• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 763
  • 737
  • 174
  • 100
  • 92
  • 71
  • 40
  • 22
  • 19
  • 12
  • 9
  • 9
  • 7
  • 4
  • 4
  • Tagged with
  • 2389
  • 361
  • 353
  • 289
  • 200
  • 177
  • 164
  • 160
  • 156
  • 129
  • 126
  • 120
  • 119
  • 105
  • 102
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Reproducibility and interpretation in tissue Doppler echocardiography

Storaa, Camilla January 2004 (has links)
As cardiovascular disease is the single most common cause ofdeath in the western world, and since there is a closeconnection between cardiovascular disease and left ventricular(LV) function, good methods for the assessment of LV functionis highly needed. A widely used tool for the diagnosis of LVdisease is echocardiography, a technique which today faces twodi_culties; the low reproducibility and the subjectiveinterpretation. The present dissertation aims to quantifyreproducibility, to study the factors that influencereproducibility and to provide tools for simplifying theinterpretation of tissue velocities measured by Dopplerultrasound. The reproducibility has been studied by letting twoindependent observers measure tissue Doppler velocities toinvestigate how well their measurements agree. To improvereproducibility an algorithm for the automatic detection ofpre-defined echocardiographic localizations is presented. Oneof the most difficult skills for the sonographer to master,thus leading to reduced reproducibility, is the transducermanipulation. The effect of poor transducer manipulation hasbeen modeled, and we show that even a poorly placed transducermay yield images which are easily mistaken for good, however,when scanning in two orthogonal planes the transducermisplacement is easily detected. Interpretation of the echocardiograms is influenced byseveral parameters. As the tissue velocities are measured byutilizing the Doppler effect, only the velocity componentdirected towards the transducer can be measured, thus thealignment of the heart within the view of the transduceraffects the tissue velocity measurements. The effect of thishas been investigated, and it is demonstrated that since themyocardium primarily has longitudinal motion and thus thevelocity vectors are mainly longitudinal, imaging in the apicalview will give little error in the velocity measurements. Filtering of the tissue velocity signals have becomecommercially available with the hope that it will improvereproducibility and simplify interpretation. One set of lowpass filters has been tested, and it is seen that there is arisk of overdoing the filtering and cause an underestimation oftissue velocity parameters. A similar effect to low passfiltering is seen when using too low sample rate when recordingthe tissue velocities. Finally a new imaging modality, tissue motion imaging, ispresented, where myocardial displacement, velocity, strain andacceleration may be interpreted from one single image, insteadof the situation today where several measurements must beperformed to get an overview of all these parameters. The thesis concludes that reproducibility can be improved bycurve smoothing and that interpretation can be simplified usingadvanced methods of parametric imaging.
2

Simulation Software and Hardware for Teaching Ultrasound

Lu, Lipin 01 January 2008 (has links)
Over the years, medical imaging modalities have evolved drastically. Accordingly, the need for conveying the basic imaging knowledge to future specialists and other trainees becomes even more crucial for devoted educators. Understanding the concepts behind each imaging modality requires a plethora of advanced physics, mathematics, mechanics and medical background. Absorbing all of this background information is a daunting task for any beginner. This thesis focuses on developing an ultrasound imaging education tutorial with the goal of easing the process of learning the principles of ultrasound. This tutorial will utilize three diverse approaches including software and hardware applications. By performing these methodologies from different perspectives, not only will the efficiency of the training be enhanced, but also the trainee?s understanding of crucial concepts will be reinforced through repetitive demonstration. The first goal of this thesis was developing an online medical imaging simulation system and deploying it on the website of the University of Miami. In order to construct an easy, understandable, and interactive environment without deteriorating the important aspects of the ultrasound principles, interactive flash animations (developed by Macromedia Director MX) were used to present concepts via graphic-oriented simulations. The second goal was developing a stand-alone MATLAB program, intended to manipulate the intensity of the pixels in the image in order to simulate how ultrasound images are derived. Additionally, a GUI (graphic user interface) was employed to maximize the accessibility of the program and provide easily adjustable parameters. The GUI window enables trainees to see the changes in outcomes by altering different parameters of the simulation. The third goal of this thesis was to incorporating an actual ultrasound demonstration into the tutorial. This was achieved by using a real ultrasound transducer with a pulse/receiver so that trainees could observe actual ultrasound phenomena, and view the results using an oscilloscope. By manually adjusting the panels on the pulse/ receiver console, basic A-mode ultrasound experiments can be performed with ease. By combining software and hardware simulations, the ultrasound education package presented in this thesis will help trainees more efficiently absorb the various concepts behind ultrasound.
3

