA. The solution annealed state is compared to aged states obtained at different heat treatment times at 750 °C. The temporal evolution of the matrix and the γ'-precipitates are analyzed in terms of chemical composition, crystallography, size, shape, and volume fraction by means of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The yield strength evolution and strength contributions are calculated by classical state-of-the-art models as well as by ab-initio-based calculations of the critical resolved shear stress. The findings indicate promising mechanical properties of the investigated alloy and provide insight not only into possible strengthening mechanisms but also into the evolution of main phases during the heat treatment.- Published
- 2023
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193. 3D nanoscale elemental mapping of precipitates in steel: Evaluation of analytical electron tomography and comparison to atom probe tomography.
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Trummer C, Mendez-Martin F, Kothleitner G, and Haberfehlner G
- Abstract
To identify different types of precipitates and their composition, analytical electron tomography analysis on a needle-shaped sample was performed. Three-dimensional chemical maps from each element in a high alloyed steel are acquired and the resulting elemental maps are jointly reconstructed. Since analytical electron tomography data suffers from noise, total generalized variation regularization is used to improve the reconstruction quality compared to conventional reconstruction techniques. We analyse the influence of regularization parameters on these reconstructions in terms of elemental quantification, and compare these results to atom probe tomography measurements done on another sample of the same material., (Copyright © 2022. Published by Elsevier Ltd.)
- Published
- 2022
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194. Assessment of Singularities in the EEG During A-Phases of Sleep Based on Wavelet Decomposition.
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Medina-Ibarra DI, Chouvarda I, Murguia JS, Alba A, Arce-Santana ER, Bianchi AM, and Mendez MO
- Subjects
- Brain, Healthy Volunteers, Humans, Sleep Stages physiology, Electroencephalography, Sleep physiology
- Abstract
Electroencephalography (EEG) signals convey information related to different processes that take place in the brain. From the EEG fluctuations during sleep, it is possible to establish the sleep stages and identify short events, commonly related to a specific physiological process or pathology. Some of these short events (called A-phases) present an organization and build up the concept of the Cyclic Alternating Pattern (CAP) phenomenon. In general, the A-phases abruptly modify the EEG fluctuations, and a singular behavior could occur. With the aim to quantify the abrupt changes during A-phases, in this work the wavelet analysis is considered to compute Hölder exponents, which measure the singularity strength. We considered time windows of 2s outside and 5s inside A-phases onset (or offset). A total number of 5121 A-phases from 9 healthy participants and 10 patients with periodic leg movements were analyzed. Within an A-phase the Hölder numerical value tends to be 0.6, which implies a less abrupt singularity. Whereas outside of A-phases, it is observed that the Hölder value is approximately equal to 0.3, which implies stronger singularities, i.e., a more evident discontinuity in the signal behavior. In addition, it seems that the number of singularities increases inside of A-phases. The numerical results suggest that the EEG naturally conveys singularities modified by the A-phase occurrence, and this information could help to conceptualize the CAP phenomenon from a new perspective based on the sharpness of the EEG instead of the oscillatory way.
- Published
- 2022
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195. Community-Driven Methods for Open and Reproducible Software Tools for Analyzing Datasets from Atom Probe Microscopy.
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Kühbach M, London AJ, Wang J, Schreiber DK, Mendez Martin F, Ghamarian I, Bilal H, and Ceguerra AV
- Abstract
Atom probe tomography, and related methods, probe the composition and the three-dimensional architecture of materials. The software tools which microscopists use, and how these tools are connected into workflows, make a substantial contribution to the accuracy and precision of such material characterization experiments. Typically, we adapt methods from other communities like mathematics, data science, computational geometry, artificial intelligence, or scientific computing. We also realize that improving on research data management is a challenge when it comes to align with the FAIR data stewardship principles. Faced with this global challenge, we are convinced it is useful to join forces. Here, we report the results and challenges with an inter-laboratory call for developing test cases for several types of atom probe microscopy software tools. The results support why defining detailed recipes of software workflows and sharing these recipes is necessary and rewarding: Open source tools and (meta)data exchange can help to make our day-to-day data processing tasks become more efficient, the training of new users and knowledge transfer become easier, and assist us with automated quantification of uncertainties to gain access to substantiated results.
