Ultrasound contrast agents have the ability to provide locally increased echogenicity, improving the sensitivity and specificity of images. Due to the unique interaction of microbubbles with the imaging ultrasound field, contrast ultrasonography offers both improved diagnostic techniques, and the potential therapeutic uses of gene and drug delivery through the use of targeted agents. By enhancing the contrast at the tissue-blood interface, an improved image of the structure of organs can be achieved, which is useful in many areas of medical ultrasound imaging. Monitoring the flow of contrast agent in the blood stream also offers information on the degree of blood perfusion into an organ or microvasculature. Present knowledge of the interaction of microbubbles with ultrasound is far from complete. The full potential of contrast agents in improving diagnostic and therapeutic techniques has therefore not yet been achieved. The nonlinear and dynamic properties of microbubble response offer potentially large improvements in contrast to tissue ratio, through intelligent pulse sequence design and/or improved signal processing. Due to various drawbacks of populations studies, only by studying the response from single microbubbles can the interaction be fully understood. The variations of microbubble size and shell parameters within a typical sample of contrast agent dictate that a large number of single scatterer data are necessary to obtain information on the variability of microbubble response, which is not possible with current optical systems. This thesis aims to be a contribution to the understanding of contrast behaviour in response to medical imaging ultrasound pulses. A fully characterized microacoustic system, employing a wide-band piezoelectric transducer from a commercial ultrasound imaging system, is introduced, which enables the measurement of single scattering events. Single microbubble signals from two commercially available contrast agents, Definity R and biSphereTM, have been measured experimentally in response to a range of clinically relevant imaging parameters. The data has been analyzed, together with the results from appropriate theoretical models, in order to gain physical insight into the evolution and dynamics of microbubble signals. A theoretical model for the lipid shelled agent Definity has been developed, and the predicted response from a real sample of single microbubbles investigated. Various characteristics of resonant scatter have been identified, and used to distinguish resonant scatter in experimental acoustic single bubble data for the first time. A clear distinction between the populations of resonant and off-resonant scatter has been observed for a range of incident frequencies and acoustic pressures. Results from consecutive imaging pulses have been used to gain understanding of how initial size, shell material and encapsulated gas may effect the lifetime of a microbubble signal. The response to a basic pulse sequence is also investigated, and an alternative processing method which takes advantage of observed behaviour is presented. Improved understanding of the contrast-ultrasound interaction will provide the basis for improved signal processing tools for contrast enhanced imaging, with potential benefits to both diagnostic techniques and microbubble manufacture.

The goal of this doctoral work was to observe and analyze the stability and dynamics of phospholipid-encapsulated microbubbles, and in particular the reaction to sudden submersion in a multi-gas medium. To accomplish this goal, first an experimental technique was developed to observe a microbubble in a single-gas environment suddenly immersed in a multi-gas environment, without perturbing the microbubble position. A modified Epstein-Plesset model was concurrently developed to account for the multiple gas species in the bulk solution. The model was used to analyze previous data for the effect of anesthesia carrier gas on microbubble ultrasound contrast agent in vivo circulation persistence. The focus of the experiments then shifted to microbubbles of different sizes encapsulated with a homologous series of saturated diacyl-chain lipid surfactants and emulsifiers. Constitutive models for the elastic and gas permeation properties of the lipid encapsulation were developed to elucidate the unique behaviors observed during the experiments. The experimental techniques employed were: (1) transmission bright field optical microscopy to obtain real-time, digital videos of microbubbles growing and dissolving in response to perturbations in the local gas environment and (2) the Langmuir trough film balance to determine the elasticity of the phospholipid monolayers during compression, expansion, and expansive relaxation. The modeling techniques employed was (1) a forward-wind finite difference method to discretize a series of non-linear differential equations and (2) a Newton-Raphson method to solve the diameter of a microbubble from the mechanical stress balance. These modeling techniques were used to determine the behavior of a microbubble a priori, whereas the fitting models implemented the iterative methods to solve for parameters without a Newton-Raphson method. Results showed that microbubbles coated with soluble surfactants and dissolving in a single gas solution could be predicted by the original Epstein-Plesset model. When subjected to a multi-gas medium, the modified Epstein-Plesset model accurately predicted microbubble growth and dissolution. The model was used to analyze the increase in microbubble circulation lifetime observed by others in anesthetized rats inhaling air rather than oxygen as the anesthesia carrier gas. The predictive capabilities of the model broke down, however, if the gas-core was encapsulated with a phospholipid monolayer. A typical, large (>40 µm diameter) lipid-coated microbubble displayed stunted growth, followed by three anomalous dissolution regimes: (1) rapid dissolution back to the initial resting diameter followed by (2) slow, steady dissolution and finally (3) stabilization, where the apparent surface tension approached a near-zero value. The model was modified to allow fitting of the radius-time curve by varying the surface tension. The analysis showed that the surface tension is dynamic, and suggested that a "break up" tension allowed for rapid expansion of the microbubble beyond the initial resting diameter. Lipid jamming was proposed as the mechanism eventually halting dissolution. Further observations of smaller microbubbles (<20 µm diameter) coated with a homologous series of saturated diacyl chain lipids gave significantly different results. Initially the microbubbles grew, but growth was severely subdued, if not eliminated, for more solid encapsulations below a threshold size (~10 µm diameter). Following growth, most microbubbles rapidly dissolved back to their original size. The microbubbles then experienced an anomalous lag time before spontaneously dissolving again. The lag times were highly variable and shown to correlate to the reduced temperature of the encapsulation, rather than the initial microbubble size. Most of the microbubbles stabilized again at a diameter of 1-2 µm, and this "stable diameter" appeared to be universal and independent of both the initial microbubble size and the rigidness of the encapsulation. Constitutive models were developed to describe these physical phenomena in the early growth and dissolution stages which were verified with independent monolayer relaxation studies.

