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Papers by fabio ayres

Research paper thumbnail of Delimitação Automática De Bacias Hidrográficas Utilizando Dados SRTM Teodorico Alves SOBRINHO1, Paulo TS OLIVEIRA2

RESUMO: A delimitação adequada de bacias hidrográficas é fundamental por ser essa a unidade terri... more RESUMO: A delimitação adequada de bacias hidrográficas é fundamental por ser essa a unidade territorial para fins de planejamento e de gerenciamento dos recursos hídricos. Desse modo, este trabalho teve objetivo de analisar uma metodologia de delimitação automática de bacias hidrográficas. Foram utilizados, para tanto, dados do projeto Shuttle Radar Topographic Mission (SRTM) integrados e processados em Sistema de Informações Geográficas (SIG). Os resultados confirmam valores de área compatíveis aos obtidos com base em cartas topográficas. Assim, a automatização da delimitação de bacias hidrográficas, por meio de dados do SRTM e ambiente SIG, apresenta-se vantajosa em relação ao custo e benefício proporcionado, além de estabelecer a padronização do traçado e posterior minimização de conflitos quanto à fixação da unidade de gestão dos recursos hídricos. PALAVRAS-CHAVE: gestão de recursos hídricos, bacia hidrográfica, delimitação automática. AUTOMATIC DELIMITATION OF WATERSHED USING DATA SRTM ABSTRACT: The adequate delimitation of watersheds is essential to be that the territorial unit for planning and management of water resources. Thus, this work aims to evaluate a methodology for automatic delimitation of drainage basins. The project Shuttle Radar Topographic Mission (SRTM) data were integrated and processed in the Geographic Information System (GIS). The results show values of area compatible to those obtained on the topographic data. So, the automation of delimitation of watersheds, through SRTM data and GIS environment, appears to be advantageous in relation to cost and benefits offered, in addition, permits the standardization of the sketch and minimization of conflicts during fixation of the elementary unit of water resources management.

Research paper thumbnail of A first principles investigation of mercuric iodide: bulk properties and intrinsic defects

Brazilian Journal of Physics, 2004

We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal... more We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. Our calculations were performed using the ab initio spin-polarized full-potential linearized augmented plane wave method, and taking into account full atomic relaxation. The results on the structural, electronic, and optical properties were compared to available experimental data.

Research paper thumbnail of Defects in mercuric iodide: an APW investigation

Physica B-condensed Matter, 2003

We carried a theoretical investigation on the structural and electronic properties of mercuric io... more We carried a theoretical investigation on the structural and electronic properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. The calculations were performed using the total energy spin-polarized augmented plane wave method and the generalized gradient approximation, considering full atomic relaxation. The results were compared to available experimental data. r

Research paper thumbnail of Defects in mercuric iodide: an APW investigation

Physica B-condensed Matter, 2003

We carried a theoretical investigation on the structural and electronic properties of mercuric io... more We carried a theoretical investigation on the structural and electronic properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. The calculations were performed using the total energy spin-polarized augmented plane wave method and the generalized gradient approximation, considering full atomic relaxation. The results were compared to available experimental data.

Research paper thumbnail of Role of intrinsic defects in the electronic and optical properties of alpha-HgI2

Applied Physics Letters, 2006

We investigated the role of intrinsic defects in the electronic and optical properties of mercuri... more We investigated the role of intrinsic defects in the electronic and optical properties of mercuric iodide using ab initio methods. The calculations were performed using the total energy all electron methodology, considering full atomic relaxation. We computed the band structure, spin, formation and transition energies, and the dielectric function of isolated iodine and mercury vacancies in several charge states. Our results were compared to available experimental data on photoluminescence and photoplasticity in HgI2. We propose a microscopic model which can explain most of the data on those luminescent centers, unifying experimental results which suggested conflicting conclusions.

Research paper thumbnail of Reduction of false positives in the detection of architectural distortion in mammograms by using a geometrically constrained phase portrait model

International Journal of Computer Assisted Radiology and Surgery, 2007

Objective One of the commonly missed signs of breast cancer is architectural distortion. We have ... more Objective One of the commonly missed signs of breast cancer is architectural distortion. We have developed techniques for the detection of architectural distortion in mammograms, based on the analysis of oriented texture through the application of Gabor filters and a linear phase portrait model. In this paper, we propose constraining the shape of the general phase portrait model as a means to reduce the false-positive rate in the detection of architectural distortion. Material and methods The methods were tested with one set of 19 cases of architectural distortion and 41 normal mammograms, and with another set of 37 cases of architectural distortion. Results Sensitivity rates of 84% with 4.5 false positives per image and 81% with 10 false positives per image were obtained for the two sets of images. Conclusion The adoption of a constrained phase portrait model with a symmetric matrix and the incorporation of its condition number in the analysis resulted in a reduction in the false-positive rate in the detection of architectural distortion. The proposed techniques, dedicated for the detection and localization of architectural distortion, should lead to efficient detection of early signs of breast cancer.

