Pressure-dependent attenuation in ultrasound contrast agents

High-Frequency Array-Based Nanobubble Nonlinear Imaging in a Phantom and In Vivo

There has been growing interest in nanobubbles (NBs) for vascular and extravascular ultrasound contrast imaging and therapeutic applications. Studies to date have generally utilized low frequencies (<12 MHz), high concentrations (>10⁹ mL^-1), and uncalibrated B-mode or contrast-mode on commercial systems without reporting investigations on NB signatures upon which the imaging protocols should be based. We recently demonstrated that low concentrations (10⁶ mL^-1) of porphyrin-lipid-encapsulated NBs scatter nonlinearly at low (2.5, 8 MHz) and high (12.5, 25, 30 MHz) frequencies in a pressure threshold-dependent manner that is advantageous for amplitude modulation (AM) imaging. Here, we implement pressure-calibrated AM at high frequency on a commercial preclinical array system to enhance sensitivity to nonlinear scattering of three phospholipid-based NB formulations. With this approach, improvements in contrast to tissue ratio relative to B-mode between 12.4 and 22.8 dB are demonstrated in a tissue-mimicking phantom, and between 6.7 and 14.8 dB in vivo.

Shell properties and concentration stability of acoustofluidic delivery agents

This paper investigates the shell elastic properties and the number-concentration stability of a new acoustofluidic delivery agent liposome in comparison to Definity™, a monolayer ultrasonic contrast agent microbubble. The frequency dependent attenuation of an acoustic beam passing through a microbubble suspension was measured to estimate the shell parameters. The excitation voltage was adjusted to ensure constant acoustic pressure at all frequencies. The pressure was kept at the lowest possible magnitude to ensure that effects from nonlinear bubble behaviour which are not considered in the analytical model were minimal. The acoustofluidic delivery agent shell stiffness S_p and friction S_f parameters were determined as (S_p = 0.11 N/m, S_f = 0.31 × 10^-6 Kg/s at 25 °C) in comparison to the Definity™ monolayer ultrasound contrast agent which were (S_p = 1.53 N/m, S_f = 1.51 × 10^-6 Kg/s at 25 °C). When the temperature was raised to physiological levels, the friction coefficient S_f decreased by 28% for the monolayer microbubbles and by only 9% for the liposomes. The stiffness parameter S_p of the monolayer microbubble decreased by 23% while the stiffness parameter of the liposome increased by a similar margin (27%) when the temperature was raised to 37 °C. The size distribution of the bubbles was measured using Tunable Resistive Pulse Sensing (TRPS) for freshly prepared microbubbles and for bubble solutions at 6 h and 24 h after activation to investigate their number-concentration stability profile. The liposome maintained >80% of their number-concentration for 24 h at physiological temperature, while the monolayer microbubbles maintained only 27% of their number-concentration over the same period. These results are important input parameters for the design of effective acoustofluidic delivery systems using the new liposomes.

Ultrasound Contrast Agent Modeling: A Review

Ultrasound is extensively used in medical imaging, being safe and inexpensive and operating in real time. Its scope of applications has been widely broadened by the use of ultrasound contrast agents (UCAs) in the form of microscopic bubbles coated by a biocompatible shell. Their increased use has motivated a large amount of research to understand and characterize their physical properties as well as their interaction with the ultrasound field and their surrounding environment. Here we review the theoretical models that have been proposed to study and predict the behavior of UCAs. We begin with a brief introduction on the development of UCAs. We then present the basics of free-gas-bubble dynamics upon which UCA modeling is based. We review extensively the linear and non-linear models for shell elasticity and viscosity and present models for non-spherical and asymmetric bubble oscillations, especially in the presence of surrounding walls or tissue. Then, higher-order effects such as microstreaming, shedding and acoustic radiation forces are considered. We conclude this review with promising directions for the modeling and development of novel agents.

Window-Modulated Compounding Nakagami Parameter Ratio Approach for Assessing Muscle Perfusion with Contrast-Enhanced Ultrasound Imaging

The assessment of microvascular perfusion is essential for the diagnosis of a specific muscle disease. In comparison with the current available medical modalities, the contrast-enhanced ultrasound imaging is the simplest and fastest means for probing the tissue perfusion. Specifically, the perfusion parameters estimated from the ultrasound time-intensity curve (TIC) and statistics-based time-Nakagami parameter curve (TNC) approaches were found able to quantify the perfusion. However, due to insufficient tolerance on tissue clutters and subresolvable effects, these approaches remain short of reproducibility and robustness. Consequently, the window-modulated compounding (WMC) Nakagami parameter ratio imaging was proposed to alleviate these effects, by taking the ratio of WMC Nakagami parameters corresponding to the incidence of two different acoustic pressures from an employed transducer. The time-Nakagami parameter ratio curve (TNRC) approach was also developed to estimate perfusion parameters. Measurements for the assessment of muscle perfusion were performed from the flow phantom and animal subjects administrated with a bolus of ultrasound contrast agents. The TNRC approach demonstrated better sensitivity and tolerance of tissue clutters than those of TIC and TNC. The fusion image with the WMC Nakagami parameter ratio and B-mode images indicated that both the tissue structures and perfusion properties of ultrasound contrast agents may be better discerned.

The relation of Bleomycin Delivery Efficiency to Microbubble Sonodestruction and Cavitation Spectral Characteristics

The concurrent assessment of principal sonoporation factors has been accomplished in a single systemic study. Microbubble sonodestruction dynamics and cavitation spectral characteristics, ultrasound scattering and attenuation, were examined in relation to the intracellular delivery of anticancer drug, bleomycin. Experiments were conducted on Chinese hamster ovary cells coadministered with Sonovue microbubbles. Detailed analysis of the scattering and attenuation temporal functions culminated in quantification of metrics, inertial cavitation dose and attenuation rate, suitable for cavitation control. The exponents, representing microbubble sonodestruction kinetics were exploited to derive dosimetric, microbubble sonodestruction rate. High intracorrelation between empirically-attained metrics defines the relations which indicate deep physical interdependencies within inherent phenomena. Subsequently each quantified metric was validated to be well-applicable to prognosticate the efficacy of bleomycin delivery and cell viability, as indicated by strong overall correlation (R² > 0.85). Presented results draw valuable insights in sonoporation dosimetry and contribute towards the development of universal sonoporation dosimetry model. Both bleomycin delivery and cell viability reach their respective plateau levels by the time, required to attain total microbubble sonodestruction, which accord with scattering and attenuation decrease to background levels. This suggests a well-defined criterion, feasible through signal-registration, universally employable to set optimal duration of exposure for efficient sonoporation outcome.

