Dissertations / Theses on the topic 'Speckle reduction'

Synthetic Aperture Radar (SAR) is a popular tool for airborne and space-borne remote sensing. Inherent to SAR imagery is a type of multiplicative noise known as speckle. There are a number of different approaches which may be taken in order to reduce the amount of speckle noise in SAR imagery. One of the approaches is termed post image formation processing and this is the main concern of this thesis. Background theory relevant to the speckle reduction problem is presented. The physical processes which lead to the formation of speckle are investigated in order to understand the nature of speckle noise. Various statistical properties of speckle noise in different types of SAR images are presented. These include Probability Distribution Functions as well as means and standard deviations. Speckle is considered as a multiplicative noise and a general model is discussed. The last section of this chapter deals with the various approaches to speckle reduction. Chapter three contains a review of the literature pertaining to speckle reduction. Multiple look methods are covered briefly and then the various classes of post image formation processing are reviewed. A number of non-adaptive, adaptive and segmentation-based techniques are reviewed. Other classes of technique which are reviewed include Morphological filtering, Homomorphic processing and Transform domain methods. From this review, insights can be gained as to the advantages and disadvantages of various methods. A number of filtering algorithms which are either promising, or are representative of a class of techniques, are chosen for implementation and analysis.

The speckle reduction is an important problem in coherent imaging, such as synthetic aperture radar or ultrasound imagery. There are several well-known speckle filters for reducing speckle in coherent images. However, different speckle filters will have their own advantages and disadvantages. The performance of speckle filter depends strongly on the speckle and scene models that are used as the basis for filter development. In this thesis, we will compare four different approaches, Lee filter, Kuan filter, Frost filter and Improved Sigma filter for speckle reduction. This thesis will further describe the Improved Sigma filter since it is a new type of speckle filter for coherent images. We will apply these four speckle filters to our synthesized one-look amplitude synthetic aperture radar (SAR) image to reduce speckle noise. In order to compare these different speckle filters, the simulated SAR image data are used quantitatively to analyze these speckle filters. In addition, we will show their advantages and disadvantages in reducing speckle noise. Unlike the original Sigma filter, which has deficiencies in causing biased estimation and in depressing strong reflected targets, the Improved Sigma filter is more efficient when applied on simulated SAR image data.

Speckle arises when coherent light is reflected from a rough screen and observed by an intensity detector with a finite aperture. Because speckle causes serious image degradation when lasers are used as light sources in e.g. projectors, methods for reducing the speckle contrast need to be developed. Different speckle contrast reduction methods are investigated in this thesis, such as a rotating diffuser and a sinusoidal rotating grating. In addition, speckle simulations with the optical system design software ZEMAX has been explored. A setup consisting of a 4-f imaging system with a rotating diffuser in the Fourier plane was developed in order to decide whether or not it is advantageous to perform speckle reduction in the Fourier plane. Hence, measurement series were performed with the rotating diffuser placed at different positions in the 4-f imaging system for comparison. Measurement series were executed both with an empty object plane and with a lens in it to spread the light in the Fourier plane. Placing a rotating diffuser in the Fourier plane does not appear to be effective for speckle contrast reduction. The last setup investigated was a transmissive spatial light modulator (SLM) placed in the beam path. Sinusoidal rotating gratings created by means of gray levels, to simulate a potential modulator based on a deformable polymer layer, were implemented on the SLM. The gratings were rotated around their centers, and in a spiral in order to reduce the speckle contrast. For the first method the modulator speckle contrast was 34% for N = 18 averaged images, and for the second method it was 31% for N = 36 averaged images, both with a grating period of 4 pixels. Due to the drawbacks of the SLM optimal results were not achieved, but the SLM is useful for a proof-of-concept. Further measurements should be performed for this promising, novel method based on a true sinusoidal grating.