Thesis (M.S.M.E.)--University of Delaware, 2006. / Principal faculty advisor: Kausik Sarkar, Dept. of Mechanical Engineering. Includes bibliographical references.

We demonstrate here that ultrasound-stimulated microbubbles can lead to enhanced cell death within tumors when combined with radiation. The aim of this study was to investigate different ultrasound parameters in conjunction with different concentrations of microbubbles with regards to this effect. Prostate xenograft tumors in Severe Combined Immunodeficient mice were subjected to ultrasound treatment that involved various peak negative pressures (250 kPa, 570 kPa, and 750 kPa), microbubble concentrations (8 µL/kg, 80 µL/kg, and 1000 µL/kg), and different radiation doses (0 Gy, 2 Gy, and 8 Gy). Twenty-four hours after treatment, tumors were excised and assessed for cell death. Histological analyses demonstrated that increases in radiation dose, microbubble concentration, and ultrasound pressure promoted apoptotic cell death and cellular disruption within tumors. Comparable increases in ceramide, a cell death mediator, were identified using immunohistochemistry. We also demonstrate that clinically-utilized microbubble concentrations combined with ultrasound can induce an enhancement in cell death.

We demonstrate here that ultrasound-stimulated microbubbles can lead to enhanced cell death within tumors when combined with radiation. The aim of this study was to investigate different ultrasound parameters in conjunction with different concentrations of microbubbles with regards to this effect. Prostate xenograft tumors in Severe Combined Immunodeficient mice were subjected to ultrasound treatment that involved various peak negative pressures (250 kPa, 570 kPa, and 750 kPa), microbubble concentrations (8 µL/kg, 80 µL/kg, and 1000 µL/kg), and different radiation doses (0 Gy, 2 Gy, and 8 Gy). Twenty-four hours after treatment, tumors were excised and assessed for cell death. Histological analyses demonstrated that increases in radiation dose, microbubble concentration, and ultrasound pressure promoted apoptotic cell death and cellular disruption within tumors. Comparable increases in ceramide, a cell death mediator, were identified using immunohistochemistry. We also demonstrate that clinically-utilized microbubble concentrations combined with ultrasound can induce an enhancement in cell death.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Micro-devices are developed for uses in targeted drug delivery and microscale manipulation. Here we numerically and analytically study two promising devices in early stages of development. Firstly, we study Armoured Microbubbles (AMBs) which can self-propel as artificial microswimmers or facilitate microfluidic mixing in a channel when held stationary on a wall. Secondly, we study an artificial cilium, which due to its unique design, when placed in an array, easily produces a metachronal wave for fluid transportation. The Armoured Microbubble was designed by our experimental collaborators (group of Philippe Marmottant, University Grenoble Alpes) and consists of a partial hollow sphere, inside which a bubble is caught. Under ultrasound the bubble oscillates, generating a streaming flow in the surrounding fluid and producing a net force. Motivated by the AMB but considering initially a general setup, using matched asymptotic expansions we calculate the streaming flow around a spherical body undergoing arbitrary, but known, small-amplitude surface shape oscillations. We then specialise back to the AMB and consider its excitation under ultrasound, using a potential flow model with mixed boundary conditions, to identify the resonant frequencies and mode shapes, including the dependence of the resonance on the AMB shape parameters. Returning to our general streaming model, we applied the mixed boundary conditions directly to this model, calculating the streaming around the AMB, in good agreement with experiments. Using hydrodynamic images and linear superposition, this model was extended to incorporate one wall, and AMB compounds. We then study the streaming flows generated by arrays of AMBs in confined channels, by modelling each AMB as its leading order behaviour (with corrections where required) and superposing the individual flow fields of all the AMBs. We identified the importance of two confining walls on the streaming flow around the array, and compared these flows to experiments in five cases. Motivated by this setup, we theoretically considered the extension of a two fluid interface passing through an AMB array to quickly identify good AMB arrays for mixing. We then studied the second artificial micro-device: an artificial cilium. Tsumori et. al. produced a cilium of PDMS containing aligned ferromagnetic filings, which beat under a rotating magnetic field. We modelled a similar cilium but assumed paramagnetic filings, using a force model balancing elastic, magnetic and hydrodynamic forces identifying the cilium beat pattern. This agreed with our equilibrium model and asymptotic analysis. We then successfully identified that the cilium applies the most force to the surrounding fluid at an intermediate value of the two dimensionless numbers quantifying the dynamics.