Theoretical estimation of attenuation coefficient of resonant ultrasound contrast agents

Acoustic characterization of ultrasound contrast agents (UCAs, coated microbubbles) relies on the attenuation theory that assumes the UCAs oscillate linearly at sufficiently low excitation pressures. Effective shell parameters of the UCAs can be estimated by fitting a theoretical attenuation curve to experimentally measured attenuation data. Depending on the excitation frequency and properties of the shell, however, an UCA may oscillate nonlinearly even at sufficiently low excitation pressures, violating the assumption in the linear attenuation theory. Notably, the concern over the estimation of the attenuation coefficient of a microbubble at resonance using linearized approximation has long been addressed. This article investigated the attenuation phenomenon through analyzing the energy dissipation of a single UCA and propagating waves in an UCA suspension, both of which employed a nonlinear Rayleigh-Plesset equation. Analytical formulas capable of estimating the attenuation coefficient due to the weakly nonlinear oscillations of the UCA were obtained with a relatively rigorous mathematical analysis. The computed results that were verified by numerical simulations showed the attenuation coefficient of the UCA at resonance was pressure-dependent and could be significantly smaller than that predicted by the linear attenuation theory. Polydispersity of the UCA population enlarged the difference in the estimation of attenuation between the linear and present second-order nonlinear theories.

Investigation of Microbubble Cavitation-Induced Calcein Release from Cells In Vitro

In the present study, microbubble (MB) cavitation signal analysis was performed together with calcein release evaluation in both pressure and exposure duration domains of the acoustic field. A passive cavitation detection system was used to simultaneously measure MB scattering and attenuation signals for subsequent extraction efficiency relative to MB cavitation activity. The results indicate that the decrease in the efficiency of extraction of calcein molecules from Chinese hamster ovary cells, as well as cell viability, is associated with MB cavitation activity and can be accurately predicted using inertial cavitation doses up to 0.18 V × s (R² > 0.9, p < 0.0001). No decrease in additional calcein release or cell viability was observed after complete MB sonodestruction was achieved. This indicates that the optimal exposure duration within which maximal sono-extraction efficiency is obtained coincides with the time necessary to achieve complete MB destruction. These results illustrate the importance of MB inertial cavitation in the sono-extraction process. To our knowledge, this study is the first to (i) investigate small molecule extraction from cells via sonoporation and (ii) relate the extraction process to the quantitative characteristics of MB cavitation acoustic spectra.

Stability of Monodisperse Phospholipid-Coated Microbubbles Formed by Flow-Focusing at High Production Rates

Monodisperse microbubble ultrasound contrast agents may dramatically increase the sensitivity and efficiency in ultrasound imaging and therapy. They can be produced directly in a microfluidic flow-focusing device, but questions remain as to the interfacial chemistry, such as the formation and development of the phospholipid monolayer coating over time. Here, we demonstrate the synthesis of monodisperse bubbles with radii of 2-10 μm at production rates ranging from 10(4) to 10(6) bubbles/s. All bubbles were found to dissolve to a stable final radius 2.55 times smaller than their initial radius, independent of the nozzle size and shear rate, indicating that the monolayer self-assembles prior to leaving the nozzle. The corresponding decrease in surface area by a factor 6.6 reveals that lipid molecules are adsorbed to the gas-liquid interface in the disordered expanded state, and they become mechanically compressed by Laplace pressure-driven bubble dissolution to a more ordered condensed state with near zero surface tension. Acoustic characterization of the stabilized microbubbles revealed that their shell stiffness gradually increased from 0.8 to 2.5 N/m with increasing number of insonations through the selective loss of the more soluble lipopolymer molecules. This work therefore demonstrates high-throughput production of clinically relevant monodisperse contrast microbubbles with excellent control over phospholipid monolayer elasticity and microbubble resonance.

Accurate measurement of microbubble response to ultrasound with a diagnostic ultrasound scanner

Ultrasound and microbubbles are often used to enhance drug delivery and the suggested mechanisms are extravasation and sonoporation. Drug delivery schemes with ultrasound and microbubbles at both low and high acoustic amplitudes have been suggested. A diagnostic ultrasound scanner may play a double role as both an imaging and a therapy device. It was not possible to accurately measure microbubble response with an ultrasound scanner for a large range of acoustic pressures and microbubble concentrations until now, mainly because of signal saturation issues. A method for continuously adjusting the receive gain of a scanner and limiting signal saturation was developed to accurately measure backscattered echoes from microbubbles for mechanical indexes (MIs) up to 2.1. The intensity of backscattered echoes from microbubbles increased quarticly with MI without reaching any limit. The signal intensity from microbubbles was found to be linear with concentration at both low and high MIs. However, at very high concentrations, acoustic shadowing occurs which limits the delivered acoustic pressure in deeper areas. The contrastto- tissue ratio was also measured and found to stay constant with MI. These results can be used to better guide drug delivery approaches and to also develop imaging techniques for therapy procedures.

Dual-frequency piezoelectric transducers for contrast enhanced ultrasound imaging

For many years, ultrasound has provided clinicians with an affordable and effective imaging tool for applications ranging from cardiology to obstetrics. Development of microbubble contrast agents over the past several decades has enabled ultrasound to distinguish between blood flow and surrounding tissue. Current clinical practices using microbubble contrast agents rely heavily on user training to evaluate degree of localized perfusion. Advances in separating the signals produced from contrast agents versus surrounding tissue backscatter provide unique opportunities for specialized sensors designed to image microbubbles with higher signal to noise and resolution than previously possible. In this review article, we describe the background principles and recent developments of ultrasound transducer technology for receiving signals produced by contrast agents while rejecting signals arising from soft tissue. This approach relies on transmitting at a low-frequency and receiving microbubble harmonic signals at frequencies many times higher than the transmitted frequency. Design and fabrication of dual-frequency transducers and the extension of recent developments in transducer technology for dual-frequency harmonic imaging are discussed.

Acoustic characterization of contrast-to-tissue ratio and axial resolution for dual-frequency contrast-specific acoustic angiography imaging

Recently, dual-frequency transducers have enabled high-spatial-resolution and high-contrast imaging of vasculature with minimal tissue artifacts by transmitting at a low frequency and receiving broadband superharmonic echoes scattered by microbubble contrast agents. In this work, we examine the imaging parameters for optimizing contrast-to-tissue ratio (CTR) for dual-frequency imaging and the relationship with spatial resolution. Confocal piston transducers are used in a water bath setup to measure the SNR, CTR, and axial resolution for ultrasound imaging of nonlinear scattering of microbubble contrast agents when transmitting at a lower frequency (1.5 to 8 MHz) and receiving at a higher frequency (7.5 to 25 MHz). Parameters varied include the frequency and peak negative pressure of transmitted waves, center frequency of the receiving transducer, microbubble concentration, and microbubble size. CTR is maximized at the lowest transmission frequencies but would be acceptable for imaging in the 1.5 to 3.5 MHz range. At these frequencies, CTR is optimized when a receiving transducer with a center frequency of 10 MHz is used, with the maximum CTR of 25.5 dB occurring when transmitting at 1.5 MHz with a peak negative pressure of 1600 kPa and receiving with a center frequency of 10 MHz. Axial resolution is influenced more heavily by the receiving center frequency, with a weak decrease in measured pulse lengths associated with increasing transmit frequency. A microbubble population containing predominately 4-μm-diameter bubbles yielded the greatest CTR, followed by 1- and then 2-μm bubbles. Varying concentration showed little effect over the tested parameters. CTR dependence on transmit frequency and peak pressure were confirmed through in vivo imaging in two rodents. These findings may lead to improved imaging of vascular remodeling in superficial or luminal cancers such as those of the breast, prostate, and colon.