Research paper thumbnail of Detection of Blood Vessels in the Retina Using Gabor Filters

Abstract Quantitative analysis of the vascular architecture of the retina as well as changes in t... more Abstract Quantitative analysis of the vascular architecture of the retina as well as changes in the shape, width, and tortuosity of the vessels could assist in the monitoring of the effects of diabetes, arteriosclerosis, hypertension, and premature birth on the visual system. The ...

[Research paper thumbnail of Characterization of architectural distortion in mammograms [EMBS 2003 Student Paper Finalists](https://a.academia-assets.com/images/blank-paper.jpg)

The analysis of mammograms is a difficult task due to the subtle appearance of some lesions. Comp... more The analysis of mammograms is a difficult task due to the subtle appearance of some lesions. Computer-aided diagnosis (CAD) have been shown to improve the sensitivity of detection of masses and calcifications; however, there is a need for dedicated methods to detect architectural distortion in the absence of a central mass. Improvement in the detection of architectural distortion may be

Research paper thumbnail of Estimation of the tissue composition of the tumour mass in neuroblastoma using segmented CT images

Medical & Biological Engineering & Computing, 2004

Neuroblastoma is the most common extra-cranial, solid, malignant tumour in children. Advances in ... more Neuroblastoma is the most common extra-cranial, solid, malignant tumour in children. Advances in radiology have made possible the detection and staging of the disease. Nevertheless, there is no method available at present that can go beyond detection and qualitative analysis, towards quantitative assessment of the tissues composition of the primary tumour mass in neuroblastoma. Such quantitative analysis could provide important information and serve as a decision-support tool to the radiologist and the oncologist, result in better treatment and follow-up and even lead to the avoidance of delayed surgery. The problem investigated was the improvement of the analysis of the primary tumour mass, in patients with neuroblastoma, using X-ray computed tomography (CT) images. A methodology was proposed for the estimation of the tissue content of the mass: it comprised a Gaussian mixture model for estimation, from segmented CT images, of the tissue composition of the primary tumour. To demonstrate the potential of the method, the results are presented of its application to ten CT examinations of four patients. The method provides quantitative information, and it was observed that the tumour in one of the patients reduced from 523 cm3 to 81 cm3 in volume, with an increase in calcification from about 20% to about 88% of the tumour volume, in response to chemotherapy over a period of five months. Results indicate that the proposed technique may be of considerable value in assessing the response to therapy of patients with neuroblastoma.

Research paper thumbnail of Detection of the Optic Nerve Head in Fundus Images of the Retina with Gabor Filters and Phase Portrait Analysis

Journal of Digital Imaging, 2010

We propose a method using Gabor filters and phase portraits to automatically locate the optic ner... more We propose a method using Gabor filters and phase portraits to automatically locate the optic nerve head (ONH) in fundus images of the retina. Because the center of the ONH is at or near the focal point of convergence of the retinal vessels, the method includes detection of the vessels using Gabor filters, detection of peaks in the node map obtained via phase portrait analysis, and an intensity-based condition. The method was tested on 40 images from the Digital Retinal Images for Vessel Extraction (DRIVE) database and 81 images from the Structured Analysis of the Retina (STARE) database. An ophthalmologist independently marked the center of the ONH for evaluation of the results. The evaluation of the results includes free-response receiver operating characteristics (FROC) and a measure of distance between the manually marked and detected centers. With the DRIVE database, the centers of the ONH were detected with an average distance of 0.36 mm (18 pixels) to the corresponding centers marked by the ophthalmologist. FROC analysis indicated a sensitivity of 100% at 2.7 false positives per image. With the STARE database, FROC analysis indicated a sensitivity of 88.9% at 4.6 false positives per image.

Research paper thumbnail of A review of computer-aided diagnosis of breast cancer: Toward the detection of subtle signs

Journal of The Franklin Institute-engineering and Applied Mathematics, 2007

Mammography is the best available tool for screening for the early detection of breast cancer. Ma... more Mammography is the best available tool for screening for the early detection of breast cancer. Mammographic screening has been shown to be effective in reducing breast cancer mortality rates: screening programs have reduced mortality rates by 30-70%.

