Acoustic droplet vaporization (ADV) is the physical process of liquid-to-gas phase transition mediated by pressure variations in an ultrasound field. In this study, the acoustic response of novel particle-stabilized perfluoropentane droplets was studied in bulk and confined media. The oil/water interface was stabilized by cellulose nanofibers. First, their acoustic responses under idealized conditions were examined to assess their susceptibility to undergo ADV. Second, the droplets were studied in a more realistic setting and placed in a confined medium. Lastly, an imaging setup was developed and tested on the droplets. The acoustic response could be seen when the amplitude of the peak negative pressure (PNP) was above 200 kPa, suggesting that this is the vaporization pressure threshold for these droplets. Increasing the PNP resulted in a decrease in signal intensity over time, suggesting a more destructive behavior. The imaging setup was able to differentiate between the droplets and the surrounding tissue. Results obtained within this study suggest that these droplets have potential in terms of ultrasound-mediated diagnostics and therapy.

Perfluoropentane droplets with cellulose nanofibers (CNF) shells have demonstrated better stability and easier surface modification as ultrasound contrast agents and drug delivery vehicles. This paper presents a theoretical model assuming a four-phase state “inverse antibubble,” with the core filled with gas perfluoropentane surrounded by liquid perfluoropentane. A continuous, incompressible, and viscoelastic stabilizing layer separates the core from the surrounding water. A parametric study is performed to predict the frequency-dependent attenuation coefficient, the speed of sound, and the resonance frequency of the droplets which have a mean diameter of 2.4760.95 lm. Results reveal that the CNF-stabilized perfluoropentane droplets can be modeled in a Rayleigh-Plesset like equation. We conclude that the shell strongly influences the acoustic behavior of the droplets and the resonance frequency largely depends on the initial gas cavity radius. More specifically, the peak attenuation coefficient and peak-to-peak speed of sound decrease with increasing shear modulus, shear viscosity, and shell thickness, while they increase with increasing gas cavity radius and concentration. The resonance frequency increases as shear modulus and shell thickness increase, while it decreases as shear viscosity and gas cavity radius increase. It is worth mentioning that droplet concentration has no effect on the resonance frequency.

Hydrodynamic cavitation is one of the major phase change phenomena and occurs with a sudden decrease in the local static pressure within a fluid. With the emergence of microelectromechanical systems (MEMS), high-speed microfluidic devices have attracted considerable attention and been implemented in many fields, including cavitation applications. In this study, a new generation of 'cavitation-on-a-chip' devices with eight parallel structured microchannels is proposed. This new device is designed with the motivation of decreasing the upstream pressure (input energy) required for facile hydrodynamic cavitation inception. Water and a poly(vinyl alcohol) (PVA) microbubble (MB) suspension are used as the working fluids. The results show that the cavitation inception upstream pressure can be reduced with the proposed device in comparison with previous studies with a single flow restrictive element. Furthermore, using PVA MBs further results in a reduction in the upstream pressure required for cavitation inception. In this new device, different cavitating flow patterns with various intensities can be observed at a constant cavitation number and fixed upstream pressure within the same device. Moreover, cavitating flows intensify faster in the proposed device for both water and the water-PVA MB suspension in comparison to previous studies. Due to these features, this next-generation 'cavitation-on-a-chip' device has a high potential for implementation in applications involving microfluidic/organ-on-a-chip devices, such as integrated drug release and tissue engineering.

Contrast agents are routinely used in ultrasound examinations. Nonlinear ultrasound imaging techniques have been developed over decades to enhance the contrast between the tissue and the blood pool after the injection of ultrasound contrast agents (UCAs). In this study, we introduce a new contrast pulse sequence, CPS4. The CPS4 combines pulse inversion (PI), subharmonic (SH), and ultraharmonic (UH) techniques to remove propagation distortion while capturing the unique SH and UH responses from UCAs. The novel CPS4 and conventional PI, SH, and UH techniques were used to detect the presence of a research-grade, thick-shell, polymer microbubble in a tissue-mimicking flow phantom. The contrast-to-tissue ratios (CTRs) obtained from the applications of all techniques were compared. The results show that the highest CTR of approximately 16 dB was obtained using CPS4, which was superior to the individual reference techniques: PI, SH, and UH techniques, in all scenarios considered in this study.