Endocardial border delineation capability of a novel multimodal polymer-shelled contrast agent

Background

A novel polymer-shelled contrast agent (CA) with multimodal and target-specific potential was developed recently. To determine its ultrasonic diagnostic features, we evaluated the endocardial border delineation as visualized in a porcine model and the concomitant effect on physiological variables.

Methods

Three doses of the novel polymer-shelled CA (1.5 ml, 3 ml, and 5 ml [5 × 10⁸ microbubbles (MBs)/ml]) and the commercially available CA SonoVue (1.5 ml [2–5 × 10⁸ MBs/ml]) were used. Visual evaluations of ultrasound images of the left ventricle were independently performed by three observers who graded each segment in a 6-segment model as either 0 = not visible, 1 = weakly visible, or 2 = visible. Moreover, the duration of clinically useful contrast enhancement and the left ventricular opacification were determined. During anesthesia, oxygen saturation, heart rate, and arterial pressure were sampled every minute and the effect of injection of CA on these physiological variables was evaluated.

Results

The highest dose of the polymer-shelled CA gave results comparable to SonoVue. Thus, no significant difference in the overall segment score distribution (2-47-95 vs. 1-39-104), time for clinically sufficient contrast enhancement (20–40 s for both) and left ventricular overall opacification was found. In contrast, when comparing the endocardial border delineation capacity for different regions SonoVue showed significantly higher segment scores for base and mid, except for the mid region when injecting 1.5 ml of the polymer-shelled CA. Neither high nor low doses of the polymer-shelled CA significantly affected the investigated physiological variables.

Conclusions

This study demonstrated that the novel polymer-shelled CA can be used in contrast-enhanced diagnostic imaging without influence on major physiological variables.

Low-amplitude non-linear volume vibrations of single microbubbles measured with an "acoustical camera"

Contrast-enhanced ultrasound imaging is based on the detection of non-linear vibrational responses of a contrast agent after its intravenous administration. Improving contrast-enhanced images requires an accurate understanding of the vibrational response to ultrasound of the lipid-coated gas microbubbles that constitute most ultrasound contrast agents. Variations in the volume of microbubbles provide the most efficient radiation of ultrasound and, therefore, are the most important bubble vibrations for medical diagnostic ultrasound imaging. We developed an "acoustical camera" that measures the dynamic volume change of individual microbubbles when excited by a pressure wave. In the work described here, the technique was applied to the characterization of low-amplitude non-linear behaviors of BR14 microbubbles (Bracco Research, Geneva, Switzerland). The amplitude dependence of the resonance frequency and the damping, the prevalence of efficient subharmonic and ultraharmonic vibrations and the amplitude dependence of the response at the fundamental frequency and at the second harmonic frequency were investigated. Because of the large number of measurements, we provide a statistical characterization of the low-amplitude non-linear properties of the contrast agent.

Visualization of multimodal polymer-shelled contrast agents using ultrasound contrast sequences: an experimental study in a tissue mimicking flow phantom

Background

A multimodal polymer-shelled contrast agent (CA) with target specific potential was recently developed and tested for its acoustic properties in a single element transducer setup. Since the developed polymeric CA has different chemical composition than the commercially available CAs, there is an interest to study its acoustic response when using clinical ultrasound systems. The aim of this study was therefore to investigate the acoustic response by studying the visualization capability and shadowing effect of three polymer-shelled CAs when using optimized sequences for contrast imaging.

Methods

The acoustic response of three types of the multimodal CA was evaluated in a tissue mimicking flow phantom setup by measuring contrast to tissue ratio (CTR) and acoustic shadowing using five image sequences optimized for contrast imaging. The measurements were performed over a mechanical index (MI) range of 0.2-1.2 at three CA concentrations (10⁶, 10⁵, 10⁴ microbubbles/ml).

Results

The CTR-values were found to vary with the applied contrast sequence, MI and CA. The highest CTR-values were obtained when a contrast sequence optimized for higher MI imaging was used. At a CA concentration of 10⁶ microbubbles/ml, acoustic shadowing was observed for all contrast sequences and CAs.

Conclusions

The CAs showed the potential to enhance ultrasound images generated by available contrast sequences. A CA concentration of 10⁶ MBs/ml implies a non-linear relation between MB concentration and image intensity.

Counter-propagating wave interaction for contrast-enhanced ultrasound imaging

Most techniques for contrast-enhanced ultrasound imaging require linear propagation to detect nonlinear scattering of contrast agent microbubbles. Waveform distortion due to nonlinear propagation impairs their ability to distinguish microbubbles from tissue. As a result, tissue can be misclassified as microbubbles, and contrast agent concentration can be overestimated; therefore, these artifacts can significantly impair the quality of medical diagnoses. Contrary to biological tissue, lipid-coated gas microbubbles used as a contrast agent allow the interaction of two acoustic waves propagating in opposite directions (counter-propagation). Based on that principle, we describe a strategy to detect microbubbles that is free from nonlinear propagation artifacts. In vitro images were acquired with an ultrasound scanner in a phantom of tissue-mimicking material with a cavity containing a contrast agent. Unlike the default mode of the scanner using amplitude modulation to detect microbubbles, the pulse sequence exploiting counter-propagating wave interaction creates no pseudoenhancement behind the cavity in the contrast image.

Can ultrasound enable efficient intracellular uptake of molecules? A retrospective literature review and analysis

Most applications of therapeutic ultrasound (US) for intracellular delivery of drugs, proteins, DNA/RNA and other compounds would benefit from efficient uptake of these molecules into large numbers of cells without killing cells in the process. In this study we tested the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro. A search of the literature for studies with quantitative data on uptake and viability yielded 26 published papers containing 898 experimental data points. Analysis of these studies showed that just 7.7% of the data points corresponded to relatively efficient uptake (>50% of cells exhibiting uptake). Closer examination of the data showed that use of Definity US contrast agent (as opposed to Optison) and elevated sonication temperature at 37°C (as opposed to room temperature) were associated with high uptake, which we further validated through independent experiments carried out in this study. Although these factors contributed to high uptake, almost all data with efficient uptake were from studies that had not accounted for lysed cells when determining cell viability. Based on retrospective analysis of the data, we showed that not accounting for lysed cells can dramatically increase the calculated uptake efficiency. We further argue that if all the data considered in this study were re-analyzed to account for lysed cells, there would be essentially no data with efficient uptake. We therefore conclude that the literature does not support the hypothesis that efficient intracellular uptake of molecules can be achieved with high cell viability after US exposure in vitro, which poses a challenge to future applications of US that require efficient intracellular delivery.