Abstract In this thesis the focus has been on laser speckle. It is done in collaboration with poLight. They are developing a projector, where laser light is the source of illumination. In such projectors, laser speckle degrades the image quality. The aim of this project is to construct a speckle reduction device to be used in the laser projector. The theory covers a description of laser speckle, how to reduce the speckle contrast, and five methods to so. We explain why speckle arises and which parameters we can manipulate to reduce the speckle contrast. The five speckle reduction methods included in this thesis are; vibrating diffuser, slowly moving diffuser, Hadamard matrices, scattering tube, and vibrating mirror. Large vibrational motions are unwanted, considering the size of the device, generation of noise, and problems with alignment of the optical components in the projector that this would lead to. The quality of the laser beam is prominent in order to produce a sharp image, thus the use of diffusers with large scattering angles is not a good solution. The scattering tubes, designed by poLight, are tubes filled with micro pearls in a polymer gel. The size of the pearls decides the nature of the scattering. Larger pearls will give less back scattering and more light transmitted in the forward direction. If the tubes are rotated in a well balanced device we can avoid generating vibrations. The Hadamard matrices is the only one of the five methods which is not based on a motion. The challenge is to find a SLM to implement the matrices. It requires a low response time in order to present enough matrices during the exposure time of the eye. The laboratory setup we use to measure the speckle contrast is an improved version of the setup constructed in the specialisation project. A screen was removed from the old setup, and the speckle is now imaged directly from the speckle reduction device. The measured speckle reduction is thus due to the device alone, and not affected by the screen. The results were reproducible and in agreement with what we expected. We implemented a vibrating diffuser, both the single and the slowly moving. A piece cut from a plastic bag and some Scotch Magic tape were used as diffusers. The tape is the strongest diffuser and gives the lowest speckle contrast, however, it also has the largest scattering angle. The single tape diffuser reduced the speckle contrast to $C = 0.112$. With two tape difusers in series the intensity in the images becomes too low to exploit the dynamic range of the CCD sensor. The result is a higher calcualted speckle contrast with two diffusers, $C=0.131$, even though it ought to be smaller. We tested five prototypes of the scattering tube with different concentrations. The tube with the highest concentration has the highest speckle reduction abilities. It also has the strongest scattering effect. The scattering is less than with the tape diffuser, and so is the speckle reduction. The speckle contrast is reduced to $C=0.320$ when the tube is rotated, and to $C=0.389$ when it is vibrated. The tubes was also tested in series with a ground glass. The ground glass acted as a second diffuser. In this setting, vibration and rotation of the tubes reduced the speckle contrast equally, $C approx 0.283$ From the measured speckle contrast of the diffusers and tubes in stationary conditions, a polarization analysis should show a depolarization of the laser beam. This were the case only for the plastic diffuser. It is assumed that the error lays with the polarization analysis. There should be a depolarization in the tape and a partial depolarization in the tubes. A calculation of the speckle size was performed as well. Based on the theory we expected the size of the speckle grains to be $sigma_s = 37.77~mu m$. From the Fourier analysis of a speckle image from the setup we calculated the speckle size to be $sigma_s = 5.35$~mm, which is approximately 140 times bigger. The expected speckle size is too small, because we did not take into account a small magnification in the setup. The Fourier analysis of discrete and limited sets of data points is probably the main explanation of the difference, but a more thorough study is needed.

In this assignment the theoretical background for the nature of speckle is presented and practical work was done to reduce the speckle effect in a display system based on a laser source. This was done without any picture modulators, or any kind of line scan or flying spot scanning. Work was done to find the right setup to be able to as easy as possible characterize the statistics of the speckle in an image. A still image of an expanded laser spot worked as an image. A series of test sets were carried out to address the different factors which could make a difference on the speckle contrast and their role in such systems.

Synthetic aperture radar (SAR) imagery is gaining increased usage as more systems become available and more applications are being developed. An "unfortunate" problem with SAR imagery is the high level of noise, often called speckle. The speckle effects reduce the utility of SAR imagery a great deal. This thesis will address the causes of speckle and many of the current methods that are used to reduce the speckle noise. Kalman filtering methods will then be adapted for use on SAR imagery taking into account the special properties of this type of filter and images.

Many clinicians regard speckle noise as an undesirable artifact in ultrasound images masking the underlying pathology within a patient. Speckle noise is a random interference pattern formed by coherent radiation in a medium containing many sub-resolution scatterers. Speckle has a negative impact on ultrasound images as the texture does not reflect the local echogenicity of the underlying scatterers. Studies have shown that the presence of speckle noise can reduce a physician's ability to detect lesions by a factor of eight. Without speckle, small high-contrast targets, low contrast objects, and image texture can be deduced quite readily.

Speckle filtering of medical ultrasound images represents a critical pre-processing step, providing clinicians with enhanced diagnostic ability. Efficient speckle noise removal algorithms may also find applications in real time surgical guidance assemblies. However, it is vital that regions of interests are not compromised during speckle removal. This research pertains to the reduction of speckle noise in ultrasound images while attempting to retain clinical regions of interest.