Microbubbles (MBs) as ultrasound contrast agents (UCAs) are increasingly accepted in the medical diagnostics. Their unique acoustic features enable the efficient detection of the MBs at a very low volume fraction. An improved understanding of the MBs dynamics could accelerate the development of UCA detection, i.e., enhanced ultrasound imaging techniques. Thereby, considerable efforts were dedicated to establishing models to interpret the dynamics of the microbubbles.

The joint endeavors of Rayleigh[1], Plesset[2], and other researchers led to the Rayleigh-Plesset equation, which describes the dynamics of the free MBs. The free MBs as a UCA has limited value because of their short lifespan in the human body. Additional coatings around the gas core with various materials were employed to extend the lifespan of the MBs. As a result, the models of the MBs evolved to explain the effects of the encapsulation. At the same time, many simplified assumptions were made. However, the diversity and the complexity of the MBs shell make some simplified assumptions invalided.

For instance, the polyvinyl alcohol (PVA) shell of the PVA MBs is heterogeneous and exhibit frequency-dependent mechanical properties, which were often neglected in previous studies.

In this study, cavitating flows inside a transparent cylindrical nozzle with an inner diameter of 0.9 mm were visualized, and the effect of cavitation on atomization characteristics of emerging sprays was investigated. Different patterns of cavitating flows inside the nozzle were visualized using a high-speed camera. In-house codes were developed to process the captured images to study the droplet size distribution and droplet velocity in different flow regimes. The results show that cavitating flows at the microscale have significant effects on atomization characteristics of the spray. Two working fluids, namely, water and poly(vinyl alcohol) microbubble (PVA MB) suspension, were employed. Accordingly, the injection pressures were detected as 690 kPa, 1035 kPa, and 1725 kPa for cavitation inception, supercavitation, and hydraulic flip flow regimes in the case of water, respectively. The corresponding pressures for the aforementioned patterns for PVA MB suspension were 590 kPa, 760 kPa, and 1070 kPa, respectively. At the microscale, as a result of a higher volume fraction of cavitation bubbles inside the nozzle, there is no large difference between the cavitation numbers corresponding to cavitating and hydraulic flip flows. Although the percentage of droplets with diameters smaller than 200 μm was roughly the same for both cases of water and PVA MB suspension, the Sauter mean diameter was considerably lower in the case of PVA MBs. Moreover, higher droplet velocities were achieved in the case of PVA MBs at lower injection pressures.

Ultrasound imaging (US) is widely used in clinical practice. Given the low cost and easy access to the ultrasound machine, US has a great potential to improve the health care condition for the majority of the population in the world. The US could be significantly improved by injecting ultrasound contrast agents to opacify the bloodstream. The polymer-shelled microbubbles (MB) are promising candidates for the next generation ultrasound contrast agent. In the current doctoral work, one of the polymer-shelled MBs, the polyvinyl alcohol (PVA) MB was investigated.

In Study I and Study II, I developed a novel contrast pulse sequence, CPS4, to efficiently detect the PVA MBs. The CPS4 is a combination of the sub-harmonic (SH), ultra-harmonic, and pulse inversion techniques. The comparison of the performance of each individual technique and CPS4 was carried out in a tissue-mimicking phantom. The CPS4 demonstrated the highest contrast-to-tissue ratio among all four imaging techniques. However, the SH response of the CPS4 was not fully excited. The high SH pressure threshold, above which the SH response is generated, was suspected to be the reason for the weak SH signal. Therefore, I wanted to optimize the performance of the CPS4 for the PVA MBs detection by boosting the SH signal. The optimization strategy was to lower the frequency-dependent SH threshold by setting the SH excitation frequency, which is the frequency of the ultrasound wave that excites the SH response, at the damped resonance frequency of the PVA MBs. To estimate the damped resonance frequency, a mathematical model based on the Church’s model with frequency-dependent material properties was proposed. The mechanical parameters of the new model were estimated by fitting the measured attenuation coefficient of the PVA MBs suspension with the simulated one. The calibrated model was employed to predict the damped resonance frequency of the PVA MBs, i.e., the optimized SH excitation frequency for the CPS4. The performance of the CPS4 was evaluated in-vitro, driving the system at four SH excitation frequencies in the proximity of the damped resonance frequency of the PVA MBs suspension. The best performance was observed at the SH excitation frequency of 11.25 MHz, which is in line with the simulated damped resonance frequency of 10.85 MHz. The in vitro experiment also revealed that the small particles constituting the artificial blood solution might interact with the PVA MBs and decreased the response echoes in a nonlinear and frequency-dependent fashion. Thus, more efforts are needed to move our model-guided optimization methods for the CPS4 towards clinical application.