Research paper thumbnail of Three-Dimensional Segmentation of the Tumor in Computed Tomographic Images of Neuroblastoma

Journal of Digital Imaging, 2006

Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost alw... more Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost always heterogeneous in nature; furthermore, viable tumor, necrosis, and normal tissue are often intermixed. Tumor definition and diagnosis require the analysis of the spatial distribution and Hounsfield unit (HU) values of voxels in computed tomography (CT) images, coupled with a knowledge of normal anatomy. Segmentation and analysis of the tissue composition of the tumor can assist in quantitative assessment of the response to therapy and in the planning of delayed surgery for resection of the tumor. We propose methods to achieve 3-dimensional segmentation of the neuroblastic tumor. In our scheme, some of the normal structures expected in abdominal CT images are delineated and removed from further consideration; the remaining parts of the image volume are then examined for the tumor mass. Mathematical morphology, fuzzy connectivity, and other image processing tools are deployed for this purpose. Expert knowledge provided by a radiologist in the form of the expected structures and their shapes, HU values, and radiological characteristics are incorporated into the segmentation algorithm. In this preliminary study, the methods were tested with 10 CT exams of four cases from the Alberta Children's Hospital. False-negative error rates of less than 12% were obtained in eight of the 10 exams; however, seven of the exams had falsepositive error rates of more than 20% with respect to manual segmentation of the tumor by a radiologist.

Research paper thumbnail of Three-Dimensional Segmentation of the Tumor in Computed Tomographic Images of Neuroblastoma

Journal of Digital Imaging, 2007

Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost alw... more Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost always heterogeneous in nature; furthermore, viable tumor, necrosis, and normal tissue are often intermixed. Tumor definition and diagnosis require the analysis of the spatial distribution and Hounsfield unit (HU) values of voxels in computed tomography (CT) images, coupled with a knowledge of normal anatomy. Segmentation and analysis of the tissue composition of the tumor can assist in quantitative assessment of the response to therapy and in the planning of delayed surgery for resection of the tumor. We propose methods to achieve 3-dimensional segmentation of the neuroblastic tumor. In our scheme, some of the normal structures expected in abdominal CT images are delineated and removed from further consideration; the remaining parts of the image volume are then examined for the tumor mass. Mathematical morphology, fuzzy connectivity, and other image processing tools are deployed for this purpose. Expert knowledge provided by a radiologist in the form of the expected structures and their shapes, HU values, and radiological characteristics are incorporated into the segmentation algorithm. In this preliminary study, the methods were tested with 10 CT exams of four cases from the Alberta Children's Hospital. False-negative error rates of less than 12% were obtained in eight of the 10 exams; however, seven of the exams had false-positive error rates of more than 20% with respect to manual segmentation of the tumor by a radiologist.

Research paper thumbnail of Effect of Pixel Resolution on Texture Features of Breast Masses in Mammograms

Journal of Digital Imaging, 2010

The effect of pixel resolution on texture features computed using the gray-level co-occurrence ma... more The effect of pixel resolution on texture features computed using the gray-level co-occurrence matrix (GLCM) was analyzed in the task of discriminating mammographic breast lesions as benign masses or malignant tumors. Regions in mammograms related to 111 breast masses, including 65 benign masses and 46 malignant tumors, were analyzed at pixel sizes of 50, 100, 200, 400, 600, 800, and 1,000 μm. Classification experiments using each texture feature individually provided accuracy, in terms of the area under the receiver operating characteristics curve (AUC), of up to 0.72. Using the Bayesian classifier and the leave-one-out method, the AUC obtained was in the range 0.73 to 0.75 for the pixel resolutions of 200 to 800 μm, with 14 GLCM-based texture features using adaptive ribbons of pixels around the boundaries of the masses. Texture features computed using the ribbons resulted in higher classification accuracy than the same features computed using the corresponding regions within the mass boundaries. The t test was applied to AUC values obtained using 100 repetitions of random splitting of the texture features from the ribbons of masses into the training and testing sets. The texture features computed with the pixel size of 200 μm provided the highest average AUC with statistically highly significant differences as compared to all of the other pixel sizes tested, except 100 μm.

Research paper thumbnail of Performance Analysis of Oriented Feature Detectors

Oriented feature detectors are fundamental tools in image understanding, as many images display r... more Oriented feature detectors are fundamental tools in image understanding, as many images display relevant information in the form of oriented features. Several oriented feature detectors have been developed; some of the important families of oriented feature detectors are steerable filters and Gabor filters. In this work, a performance analysis is presented of the following oriented feature detectors: the Gaussian second-derivative steerable filter, the quadrature-pair Gaussian second-derivative steerable filter, the real Gabor filter, the complex Gabor filter, and a line operator that has been shown to outperform the Gaussian second-derivative steerable filter in the detection of linear structures in mammograms. The detectors are assessed in terms of their capability to detect the presence of oriented features, as well as their accuracy in the estimation of the angle of the oriented features present in the image. It is shown that the Gabor filters yield the best detection performance and angular accuracy, whereas the steerable filters have the best performance in terms of computational speed.

