Recent Experiences and Advances in Contrast-Enhanced Subharmonic Ultrasound

Acoustic response and ambient pressure sensitivity characterization of SonoVue for noninvasive pressure estimation

Subharmonic aided pressure estimation (SHAPE) is a noninvasive pressure measurement technique based on the pressure dependent subharmonic signal from contrast microbubbles. Here, SonoVue microbubble with a sulfur hexafluoride (SF6) core, was investigated for use in SHAPE. The study uses excitations of 25-700 kPa peak negative pressure (PNP) and 3 MHz frequency over eight pressurization cycles between atmospheric pressure and overpressures, ranging from 0 to 25 kPa (0 to 186 mm Hg). The SonoVue subharmonic response was characterized into two types. Unlike other microbubbles, SonoVue showed significant subharmonic signals at low excitations (PNPs, 25-400 kPa), denoted here as type I subharmonic. It linearly decreased with increasing overpressure (-0.52 dB/kPa at 100 kPa PNP). However, over multiple pressurization-depressurization cycles, type I subharmonic changed; its value at atmospheric pressure decreased over multiple cycles, and at later cycles, it recorded an increase in amplitude with overpressure (highest, +13 dB at 50 kPa PNP and 10 kPa overpressure). The subharmonic at higher excitations (PNP > 400 kPa), denoted here as type II subharmonic, showed a consistent decrease with the ambient pressure increase with strongest sensitivity of -0.4 dB/kPa at 500 kPa PNP.

Noninvasive Estimation of Tumor Interstitial Fluid Pressure from Subharmonic Scattering of Ultrasound Contrast Microbubbles

The noninvasive estimation of interstitial fluid pressure (IFP) using ultrasound contrast agent (UCA) microbubbles as pressure sensors will provide tumor treatments and efficacy assessments with a promising tool. This study aimed to verify the efficacy of the optimal acoustic pressure in vitro in the prediction of tumor IFPs based on UCA microbubbles' subharmonic scattering. A customized ultrasound scanner was used to generate subharmonic signals from microbubbles' nonlinear oscillations, and the optimal acoustic pressure was determined in vitro when the subharmonic amplitude reached the most sensitive to hydrostatic pressure changes. This optimal acoustic pressure was then applied to predict IFPs in tumor-bearing mouse models, which were further compared with the reference IFPs measured using a standard tissue fluid pressure monitor. An inverse linear relationship and good correlation (r = -0.853, p < 0.001) existed between the subharmonic amplitude and tumor IFPs at the optimal acoustic pressure of 555 kPa, and pressure sensitivity was 1.019 dB/mmHg. No statistical differences were found between the pressures measured by the standard device and those estimated via the subharmonic amplitude, as confirmed by cross-validation (mean absolute errors from 2.00 to 3.09 mmHg, p > 0.05). Our findings demonstrated that in vitro optimized acoustic parameters for UCA microbubbles' subharmonic scattering can be applied for the noninvasive estimation of tumor IFPs.

Theoretical prediction of the scattering of spherical bubble clusters under ultrasonic excitation

Due to the nonlinear vibration of ultrasound contrast agent bubbles, a nonlinear scattered sound field will be generated when bubbles are driven by ultrasound. A bubble cluster consists of numerous bubbles gathering in a spherical space. It has been noted that the forward scattering of a bubble cluster is larger than its backscattering, and some studies have experimentally found the angular dependence of a bubble cluster's scattering signal. In this paper, a theory is proposed to explain the difference of acoustic scattering at different directions of a bubble cluster when it is driven by ultrasound, and predicts the angular distribution of scattered acoustic pressure under different parameters. The theory is proved to be correct under circumstances of small clusters and weak interactions by comparing theoretical results with numerical simulations. This theory not only sheds light on the physics of bubble cluster scattering, but also may contribute to the improvement of ultrasound imaging technology, including ultrasonic harmonic imaging and contrast-enhanced ultrasonography.