Adaptive control of contrast agent microbubbles for shell parameter identification

An adaptive controller design is proposed and simulated for parameter identification and oscillation control in microbubble systems. Lyapunov's direct method and a Lyapunov-like analysis are used to show stability and convergence of trajectory tracking and parameter adaptation. The method allows for the determination of microbubble contrast agent shell thickness or material parameters in a nondestructive manner.

Far-wall pseudoenhancement during contrast-enhanced ultrasound of the carotid arteries: clinical description and in vitro reproduction

The present study describes the presence of pseudoenhancement during contrast-enhanced ultrasound (CEUS) imaging of human carotid arteries and the reproduction of this pseudoenhancement in vitro. Seventy patients underwent bilateral CEUS examination of the carotid arteries using a Philips iU22 ultrasound system equipped with a L9-3 ultrasound probe and SonoVue microbubble contrast. During CEUS of the carotid arteries, we identified enhancement in close proximity to the far wall, parallel to the main lumen. The location of this enhancement does not correlate to the anatomical location of a parallel vessel. To corroborate the hypothesis that this is a pseudoenhancement artifact, the enhancement was recreated in a tissue-mimicking material phantom, using the same ultrasound system, settings and contrast agent as the patient study. The phantom study showed that pseudoenhancement may be present during vascular CEUS and that the degree of pseudoenhancement is influenced by the size and concentration of the microbubbles. During vascular CEUS, identification of the artifact is important to prevent misinterpretation of enhancement in and near the far wall.

Improving the sensitivity of high-frequency subharmonic imaging with coded excitation: a feasibility study

PURPOSE

Subharmonic intravascular ultrasound imaging (S-IVUS) could visualize the adventitial vasa vasorum, but the high pressure threshold required to incite subharmonic behavior in an ultrasound contrast agent will compromise sensitivity-a trait that has hampered the clinical use of S-IVUS. The purpose of this study was to assess the feasibility of using coded-chirp excitations to improve the sensitivity and axial resolution of S-IVUS.

METHODS

The subharmonic response of Targestar-p(TM), a commercial microbubble ultrasound contrast agent (UCA), to coded-chirp (5%-20% fractional bandwidth) pulses and narrowband sine-burst (4% fractional bandwidth) pulses was assessed, first using computer simulations and then experimentally. Rectangular windowed excitation pulses with pulse durations ranging from 0.25 to 3 μs were used in all studies. All experimental studies were performed with a pair of transducers (20 MHz/10 MHz), both with diameter of 6.35 mm and focal length of 50 mm. The size distribution of the UCA was measured with a Casy(TM) Cell counter.

RESULTS

The simulation predicted a pressure threshold that was an order of magnitude higher than that determined experimentally. However, all other predictions were consistent with the experimental observations. It was predicted that: (1) exciting the agent with chirps would produce stronger subharmonic response relative to those produced by sine-bursts; (2) increasing the fractional bandwidth of coded-chirp excitation would increase the sensitivity of subharmonic imaging; and (3) coded-chirp would increase axial resolution. The experimental results revealed that subharmonic-to-fundamental ratios obtained with chirps were 5.7 dB higher than those produced with sine-bursts of similar duration. The axial resolution achieved with 20% fractional bandwidth chirps was approximately twice that achieved with 4% fractional bandwidth sine-bursts.

CONCLUSIONS

The coded-chirp method is a suitable excitation strategy for subharmonic IVUS imaging. At the 20 MHz transmission frequency and 20% fractional bandwidth, coded-chirp excitation appears to represent the ideal tradeoff between subharmonic strength and axial resolution.

Estimating concentration of ultrasound contrast agents with backscatter coefficients: experimental and theoretical aspects

Ultrasound contrast agents (UCAs) have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to estimate UCA concentration noninvasively. In this study, a noninvasive method for estimating UCA concentration was developed in vitro. Backscatter coefficients (BSCs) estimated from measurements of Definity(®) UCAs were fitted to a theoretical scattering model in the 15-25 MHz range using a Levenberg-Marquardt regression technique. The model was defined by the UCA size distribution and concentration, and therefore concentration estimates were extracted directly from the fit. Calculation of the BSC was accomplished using planar reference measurements from the back wall of a Plexiglas(®) chamber and an average of 500 snapshots of ultrasonic backscatter from UCAs flowing through the chamber. In order to verify the ultrasonically derived UCA concentration estimates, a sample of the UCAs was extracted from the flow path and the concentration was estimated with a hemacytometer. UCA concentrations of 1, 2, and 5 times the dose recommended by the manufacturer were used in experiments. All BSC-based estimates were within one standard deviation of hemacytometer based estimates for peak rarefactional pressures of 100-400 kPa.

Optimal dosage of ultrasound contrast agent for ultrasound surgery: thermal effect of linear plane wave

The optimal dosage of ultrasound contrast agent model for ultrasound surgery was explored. A specific ultrasound contrast agent Albunex was chosen for simulation. The model was developed based on a dilute bubbly liquid model proposed by Ye and Ding [Z. Ye, L. Ding, Acoustic dispersion and attenuation relations in bubbly mixture, J. Acoust. Soc. Am. 98 (3) (1995) 1629-1636]. The numerical simulation suggests that 2 MHz is more efficient than 1 MHz to thermally treat cancer in deep tissue with the optimal dosage of 3 ml. On the other hand, the simulation also suggests 3 MHz center frequency with the optimal dosage of 1.6 ml is adequate for prostate cancer treatment with transrectal equipment. The simulation is expected to valid up to 2 MPa incident pressure due to the limitation of the linearized UCA model. Even though it is developed from a single ultrasound contrast agent, this model is expected to be useful for any ultrasound contrast agent as long as the necessary parameters are provided.

Toward contrast-enhanced microwave-induced thermoacoustic imaging of breast cancer: an experimental study of the effects of microbubbles on simple thermoacoustic targets

Microwave-induced thermoacoustic tomography (MI-TAT) is an imaging technique that exploits dielectric contrast at microwave frequencies while creating images with ultrasound resolution. We propose the use of microbubbles as a dielectric contrast agent for enhancing the sensitivity of MI-TAT for breast cancer detection. As an initial investigation of this concept, we experimentally studied the extent to which the microwave-induced thermoacoustic response of a dielectric target is modified by the presence of air-filled glass microbubbles. We created mixtures of ethylene glycol with varying weight percentages of microbubbles and characterized both their microwave properties (0.5-6 GHz) and thermoacoustic response when irradiated with microwave energy at 3 GHz. Our data show that the microbubbles considerably lowered the relative permittivity, electrical conductivity and thermoacoustic response of the ethylene glycol mixtures. We hypothesize that the interstitial infusion of microbubbles to a tumor site will similarly create a smaller thermoacoustic response compared to the pre-contrast-agent response, thereby enhancing sensitivity through the use of differential imaging techniques.