Research paper thumbnail of Um Algoritmo Rápido para Iluminação Volumétrica

Direct Volume Rendering algorithms are very important tools in the analysis of volumetric data in... more Direct Volume Rendering algorithms are very important tools in the analysis of volumetric data in scientific visualization applications. Volume shading is a critical step regarding the computational cost of these algorithms. This paper presents a fast algorithm for volume shading. The proposed technique is based on decomposing the illumination equation into a sum of functions that can be pre-computed in look-up tables. Preliminary results showed the new algorithm is about six times faster than the conventional method.

Research paper thumbnail of Segmentation and estimation of the histological composition of the tumor mass in computed tomographic images of neuroblastoma

The problem that we investigate in the present paper is the improvement of the analysis of the pr... more The problem that we investigate in the present paper is the improvement of the analysis of the primary tumor mass, in patients with advanced neuroblastoma, using X-ray computed tomography (CT) exams. To achieve this goal, we propose a methodology for the estimation of the histological content of the mass that comprises a technique for semi-automatic segmentation of the primary tumor mass in CT images of neuroblastoma and a statistical method to estimate, from segmented CT images, the histological composition of the primary tumor. The results of the method are compared with the results of histological analysis of surgically resected tumor mass.

Research paper thumbnail of Detection of architectural distortion in prior screening mammograms using Gabor filters, phase portraits, fractal dimension, and texture analysis

International Journal of Computer Assisted Radiology and Surgery, 2008

Objective Mammography is a widely used screening tool for the early detection of breast cancer. O... more Objective Mammography is a widely used screening tool for the early detection of breast cancer. One of the commonly missed signs of breast cancer is architectural distortion. The purpose of this study is to explore the application of fractal analysis and texture measures for the detection of architectural distortion in screening mammograms taken prior to the detection of breast cancer. Materials and methods A method based on Gabor filters and phase portrait analysis was used to detect initial candidates for sites of architectural distortion. A total of 386 regions of interest (ROIs) were automatically obtained from 14 “prior mammograms”, including 21 ROIs related to architectural distortion. From the corresponding set of 14 “detection mammograms”, 398 ROIs were obtained, including 18 related to breast cancer. For each ROI, the fractal dimension and Haralick’s texture features were computed. The fractal dimension of the ROIs was calculated using the circular average power spectrum technique. Results The average fractal dimension of the normal (false-positive) ROIs was significantly higher than that of the ROIs with architectural distortion (p = 0.006). For the “prior mammograms”, the best receiver operating characteristics (ROC) performance achieved, in terms of the area under the ROC curve, was 0.80 with a Bayesian classifier using four features including fractal dimension, entropy, sum entropy, and inverse difference moment. Analysis of the performance of the methods with free-response receiver operating characteristics indicated a sensitivity of 0.79 at 8.4 false positives per image in the detection of sites of architectural distortion in the “prior mammograms”. Conclusion Fractal dimension offers a promising way to detect the presence of architectural distortion in prior mammograms.

Research paper thumbnail of Detection of Blood Vessels in Fundus Images of the Retina using Gabor Wavelets

The monitoring of the effects of diabetes, hypertension, and premature birth on the visual system... more The monitoring of the effects of diabetes, hypertension, and premature birth on the visual system can be assisted by quantitative analysis of the vascular architecture of the retina. The application of image analysis techniques in ophthalmology becomes possible after the desired features have been detected through the use of an appropriate method. We propose image processing techniques for the detection of blood vessels in the retina. The methods include the design of a bank of directionally sensitive Gabor filters for several values of the scale and elongation parameters. Forty images of the retina from the DRIVE database were used to evaluate the performance of the methods. High efficiency in the detection of blood vessels with the area under the receiver operating characteristics curve of up to 0.96 was achieved.

Research paper thumbnail of Reduction of false positives in the detection of architectural distortion in mammograms by using a geometrically constrained phase portrait model

International Journal of Computer Assisted Radiology and Surgery, 2007

Objective One of the commonly missed signs of breast cancer is architectural distortion. We have ... more Objective One of the commonly missed signs of breast cancer is architectural distortion. We have developed techniques for the detection of architectural distortion in mammograms, based on the analysis of oriented texture through the application of Gabor filters and a linear phase portrait model. In this paper, we propose constraining the shape of the general phase portrait model as a means to reduce the false-positive rate in the detection of architectural distortion. Material and methods The methods were tested with one set of 19 cases of architectural distortion and 41 normal mammograms, and with another set of 37 cases of architectural distortion. Results Sensitivity rates of 84% with 4.5 false positives per image and 81% with 10 false positives per image were obtained for the two sets of images. Conclusion The adoption of a constrained phase portrait model with a symmetric matrix and the incorporation of its condition number in the analysis resulted in a reduction in the false-positive rate in the detection of architectural distortion. The proposed techniques, dedicated for the detection and localization of architectural distortion, should lead to efficient detection of early signs of breast cancer.