Reproducibility and interpretation in tissue Doppler echocardiography

Storaa, Camilla January 2004 (has links)
<p>As cardiovascular disease is the single most common cause ofdeath in the western world, and since there is a closeconnection between cardiovascular disease and left ventricular(LV) function, good methods for the assessment of LV functionis highly needed. A widely used tool for the diagnosis of LVdisease is echocardiography, a technique which today faces twodi_culties; the low reproducibility and the subjectiveinterpretation. The present dissertation aims to quantifyreproducibility, to study the factors that influencereproducibility and to provide tools for simplifying theinterpretation of tissue velocities measured by Dopplerultrasound.</p><p>The reproducibility has been studied by letting twoindependent observers measure tissue Doppler velocities toinvestigate how well their measurements agree. To improvereproducibility an algorithm for the automatic detection ofpre-defined echocardiographic localizations is presented. Oneof the most difficult skills for the sonographer to master,thus leading to reduced reproducibility, is the transducermanipulation. The effect of poor transducer manipulation hasbeen modeled, and we show that even a poorly placed transducermay yield images which are easily mistaken for good, however,when scanning in two orthogonal planes the transducermisplacement is easily detected.</p><p>Interpretation of the echocardiograms is influenced byseveral parameters. As the tissue velocities are measured byutilizing the Doppler effect, only the velocity componentdirected towards the transducer can be measured, thus thealignment of the heart within the view of the transduceraffects the tissue velocity measurements. The effect of thishas been investigated, and it is demonstrated that since themyocardium primarily has longitudinal motion and thus thevelocity vectors are mainly longitudinal, imaging in the apicalview will give little error in the velocity measurements.</p><p>Filtering of the tissue velocity signals have becomecommercially available with the hope that it will improvereproducibility and simplify interpretation. One set of lowpass filters has been tested, and it is seen that there is arisk of overdoing the filtering and cause an underestimation oftissue velocity parameters. A similar effect to low passfiltering is seen when using too low sample rate when recordingthe tissue velocities.</p><p>Finally a new imaging modality, tissue motion imaging, ispresented, where myocardial displacement, velocity, strain andacceleration may be interpreted from one single image, insteadof the situation today where several measurements must beperformed to get an overview of all these parameters.</p><p>The thesis concludes that reproducibility can be improved bycurve smoothing and that interpretation can be simplified usingadvanced methods of parametric imaging.</p>
4

The influence of ultrasonic radiation on polymers

Kanwal, Farah January 1994 (has links)
No description available.
5

Quantitative simulation of backscatter from tissue and blood flow for ultrasonic transducers

