Examinando por Autor "Risk, Marcelo"
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Artículo de Publicación Periódica Bandt-Pompe symbolization dynamics for time series with tied values: a data-driven approach(2018-07) Traversaro Varela, Francisco; Redelico, Francisco; Risk, Marcelo; Frery, Alejandro C.; Rosso, Osvaldo A."In 2002, Bandt and Pompe [Phys. Rev. Lett. 88, 174102 (2002)] introduced a successfully symbolic encoding scheme based on the ordinal relation between the amplitude of neighboring values of a given data sequence, from which the permutation entropy can be evaluated. Equalities in the analyzed sequence, for example, repeated equal values, deserve special attention and treatment as was shown recently by Zunino and co-workers [Phys. Lett. A 381, 1883 (2017)]. A significant number of equal values can give rise to false conclusions regarding the underlying temporal structures in practical contexts. In the present contribution, we review the different existing methodologies for treating time series with tied values by classifying them according to their different strategies. In addition, a novel data-driven imputation is presented that proves to outperform the existing methodologies and avoid the false conclusions pointed by Zunino and co-workers."Artículo de Publicación Periódica Classification of normal and pre-ictal EEG signals using permutation entropies and a generalized linear model as a classifier(2017-02) Redelico, Francisco; Traversaro Varela, Francisco; García, María del Carmen; Silva, Walter; Rosso, Osvaldo A.; Risk, Marcelo"In this contribution, a comparison between different permutation entropies as classifiers of electroencephalogram (EEG) records corresponding to normal and pre-ictal states is made. A discrete probability distribution function derived from symbolization techniques applied to the EEG signal is used to calculate the Tsallis entropy, Shannon Entropy, Renyi Entropy, and Min Entropy, and they are used separately as the only independent variable in a logistic regression model in order to evaluate its capacity as a classification variable in a inferential manner. The area under the Receiver Operating Characteristic (ROC) curve, along with the accuracy, sensitivity, and specificity are used to compare the models. All the permutation entropies are excellent classifiers, with an accuracy greater than 94.5% in every case, and a sensitivity greater than 97%. Accounting for the amplitude in the symbolization technique retains more information of the signal than its counterparts, and it could be a good candidate for automatic classification of EEG signals."Artículo de Publicación Periódica Electropore formation in mechanically constrained phospholipid bilayers(2018-04) Fernández, María Laura; Risk, Marcelo; Vernier, P. Thomas"Molecular dynamics simulations of lipid bilayers in aqueous systems reveal how an applied electric field stabilizes the reorganization of the water–membrane interface into water-filled, membrane-spanning, conductive pores with a symmetric, toroidal geometry. The pore formation process and the resulting symmetric structures are consistent with other mathematical approaches such as continuum models formulated to describe the electroporation process. Some experimental data suggest, however, that the shape of lipid electropores in living cell membranes may be asymmetric. We describe here the axially asym-metric pores that form when mechanical constraints are applied to selected phospholipid atoms. Electropore formation pro-ceeds even with severe constraints in place, but pore shape and pore formation time are affected. Since lateral and transverse movement of phospholipids may be restricted in cell membranes by covalent attachments to or non-covalent associations with other components of the membrane or to membrane-proximate intracellular or extracellular biomolecular assemblies, these lipid-constrained molecular models point the way to more realistic representations of cell membranes in electric fields."Artículo de Publicación Periódica Fast statistical model-based classification of epileptic EEG signals(2018-01) Quintero-Rincón, Antonio; Pereyra, Marcelo; D'Giano, Carlos; Risk, Marcelo; Batatia, Hadj"This paper presents a supervised classification method to accurately detect epileptic brain activity in real-time from electroencephalography (EEG) data. The proposed method has three main strengths: it has low computational cost, making it suitable for real-time implementation in EEG devices; it performs detection separately for each brain rhythm or EEG spectral band, following the current medical practices; and it can be trained with small datasets, which is key in clinical problems where there is limited annotated data available. This is in sharp contrast with modern approaches based on machine learning techniques, which achieve very high sensitivity and specificity but require large training sets with expert annotations that may not be available. The proposed method proceeds by first separating EEG signals into their five brain rhythms by using awavelet filter bank. Each brain rhythm signal is then mapped to a low-dimensional manifold by using a generalized Gaussian statistical model; this dimensionality reduction step is