Multiparametric ultrasound and micro-ultrasound in prostate cancer: a comprehensive review

Prostate cancer (PCa) is the most common non-cutaneous cancer diagnosed in males. Traditional tools for screening and diagnosis, such as prostate-specific antigen, digital rectal examination and conventional transrectal ultrasound (TRUS), present low accuracy for PCa detection. Multiparametric MRI has become a game changer in the PCa diagnosis pathway and MRI-targeted biopsies are currently recommended for males at risk of clinically significant PCa, even in biopsy-naïve patients. Recent advances in ultrasound have also emerged with the goal to provide a readily accessible and cost-effective tool for detection of PCa. These newer techniques include elastography and contrast-enhanced ultrasound, as well as improved B-mode and Doppler techniques. These modalities can be combined to define a novel ultrasound approach, multiparametric ultrasound. High frequency Micro-ultrasound has emerged as a promising imaging technology for PCa diagnosis. Initial results have shown high sensitivity of Micro-ultrasound in detecting PCa in addition to its potential in improving the accuracy of targeted biopsies, based on targeting under real-time visualization, rather than relying on cognitive/fusion software MRI-transrectal ultrasound-guided biopsy.

Emerging Applications of Ultrasound-Contrast Agents in Radiation Therapy

Radiation therapy (RT) causes DNA damage through ionization, leading to double-strand breaks. In addition, it generates reactive oxygen species (ROS), which are toxic to tumor cells and the vasculature. However, hypoxic regions in the tumor have been shown to not only decrease treatment response but also increase the likelihood of recurrence and metastasis. Ultrasound-sensitive micro-bubbles are emerging as a useful diagnostic and therapeutic tool within RT. Contrast-enhanced ultrasound (CEUS) has shown great promise in early prediction of tumor response to RT. Ultrasound-triggered micro-bubble cavitation has also been shown to induce bio-effects that can sensitize angiogenic tumor vessels to RT. Additionally, ultrasound can trigger the release of drugs from micro-bubble carriers via localized micro-bubble destruction. This approach has numerous applications in RT, including targeted oxygen delivery before radiotherapy. Furthermore, micro-bubbles can be used to locally create ROS without radiation. Sonodynamic therapy uses focused ultrasound and a sonosensitizer to selectively produce ROS in the tumor region and has been explored as a treatment option for cancer. This review summarizes emerging applications of ultrasound contrast agents in RT and ROS augmentation.

A Horseshoe Kidney With Solid Mass: A Case Report

Sonography can be used to detect horseshoe kidneys. Sonographic findings for horseshoe kidney commonly include an isthmus connection occurring anterior to the aorta and just inferior to the inferior mesenteric artery. This anomaly can occur with or without complications. Each kidney is associated with its separate collecting system, leading to increased risk for complications. This case study describes the importance of identifying a horseshoe kidney, isthmus, and blood supply to ensure prompt intervention if needed. A case report of a horseshoe kidney with a solid mass, in the isthmus, is provided.

Contrast-enhanced ultrasound for quantification of tissue perfusion in humans

Contrast-enhanced ultrasound is an imaging technique that can be used to quantify microvascular blood volume and blood flow of vital organs in humans. It relies on the use of microbubble contrast agents and ultrasound-based imaging of microbubbles. Over the past decades, both ultrasound contrast agents and experimental techniques to image them have rapidly improved, as did experience among investigators and clinicians. However, these improvements have not yet resulted in uniform guidelines for CEUS when it comes to quantification of tissue perfusion in humans, preventing its uniform and widespread use in research settings. The objective of this review is to provide a methodological overview of CEUS and its development, the influences of hardware and software settings, type and dosage of ultrasound contrast agent, and method of analysis on CEUS-derived perfusion data. Furthermore, we will discuss organ-specific imaging challenges, advantages, and limitations of CEUS.