Shieh, Bernard D. 21 September 2015 (has links)
Ultrasound imaging is a ubiquitous part of the modern medical diagnostics toolbox. It has widespread applications to many areas of medicine, including angiology, cardiology, nephrology, urology, and obstetrics. It is often preferred over other imaging modalities, such as x-ray computed tomography (CAT) and magnetic resonance imaging (MRI) because it is non-invasive, non-ionizing, inexpensive, and has excellent penetration depth in the body. The design, optimization, and manufacturing of ultrasound transducers used in ultrasound imaging is a challenging engineering problem. Faced with a variety of different imaging environments, ultrasound transducers must often be optimized for performance in very specific applications. This is especially true for catheter-based solutions, such as intracardiac and intravascular ultrasound, where imaging performance is strongly dependent on the strength of backscatter from tissue due to significant limitations in device size, electronics, and signal-to-noise ratio. Currently, there is a need for the accurate and fast simulation of the imaging process used in ultrasound imaging, including the ability to capture the effects of backscatter from a variety of different tissues. This thesis discusses the development of simulation tools for the quantitative simulation of tissue backscatter and blood motion from acoustic fields coupled to spatial array transducers, based on an application of the Rayleigh speckle model to the linear systems model for acoustic diffraction from spatial array transducers. These simulation tools have potential applications in the field of medical ultrasonics, with particular attention to the areas of transducer design and optimization, beamforming and array processing, and image reconstruction. We demonstrate how the simulation tools developed here can be used to characterize array imaging performance and to investigate reconstruction performance of common flow algorithms for Doppler ultrasound imaging.
6

Development and Evaluation of a Close-Proximity, Real Time Thermoacoustic Ultrasound Sensor

Choi, Michael Unknown Date
No description available.
7

The effect of ultrasound on organic synthesis and processing from laboratory to large scale test

Paniwnyk, Larysa January 1993 (has links)
The research programme involved the exploitation of ultrasound with a view to applications within commercial and processing industries. This was accomplished by employing dosimetry and calorimetry to study the efficiency of several sonochemical reactors. The effect of factors such as reaction vessel geometry and volume was investigated and the general trends obtained for all three dosimeters were comparable. The effects of various parameters e.g. power, solvent, volume etc etc on the sonochemical 0-allcylation of 2,6-dimethylphenol was examined. Decreases in temperature and volume, and increases in concentration and power, led to increases in the sonochemical effect. A study of the allcylation products from a reaction between 5-hydroxychromone-2-carboxylic acid ethyl ester with less reactive alkyl halides such as 1- and 2-bromobutane resulted in comparisons with phase transfer catalysis and conventional thermal methods. An investigation of the dehydrogenation of tetrahydronaphthalene under the influence of sonication was also attempted. Dehydrogenation was enhanced by sonication with sonochemical dehydrogenation occurring 20-40°C below the corresponding thermal reaction. Sonication as a processing aid was studied using examples taken from the food industry. The applications of particle size reduction, emulsification and crystallisation of various foodstuffs such as rice, sugar and cocoa grains were examined. The effect of sonication on the viscosity of gelled starch was also monitored with a view to achieving either a permanent or temporary reduction in viscosity.
8

Investigation of bubble dynamics and heating during focused ultrasound insonation in tissue-mimicking materials

Yang, Xinmai 10 November 2010 (has links)
The deposition of ultrasonic energy in tissue can cause tissue damage due to local heating. For pressures above a critical threshold, cavitation will occur in tissue and bubbles will be created. These oscillating bubbles can induce a much larger thermal energy deposition in the local region. Traditionally, clinicians and researchers have not exploited this bubble-enhanced heating since cavitation behavior is erratic and very difficult to control. The present work is an attempt to control and utilize this bubble-enhanced heating. First, by applying appropriate bubble dynamic models, limits on the asymptotic bubble size distribution are obtained for different driving pressures at 1 MHz. The size distributions are bounded by two thresholds: the bubble shape instability threshold and the rectified diffusion threshold. The growth rate of bubbles in this region is also given, and the resulting time evolution of the heating in a given insonation scenario is modeled. In addition, some experimental results have been obtained to investigate the bubble-enhanced heating in an agar and graphite based tissue- mimicking material. Heating as a function of dissolved gas concentrations in the tissue phantom is investigated. Bubble-based contrast agents are introduced to investigate the effect on the bubble-enhanced heating, and to control the initial bubble size distribution. The mechanisms of cavitation-related bubble heating are investigated, and a heating model is established using our understanding of the bubble dynamics. By fitting appropriate bubble densities in the ultrasound field, the peak temperature changes are simulated. The results for required bubble density are given. Finally, a simple bubbly liquid model is presented to estimate the shielding effects which may be important even for low void fraction during high intensity focused ultrasound (HIFU) treatment.
9