- Published
- 2021
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196. Fluorescence background removal method for biological Raman spectroscopy based on empirical mode decomposition.
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Leon-Bejarano M, Dorantes-Mendez G, Ramirez-Elias M, Mendez MO, Alba A, Rodriguez-Leyva I, and Jimenez M
- Subjects
- Algorithms, Fluorescence, Humans, Signal-To-Noise Ratio, Skin chemistry, Signal Processing, Computer-Assisted, Spectrum Analysis, Raman methods
- Abstract
Raman spectroscopy of biological tissue presents fluorescence background, an undesirable effect that generates false Raman intensities. This paper proposes the application of the Empirical Mode Decomposition (EMD) method to baseline correction. EMD is a suitable approach since it is an adaptive signal processing method for nonlinear and non-stationary signal analysis that does not require parameters selection such as polynomial methods. EMD performance was assessed through synthetic Raman spectra with different signal to noise ratio (SNR). The correlation coefficient between synthetic Raman spectra and the recovered one after EMD denoising was higher than 0.92. Additionally, twenty Raman spectra from skin were used to evaluate EMD performance and the results were compared with Vancouver Raman algorithm (VRA). The comparison resulted in a mean square error (MSE) of 0.001554. High correlation coefficient using synthetic spectra and low MSE in the comparison between EMD and VRA suggest that EMD could be an effective method to remove fluorescence background in biological Raman spectra.
- Published
- 2016
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197. Analysis of A-phase transitions during the cyclic alternating pattern under normal sleep.
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Mendez MO, Chouvarda I, Alba A, Bianchi AM, Grassi A, Arce-Santana E, Milioli G, Terzano MG, and Parrino L
- Subjects
- Adult, Electroencephalography, Female, Healthy Volunteers, Humans, Male, Middle Aged, Sleep physiology
- Abstract
An analysis of the EEG signal during the B-phase and A-phases transitions of the cyclic alternating pattern (CAP) during sleep is presented. CAP is a sleep phenomenon composed by consecutive sequences of A-phases (each A-phase could belong to a possible group A1, A2 or A3) observed during the non-REM sleep. Each A-phase is separated by a B-phase which has the basal frequency of the EEG during a specific sleep stage. The patterns formed by these sequences reflect the sleep instability and consequently help to understand the sleep process. Ten recordings from healthy good sleepers were included in this study. The current study investigates complexity, statistical and frequency signal properties of electroencephalography (EEG) recordings at the transitions: B-phase--A-phase. In addition, classification between the onset-offset of the A-phases and B-phase was carried out with a kNN classifier. The results showed that EEG signal presents significant differences (p < 0.05) between A-phases and B-phase for the standard deviation, energy, sample entropy, Tsallis entropy and frequency band indices. The A-phase onset showed values of energy three times higher than B-phase at all the sleep stages. The statistical analysis of variance shows that more than 80% of the A-phase onset and offset is significantly different from the B-phase. The classification performance between onset or offset of A-phases and background showed classification values over 80% for specificity and accuracy and 70% for sensitivity. Only during the A3-phase, the classification was lower. The results suggest that neural assembles that generate the basal EEG oscillations during sleep present an over-imposed coordination for a few seconds due to the A-phases. The main characteristics for automatic separation between the onset-offset A-phase and the B-phase are the energy at the different frequency bands.