computationally straightforward and greatly improves supervised classification performance in problems with little training data available. Finally, this is followed by parallel linear classifications on the statistical manifold to detect if the signals exhibit healthy or abnormal brain activity in each spectral band. The good performance of the proposed method is demonstrated with an application to paediatric neurology using 39 EEG recordings from the Children's Hospital Boston database, where it achieves an average sensitivity of 98%, specificity of 88%, and detection latency of 4 s, performing similarly to the best approaches from the literature."Artículo de Publicación Periódica Herramienta web para post-análisis de simulaciones de dinámica molecular(2013) Borgna, Karina Giselle; Fernández, María Laura; Risk, Marcelo"En este trabajo se busca desarrollar una aplicación web interactiva y personalizada que permita analizar resultados generados mediante simulaciones de dinámica molecular. Con esta herramienta se pretende analizar las propiedades cada átomo o molécula de manera individual o conjunta a partir de una trayectoria, permitiendo segmentaciones tridimensionales que el usuario puede personalizar, así como análisis estadísticos no disponibles en las aplicaciones de post análisis que brindan los paquetes de dinámica molecular. Una herramienta de estas características es de gran ayuda en el estudio de los datos obtenidos permitiendo al usuario proponer el análisis o representación de los mismos que considere necesarios. Para este trabajo, como ejemplo, se tomaron datos generados por el programa Gromacs y fueron incorporados a una base de datos desarrollada en un entorno web. Se seleccionaron simulaciones de una bicapa lipídica expuesta a un campo eléctrico por el gran costo computacional que presenta de modo de generar una aplicación que pueda ser capaz de analizar datos independientemente de la demanda que éstos requieran. El sitio web se desarrolló con el framework Django, el cual permite utilizar Python, mientras que la base de datos se implementó en PostgreSQL. Se obtuvo una aplicación web que permite cargar datos de una trayectoria y generar un análisis estadístico, así como gráficos y representaciones espaciales. Se propone generar modelos 3D que permitan estimar superficie o volumen de la selección de átomos que el usuario considere necesarios."Ponencia en Congreso Impact of participatory design for drug-drug interaction alerts: a comparison study between two interfaces(2017) Luna, Daniel; Otero, Carlos; Risk, Marcelo; Stanziola, Enrique; González Bernaldo de Quirós, Fernán"Decision support systems for alert drug-drug interactions have been shown as valid strategy to reduce medical error. Even so the use of these systems has not been as expected, probably due to the lack of a suitable design. This study compares two interfaces, one of them developed using participatory design techniques (based on user centered design processes). This work showed that the use of these techniques improves satisfaction, effectiveness and efficiency in an alert system for drug-drug interactions, a fact that was evident in specific situations such as the decrease of errors to meet the specified task, the time, the workload optimization and users overall satisfaction with the system."Póster In vitro comparative study between conventional and computer-assisted surgery methods for planning and resection of bone sarcomas(2017-08) Ritacco, Lucas E.; Aponte-Tinao, Luis A.; Múscolo, D. Luis; Ayerza, Miguel A.; Albergo, José I.; Farfalli, Germán L.; Risk, Marcelo; González Bernaldo de Quirós, Fernán; Milano, Federico E."This poster aims to achieve an “in vitro” comparative study between three methods: 2D digital images planning and execution without navigation (freehand with ruler and caliper), 3D planning and execution without navigation (freehand with ruler and caliper) and 3D planning and execution guided with navigation. 3D planning and navigated procedures potentially improve sarcoma resection."Capítulo de libro Lipid electropore stabilization(2017-08) Fernández, María Laura; Risk, Marcelo"The stabilization of pores can be studied by different approaches such as simulations in silico or experimental procedures in vivo or in vitro. The energy to open a pore in a lipid membrane can be delivered by different external stimuli. To disrupt the membrane and initiate the pore opening, this energy has to reach a threshold. Then, once the pore is open, the external stimulus can be modulated to maintain the pore stable in time. This chapter first describes the basics of electropermeabilization, a process also called electroporation, and the basics of molecular dynamics in electropermeabilization. The chapter then describes in detail the molecular changes that lead to the pore opening and evolution by molecular dynamics. The chapter focuses on molecular dynamics because this technique allows the study of pore stabilization at molecular level, the interpretation of the lipid and water molecule rearrangements that are behind this phenomenon, and the visualization of the pore at the scale of size and time, in the order of nanometers and nanoseconds, respectively. Finally, the chapter also describes other