Recently, the advance of wavelet theory has lead to many applications in noise reduction and compression. Upon investigation of these two divergent fields, it was found that the speckle noise tends to rotate an image's homomorphic complex-wavelet coefficients. This work proposes a new speckle reduction filter involving a counter-rotation of these complex-wavelet coefficients to mitigate the presence of speckle noise. Simulations suggest the proposed denoising technique offers superior visual quality, though its signal-to-mean-square-error ratio (S/MSE) is numerically comparable to adaptive frost and kuan filtering.

This research improves the quality of ultrasound medical images, leading to improved diagnosis for one of the most popular and cost effective imaging modalities used in clinical medicine.

Le bruit de speckle acoustique est une consequence de la nature stochastique de la reflectivite des milieux multidiffuseurs. Ce bruit peut etre reduit par des traitements incoherents. Le transducteur a phase aleatoire est propose comme une technique de traitement incoherent du signal acoustique, dans le but de reduire le bruit de speckle. Un ecran de phase aleatoire en mouvement a travers le faisceau ultrasonore, permet de controler la coherence spatiale du transducteur. Nous montrons que le degre de coherence spatiale du faisceau transmis est determine par la fonction d'autocorrelation de la transmittance de l'ecran. Nous avons etudie la dependance du changement du speckle, en fonction de la coherence partielle du transducteur a phase aleatoire. Nous decrivons le compromis existant entre la degradation de la resolution et l'amelioration du rapport signal sur bruit, dans le cas d'un traitement incoherent ou partiellement coherent. Nous presentons des configurations d'emploi d'un tel systeme a l'imagerie, a la reduction du speckle, et a la detection de reflecteurs speculaires, pouvant trouver des applications aussi bien dans le medical que dans le controle non destructif

The thesis deals with speckle suppression in images acquired by optical coherence tomograph. It is divided into four parts. The first part describes basic information about the medical imaging method. It also deals with principle of measurement. The second section discusses the formation of image speckle and selected methods to reduce them. Next part is practical and consists of data acquisition, determination of the evaluation methodology and the implementation of speckle reduction methods. The last part is focused on testing and reviews of algorithms and discussion about their results.

Orientador: José Vicente Hallak d'Angelo
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química
Made available in DSpace on 2018-08-18T13:13:04Z (GMT). No. of bitstreams: 1 Moreira_CintiaCristinaGarcia_M.pdf: 1646345 bytes, checksum: 6924d108b277ad4e1638c59be0e3def6 (MD5) Previous issue date: 2007
Resumo: A sutura de categute é um material composto basicamente de colágeno, o qual é uma proteína obtida da camada serosa do intestino delgado de bovinos e é facilmente absorvido pelo corpo humano. Devido a esta característica, fios de categute são muito utilizados em vários tipos de cirurgias, como por exemplo: gastrintestinais, amarraduras de vasos na tela subcutânea, cirurgias ginecológicas e urológicas. De fácil manipulação, a sutura de categute, comporta-se como um corpo estranho, desencadeando uma reação inflamatória intensa ao seu redor. O categute possui uma sólida aceitação no mercado de produtos médicos hospitalares. Seu processo de produção ainda apresenta um alto índice de rejeição (cerca de 25%) do material semi-acabado. Deste percentual rejeitado, mais da metade é devido ao aparecimento de manchas no fio. Esta rejeição se dá, num primeiro momento, exclusivamente em função do aspecto visual, uma vez que fios com manchas são tomados como sendo fios defeituosos. Este trabalho teve por finalidade identificar a influência dos defeitos de manchas no material semi-acabado e também no produto final, comparando o desempenho dos fios com e sem manchas. Nesta comparação entre os fios foram realizados testes visuais (aparência); químicos (teor de cromo e de formol); físicos (resistência à tração com nó, resistência ao encastoamento, diâmetro) e estudos in-vivo (resistência tênsil, reação tissular). Os resultados obtidos demonstraram que os fios manchados possuem um desempenho tão bom quanto os sem mancha e podem ser utilizados sem comprometer a qualidade do produto. Isto possibilitou uma redução significativa das perdas totais no processo, de 25% para 17% de todo o material processado
Abstract: Catgut suture is a material composed basically of collagen which is a fibrous protein obtained from the serosa of bovines intestine and is easily absorbed by the human body. Due this characteristic, catgut sutures can be used in several types of surgeries like: endosurgery, ophthalmology, gynecology and urology. Catgut suture is easy to manipulate, however it can produce an intense inflammatory reaction around it. Catgut is considered a very good product in the market of medical devices. In the catgut raw material process there is a great loss of material (approximately 25%). About half of this rejected material is lost because of the presence of speckles. This rejection is, at first time, because of its visual appearance, once the strands with speckles are considered as a damaged suture. The aim of this work is to study the influence of the presence of speckles in the raw material (semi-finished) as well as in the final product, comparing the performance between the strands with and without speckles. In this comparison the tests performed were: visual (appearance); chemicals (chromium an formaldehyde content); physics (knot tensile strength, needle attachment tensile, diameter); "in-vivo" tests (tensile strength, tissue reaction). The results obtained have shown that it is possible to use the suture with speckles because its performance is as good as the ones without speckle and the quality of the product is not affected by their presence. After this study the total losses in the process were significantly reduced from 25% to 17% of all processed material
Mestrado
Sistemas de Processos Quimicos e Informatica
Mestre em Engenharia Química