Pressure-dependent attenuation and scattering of phospholipid-coated microbubbles at low acoustic pressures

Previous optical studies have shown threshold behavior of single-contrast agent microbubbles. Below the acoustic pressure threshold, phospholipid-coated microbubbles with sizes <5.0 mum in diameter oscillate significantly less than above the threshold pressure. Previous studies also revealed an acoustic pressure-dependent attenuation of ultrasound by microbubble contrast agents. In this study, we investigated whether pressure-dependent acoustic behavior may be explained by threshold behavior. For this purpose, pressure-dependent attenuation and scattering of a phospholipid-coated contrast agent were measured. Transmit frequencies between 1.5 and 6.0 MHz and acoustic pressures between 5 and 200 kPa were applied. Unlike the galactose-based contrast agent Levovist, the phospholipid-coated contrast agent BR14 showed a pressure-dependent attenuation. In addition, it was found that filtered suspensions with only microbubbles <3.0 mum in diameter show more pressure-dependent attenuation behavior than native suspensions of phospholipid-coated microbubbles. For the scattering measurements conducted at 3.0 MHz, the native suspension did not show any pressure-dependent behavior. However, the filtered suspension responded highly nonlinearly. Between 30 and 150 kPa, 16 dB additional scattered power was obtained. We concluded that threshold behavior of phospholipid-coated microbubbles results in pressure-dependent attenuation and scattering.

Evaluation of tumor angiogenesis with a second-generation US contrast medium in a rat breast tumor model

Objective

Tumor angiogenesis is an important factor for tumor growth, treatment response and prognosis. Noninvasive imaging methods for the evaluation of tumor angiogenesis have been studied, but a method for the quantification of tumor angiogenesis has not been established. This study was designed to evaluate tumor angiogenesis in a rat breast tumor model by the use of a contrast-enhanced ultrasound (US) examination with a second-generation US contrast agent.

Materials and Methods

The alkylating agent 19N-ethyl-N-nitrosourea (ENU) was injected into the intraperitoneal cavity of 30-day-old female Sprague-Dawley rats. Three to four months later, breast tumors were detected along the mammary lines of the rats. A total of 17 breast tumors larger than 1 cm in nine rats were evaluated by gray-scale US, color Doppler US and contrast-enhanced US using SonoVue. The results were recorded as digital video images; time-intensity curves and hemodynamic parameters were analyzed. Pathological breast tumor specimens were obtained just after the US examinations. The tumor specimens were stained with hematoxylin and eosin (H & E) and the expression of CD31, an endothelial cell marker, was determined by immunohistochemical staining. We also evaluated the pathological diagnosis of the tumors and the microvessel density (MVD). Spearman's correlation and the Kruskal-Wallis test were used for the analysis.

Results

The pathological diagnoses were 11 invasive ductal carcinomas and six benign intraductal epithelial proliferations. The MVD did not correlate with the pathological diagnosis. However, blood volume (BV) showed a statistically significant correlation with MVD (Spearman's correlation, p < 0.05).

Conclusion

Contrast-enhanced US using a second-generation US contrast material was useful for the evaluation of tumor angiogenesis of breast tumors in the rat.

Response of contrast agents to ultrasound

Microbubbles are used as ultrasonic contrast agents that enhance the ultrasound signals of the vascular bed. The recent development of site-targeted microbubbles opened up the possibility for molecular imaging as well as localised drug and gene delivery. Initially the microbubbles' physical properties and their response to the ultrasound beam were not fully understood. However, the introduction of fast acquisition microscopy has allowed the observation of the microbubble behaviour in the presence of ultrasound. In addition, acoustical techniques can determine the scatter of single microbubbles. Sonoporation experiments promise high-specificity drug and gene delivery, but the responsible physical mechanisms, particularly for in vivo applications, are not fully understood. An improvement of microbubble technology may address variability related problems in both imaging and drug/gene delivery.

Regularized estimation of contrast agent attenuation to improve the imaging of microbubbles in small animal studies

Quantitative analysis of tissue perfusion using contrast-enhanced ultrasound is still limited by shadowing, which is caused by inadequate compensation for microbubble contrast agent attenuation. Many previous methods have been developed for attenuation correction in soft tissues. However, no method has been proposed to correct for microbubble attenuation in vivo. In this article, a model to estimate microbubble attenuation is presented, using the time-intensity variation in a highly echogenic distal area without contrast uptake. This model is based on the assumption that a linear relationship holds between local microbubble attenuation and local backscatter. The model was applied to 12 murine renal perfusion studies. Parametric images of microbubble attenuation were generated, corresponding to dynamic contrast agent-specific sequences without shadowing. Contrast uptake kinetics consistent with the physiology were retrieved in all perfused areas. This method therefore proved to be of potential interest in the quantification of tissue perfusion in small animal studies.

Clinical relevance of pressure-dependent scattering at low acoustic pressures

Recent optical and acoustical studies have shown a threshold behaviour in the response of phospholipid-coated contrast agents, for a certain range of sizes. Below the acoustic pressure threshold, the microbubbles' scattering efficacy is significantly reduced compared to that above the threshold. Here we investigate the clinical relevance of the observed threshold behaviour. A cardiac ultrasound scanner system was used to analyse the pressure-dependence of the scatter intensity. The scattering of a native suspension of a phospholipid-coated contrast agent was compared to that of a suspension in which microbubbles with a size larger than 3.0 microm in diameter were extracted. A power modulation scheme at the fundamental frequency was applied. After linearly scaling and subtracting the B-mode images recorded at 70 and 200 kPa, the contrast-to-tissue ratio (CTR) of the native suspension was 3.2dB, whereas the CTR of the filtered suspension was 20 dB. The 17 dB difference is attributed to the threshold behaviour. Well-established ultrasound imaging techniques such as fundamental power modulation imaging could benefit from the pressure-dependent scattering properties of this type of contrast microbubbles.

Subharmonic response of encapsulated microbubbles: conditions for existence and amplification

The response of encapsulated microbubbles at half the ultrasound insonation frequency, termed subharmonic response, may have potential applications in diagnosis and therapy. The subharmonic signal, emitted by Definity microbubble cloud sonicated by ultrasound was studied theoretically and experimentally. The size distribution of the microbubbles was optically analyzed and resonance frequency of 2.7 MHz was determined. An asymptotic model has been developed that generates subharmonic response of a single and of a cloud of bubbles of various sizes. Threshold conditions for existence and the intensity of the subharmonic signal are predicted to depend on microbubbles size distribution and shell properties, as well as on the driving field frequency and pressure. Thin tubes filled with Definity solution were insonated at acoustic pressures from 100 to 630 kPa. The intensities of the emitted fundamental harmonics and subharmonics were measured. At frequency 5.5MHz, twice the resonance frequency, the subharmonic signals were observed only at pressures greater than 190 kPa. The subharmonic to fundamental harmonics intensity ratio was within -12 to -1 dB. The experimental results showed good correlation with the theoretical results in the range of validity of the asymptotic solution, thus supporting the model assumptions.