approaches where pores remain open or the permeabilized state remains stable for a period of time, such as continuum modeling, experiments in planar membranes, and experiments in cells. The objective of this selection is to relate the results obtained by molecular dynamics with those obtained experimentally, or by other types of modeling, aiming to connect the mechanisms of pore stabilization by molecular dynamics at different scales."Artículo de Publicación Periódica Nanobioingeniería: historia y futuro(2015) Risk, Marcelo"Las aplicaciones de la nanotecnología desde la ingeniería a los ámbitos de la biología y la medicina constituyen la nanobioingeniería. Entonces la nanobioingeniería se puede describir como un área de trabajo multidisciplinaria, donde convergen las ciencias básicas como biología, matemática, química y física, con la medicina, la electrónica, la mecánica, informática y los aspectos éticos y legales."Artículo de Publicación Periódica Obtaining accurate and calibrated coil models for transcranial magnetic stimulation using magnetic field measurements(2020) Mancino, Axel; Milano, Federico E.; Martín-Bertuzzi, Fiorella; Yampolsky, C. G.; Ritacco, Lucas E.; Risk, Marcelo"Currently, simulations of the induced currents in the brain produced by transcranial magnetic stimulation (TMS) are used to elucidate the regions reached by stimuli. However, models commonly found in the literature are too general and neglect imperfections in the windings. Aiming to predict the stimulation sites in patients requires precise modeling of the electric field (E-field), and a proper calibration to adequate to the empirical data of the particular coil employed. Furthermore, most fabricators do not provide precise information about the coil geometries, and even using X-ray images may lead to subjective interpretations. We measured the three components of the vector magnetic field induced by a TMS figure-8 coil with spatial resolutions of up to 1 mm. Starting from a computerized tomography-based coil model, we applied a multivariate optimization algorithm to automatically modify the original model and obtain one that optimally fits the measurements. Differences between models were assessed in a human brain mesh using the finite-elements method showing up to 6% variations in the E-field magnitude. Our calibrated model could increase the precision of the estimated E-field induced in the brain during TMS, enhance the accuracy of delivered stimulation during functional brain mapping, and improve dosimetry for repetitive TMS."Artículo de Publicación Periódica Predicción de crisis epilépticas utilizando el coeficiente de correlación producto-momento de Pearson a partir de un clasificador lineal de la distribución Gaussiana generalizada(2018) Quintero-Rincón, Antonio; D'Giano, Carlos; Risk, Marcelo"Predecir una crisis epiléptica significa la capacidad de determinar de antemano el momento de una crisis con la mayor precisión posible. Un pronóstico correcto de un evento epiléptico en aplicaciones clínicas es un problema típico en procesamiento de senales biomédicas, lo cual ayuda a un diagnóstico y tratamiento apropiado de esta enfermedad. En este trabajo, utilizamos el coeficiente de correlación producto-momento de Pearson a partir de las clases estimadas con un clasificador lineal, usando los parámetros de la distribución Gaussiana generalizada. Esto con el fin de poder pronosticar eventos con crisis y eventos con no-crisis en senales epilépticas. El desempeño en 36 eventos epilépticos de 9 pacientes muestra un buen rendimiento, con un 100% de efectividad para sensibilidad y especificidad superior al 83% para eventos con crisis en todos los ritmos cerebrales. El test de Pearson indica que todos los ritmos cerebrales están altamente correlacionados en los eventos con no-crisis, más no durante los eventos con crisis. Esto indica que nuestro modelo puede escalarse con el coeficiente de correlación producto-momento de Pearson para la detección de crisis en senales epilépticas."Ponencia en Congreso Time Domain Estimation of Arterial Parameters using the Windkessel Model and the Monte Carlo Method(IOP Publishing Ltd, 2016) Gostuski, Vladimir; Pastore, Ignacio; Rodriguez Palacios, Gaspar; Vaca Diez, Gustavo; Moscoso-Vasquez, Marcela; Risk, Marcelo"Numerous parameter estimation techniques exist for characterizing the arterial system using electrical circuit analogs. However, they are often limited by their requirements and usually high computational burdain. Therefore, a new method for estimating arterial parameters based on Monte Carlo simulation is proposed. A three element Windkessel model was used to represent the arterial system. The approach was to reduce the error between the calculated and physiological aortic pressure by randomly generating arterial parameter values, while keeping constant the arterial resistance. This last value was obtained for each subject using the arterial flow, and was a necessary consideration in order to obtain a unique set of values for the arterial compliance and peripheral resistance. The estimation technique was applied to in vivo data containing steady beats in mongrel dogs, and it reliably estimated Windkessel arterial parameters. Further, this method appears to be