- Published
- 2016
- Full Text
- View/download PDF
198. Robot-assisted gastroesophageal surgery: usefulness and limitations.
- Author
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Diez Del Val I, Martinez Blazquez C, Loureiro Gonzalez C, Vitores Lopez JM, Sierra Esteban V, Barrenetxea Asua J, Del Hoyo Aretxabala I, Perez de Villarreal P, Bilbao Axpe JE, and Mendez Martin JJ
- Abstract
Robot-assisted surgery overcomes some of the limitations of traditional laparoscopic surgery. We present our experience and lessons learned in two surgical units dedicated to gastro-esophageal surgery. From June 2009 to January 2013, we performed 130 robot-assisted gastroesophageal procedures, including Nissen fundoplication (29), paraesophageal hernia repair (18), redo for failed antireflux surgery (11), esophagectomy (19), subtotal (5) or wedge (4) gastrectomy, Heller myotomy for achalasia (22), gastric bypass for morbid obesity (12), thoracoscopic leiomyomectomy (4), Morgagni hernia repair (3), lower-third esophageal diverticulectomy (1) and two diagnostic procedures. There were 80 men and 50 women with a median age of 54 years (interquartile range: 46-65). Ten patients (7.7 %) had severe postoperative complications: eight after esophagectomy (three leaks-two cervical and one thoracic-managed conservatively), one stapler failure, one chylothorax, one case of gastric migration to the thorax, one case of biliary peritonitis, and one patient with a transient ventricular dyskinesia. One redo procedure needed reoperation because of port-site bleeding, and one patient died of pulmonary complications after a giant paraesophageal hernia repair; 30-day mortality was, therefore, 0.8 %. There were six elective and one forced conversions (hemorrhage), so total conversion was 5.4 %. Median length of stay was 4 days (IQ range 3-7). Robot-assisted gastroesophageal surgery is feasible and safe, and may be applied to most common procedures. It seems of particular value for Heller myotomy, large paraesophageal hernias, redo antireflux surgery, transhiatal dissection, and hand-sewn intrathoracic anastomosis.
- Published
- 2014
- Full Text
- View/download PDF
199. Contribution of robotics to minimally invasive esophagectomy.
- Author
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Diez Del Val I, Loureiro Gonzalez C, Larburu Etxaniz S, Barrenetxea Asua J, Leturio Fernandez S, Ruiz Carballo S, Etxebarria Beitia E, Perez de Villarreal P, Hierro-Olabarria L, Bilbao Axpe JE, and Mendez Martin JJ
- Abstract
Robot-assisted surgery has the advantages of a three-dimensional view, versatility of instruments and better ergonomics. It allows fine dissection and difficult anastomoses in deep fields. Based on our experience, we try to define what are the main contributions of robotics to minimally invasive esophagectomy. From December 2009 to July 2012, we performed 24 minimally invasive esophagectomies (9 transhiatal, 5 Ivor-Lewis and 10 three-field), 16 of them robotically (8, 5 and 3, respectively). Eighteen patients (18/24 = 75 %) received neoadjuvant therapy. Nine patients (9/24 = 37.5 %) had symptomatic complications: 4 anastomotic leaks treated conservatively, one staple failure of the gastric plasty needing reoperation, one biliary peritonitis secondary to a gangrenous cholecystitis, one intrathoracic gastric migration after the only nonresectable case, one chylothorax and one patient with major cardiopulmonary complications. The median number of lymph nodes harvested was 12 ± 7. Median length of stay was 14 ± 13.5 days. Thirty-day mortality was nil. Complications were not related to the robot itself but to the complexity of both the technique and the patient. Although we found no advantages for the use of robotics during threefield minimally invasive esophagectomy, robotic mediastinal dissection during transhiatal esophagectomy can be performed safely under direct vision. Moreover, hand-sewn robotic-assisted technique in the prone position is promising and maybe the simplest way to carry out thoracic anastomosis during Ivor-Lewis esophagectomy.
- Published
- 2013
- Full Text
- View/download PDF
200. Methods for heart rate variability analysis during sleep.
- Author
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Bianchi AM and Mendez MO
- Subjects
- Data Interpretation, Statistical, Female, Humans, Polysomnography, Heart Rate, Sleep physiology
- Abstract
In the last years we have witnessed the growing interest in the heart rate variability (HRV) signal analysis during sleep. The study of the autonomic regulation during sleep allowed developing methods for automatic detection and classification of some sleep characteristics, both in physiological and pathological conditions. The main problems which require to be faced are the presence of frequent non-stationarities in the signal and the need of dealing with long term analysis, in order to provide reliable indices able to describe the whole night of sleep. In the present paper we are presenting some of the methodologies we recently employed in the study of the heart rate variability during sleep, ranging from time-frequency analysis to long time correlation. Some results are also presented, related to different applications, dealing with both physiological and pathological conditions.
- Published
- 2013
- Full Text
- View/download PDF
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