[Tissue attenuation in small animals on contrast enhanced ultrasound]

Despite recent advances in contrast-enhanced ultrasound imaging, evaluation of tissue perfusion with contrast-enhanced ultrasound is still impaired by shadowing effects. These effects are particularly relevant in small animal studies due to high frequency imaging. Current methods of tissue attenuation correction are not suited for contrast-enhanced ultrasound examinations, because microbubble acoustic response to ultrasound waves is far more complex than that of tissues. A method allowing in vivo tissue attenuation correction in the presence of contrast agents is presented.

Attenuation and size distribution measurements of Definity and manipulated Definity populations

The contrast agent Definity has recently been shown to have substantial nonlinear activity at high ultrasound frequencies (>10 MHz). In this study, measurements were performed to characterize the frequency dependent attenuation properties of Definity and populations of Definity that had been modified to preferentially isolate smaller bubbles through decantation or mechanical filtration. A narrowband pulse-echo substitution method was employed with a series of four transducers covering the frequency range from 2 to 50 MHz. "Native" Definity has peak in attenuation in the vicinity of 10 MHz and remains high until 50 MHz. This pattern is significantly different from other clinically approved agents and is consistent with recent reports of nonlinear activity at high frequencies. With increasing decantation times, the attenuation peak becomes more diffuse and occurs at progressively higher frequencies. By 3 h for example, attenuation continues to rise until 30 MHz. The bubble size distribution undergoes preferential skewing toward smaller bubbles with increasing decantation time. Between 30 s and 3 h, the mean bubble diameter goes from 3.99 to 0.98 micrometers. Mechanical filtration with 2 and 1 microm pores causes attenuation to rise until 15 and 40 MHz, respectively. Definity can therefore be manipulated to improve its relative activity at higher frequencies (>10 MHz), which has implications for ultrasound biomicroscopy and intravascular ultrasound applications. Further, these results suggest that agent handling can have a substantial impact on size distributions affecting lower frequency applications. Shell friction estimates derived from these data are lower than those reported for larger bubbles at lower frequencies.

The onset of microbubble vibration

A linear relationship between the relative expansion of an off-resonance ultrasound contrast microbubble and low acoustic pressures is expected. In this study, high-speed optical recordings of individual phospholipid-coated microbubbles were used to investigate this relationship for microbubbles ranging from 2 to 11 microm and for acoustic pressures ranging from 20 to 250 kPa at a driving frequency of 1.7 MHz. For microbubbles larger than 5 microm, the relative expansion (DeltaD/D0) increased linearly with applied acoustic pressure, starting at the origin. The response of smaller microbubbles (<5 microm) also increased linearly with the applied acoustic pressure. However, linearity started at an acoustic pressure threshold value of 30 to 120 kPa for the different individual microbubbles. Below these pressure values, little or no oscillation was observed. The results may be explained by size-dependent mechanical properties of the phospholipid shells. An imaging technique such as power modulation imaging could profit from the presence of an acoustic pressure threshold in the microbubble response.

Experimental investigations of nonlinearities and destruction mechanisms of an experimental phospholipid-based ultrasound contrast agent

OBJECTIVES

We sought to characterize the acoustical behavior of the experimental ultrasound contrast agent BR14 by determining the acoustic pressure threshold above which nonlinear oscillation becomes significant and investigating microbubble destruction mechanisms.

MATERIALS AND METHODS

We used a custom-designed in vitro setup to conduct broadband attenuation measurements at 3.5 MHz varying acoustic pressure (range, 50-190 kPa). We also performed granulometric analyses on contrast agent solutions to accurately measure microbubble size distribution and to evaluate insonification effects.

RESULTS

Attenuation did not depend on acoustic pressure less than 100 kPa, indicating this pressure as the threshold for the appearance of microbubble nonlinear behavior. At the lowest excitation amplitude, attenuation increased during insonification, while, at higher excitation levels, the attenuation decreased over time, indicating microbubble destruction. The destruction rate changed with pressure amplitude suggesting different destruction mechanisms, as it was confirmed by granulometric analysis.

CONCLUSIONS

Microbubbles showed a linear behavior until 100 kPa, whereas beyond this value significant nonlinearities occurred. Observed destruction phenomena seem to be mainly due to gas diffusion and bubble fragmentation mechanisms.

Frequency and pressure dependent attenuation and scattering by microbubbles

The aim of this study was to evaluate experimentally the degree of pressure dependence of attenuation and scattering by microbubbles at low acoustic pressures with an empirical nonlinear model. In addition, the pressure dependency over a range of frequencies (1 to 5 MHz) has been studied. A series of transmission and scattering measurements were made with the microbubble SonoVue, using an automated system. Results show that, within the pressure range studied, attenuation as a result of the microbubble is pressure-dependent, whereas no such dependence of scattering was detectable. The pressure dependence of attenuation for SonoVue was found to be most significant at 1.5 MHz. The scattering is shown to be the highest at the lowest insonation frequency, around 1 approximately 1.25 MHz, and then decreases with frequency.

Nonlinear propagation of ultrasound through microbubble contrast agents and implications for imaging

Microbubble contrast agents produce nonlinear echoes under ultrasound insonation, and current imaging techniques detect these nonlinear echoes to generate contrast agent images accordingly. For these techniques, there is a potential problem in that bubbles along the ultrasound transmission path between transducer and target can alter the ultrasound transmission nonlinearly and contribute to the nonlinear echoes. This can lead to imaging artefacts, especially in regions at depth. In this paper we provide insight, through both simulation and experimental measurement, into the nonlinear propagation caused by microbubbles and the implications for current imaging techniques. A series of investigations at frequencies below, at, and above the resonance frequency of microbubbles were performed. Three specific effects on the pulse propagation (i.e., amplitude attenuation, phase changes, and harmonic generation) were studied. It was found that all these effects are dependent on the initial pulse amplitude, and their dependence on the initial phase of the pulse is shown to be insignificant. Two types of imaging errors are shown to result from this nonlinear propagation: first, that tissue can be misclassified as microbubbles; second, the concentration of microbubbles in the image can be misrepresented. It is found that these imaging errors are significant for all three pulse frequencies when the pulses transmit through a microbubble suspension of 6 cm in path length. It also is found that the first type of error is larger at the bubble resonance frequency.