In Study III, I modified the PVA MBs to support the dual-modal imaging of CT and US. The main idea of the modification is to incorporate the gold nanoparticles with the PVA MBs. The success of the modification is dependent on the amount of the gold nanoparticles carried by the modified PVA MBs. Two routes were proposed to fabricate candidates that support dual-modal imaging. In the first route, the gold nanoparticles were added during the fabrication of PVA MBs. Thus, the gold nanoparticles were embedded in the PVA shell during its formation (candidate named AuNP-S-MB). In the second route, the gold nanoparticles were loaded into the core of the PVA MBs, substituting air by increasing the permeability (candidate named AuNP-Capsule). The CT revealed an insignificant amount of gold nanoparticles was embedded in the shell of AuNP-S-MB, while detectable gold nanoparticles were loaded into AuNP-Capsule. Moreover, the CT-number of the surrounding liquid of AuNP-Capsule is low, i.e., the gold nanoparticles were locked in the AuNP-Capsule, making the second route a promising step towards the further development of the dual-modal contrast agent for CT and US.

In Study IV, I studied the effect of PVA MBs on the cavitation flows in microscale. The cavitation in clinical practices generates great pressure, which might be harmful and damage cells or beneficial and facilitate the treatment. A better understanding of cavitation generation mechanisms could avoid harmful cavitation, increase the safety of the clinical protocol, and increase the therapeutic cavitation, empower the treatments. Therefore, the effect of PVA MBs on cavitation is of great interest. More specifically, the effect of PVA MBs on the hydrodynamic cavitation was studied. Three microfluidic devices with different wall roughness and structure were fabricated. Two working fluids, PVA MBs suspension and water, were driven with controlled pressure through different microfluidic devices. The high-speed visualization revealed that the PVA MBs trigger the inception of hydrodynamic cavitation at a lower upstream pressure and enhance the cavitation flow in all three microfluidic devices. Furthermore, it takes a longer time for the cavitation bubbles to disappear in the PVA MB suspension.

To conclude the doctoral work, I developed a novel detection sequence, CPS4, optimized it for PVA MBs with a model-guided method, modified the PVA MB to extend its application, and studied the effect of PVA MB on hydrodynamic cavitation. The work promotes the PVA MBs for pre-clinical study, as well as provides an insight into the studies of other clinically approved ultrasound contrast agents. The methodology developed and presented within the thesis can be transferred to other clinically approved ultrasound contrast agents. For instance, the CPS4 and model-guided optimization method could be employed to improve CPS4 to other ultrasound contrast agents.

Ultraljudsavbildning (US) används ofta inom klinisk praxis. Med tanke påultraljudsmaskinens låga kostnad och enkla åtkomst har den förbättrade US en stor potentialför att förbättra hälsovården för majoriteten av världsbefolkningen. US kan förbättrasavsevärt genom att injicera ultraljudskontrastmedel för att göra blodflödet opakt.Mikrobubblor (MB) med polymerskal är lovande kandidater för nästa generationsultraljudskontrastmedel. I det aktuella doktorandprojektet undersöktes MB med en viss typav polymerskal, polyvinylalkohol (PVA) MB.

