ABSTRACT A contribution is presented, intended to provide theoretical foundations for the ongoing... more ABSTRACT A contribution is presented, intended to provide theoretical foundations for the ongoing efforts to employ global instability theory for the analysis of the classic boundary-layer flow, and address the associated issue of appropriate inflow/outflow boundary conditions to close the PDE-based global eigenvalue problem in open flows. Starting from a theoretically clean and numerically simple application, in which results are also known analytically and thus serve as a guidance for the assessment of the performance of the numerical methods employed herein, a sequence of issues is systematically built into the target application, until we arrive at one representative of open systems whose instability is presently addressed by global linear theory applied to open flows, the latter application being neither tractable theoretically nor straightforward to solve by numerical means. Experience gained along the way is documented. It regards quantification of the departure of the numerical solution from the analytical one in the simple problem, the generation of numerical boundary layers at artificially truncated boundaries, no matter how far the latter are placed from the region of highest flow gradients and, ultimately the impracti-cally large number of (direct and adjoint) modes necessary to project an arbitrary initial perturbation and follow its temporal evolution by a global analysis approach, a finding which may question the purported robustness reported in the literature of the recovery of optimal perturbations as part of global analyses yielding under-resolved eigenspectra.
Approximately one third of all HIV-infected people are coinfected with hepatitis C virus (HCV). T... more Approximately one third of all HIV-infected people are coinfected with hepatitis C virus (HCV). The progression from initial HCV infection to cirrhosis of the liver is accelerated in coinfected patients compared with HCV-monoinfected patients. Because of improved therapies for HIV/AIDS, one of the leading causes of death for HIV-infected patients is liver disease. It is therefore important to screen all persons with HIV infection for the presence of HCV and to treat the HCV infection as appropriate. Treatment options need to be tailored to each patient, depending on the severity of liver damage, the HCV genotype, and the willingness of the patient to receive therapy. Current first-line therapy for hepatitis C consists of pegylated interferon plus ribavirin. However, this therapy has only limited success in persons with HCV genotype 1 and in African Americans. In addition, the side effects of anti-HCV therapy can be considerable and require careful management. Improvement or delayed progression of fibrosis should be the main goal of therapy in most patients.
The prognosis and management of chronic liver disease largely depends on the extent and progressi... more The prognosis and management of chronic liver disease largely depends on the extent and progression of liver fibrosis. Unfortunately, liver biopsy, an invasive and painful technique with several limitations, continues to be the gold standard for the staging and grading of fibrosis. Therefore, accurate noninvasive tests for liver injury are urgently needed. During the last years, transient elastography (Fibroscan®) has been proposed for the assessment of hepatic fibrosis in patients with chronic liver disease, by measuring liver stiffness. The aim of this study was to evaluate the effectiveness, objectivity and safety of this technique. We included 68 patients who underwent a liver biopsy in the last 18 months with a wide spectrum of chronic liver diseases. All procedures as well as the liver biopsies according to the METAVIR scoring system were analyzed by the same sonographer and the same specialist in pathology, respectively. Median value of stiffness with none or mild fibrosis (F0 and FI), and severe fibrosis or cirrhosis (F3 and F4) was 6.8 ± 3.0 kPa and 21.0 ± 15.1 kPa, respectively, with a significant difference between them (p < 0.01). The areas under the receiver operating characteristic curves showed the optimal liver stiffness cut-off values for each group. We found also a positive correlation between liver stiffness found by transient elastography and fibrosis stage on biopsy in all patients, independently of the liver disease etiology. Fibroscan® is an easy, quick to perform and safe non-invasive method, reliable for assessing liver fibrosis.
The aim of this study was to develop a Brain Computer Interface (BCI) application to control domo... more The aim of this study was to develop a Brain Computer Interface (BCI) application to control domotic devices usually present at home. Electroencephalographic (EEG) activity was recorded from users' scalp and sensorimotor rhythms were used to control the BCI. Our ...
This study is focused on the classification of patients suspected of suffering from obstructive s... more This study is focused on the classification of patients suspected of suffering from obstructive sleep apnea (OSA) by means of cluster analysis. We assessed the diagnostic ability of three clustering algorithms: k-means, hierarchical and fuzzy c-means (FCM). Nonlinear features of blood oxygen saturation (SaO2) from nocturnal oximetry were used as inputs to the clustering methods. Three nonlinear methods were used: approximate entropy (ApEn), central tendency measure (CTM) and Lempel-Ziv (LZ) complexity. A population of 74 subjects (44 OSA positive and 30 OSA negative) was studied. 90.5%, 87.8% and 86.5% accuracies were reached with k-means, hierarchical and FCM algorithms, respectively. The diagnostic accuracy values improved those obtained with each nonlinear method individually. Our results suggest that nonlinear analysis and clustering classification could provide useful information to help in the diagnosis of OSA syndrome.
Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 2011
This study performed an analysis of several feature extraction methods and a genetic algorithm ap... more This study performed an analysis of several feature extraction methods and a genetic algorithm applied to a motor imagery-based Brain Computer Interface (BCI) system. Several features can be extracted from EEG signals to be used for classification in BCIs. However, it is necessary to select a small group of relevant features because the use of irrelevant features deteriorates the performance of the classifier. This study proposes a genetic algorithm (GA) as feature selection method. It was applied to the dataset IIb of the BCI Competition IV achieving a kappa coefficient of 0.613. The use of a GA improves the classification results using extracted features separately (kappa coefficient of 0.336) and the winner competition results (kappa coefficient of 0.600). These preliminary results demonstrated that the proposed methodology could be useful to control motor imagery-based BCI applications.