[Truncated abstract] Transrectal ultrasound (TRUS) has become the urologist’s primary tool for diagnosing and staging prostate cancer due to its real-time and non-invasive nature, low cost, and minimal discomfort. However, the interpretation of a prostate ultrasound image depends critically on the experience and expertise of a urologist and is still difficult and subjective. To overcome the subjective interpretation and facilitate objective diagnosis, computer aided analysis of ultrasound images of the prostate would be very helpful. Computer aided analysis of images may improve diagnostic accuracy by providing a more reproducible interpretation of the images. This thesis is an attempt to address several key elements of computer aided analysis of ultrasound images of the prostate. Specifically, it addresses the following tasks: 1. modelling B-mode ultrasound image formation and statistical properties; 2. reducing ultrasound speckle; and 3. extracting prostate contour. Speckle refers to the granular appearance that compromises the image quality and resolution in optics, synthetic aperture radar (SAR), and ultrasound. Due to the existence of speckle the appearance of a B-mode ultrasound image does not necessarily relate to the internal structure of the object being scanned. A computer simulation of B-mode ultrasound imaging is presented, which not only provides an insight into the nature of speckle, but also a viable test-bed for any ultrasound speckle reduction methods. Motivated by analysis of the statistical properties of the simulated images, the generalised Fisher-Tippett distribution is empirically proposed to analyse statistical properties of ultrasound images of the prostate. A speckle reduction scheme is then presented, which is based on Mallat and Zhong’s dyadic wavelet transform (MZDWT) and modelling statistical properties of the wavelet coefficients and exploiting their inter-scale correlation. Specifically, the squared modulus of the component wavelet coefficients are modelled as a two-state Gamma mixture. Interscale correlation is exploited by taking the harmonic mean of the posterior probability functions, which are derived from the Gamma mixture. This noise reduction scheme is applied to both simulated and real ultrasound images, and its performance is quite satisfactory in that the important features of the original noise corrupted image are preserved while most of the speckle noise is removed successfully. It is also evaluated both qualitatively and quantitatively by comparing it with median, Wiener, and Lee filters, and the results revealed that it surpasses all these filters. A novel contour extraction scheme (CES), which fuses MZDWT and snakes, is proposed on the basis of multiresolution analysis (MRA). Extraction of the prostate contour is placed in a multi-scale framework provided by MZDWT. Specifically, the external potential functions of the snake are designated as the modulus of the wavelet coefficients at different scales, and thus are “switchable”. Such a multi-scale snake, which deforms and migrates from coarse to fine scales, eventually extracts the contour of the prostate

Le laplacien d'une variete riemannienne compacte a un spectre discret qui s'appelle le spectre de la variete. Un probleme classique est de savoir si deux varietes isospectrales sont isometriques. Depuis le celebre contre-exemple de milnor, on sait que la reponse a ce probleme est negative. Dans la premiere partie, on donne une borne explicite du nombre de tores plats isospectraux a un tore donne et non deux a deux isometriques, borne dependant de la geometrie du tore donne. On utilise des resultats sur les formes quadratiques. En 1984, c. Gordon et e. Wilson ont construit les premiers exemples de deformations isospectrales. Les varietes qu'ils considerent sont des nilvarietes. Le but des deux dernieres parties est de montrer que toutes les deformations isospectrales des nilvarietes de rang deux sont celles construites par c. Gordon et e. Wilson. Dans la deuxieme partie, on demontre le resultat voulu dans le cas ou la nilvariete est non singuliere, puis on demontre qu'il n'y a qu'un nombre fini de classes d'isometrie de varietes de heisenberg isospectrales. Dans la troisieme partie, on calcule le spectre d'une nilvariete de rang deux en utilisant la theorie de kirillov et on caracterise les deformations isospectrales des nilvarietes de rang deux

Ces bronzes de titane de type hollandite sont des composes a vfalence mixte de formule generale a::(x)tio::(2), avec a=k, rb, cs et 0,125