I Studie I och II utvecklade jag en ny kontrastpulssekvens, CPS4, för att effektivt detekteraPVA MB. CPS4 är en kombination av subharmoniska (SH), ultraharmoniska ochpulsinverteringstekniker. Jämförelsen av prestandan för varje enskild teknik och CPS4utfördes i en vävnadsfantom. CPS4 visade det högsta förhållandet mellan kontrast ochvävnad av alla fyra avbildningstekniker. SH- svar i CPS4 var dock inte helt exciterad. Dethöga SH-tröskelvärdet, över vilket ett SH-svar genereras, var misstänkt för att vara orsakentill den svaga SH-signalen. Därför ville jag optimera prestandan för CPS4 för detektering avPVA MB genom att öka SH-signalen. Optimeringsstrategin var att sänka detfrekvensberoende SH-tröskelvärdet genom att ställa in sändningsfrekvensen för CPS4, somär frekvensen hos den ultraljudsvåg som exciterar SH-svaret, vid den dämpaderesonansfrekvensen för PVA MB. För att uppskatta den dämpade resonansfrekvensenföreslogs en matematisk modell baserad på Churchs modell med frekvensberoendematerialegenskaper. De mekaniska parametrarna i den nya modellen estimerades genom attanpassa den uppmätta dämpningskoefficienten för PVA MB-suspensionen med densimulerade. Den kalibrerade modellen användes för att förutspå den dämpaderesonansfrekvensen för PVA MB, dvs. den optimerade SH-frekvensen för CPS4. CPS4-algoritmens prestanda utvärderades in vitro genom att köra systemet vid fyra olika frekvensersom ligger i närheten av den dämpade PVA MB-resonansfrekvensen. Den bästa prestandanobserverades vid 11.25 MHz, vilket är i linje med den simulerade dämpaderesonansfrekvensen på 10.85 MHz. In vitro-experimentet avslöjade också att de småpartiklarna som utgör den konstgjorda blodfantomen kan interagera med PVA MB ochminskade svarsekon på ett icke-linjärt och frekvensberoende sätt. Således behövs fler studierför att våra modellstyrda optimeringsmetoder för att CPS4 ska kunna tillämpas kliniskt.

I Studie III modifierade jag PVA MB:na för att stödja dubbelmodal avbildning för CT ochUS. Huvudidén med modifieringen är att införliva guldnanopartiklar i PVA MB. Huruvidamodifieringen är framgångsrik beror på storleken på nyttolasten som bärs av PVA MB. Tvåolika tillvägagångssätt föreslogs för att tillverka kandidater som stöder dubbelmodalavbildning. I det första tillvägagångssättet tillsattes guldnanopartiklarna under tillverkningenav PVA MB, så att guldnanopartiklarna bäddades in i PVA-skalet under dess bildande(kandidat som heter AuNP-S-MB). I det andra tillvägagångssättet laddadesguldnanopartiklarna in i kärnan i PVA MB som ersatte luft genom att öka permeabiliteten(kandidat som heter AuNP-kapsel). CT avslöjade att en obetydlig mängd guldnanopartiklarvar inbäddade i skalet av AuNP-S-MB, medan en detekterbar mängd guldnanopartiklarpackades in i PVA MB. Dessutom är CT-numret för den omgivande vätskan kring AuNPkapseln lågt, dvs. guldnanopartiklarna var bundna inuti AuNP-kapseln, vilket gör det andratillvägagångssättet till ett lovande steg mot den vidare utvecklingen av dubbelmodaltkontrastmedel.

