Measurements of the nonlinearity parameter B/A of contrast agents

Acoustic characterization of tissue-mimicking materials for ultrasound perfusion imaging research

Materials with well-characterized acoustic properties are of great interest for the development of tissue-mimicking phantoms with designed (micro)vasculature networks. These represent a useful means for controlled in-vitro experiments to validate perfusion imaging methods such as Doppler and contrast-enhanced ultrasound (CEUS) imaging. In this work, acoustic properties of seven tissue-mimicking phantom materials at different concentrations of their compounds and five phantom case materials are characterized and compared at room temperature. The goal of this research is to determine the most suitable phantom and case material for ultrasound perfusion imaging experiments. The measurements show a wide range in speed of sound varying from 1057 to 1616 m/s, acoustic impedance varying from 1.09 to 1.71 × 10⁶ kg/m²s, and attenuation coefficients varying from 0.1 to 22.18 dB/cm at frequencies varying from 1 MHz to 6 MHz for different phantom materials. The nonlinearity parameter B/A varies from 6.1 to 12.3 for most phantom materials. This work also reports the speed of sound, acoustic impedance and attenuation coefficient for case materials. According to our results, polyacrylamide (PAA) and polymethylpentene (TPX) are the optimal materials for phantoms and their cases, respectively. To demonstrate the performance of the optimal materials, we performed power Doppler ultrasound imaging of a perfusable phantom, and CEUS imaging of that phantom and a perfusion system. The obtained results can assist researchers in the selection of the most suited materials for in-vitro studies with ultrasound imaging.

A review on B/A measurement methods with a clinical perspective

The nonlinear parameter of ultrasound B/A has shown to be a useful diagnostic parameter, reflecting medium content, structure, and temperature. Despite its recognized values, B/A is not yet used as a diagnostic tool in the clinic due to the limitations of current measurement and imaging techniques. This review presents an extensive and comprehensive overview of the techniques developed for B/A measurement of liquid and liquid-like media (e.g., tissue), identifying the methods that are most promising from a clinical perspective. This work summarizes the progress made in the field and the typical challenges on the way to B/A estimation. Limitations and problems with the current techniques are identified, suggesting directions that may lead to further improvement. Since the basic theory of the physics behind the measurement strategies is presented, it is also suited for a reader who is new to nonlinear ultrasound.

Acoustic Distortion Ratio Enhancement Using Multiple Pulse-Echo Method (MPEM) for Evaluation of B/A Nonlinear Parameter

In this article, we report on the analysis of the extended acoustic signature obtained from the pulse-echo method to evaluate the B/A nonlinear parameter in fluids. In the known form of the method, the first acoustic tone burst from the reflector is used for the parameter measurement. The multiple pulse-echo method (MPEM) makes use of several tone bursts coming from the reflector back wall. The distortion ratio can be increased when the source frequency is tuned to a reflector resonance. The repercussion of this increase in the measurement of the nonlinear parameter B/A is investigated. As a practical result, this work suggests that the fluid volume required for the measurement can be reduced.