Prolonging the ultrasound signal enhancement from thrombi using targeted microbubbles based on sulfur-hexafluoride-filled gas

RATIONALE AND OBJECTIVES

The objective of this study is to develop and characterize new microbubbles based on lipids and sulfur hexafluoride (SF6) for targeting thrombi as an improved ultrasound contrast agent.

MATERIALS AND METHODS

Bioconjugate ligands were inserted into the lipid-coated membranes of SF6 gas microbubbles, and their physicochemical properties were determined. Diagnostic efficacies of SF6-filled microbubbles and the contrast agent SonoVue (Bracco Imaging, Geneve, Switzerland) were compared in dogs.

RESULTS

Suspensions of lyophilized powder were reconstituted by injecting saline containing 3.1 x 10(8) SF6 microbubbles/mL with a mean diameter of 4.4 microm. More than 90% of microbubbles had diameters between 1 and 10 microm. After reconstitution, echogenicity and microbubble characteristics were unchanged for 8 hours. Targeted microbubbles increased the echogenicity of thrombi significantly and provided a longer period of optimal signal enhancement compared with nontargeted microbubbles.

CONCLUSIONS

Our thrombus-targeting microbubble contrast agent shows high echogenicity and stability and thereby enhances the visualization of intravascular thrombi and prolongs the duration of the diagnostic window.

Investigating perfluorohexane particles with high-frequency ultrasound

Submicron particles filled with liquid perfluorocarbon are currently being studied as a potential ultrasound-targeted contrast agent. The objective of this study was to evaluate the scattering properties of these particles. Sets of perfluorohexane-filled particles of different average sizes (300 nm to 1000 nm) were produced with a constant total volume fraction. The attenuation coefficient was measured in the 15- to 50-MHz frequency range and was found to increase smoothly with frequency and to be independent of the amplitude and bandwidth of the transmitted pulse. The values range from 0.31 to 0.64 dB/mm at 30 MHz for mean particle size ranging from 970 to 310 nm, respectively. The backscattering spectra of the particle solutions were measured and showed no sign of nonlinear scattering. The backscattering coefficient increased with the power 3.9 +/- 0.3 of the frequency. These results confirm that liquid perfluorocarbon droplets behave as linear Rayleigh scatterers.

Enhanced heat deposition using ultrasound contrast agent--modeling and experimental observations

Ultrasound contrast agents (UCA), created originally for visualization and diagnostic purposes, recently have been suggested as efficient enhancers of ultrasonic power deposition in tissue. The ultrasonic energy absorption by the contrast agents, considered as problematic in diagnostic imaging, might have beneficial impact in therapeutic applications such as targeted hyperthermia-based or ablation treatments. Introduction of gas microbubbles into the tissue to be treated can improve the effectiveness of current treatments by limiting the temperature rise to the treated site and minimizing the damage to the surrounding healthy tissues. To this end, proper assessment of the governing parameters of energy absorption by ultrasonically induced stabilized bubbles is important for both diagnostic and therapeutic ultrasound applications. The current study was designed to predict theoretically and measure experimentally the dissipation and heating effects of encapsulated UCA in a well-controlled and calibrated environment. The ultrasonic effects of the microbubble concentration, transmitted intensity, and frequency on power dissipation and stability of the UCA have been studied. The maximal temperature elevation obtained during 300 s experiments was 21 degrees C, in a 10 ml volume target containing UCA, insonifled by unfocused 3.2 MHz continuous wave (CW) at spatial average intensity of 1.1 W/cm2 (182 kPa). The results also suggest that higher frequencies are more efficiently absorbed by commonly used UCA. In particular, for spatial average intensity of 1.1 W/cm2 and concentration of 5 x 10(6) microspheres/cm3, no significant reduction of UCA absorption was noticed during the first 150 s for insonation at 3.2 MHz and the first 100 s for insonation at 1 MHz. In addition, when lower average intensity of 0.5 W/cm2 (160 kPa) at 3.2 MHz was used, the UCA absorptivity sustained for almost 200 s. Thus, when properly activated, UCA may be suitable for localized hyperthermic therapies.

Ultrasound attenuation measurement in the presence of scatterer variation for reduction of shadowing and enhancement

Pulse-echo ultrasound display relies on many assumptions that are known to be incorrect. Departure from these makes interpretation of conventional ultrasound images difficult, and three-dimensional (3-D) visualizations harder still. For instance, shadowing and enhancement are the result of an incorrect assumption that sound attenuation is a function only of depth. Attempts to reduce such artefacts by estimating attenuation locally have been frustrated by large statistical variations and the influence of scatterer type. We address the latter by examining the influence of scatterer type on two existing attenuation estimation algorithms. This analysis is novel for one of the algorithms, and contains a correction to previously published work for the other. We then propose a novel algorithm that is less sensitive to scatterer variation. We also present a novel technique for handling large statistical variations based on combined assumptions of monotonicity and smoothness. We then assess the performance of each algorithm for correcting shadowing and enhancement in in vitro data, using a real time 3-D radio frequency (RF) ultrasound acquisition system developed for this purpose. The results show visible differences in attenuation estimates from each technique, which are supported by the theoretical analysis. The novel attenuation estimation algorithm does show less sensitivity to scatterer variation, though it results in a more noisy estimate. Nevertheless, the novel technique for reducing statistical variations is sufficient to allow some degree of correction of shadowing and enhancement in each case.

Investigating the significance of multiple scattering in ultrasound contrast agent particle populations

The majority of the existing models describing the behavior of microbubble ultrasound contrast agents consider single, isolated microbubbles suspended in infinite media. The behavior of a microbubble population is predicted by summing the results for single microbubbles and ignoring multiple scattering effects. The aim of this investigation is to determine the significance of multiple scattering in microbubble populations and establish whether an alternative approach is required. In the first part of the work, linear models are derived to identify approximately the conditions under which multiple scattering may be expected. A nonlinear model for sound propagation in a microbubble suspension then is developed and used to examine multiple scattering at higher insonation pressures. Broadband attenuation measurements are described for two different types of microbubble suspension (albumin encapsulated octofluropropane and copolymer encapsulated isobutane) to ascertain whether or not multiple scattering may be observed experimentally. The results from the simulation work indicate that multiple scattering effects would be discernible at moderate concentrations (10(6) microbubbles/ml) such as may be present in vivo. The effect upon attenuation in the suspension would be pronounced, however, only if the population contained a sufficient proportion of relatively large (> 4 microm radius) microbubbles excited at their resonance frequency. This also is found to be the case experimentally. These findings may have important implications for the characterization of ultrasound contrast agents and their use in quantitative diagnostic techniques.