Research paper thumbnail of Delimitação Automática De Bacias Hidrográficas Utilizando Dados SRTM Teodorico Alves SOBRINHO1, Paulo TS OLIVEIRA2

RESUMO: A delimitação adequada de bacias hidrográficas é fundamental por ser essa a unidade terri... more RESUMO: A delimitação adequada de bacias hidrográficas é fundamental por ser essa a unidade territorial para fins de planejamento e de gerenciamento dos recursos hídricos. Desse modo, este trabalho teve objetivo de analisar uma metodologia de delimitação automática de bacias hidrográficas. Foram utilizados, para tanto, dados do projeto Shuttle Radar Topographic Mission (SRTM) integrados e processados em Sistema de Informações Geográficas (SIG). Os resultados confirmam valores de área compatíveis aos obtidos com base em cartas topográficas. Assim, a automatização da delimitação de bacias hidrográficas, por meio de dados do SRTM e ambiente SIG, apresenta-se vantajosa em relação ao custo e benefício proporcionado, além de estabelecer a padronização do traçado e posterior minimização de conflitos quanto à fixação da unidade de gestão dos recursos hídricos. PALAVRAS-CHAVE: gestão de recursos hídricos, bacia hidrográfica, delimitação automática. AUTOMATIC DELIMITATION OF WATERSHED USING DATA SRTM ABSTRACT: The adequate delimitation of watersheds is essential to be that the territorial unit for planning and management of water resources. Thus, this work aims to evaluate a methodology for automatic delimitation of drainage basins. The project Shuttle Radar Topographic Mission (SRTM) data were integrated and processed in the Geographic Information System (GIS). The results show values of area compatible to those obtained on the topographic data. So, the automation of delimitation of watersheds, through SRTM data and GIS environment, appears to be advantageous in relation to cost and benefits offered, in addition, permits the standardization of the sketch and minimization of conflicts during fixation of the elementary unit of water resources management.

Research paper thumbnail of A first principles investigation of mercuric iodide: bulk properties and intrinsic defects

Brazilian Journal of Physics, 2004

We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal... more We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. Our calculations were performed using the ab initio spin-polarized full-potential linearized augmented plane wave method, and taking into account full atomic relaxation. The results on the structural, electronic, and optical properties were compared to available experimental data.

Research paper thumbnail of Defects in mercuric iodide: an APW investigation

Physica B-condensed Matter, 2003

We carried a theoretical investigation on the structural and electronic properties of mercuric io... more We carried a theoretical investigation on the structural and electronic properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. The calculations were performed using the total energy spin-polarized augmented plane wave method and the generalized gradient approximation, considering full atomic relaxation. The results were compared to available experimental data. r

Research paper thumbnail of Defects in mercuric iodide: an APW investigation

Physica B-condensed Matter, 2003

We carried a theoretical investigation on the structural and electronic properties of mercuric io... more We carried a theoretical investigation on the structural and electronic properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. The calculations were performed using the total energy spin-polarized augmented plane wave method and the generalized gradient approximation, considering full atomic relaxation. The results were compared to available experimental data.

Research paper thumbnail of Role of intrinsic defects in the electronic and optical properties of alpha-HgI2

Applied Physics Letters, 2006

We investigated the role of intrinsic defects in the electronic and optical properties of mercuri... more We investigated the role of intrinsic defects in the electronic and optical properties of mercuric iodide using ab initio methods. The calculations were performed using the total energy all electron methodology, considering full atomic relaxation. We computed the band structure, spin, formation and transition energies, and the dielectric function of isolated iodine and mercury vacancies in several charge states. Our results were compared to available experimental data on photoluminescence and photoplasticity in HgI2. We propose a microscopic model which can explain most of the data on those luminescent centers, unifying experimental results which suggested conflicting conclusions.

Research paper thumbnail of Reduction of false positives in the detection of architectural distortion in mammograms by using a geometrically constrained phase portrait model

International Journal of Computer Assisted Radiology and Surgery, 2007

Objective One of the commonly missed signs of breast cancer is architectural distortion. We have ... more Objective One of the commonly missed signs of breast cancer is architectural distortion. We have developed techniques for the detection of architectural distortion in mammograms, based on the analysis of oriented texture through the application of Gabor filters and a linear phase portrait model. In this paper, we propose constraining the shape of the general phase portrait model as a means to reduce the false-positive rate in the detection of architectural distortion. Material and methods The methods were tested with one set of 19 cases of architectural distortion and 41 normal mammograms, and with another set of 37 cases of architectural distortion. Results Sensitivity rates of 84% with 4.5 false positives per image and 81% with 10 false positives per image were obtained for the two sets of images. Conclusion The adoption of a constrained phase portrait model with a symmetric matrix and the incorporation of its condition number in the analysis resulted in a reduction in the false-positive rate in the detection of architectural distortion. The proposed techniques, dedicated for the detection and localization of architectural distortion, should lead to efficient detection of early signs of breast cancer.