I Studie IV studerade jag effekten av PVA MB på kavitationsflöden i mikroskala.Kavitationen i klinisk praxis genererar stora tryck, vilket kan vara ogynnsamt och skada cellereller vara till nytta och underlätta en behandling. En bättre förståelse för mekanismen förkavitationsgenerering kan undvika skadlig kavitation, öka säkerheten för det kliniskaprotokollet och öka den terapeutiska kavitationen. Därför är effekten av PVA MB påkavitationen av stort intresse. Mer specifikt studerades effekten av PVA MB på denhydrodynamiska kavitationen. Tre mikrofluidanordningar med olika strukturer ochytjämnheter på väggarna tillverkades. Två arbetsvätskor, PVA MB-suspension och vatten,kördes med kontrollerat tryck genom de olika mikrofluidanordningarna.Höghastighetsvisualiseringen avslöjade att PVA MB:na utlöser starten av hydrodynamiskkavitation vid ett lägre tryck uppströms och förbättrar kavitationsflödet i alla tremikrofluidanordningarna. Dessutom tar det längre tid för kavitationsbubblorna att försvinnai PVA MB-suspensionen.

För att sammanfatta mitt doktorandarbete har jag utvecklat en ny detektionssekvens, CPS4,optimerat den för PVA MB med en modellstyrd metod, modifierat PVA MB för att utvidgadess tillämpning, och studerat effekten av PVA MB på hydrodynamisk kavitation. Arbetetfrämjar PVA MB för prekliniska studier, samt ger en inblick i studierna av de andra klinisktgodkända ultraljudskontrastmedlen. Metoderna som utvecklats och presenterats inomavhandlingen kan tillämpas på andra kliniskt godkända ultraljudskontrastmedel. Till exempelkan CPS4 och modellstyrd optimeringsmetod användas för att detektera andraultraljudskontrastmedel med hög effektivitet, vilket erbjuder en överlägsen upplösning.

Microbubbles (MBs) with size below 10 μm are commonly used as an ultrasound contrast agent (UCA). The aim of the novel UCA developed in our lab is to support imaging modalities other than ultrasound to form hybrid contrast agents. The hybrid contrast agents through the synergistic effect can potentially improve the diagnostic outcome of the combined multimodal imaging technique. In this study, we modified the polyvinyl alcohol (PVA) MB fabrication protocol to encapsulate the gold nanoparticles into the shell and also in the core of the MBs. Furthermore, we evaluated the morphology, nonlinear ultrasound response, and X-ray property of dual modal contrast agents. The results revealed that the loading of the gold nanoparticles into the PVA MB core is a promising step towards the development of the dual modal contrast agent.

In the previous study [1], a novel contrast pulse sequence, CPS4, was introduced. The CPS4 combined sub-harmonic, ultra-harmonic and pulse inverse imaging to provide an improved contrast-to-tissue ratio (CTR). The CPS4 emits two pairs of transmitting waves at frequencies of f0/2 and 2*f0 with inversed phase within each pair and filters the received echoes at the frequency of f0. However, the performance of CPS4 was not optimized. Simulation study [2] shows that there is a pressure threshold for the sub-harmonic response generation of the ultrasound contrast agent (UCA). The threshold is expected to reach its local minima with the transmitting frequency around the resonance frequency. By lowering the threshold, more MBs could be excited to response sub-harmonic signal which could improve the CTR of CPS4.

The current study aims to investigate frequency-dependent performance of CPS4 with the polyvinyl alcohol microbubbles (PVA MBs). First a linear oscillator model adapted from Hoff and Church[3, 4] was built for single PVA MB. The attenuation and phase velocity of a PVA MB suspension were obtained to calibrate the linear oscillator. The model was used to estimate the resonance frequency of the MBs. The transmitting frequency of CPS4 for sub-harmonic was set at four frequency points around the local minima, i.e. resonance frequency. The performance of CPS4 at different frequencies were evaluated.

Nonlinear behavior of the ultrasound contrast agent (UCA) offers a unique feature to be distinguished from the surrounding tissue. In a recent years several methods were developed to enhance the nonlinear response of UCA. Crucial for efficient differentiation of the nonlinear response of UCA from the surrounding tissue is to design the contrast pulse sequence specific to the unique nonlinear properties that the particular UCA is offering.

In the previous study, the nonlinear response from a novel polyvinyl alcohol (PVA) microbubbles (MB), in ultra-harmonic region was investigated over a pressure range from 50 kPa to 300 kPa. In this study, five contrast pulse sequences and reference B-mode sequence were designed to visualize PVA MB. The performance of those sequences were evaluated and compared.