Theoretical modelling of ultrasound contrast agents

Looney, Padraig January 2011 (has links)
This thesis compares theoretical models of ultrasound contrast agents to the acoustic response from single Microbubbles(MBs). The acoustic response was compared using a range of driving parameters. A rigid shelled contrast agent and a lipid shelled contrast agent were used in the comparison. While attempts to model the behaviour of some contrast agents at low mechanical index (MI) have been successful at higher MI the behaviour of MBs is still not well understood. Understanding and predicting the response ofMBs to medical ultrasound can lead to improvements in the clinical use of MBs through improved contrast agent design or improved signal processing. Numerical models were developed to compare to three specific cases; 1) Rigid shelled contrast agents 2) Lipid shelled contrast agents 3) Responses from lipid shelled contrast agents that are hit by subsequent driving pulses. Three models were used to compare to the responses from single rigid shelled contrast agents. Two of these models have been used before and the third was developed based on the optical observations of the responses of these rigid shelled agents at these MI. Two shelled models were used to compare to the response of single lipid shelled MBs. Using statistical methods the parameters defining the shell properties were found. The parameters that gave best agreement with the lipid shelled data was then used with a model to account for the molecular diffusion of gas from a MB and a new model to account for the optically observed shedding of the shell from a MB to compare to the multiple response from single MBs. While the theoretical prediction of an acoustic response of a suspension of MBs or the radial oscillation of single MBs has been compared before to experimental data, the successful comparison of the acoustic response of single MBs to the theoretical prediction is the first of it’s kind known to the author. The new theoretical model of the rigid shelled MB that was developed in this thesis gave better agreement with the experimental data than the other previously used models. The shell parameters of the lipid shelled MB were determined for the lowest driving amplitude and were in agreement with those measured previously from optical observations. Finally, the model for the shedding of the shell was shown to give quantitative agreement with the multiple acoustic responses from single MBs. When shedding of the shell was included the choice of constitutive equation for the shell was shown to strongly affect subsequent responses from the MB.
10

Convergent field elastography

Gray, Michael D. 08 June 2015 (has links)
A new approach to soft tissue elastography is presented. The work was motivated by the need to understand and mitigate the effects of anthropogenic sound on marine mammals. These efforts have been hampered by a lack of knowledge of in vivo tissue viscoelastic moduli. To address this problem, a measurement system concept was developed to non-invasively determine shear viscoelastic properties at tissue depths of over 12 cm – well beyond the capabilities of existing systems. The central design feature of the measurement system is a focused, sectored, annular ultrasonic source that generates a ring-like pressure field. This in turn produces a ring-like radiation force distribution in soft tissue, the response of which is primarily observable as a shear wave field that converges to the center of the force pattern. A second confocal transducer nested inside the shear wave generation source is used to measure the component of the shear wave motion along the beam axis. Propagation speed is estimated from displacement phase changes resulting from drive frequency induced dilation of the forcing radius. Forcing beams are modulated in order to establish shear speed frequency dependence, allowing quantification of shear speed dispersion. This concept for convergent field elastography (CFE) is intended to significantly improve the overall ability to estimate soft tissue shear speeds in thick, complex tissues while keeping within FDA-mandated ultrasound exposure limits. A prototype system was developed and tested in tissue mimicking materials for which properties were independently determined. Experiments were first carried out in a homogeneous material, and subsequently in a material containing elastic contrast inclusions. Transmission experiments with re-hydrated samples of bottlenose dolphin skull and mandibular bone samples were conducted to quantify ultrasonic beam attenuation and distortion effects, and their cumulative impact on CFE shear estimation performance. In addition to supporting marine mammal studies, the techniques developed in this thesis may enable or extend a wide range of human medical diagnostics.

Page generated in 0.1427 seconds