Research paper thumbnail of Detection of Blood Vessels in the Retina Using Gabor Filters

Abstract Quantitative analysis of the vascular architecture of the retina as well as changes in t... more Abstract Quantitative analysis of the vascular architecture of the retina as well as changes in the shape, width, and tortuosity of the vessels could assist in the monitoring of the effects of diabetes, arteriosclerosis, hypertension, and premature birth on the visual system. The ...

[Research paper thumbnail of Characterization of architectural distortion in mammograms [EMBS 2003 Student Paper Finalists](https://a.academia-assets.com/images/blank-paper.jpg)

The analysis of mammograms is a difficult task due to the subtle appearance of some lesions. Comp... more The analysis of mammograms is a difficult task due to the subtle appearance of some lesions. Computer-aided diagnosis (CAD) have been shown to improve the sensitivity of detection of masses and calcifications; however, there is a need for dedicated methods to detect architectural distortion in the absence of a central mass. Improvement in the detection of architectural distortion may be

Research paper thumbnail of Estimation of the tissue composition of the tumour mass in neuroblastoma using segmented CT images

Medical & Biological Engineering & Computing, 2004

Neuroblastoma is the most common extra-cranial, solid, malignant tumour in children. Advances in ... more Neuroblastoma is the most common extra-cranial, solid, malignant tumour in children. Advances in radiology have made possible the detection and staging of the disease. Nevertheless, there is no method available at present that can go beyond detection and qualitative analysis, towards quantitative assessment of the tissues composition of the primary tumour mass in neuroblastoma. Such quantitative analysis could provide important information and serve as a decision-support tool to the radiologist and the oncologist, result in better treatment and follow-up and even lead to the avoidance of delayed surgery. The problem investigated was the improvement of the analysis of the primary tumour mass, in patients with neuroblastoma, using X-ray computed tomography (CT) images. A methodology was proposed for the estimation of the tissue content of the mass: it comprised a Gaussian mixture model for estimation, from segmented CT images, of the tissue composition of the primary tumour. To demonstrate the potential of the method, the results are presented of its application to ten CT examinations of four patients. The method provides quantitative information, and it was observed that the tumour in one of the patients reduced from 523 cm3 to 81 cm3 in volume, with an increase in calcification from about 20% to about 88% of the tumour volume, in response to chemotherapy over a period of five months. Results indicate that the proposed technique may be of considerable value in assessing the response to therapy of patients with neuroblastoma.

Research paper thumbnail of Detection of the Optic Nerve Head in Fundus Images of the Retina with Gabor Filters and Phase Portrait Analysis

Journal of Digital Imaging, 2010

We propose a method using Gabor filters and phase portraits to automatically locate the optic ner... more We propose a method using Gabor filters and phase portraits to automatically locate the optic nerve head (ONH) in fundus images of the retina. Because the center of the ONH is at or near the focal point of convergence of the retinal vessels, the method includes detection of the vessels using Gabor filters, detection of peaks in the node map obtained via phase portrait analysis, and an intensity-based condition. The method was tested on 40 images from the Digital Retinal Images for Vessel Extraction (DRIVE) database and 81 images from the Structured Analysis of the Retina (STARE) database. An ophthalmologist independently marked the center of the ONH for evaluation of the results. The evaluation of the results includes free-response receiver operating characteristics (FROC) and a measure of distance between the manually marked and detected centers. With the DRIVE database, the centers of the ONH were detected with an average distance of 0.36 mm (18 pixels) to the corresponding centers marked by the ophthalmologist. FROC analysis indicated a sensitivity of 100% at 2.7 false positives per image. With the STARE database, FROC analysis indicated a sensitivity of 88.9% at 4.6 false positives per image.

Research paper thumbnail of A review of computer-aided diagnosis of breast cancer: Toward the detection of subtle signs

Journal of The Franklin Institute-engineering and Applied Mathematics, 2007

Mammography is the best available tool for screening for the early detection of breast cancer. Ma... more Mammography is the best available tool for screening for the early detection of breast cancer. Mammographic screening has been shown to be effective in reducing breast cancer mortality rates: screening programs have reduced mortality rates by 30-70%.