computationally efficient for on-line time-domain estimation of these parameters."Ponencia en Congreso Two-dimensional posture evaluation in Parkinson’s disease: effect of loads on the spinal angle during gait(2016) Celoria, Paula; Nanni, Federico; Pastore, Flavia Carina; Pulenta, Sebastián; Tajerian, Matías Nazareth; Pantazis, Lucio; Moscoso-Vásquez, Marcela; Cerquetti, Daniel; Merello, Marcelo; Risk, Marcelo"Parkinson’s Disease patients present diminished coordination caused by neural degeneration. This leads to large motor difficulties during gait such as balance loss and pronounced forward inclination of the upper body. This work assessed the spinal sagittal plane angle alterations in two groups: six parkinsonian patients and six control healthy subjects. This parameter was analyzed during gait under three conditions: without external loads and with external loads applied either on the chest or on the lower back area. Results were statistically compared by means of t-test of paired samples in both groups. For patients, a significant effect was found when loads were applied on the chest. On the other hand, healthy subjects showed no significant differences in either case."Ponencia en Congreso User-centered design improves the usability of drug-drug interaction alerts: a validation study in the real scenario(2017-08) Luna, Daniel; Rizzato Lede, Daniel; Rubin, Luciana; Otero, Carlos; Ortiz, Juan M.; García, Mónica G.; Rapisarda, Romina P.; Risk, Marcelo; González Bernaldo de Quirós, Fernán"Decision support systems can alert physicians to the existence of drug interactions. The Hospital Italiano de Buenos Aires, Argentina, has an in-house electronic health record with computerized physician order entry and clinical decision support. It includes a drug-drug interaction alert system, initially developed under traditional engineering techniques. As we detected a high alert override rate, we rebuilt the knowledge database and redesigned the alert interface with User-Centered Design techniques. A laboratory crossover study using clinical vignettes showed that new alerts were more usable than traditional ones. This paper aimed to validate these results through a controlled and randomized experimental study with two branches (old vs. new design) in a real setting. We analyzed, quantitatively, every fired alert between April 2015 and September 2016. Finally, we performed user surveys and qualitative interviews to inquire about their satisfaction and perceptions. In real scenarios, user-centered design alerts were more usable, being more effective and satisfactory, but less efficient than traditional alerts. "Safe omission", as a new concept, emerged from our stratified analyses and interviews."Artículo de Publicación Periódica User-centered design improves the usability of drug-drug interaction alerts: experimental comparison of interfaces(2017) Luna, Daniel; Rizzato Lede, Daniel; Otero, Carlos; Risk, Marcelo; González Bernaldo de Quirós, Fernán"Clinical Decision Support Systems can alert health professionals about drug interactions when they prescribe medications. The Hospital Italiano de Buenos Aires in Argentina developed an electronic health record with drug-drug interaction alerts, using traditional software engineering techniques and requirements. Despite enhancing the drug-drug interaction knowledge database, the alert override rate of this system was very high. We redesigned the alert system using user-centered design (UCD) and participatory design techniques to enhance the drug-drug interaction alert interface. This paper describes the methodology of our UCD. We used crossover method with realistic, clinical vignettes to compare usability of the standard and new software versions in terms of efficiency, effectiveness, and user satisfaction. Our study showed that, compared to the traditional alert system, the UCD alert system was more efficient (alerts faster resolution), more effective (tasks completed with fewer errors), and more satisfying. These results indicate that UCD techniques that follow ISO 9241-210 can generate more usable alerts than traditional design."Ponencia en Congreso A visual EEG epilepsy detection method based on a wavelet statistical representation and the Kullback-Leibler divergence(2017-10) Quintero-Rincón, Antonio; Pereyra, Marcelo; D'Giano, Carlos; Batatia, Hadj; Risk, Marcelo"This paper presents a statistical signal processing method for the characterization of EEG of patients suffering from epilepsy. A statistical model is proposed for the signals and the Kullback-Leibler divergence is used to study the differences between Seizure/Non-Seizure in patients suffering from epilepsy. Precisely, EEG signals are transformed into multivariate coefficients through multilevel 1D wavelet decomposition of different brain frequencies. The generalized Gaussian distribution (GGD) is shown to model precisely these coefficients. Patients are compared based on the analytical development of Kullback-Leibler divergence (KLD) of their corresponding GGD distributions. The method has been applied to a dataset of 18 epileptic signals of 9 patients. Results show a clear discrepancy between Seizure/Non-Seizure in epileptic signals, which helps in determining the onset of the seizure."