Several studies suggest that the use of ultrasound in conjunction with microbubbles (MBs) can induce the lysis of the blood clots through acoustic cavitation and through the production of microjets and microstreaming. However, there is no accordance about the optimal ultrasound parameters that have to be considered in order to achieve the maximum thrombolytic effect, neither a clear agreement about the type of MBs that have to be used. This project had two main goals: the design and optimization of an in-vitro set-up for the study of clot lysis within coronary arteries and its testing with ultrasound in conjunction with two different types of MBs. The MBs considered were the 3MiCRON MBs and the SonoVue MBs. The ultrasound sequence was developed using a programmable ultrasound architecture (Verasonics, Inc) and was tested using commercially available clinical transducers. Using the designed set-up and varying the ultrasound parameters (frequency, pulse length and pulse amplitude) it was possible to study the clot lysis effciency in conjunction with the two types of MBs. For the 3MiCRON MBs no increase in clot lysis was found with the implemented ultrasound parameters, while considering the SonoVue MBs, a 10% increase in clot lysis was found with 10ms long pulse delivered at 50V (peak-to peak value). The obtained set-up had several aspects in common with the real situation of occluded coronary arteries, although some limitations were present and further optimizations are required. Further work is required in order to assess if different combination of ultrasound parameters are able to lead to an increase in clot lysis when delivered with 3MiCRON or SonoVue MBs.

No description available.

Targeted therapy is a significant area of research in pharmaceutical and biomedical science. Its overall aim is to achieve maximum impact on malignant cells with minimum side effects to healthy tissue. In this thesis the capabilities of magnetic microbubbles as targeted therapeutic delivery vehicles are explored. New characterisation techniques were developed in order to understand and improve the current magnetic microbubble formulation. Electron microscopy was used to analyse the nanoscale structure of microbubble shells and observe nanoparticles attached to the shell surface. A new flow phantom was developed and the targeting of magnetic microbubbles against flow conditions corresponding to those in the human body was found to be feasible in numerous vessel sizes and flow conditions. Magnetic targeting of microbubbles was also observed in a perfused porcine liver model. Magnetic targeting was then attempted against flowing blood and a decrease in targeting efficiency observed. This was also seen for biochemical targeting and collisions with red blood cells identified as the most likely cause. Importantly, the number of magnetically targeted microbubbles significantly exceeded those targeted via biochemical interactions in both blood and water. In the second part of the thesis new types of magnetic microbubble were developed. The first exploits the fusion of nano-scale magnetic droplets with phospholipid microbubbles. In the second magnetic nanoparticles were incorporated directly into the lipid shell. The new magnetic microbubble formulation could be magnetically targeted, observed via contrast ultrasound and was successfully used to deliver siRNA to neuroblastoma cells.