Research paper thumbnail of Three-Dimensional Segmentation of the Tumor in Computed Tomographic Images of Neuroblastoma

Journal of Digital Imaging, 2006

Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost alw... more Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost always heterogeneous in nature; furthermore, viable tumor, necrosis, and normal tissue are often intermixed. Tumor definition and diagnosis require the analysis of the spatial distribution and Hounsfield unit (HU) values of voxels in computed tomography (CT) images, coupled with a knowledge of normal anatomy. Segmentation and analysis of the tissue composition of the tumor can assist in quantitative assessment of the response to therapy and in the planning of delayed surgery for resection of the tumor. We propose methods to achieve 3-dimensional segmentation of the neuroblastic tumor. In our scheme, some of the normal structures expected in abdominal CT images are delineated and removed from further consideration; the remaining parts of the image volume are then examined for the tumor mass. Mathematical morphology, fuzzy connectivity, and other image processing tools are deployed for this purpose. Expert knowledge provided by a radiologist in the form of the expected structures and their shapes, HU values, and radiological characteristics are incorporated into the segmentation algorithm. In this preliminary study, the methods were tested with 10 CT exams of four cases from the Alberta Children's Hospital. False-negative error rates of less than 12% were obtained in eight of the 10 exams; however, seven of the exams had falsepositive error rates of more than 20% with respect to manual segmentation of the tumor by a radiologist.

Research paper thumbnail of Three-Dimensional Segmentation of the Tumor in Computed Tomographic Images of Neuroblastoma

Journal of Digital Imaging, 2007

Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost alw... more Segmentation of the tumor in neuroblastoma is complicated by the fact that the mass is almost always heterogeneous in nature; furthermore, viable tumor, necrosis, and normal tissue are often intermixed. Tumor definition and diagnosis require the analysis of the spatial distribution and Hounsfield unit (HU) values of voxels in computed tomography (CT) images, coupled with a knowledge of normal anatomy. Segmentation and analysis of the tissue composition of the tumor can assist in quantitative assessment of the response to therapy and in the planning of delayed surgery for resection of the tumor. We propose methods to achieve 3-dimensional segmentation of the neuroblastic tumor. In our scheme, some of the normal structures expected in abdominal CT images are delineated and removed from further consideration; the remaining parts of the image volume are then examined for the tumor mass. Mathematical morphology, fuzzy connectivity, and other image processing tools are deployed for this purpose. Expert knowledge provided by a radiologist in the form of the expected structures and their shapes, HU values, and radiological characteristics are incorporated into the segmentation algorithm. In this preliminary study, the methods were tested with 10 CT exams of four cases from the Alberta Children's Hospital. False-negative error rates of less than 12% were obtained in eight of the 10 exams; however, seven of the exams had false-positive error rates of more than 20% with respect to manual segmentation of the tumor by a radiologist.

Research paper thumbnail of Effect of Pixel Resolution on Texture Features of Breast Masses in Mammograms

Journal of Digital Imaging, 2010

The effect of pixel resolution on texture features computed using the gray-level co-occurrence ma... more The effect of pixel resolution on texture features computed using the gray-level co-occurrence matrix (GLCM) was analyzed in the task of discriminating mammographic breast lesions as benign masses or malignant tumors. Regions in mammograms related to 111 breast masses, including 65 benign masses and 46 malignant tumors, were analyzed at pixel sizes of 50, 100, 200, 400, 600, 800, and 1,000 μm. Classification experiments using each texture feature individually provided accuracy, in terms of the area under the receiver operating characteristics curve (AUC), of up to 0.72. Using the Bayesian classifier and the leave-one-out method, the AUC obtained was in the range 0.73 to 0.75 for the pixel resolutions of 200 to 800 μm, with 14 GLCM-based texture features using adaptive ribbons of pixels around the boundaries of the masses. Texture features computed using the ribbons resulted in higher classification accuracy than the same features computed using the corresponding regions within the mass boundaries. The t test was applied to AUC values obtained using 100 repetitions of random splitting of the texture features from the ribbons of masses into the training and testing sets. The texture features computed with the pixel size of 200 μm provided the highest average AUC with statistically highly significant differences as compared to all of the other pixel sizes tested, except 100 μm.

Research paper thumbnail of Performance Analysis of Oriented Feature Detectors

Oriented feature detectors are fundamental tools in image understanding, as many images display r... more Oriented feature detectors are fundamental tools in image understanding, as many images display relevant information in the form of oriented features. Several oriented feature detectors have been developed; some of the important families of oriented feature detectors are steerable filters and Gabor filters. In this work, a performance analysis is presented of the following oriented feature detectors: the Gaussian second-derivative steerable filter, the quadrature-pair Gaussian second-derivative steerable filter, the real Gabor filter, the complex Gabor filter, and a line operator that has been shown to outperform the Gaussian second-derivative steerable filter in the detection of linear structures in mammograms. The detectors are assessed in terms of their capability to detect the presence of oriented features, as well as their accuracy in the estimation of the angle of the oriented features present in the image. It is shown that the Gabor filters yield the best detection performance and angular accuracy, whereas the steerable filters have the best performance in terms of computational speed.