A Narrative Review on Contrast-Enhanced Ultrasound in Aortic Endograft Endoleak Surveillance

Endovascular repair of abdominal aortic aneurysms have been performed successfully since 1991. However, 20% to 50% of these patients may develop an endoleak or continued aneurysmal sac expansion or perfusion despite stent graft coverage. Current recommendations suggest lifelong surveillance with computed tomographic angiography (CTA) at least 1 month after intervention and yearly after that. In select patients with a stable aneurysm sac on computed tomography performed 1 year after treatment, future screening could be performed with ultrasonography. However, color Doppler ultrasound can fail to detect as many as 31% of endoleaks. Contrast-enhanced ultrasound (CEUS) provides an alternative approach to excluded aneurysm sac follow-up imaging. The Society for Vascular Surgery notes a need for further research on the role of CEUS in endovascular aortic repair surveillance. The European Federation of Societies for Ultrasound in Medicine and Biology suggests that early results are promising. Meta-analyses report pooled sensitivities and specificities of CEUS compared with CTA for the detection of endoleak between 89% and 98% and 86% and 88%, respectively. Owing to the dynamic flow information it provides, CEUS may actually be more sensitive than CTA at detection and characterization in select circumstances. Challenges with adoption, patient selection, and operator dependency remain, but current and future research suggests a role for CEUS in endoleak surveillance.

Contrast-enhanced ultrasonography in interventional oncology

Contrast-enhanced ultrasound (CEUS) has evolved from the use of agitated saline to second generation bioengineered microbubbles designed to withstand insonation with limited destruction. While only one of these newer agents is approved by the Food and Drug Administration for use outside echocardiography, interventional radiologists are increasingly finding off-label uses for ultrasound contrast agents. Notably, these agents have an extremely benign safety profile with no hepatic or renal toxicities and no radiation exposure. Alongside diagnostic applications, CEUS has begun to develop its own niche within the realm of interventional oncology. Certainly, the characterization of focal solid organ lesions (such as hepatic and renal lesions) by CEUS has been an important development. However, interventional oncologists are finding that the dynamic and real-time information afforded by CEUS can improve biopsy guidance, ablation therapy, and provide early evidence of tumor viability after locoregional therapy. Even more novel uses of CEUS include lymph node mapping and sentinel lymph node localization. Critical areas of research still exist. The purpose of this article is to provide a narrative review of the emerging roles of CEUS in interventional oncology.

A Novel Microflow Phantom Dedicated to Ultrasound Microvascular Measurements

Tumor microvascularization is a biomarker of response to antiangiogenic treatments and is accurately assessed by ultrasound imaging. Imaging modes used to visualize slow flows include Power Doppler imaging, dynamic contrast-enhanced ultrasonography, and more recently, microvascular Doppler. Flow phantoms are used to evaluate the performance of Doppler imaging techniques, but they do not have a steady flow and sufficiently small channels. We report a novel device for robust and stable microflow measurements and the study of the microvascularization. Based on microfluidics technology, the prototype features wall-less cylindrical channels of diameters ranging from as small as 147 up to 436 µm, cast in a soft silicone polymer and perfused via a microfluidic flow pressure controller. The device was assessed using flow rates from 49 to 146 µL/min, with less than 1% coefficient of variation over three minutes, corresponding to velocities of 6 to 142 mm/s. This enabled us to evaluate and confirm the reliability of the Superb Microvascular Imaging Doppler mode compared with the Power Doppler mode at these flow rates in the presence of vibrations mimicking physiological motion.

Dependence of the subharmonic signal from contrast agent microbubbles on ambient pressure: A theoretical analysis

This paper investigates the dependence of the subharmonic response in a signal scattered by contrast agent microbubbles on ambient pressure to provide quantitative estimations of local blood pressure. The problem is formulated by assuming a gas bubble encapsulated by a shell of finite thickness with dynamic behavior modeled by a nonlinear viscoelastic constitutive equation. For ambient overpressure compatible with the clinical range, the acoustic pressure intervals where the subharmonic signal may be detected (above the threshold for the onset and below the limit value for the first chaotic transition) are determined. The analysis shows that as the overpressure is increased, all harmonic components are displaced to higher frequencies. This displacement is significant for the subharmonic of order 1/2 and explains the increase or decrease in the subharmonic amplitude with ambient pressure described in previous works. Thus, some questions related to the monotonic dependence of the subharmonic amplitude on ambient pressure are clarified. For different acoustic pressures, quantitative conditions for determining the intervals where the subharmonic amplitude is a monotonic or non-monotonic function of the ambient pressure are provided. Finally, the influence of the ambient pressure on the subharmonic resonance frequency is analyzed.