ABSTRACT A contribution is presented, intended to provide theoretical foundations for the ongoing... more ABSTRACT A contribution is presented, intended to provide theoretical foundations for the ongoing efforts to employ global instability theory for the analysis of the classic boundary-layer flow, and address the associated issue of appropriate inflow/outflow boundary conditions to close the PDE-based global eigenvalue problem in open flows. Starting from a theoretically clean and numerically simple application, in which results are also known analytically and thus serve as a guidance for the assessment of the performance of the numerical methods employed herein, a sequence of issues is systematically built into the target application, until we arrive at one representative of open systems whose instability is presently addressed by global linear theory applied to open flows, the latter application being neither tractable theoretically nor straightforward to solve by numerical means. Experience gained along the way is documented. It regards quantification of the departure of the numerical solution from the analytical one in the simple problem, the generation of numerical boundary layers at artificially truncated boundaries, no matter how far the latter are placed from the region of highest flow gradients and, ultimately the impracti-cally large number of (direct and adjoint) modes necessary to project an arbitrary initial perturbation and follow its temporal evolution by a global analysis approach, a finding which may question the purported robustness reported in the literature of the recovery of optimal perturbations as part of global analyses yielding under-resolved eigenspectra.
Approximately one third of all HIV-infected people are coinfected with hepatitis C virus (HCV). T... more Approximately one third of all HIV-infected people are coinfected with hepatitis C virus (HCV). The progression from initial HCV infection to cirrhosis of the liver is accelerated in coinfected patients compared with HCV-monoinfected patients. Because of improved therapies for HIV/AIDS, one of the leading causes of death for HIV-infected patients is liver disease. It is therefore important to screen all persons with HIV infection for the presence of HCV and to treat the HCV infection as appropriate. Treatment options need to be tailored to each patient, depending on the severity of liver damage, the HCV genotype, and the willingness of the patient to receive therapy. Current first-line therapy for hepatitis C consists of pegylated interferon plus ribavirin. However, this therapy has only limited success in persons with HCV genotype 1 and in African Americans. In addition, the side effects of anti-HCV therapy can be considerable and require careful management. Improvement or delayed progression of fibrosis should be the main goal of therapy in most patients.
The prognosis and management of chronic liver disease largely depends on the extent and progressi... more The prognosis and management of chronic liver disease largely depends on the extent and progression of liver fibrosis. Unfortunately, liver biopsy, an invasive and painful technique with several limitations, continues to be the gold standard for the staging and grading of fibrosis. Therefore, accurate noninvasive tests for liver injury are urgently needed. During the last years, transient elastography (Fibroscan®) has been proposed for the assessment of hepatic fibrosis in patients with chronic liver disease, by measuring liver stiffness. The aim of this study was to evaluate the effectiveness, objectivity and safety of this technique. We included 68 patients who underwent a liver biopsy in the last 18 months with a wide spectrum of chronic liver diseases. All procedures as well as the liver biopsies according to the METAVIR scoring system were analyzed by the same sonographer and the same specialist in pathology, respectively. Median value of stiffness with none or mild fibrosis (F0 and FI), and severe fibrosis or cirrhosis (F3 and F4) was 6.8 ± 3.0 kPa and 21.0 ± 15.1 kPa, respectively, with a significant difference between them (p < 0.01). The areas under the receiver operating characteristic curves showed the optimal liver stiffness cut-off values for each group. We found also a positive correlation between liver stiffness found by transient elastography and fibrosis stage on biopsy in all patients, independently of the liver disease etiology. Fibroscan® is an easy, quick to perform and safe non-invasive method, reliable for assessing liver fibrosis.
The aim of this study was to develop a Brain Computer Interface (BCI) application to control domo... more The aim of this study was to develop a Brain Computer Interface (BCI) application to control domotic devices usually present at home. Electroencephalographic (EEG) activity was recorded from users' scalp and sensorimotor rhythms were used to control the BCI. Our ...
This study is focused on the classification of patients suspected of suffering from obstructive s... more This study is focused on the classification of patients suspected of suffering from obstructive sleep apnea (OSA) by means of cluster analysis. We assessed the diagnostic ability of three clustering algorithms: k-means, hierarchical and fuzzy c-means (FCM). Nonlinear features of blood oxygen saturation (SaO2) from nocturnal oximetry were used as inputs to the clustering methods. Three nonlinear methods were used: approximate entropy (ApEn), central tendency measure (CTM) and Lempel-Ziv (LZ) complexity. A population of 74 subjects (44 OSA positive and 30 OSA negative) was studied. 90.5%, 87.8% and 86.5% accuracies were reached with k-means, hierarchical and FCM algorithms, respectively. The diagnostic accuracy values improved those obtained with each nonlinear method individually. Our results suggest that nonlinear analysis and clustering classification could provide useful information to help in the diagnosis of OSA syndrome.
Annual International Conference of the IEEE Engineering in Medicine and Biology Society, 2011
This study performed an analysis of several feature extraction methods and a genetic algorithm ap... more This study performed an analysis of several feature extraction methods and a genetic algorithm applied to a motor imagery-based Brain Computer Interface (BCI) system. Several features can be extracted from EEG signals to be used for classification in BCIs. However, it is necessary to select a small group of relevant features because the use of irrelevant features deteriorates the performance of the classifier. This study proposes a genetic algorithm (GA) as feature selection method. It was applied to the dataset IIb of the BCI Competition IV achieving a kappa coefficient of 0.613. The use of a GA improves the classification results using extracted features separately (kappa coefficient of 0.336) and the winner competition results (kappa coefficient of 0.600). These preliminary results demonstrated that the proposed methodology could be useful to control motor imagery-based BCI applications.
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Papers by daniel alvarez