Research paper thumbnail of Um Algoritmo Rápido para Iluminação Volumétrica

Direct Volume Rendering algorithms are very important tools in the analysis of volumetric data in... more Direct Volume Rendering algorithms are very important tools in the analysis of volumetric data in scientific visualization applications. Volume shading is a critical step regarding the computational cost of these algorithms. This paper presents a fast algorithm for volume shading. The proposed technique is based on decomposing the illumination equation into a sum of functions that can be pre-computed in look-up tables. Preliminary results showed the new algorithm is about six times faster than the conventional method.

Research paper thumbnail of Segmentation and estimation of the histological composition of the tumor mass in computed tomographic images of neuroblastoma

The problem that we investigate in the present paper is the improvement of the analysis of the pr... more The problem that we investigate in the present paper is the improvement of the analysis of the primary tumor mass, in patients with advanced neuroblastoma, using X-ray computed tomography (CT) exams. To achieve this goal, we propose a methodology for the estimation of the histological content of the mass that comprises a technique for semi-automatic segmentation of the primary tumor mass in CT images of neuroblastoma and a statistical method to estimate, from segmented CT images, the histological composition of the primary tumor. The results of the method are compared with the results of histological analysis of surgically resected tumor mass.

Research paper thumbnail of Detection of architectural distortion in prior screening mammograms using Gabor filters, phase portraits, fractal dimension, and texture analysis

International Journal of Computer Assisted Radiology and Surgery, 2008

Objective Mammography is a widely used screening tool for the early detection of breast cancer. O... more Objective Mammography is a widely used screening tool for the early detection of breast cancer. One of the commonly missed signs of breast cancer is architectural distortion. The purpose of this study is to explore the application of fractal analysis and texture measures for the detection of architectural distortion in screening mammograms taken prior to the detection of breast cancer. Materials and methods A method based on Gabor filters and phase portrait analysis was used to detect initial candidates for sites of architectural distortion. A total of 386 regions of interest (ROIs) were automatically obtained from 14 “prior mammograms”, including 21 ROIs related to architectural distortion. From the corresponding set of 14 “detection mammograms”, 398 ROIs were obtained, including 18 related to breast cancer. For each ROI, the fractal dimension and Haralick’s texture features were computed. The fractal dimension of the ROIs was calculated using the circular average power spectrum technique. Results The average fractal dimension of the normal (false-positive) ROIs was significantly higher than that of the ROIs with architectural distortion (p = 0.006). For the “prior mammograms”, the best receiver operating characteristics (ROC) performance achieved, in terms of the area under the ROC curve, was 0.80 with a Bayesian classifier using four features including fractal dimension, entropy, sum entropy, and inverse difference moment. Analysis of the performance of the methods with free-response receiver operating characteristics indicated a sensitivity of 0.79 at 8.4 false positives per image in the detection of sites of architectural distortion in the “prior mammograms”. Conclusion Fractal dimension offers a promising way to detect the presence of architectural distortion in prior mammograms.

Research paper thumbnail of Detection of Blood Vessels in Fundus Images of the Retina using Gabor Wavelets

The monitoring of the effects of diabetes, hypertension, and premature birth on the visual system... more The monitoring of the effects of diabetes, hypertension, and premature birth on the visual system can be assisted by quantitative analysis of the vascular architecture of the retina. The application of image analysis techniques in ophthalmology becomes possible after the desired features have been detected through the use of an appropriate method. We propose image processing techniques for the detection of blood vessels in the retina. The methods include the design of a bank of directionally sensitive Gabor filters for several values of the scale and elongation parameters. Forty images of the retina from the DRIVE database were used to evaluate the performance of the methods. High efficiency in the detection of blood vessels with the area under the receiver operating characteristics curve of up to 0.96 was achieved.

Research paper thumbnail of Reduction of false positives in the detection of architectural distortion in mammograms by using a geometrically constrained phase portrait model

International Journal of Computer Assisted Radiology and Surgery, 2007

Objective One of the commonly missed signs of breast cancer is architectural distortion. We have ... more Objective One of the commonly missed signs of breast cancer is architectural distortion. We have developed techniques for the detection of architectural distortion in mammograms, based on the analysis of oriented texture through the application of Gabor filters and a linear phase portrait model. In this paper, we propose constraining the shape of the general phase portrait model as a means to reduce the false-positive rate in the detection of architectural distortion. Material and methods The methods were tested with one set of 19 cases of architectural distortion and 41 normal mammograms, and with another set of 37 cases of architectural distortion. Results Sensitivity rates of 84% with 4.5 false positives per image and 81% with 10 false positives per image were obtained for the two sets of images. Conclusion The adoption of a constrained phase portrait model with a symmetric matrix and the incorporation of its condition number in the analysis resulted in a reduction in the false-positive rate in the detection of architectural distortion. The proposed techniques, dedicated for the detection and localization of architectural distortion, should lead to efficient detection of early signs of breast cancer.