On the suitability of broadband attenuation measurement for characterizing contrast microbubbles

Broadband attenuation measurement has been widely used for characterizing ultrasound contrast agents. Chen et al. (2002) recently suggested that broadband attenuation data depend on the center frequency of the broadband excitation pulse and, therefore, that they are not a reliable measure of the bubble behavior. We investigated the suitability of measurement of broadband attenuation as a characterizing tool using the contrast agent Definity as a test case. Analyzing the attenuation data obtained with three broadband unfocused transducers with different center frequencies (2.25, 3.5 and 5 MHz), we found that attenuation is independent of the transducer used and matches in the overlap regions of any two transducers. Attenuation does not depend on excitation pressure amplitude as long as the excitation amplitude remains below a critical value ( approximately 0.26 MPa), indicating that the measurement of broadband attenuation below critical excitation can, indeed, be used for characterization. Furthermore, the linear relationship of attenuation with concentrations of Definity is also investigated.

On the relationship between encapsulated ultrasound contrast agent and pressure

Noninvasive measurement of pressure within the heart cavities and other internal organs (e.g., kidney, liver) has significant clinical value, but currently is not feasible. Noninvasive pressure estimation using encapsulated ultrasound (US) contrast agents (UCA) as sensors is a challenge because they supposedly respond to their ambient pressure, but they are more rigid and less sensitive to pressure than gas microbubbles. Here, Optison sensitivity was studied (f(resonance) = approximately 2 MHz) to varying pressures, when excited at 2 times and also at 0.5 times f(resonance). Cyclic momentary increases in ambient pressure of 0 to 5, 0 to 10, 0 to 15 or 0 to 20 kPa at 1.0 Hz, mimicking left ventricular (LV) pressure changes, caused amplitude decrease of echoes at 0.5, 1 and 2 times the transmitted frequency and decrease of attenuation. Changes at 0.5 times the transmitted frequency correlated best, but only after 70 to 150 s. The correlations (mean +/- SD) during 150 to 300 s were 0.706 +/- 0.072 for 0 to 10 kPa, 0.844 +/- 0.042 for 0 to 15 kPa and 0.859 +/- 0.031 for 0 to 20 kPa. Attenuation presented less correlation. For 1.0 Hz, 10 to 15 kPa or 15 to 20 kPa pressures, mimicking systemic pressures, the attenuation decayed fast and even faster for slow (0.05 Hz) cyclic varying pressures, or elevated steady-state pressures (of 10 kPa and 20 kPa). Thus, cyclic pressure effects on UCA are demonstrated to be reversible, but elevated static pressures cause UCA destruction. This allows cyclic pressure variations to be detected, using the subharmonics of the transmitted frequency, down to 10 kPa.

Pressure-dependent attenuation with microbubbles at low mechanical index

It has previously been shown that the attenuation of ultrasound (US) by microbubble contrast agents is dependent on acoustic pressure (Chen et al. 2002). Although previous studies have modelled the pressure-dependence of attenuation in single bubbles, this paper investigates this subject by considering a bulk volume of bubbles together with other linear attenuators. Specifically, a new pressure-dependent attenuation model for an inhomogeneous volume of attenuators is proposed. In this model, the effect of the attenuation on US propagation is considered. The model was validated using experimental measurements on the US contrast agent Sonovue. The results indicate, at low acoustic pressures, a linear relationship between the attenuation of Sonovue, measured in dB, and the insonating acoustic pressure.

Theoretical and experimental investigation of the behaviour of ultrasound contrast agent particles in whole blood

The majority of the existing models for the behaviour of ultrasound (US) contrast agents consider a single contrast agent particle (CAP) surrounded by an infinite, homogeneous and Newtonian fluid. In vivo, however, CAPs are suspended within the confines of blood vessels, in fluid containing both other CAPs and a high volume fraction of cells of comparable size. The aim of this work was to investigate the influence of blood cells upon CAP acoustic response to determine how existing models should be modified for the purposes of improving CAP design. A new model for a CAP surrounded by a cluster of cells was derived and solved numerically. Broadband US attenuation measurements were then made in suspensions of Optison (Amersham PLC, Bucks, UK) in plasma and in whole blood. Both the theoretical and experimental results indicate that the presence of blood cells has a relatively small effect upon CAP dynamics and hence acoustic response. This implies that it is justifiable to model blood as homogeneous and Newtonian for the purposes of CAP design.

Spectral normalization for ultrasonic contrast microbubble detection

Ultrasonic contrast agents consisting of microbubbles are used to assess tissue perfusion. The microbubbles are highly reflective and nonlinear and thus produce harmonics that are stronger than those from tissues. However, the magnitude of harmonic signals resulting from a region with microbubbles also depends on the acoustic pressure of incident ultrasound and the attenuation of intervening tissues in the ultrasound path. Therefore, the harmonic magnitude, as used in traditional harmonic imaging, may not be a reliable indicator of the presence or absence of microbubbles, and hence, tissue perfusion. To compensate for these effects, we present two parameters defined as the ratio of the harmonic to the fundamental component (HFR) and the ratio of the harmonic to squared fundamental (HSFR). A simplified model is used to illustrate the usefulness of these two parameters. Experiments show that both parameters improve detection of microbubbles and that HSFR performs better than HFR.

The potential for thermal damage posed by microbubble ultrasound contrast agents

The development of coated microbubble ultrasound contrast agents for use in imaging applications and as carriers in drug and gene delivery applications has intensified the need for a clear understanding of their behaviour and potential bioeffects. Previous studies have focused on the risks posed by unencapsulated bubbles as representing the "worst case scenario". They have concluded that the risk of thermal damage should be minimal provided the threshold for inertial cavitation is not exceeded. However, these treatments have ignored the heating effects due to viscous dissipation in the coatings of contrast agent particles. Simulations indicate that the temperature rise due to this process may be sufficient to generate harmful bioeffects even under conventionally "safe" insonation conditions. The implications of these findings and strategies for addressing the risks posed by contrast agents are discussed.

Nonlinear behaviors of contrast agents relevant to diagnostic and therapeutic applications

The nonlinear properties of an encapsulated microbubble of a contrast agent were studied theoretically and experimentally. A modified nonlinear differential equation (Herring equation) was used to describe the radial oscillation of the microbubble and solved numerically. It was found that the nonlinear resonance frequency, at which the peak radial oscillation amplitude occurs, was a decreasing function of the acoustic amplitude of a driving ultrasonic pulse. Optical images of the contrast agent microbubbles under various ultrasonic exposure conditions: 1. sham exposure; 2. 2-MHz spatial peak acoustic pressure = 200 kPa, I(SATA) = 260 mW/cm(2), duty cycle = 7.5%, repetition period = 0.0266 ms; 3. 0.5-MHz spatial peak acoustic pressure = 200 kPa, I(SATA) = 130 mW/cm(2), duty cycle = 7.5%, repetition period = 0.1067 ms; have also shown that the lower-frequency ultrasound (US) excitation (0.5 MHz) is more effective in disruption of the microbubbles due to acoustic inertial cavitation than the higher frequency US (2 MHz).