Focal areas of increased lipid concentration on the coating of microbubbles during short tone-burst ultrasound insonification

Acoustic behavior of lipid-coated microbubbles has been widely studied, which has led to several numerical microbubble dynamics models that incorporate lipid coating behavior, such as buckling and rupture. In this study we investigated the relationship between microbubble acoustic and lipid coating behavior on a nanosecond scale by using fluorescently labeled lipids. It is hypothesized that a local increased concentration of lipids, appearing as a focal area of increased fluorescence intensity (hot spot) in the fluorescence image, is related to buckling and folding of the lipid layer thereby highly influencing the microbubble acoustic behavior. To test this hypothesis, the lipid microbubble coating was fluorescently labeled. The vibration of the microbubble (n = 177; 2.3–10.3 μm in diameter) upon insonification at an ultrasound frequency of 0.5 or 1 MHz at 25 or 50 kPa acoustic pressure was recorded with the UPMC Cam, an ultra-high-speed fluorescence camera, operated at ~4–5 million frames per second. During short tone-burst excitation, hot spots on the microbubble coating occurred at relative vibration amplitudes > 0.3 irrespective of frequency and acoustic pressure. Around resonance, the majority of the microbubbles formed hot spots. When the microbubble also deflated acoustically, hot spot formation was likely irreversible. Although compression-only behavior (defined as substantially more microbubble compression than expansion) and subharmonic responses were observed in those microbubbles that formed hot spots, both phenomena were also found in microbubbles that did not form hot spots during insonification. In conclusion, this study reveals hot spot formation of the lipid monolayer in the microbubble’s compression phase. However, our experimental results show that there is no direct relationship between hot spot formation of the lipid coating and microbubble acoustic behaviors such as compression-only and the generation of a subharmonic response. Hence, our hypothesis that hot spots are related to acoustic buckling could not be verified.

Testing the Efficacy of Contrast-Enhanced Ultrasound in Detecting Transplant Rejection Using a Murine Model of Heart Transplantation

One of the key unmet needs to improve long-term outcomes of heart transplantation is to develop accurate, noninvasive, and practical diagnostic tools to detect transplant rejection. Early intragraft inflammation and endothelial cell injuries occur prior to advanced transplant rejection. We developed a novel diagnostic imaging platform to detect early declines in microvascular perfusion (MP) of cardiac transplants using contrast-enhanced ultrasonography (CEUS). The efficacy of CEUS in detecting transplant rejection was tested in a murine model of heart transplants, a standard preclinical model of solid organ transplant. As compared to the syngeneic groups, a progressive decline in MP was demonstrated in the allografts undergoing acute transplant rejection (40%, 64%, and 92% on days 4, 6, and 8 posttransplantation, respectively) and chronic rejection (33%, 33%, and 92% on days 5, 14, and 30 posttransplantation, respectively). Our perfusion studies showed restoration of MP following antirejection therapy, highlighting its potential to help monitor efficacy of antirejection therapy. Our data suggest that early endothelial cell injury and platelet aggregation contributed to the early MP decline observed in the allografts. High-resolution MP mapping may allow for noninvasive detection of heart transplant rejection. The data presented have the potential to help in the development of next-generation imaging approaches to diagnose transplant rejection.

Characterisation of Liposome-Loaded Microbubble Populations for Subharmonic Imaging

Therapeutic microbubbles could make an important contribution to the diagnosis and treatment of cancer. Acoustic characterisation was performed on microfluidic generated microbubble populations that either were bare or had liposomes attached. Through the use of broadband attenuation techniques (3-8 MHz), the shell stiffness was measured to be 0.72 ± 0.01 and 0.78 ± 0.05 N/m and shell friction was 0.37 ± 0.05 and 0.74 ± 0.05 × 10^-6 kg/s for bare and liposome-loaded microbubbles, respectively. Acoustic scatter revealed that liposome-loaded microbubbles had a lower subharmonic threshold, occurring from a peak negative pressure of 50 kPa, compared with 200 kPa for equivalent bare microbubbles. It was found that liposome loading had a negligible effect on the destruction threshold for this microbubble type, because at a mechanical index >0.4 (570 kPa), 80% of both populations were destroyed.