Nonlinear X-wave ultrasound imaging of acoustic biomolecules

The basic physics of sound waves enables ultrasound to visualize biological tissues with high spatial and temporal resolution. Recently, this capability was enhanced with the development of acoustic biomolecules - proteins with physical properties enabling them to scatter sound. The expression of these unique air-filled proteins, known as gas vesicles (GVs), in cells allows ultrasound to image cellular functions such as gene expression in vivo, providing ultrasound with its analog of optical fluorescent proteins. Acoustical methods for the in vivo detection of GVs are now required to maximize the impact of this technology in biology and medicine. We previously engineered GVs exhibiting a nonlinear scattering behavior in response to acoustic pressures above 300 kPa, and showed that amplitude-modulated (AM) ultrasound pulse sequences that both excite the linear and nonlinear GV scattering regimes were highly effective at distinguishing GVs from linear scatterers like soft biological tissues. Unfortunately, the in vivo specificity of AM ultrasound imaging is systematically compromised by the nonlinearity added by the GVs to propagating waves, resulting in strong image artifacts from linear scatterers downstream of GV inclusions. To address this issue, we present an imaging paradigm, cross-amplitude modulation (xAM), which relies on cross-propagating plane-wave transmissions of finite aperture X-waves to achieve quasi artifact-free in vivo imaging of GVs. The xAM method derives from counter-propagating wave interaction theory which predicts that, in media exhibiting quadratic elastic nonlinearity like biological tissue, the nonlinear interaction of counter-propagating acoustic waves is inefficient. By transmitting cross-propagating plane-waves, we minimize cumulative nonlinear interaction effects due to collinear wave propagation, while generating a transient wave-amplitude modulation at the two plane-waves' intersection. We show in both simulations and experiments that residual xAM nonlinearity due to wave propagation decreases as the plane-wave cross-propagation angle increases. We demonstrate in tissue-mimicking phantoms that imaging artifacts distal to GV inclusions decrease as the plane-wave cross-propagation angle opens, nearing complete extinction at angles above 16.5 degrees. Finally, we demonstrate that xAM enables highly specific in vivo imaging of GVs located in the gastrointestinal tract, a target of prime interest for future cellular imaging. These results advance the physical facet of the emerging field of biomolecular ultrasound, and are also relevant to synthetic ultrasound contrast agents.

Multiple scattering of an ultrasonic shock wave in bubbly media

This experimental study deals with the propagation of an ultrasonic shock wave in a random heterogeneous medium, constituted of identical 75μm radius bubbles, trapped in a yield-stress fluid. The fundamental frequency of the incident wave (in the MHz range) was much larger than the resonance frequency of bubbles (38kHz). A well-expanded coda, resulting from the multiple scattering of the incident shock wave through the heterogeneous medium, was experimentally measured in transmission. Despite the significant amplitude of the shock wave (90kPa), no sign of nonlinear response of the bubbles was detected. Both the coherent and incoherent fields were successfully described by linear theories. Using a shock wave presents the advantage of characterizing the medium over a large frequency range (1.5-15MHz).

Fast Nonlinear Ultrasound Propagation Simulation Using a Slowly Varying Envelope Approximation

Medical systems usually consider linear propagation of ultrasound, an approximation of reality. However, numerous studies have attempted to accurately simulate the nonlinear pressure wave distortion and to evaluate the contribution of harmonic frequencies. In such simulations, the computation time is very large, except for the method based on the angular spectrum scheme where the derivative order is reduced using the Fourier transform. However, the harmonic computation is usually limited to the second harmonic because of quasi-linear approximation. In this paper, a slowly varying envelope approximation (SVEA) is used in the Fourier domain to compute the entire nonlinear distortion induced, including high harmonics and nonlinear mixing frequencies. The simulation by SVEA is evaluated by comparison with other simulation tools. The obtained deviation and difference remain low enough to fully validate such an approximation. Moreover, the simulator is implemented on a GPU to obtain a very fast tool, where the full nonlinear distorted [Formula: see text] field is computed in less than 10 s.

Increasing specificity of contrast-enhanced ultrasound imaging using the interaction of quasi counter-propagating wavefronts: a proof of concept

Detection methods implemented in present clinical ultrasound scanners for contrast-enhanced ultrasound imaging show high sensitivity but a rather poor specificity due to pseudo-enhancement (false detection of contrast agent) produced by nonlinear wave propagation. They all require linear ultrasound propagation to detect nonlinear scattering of contrast agent microbubbles. Even at low transmit pressure, nonlinear wave propagation occurs in regions perfused with contrast agent because contrast agent microbubbles can dramatically enhance the nonlinear elastic behavior of the medium. This image artifact hinders further development of contrast-enhanced ultrasound imaging toward reliable quantitative measurement of local concentration of contrast agent and blood perfusion kinetics. We propose in this manuscript a new detection method, with specific beamforming and pulsing scheme, that produces contrast images with highly reduced pseudo-enhancement. It is based on the interaction of two diverging wavefronts broadcasted by two single elements of a conventional probe array. The contrast image is formed line by line; one single image line is the line segment bisector defined by the centers of the two transmitting elements. Each image line is formed by a three-step pulse sequence: (1) transmission with one element, (2) transmission with the other element, and (3) transmission with both elements. The proof of principle is shown with numerical simulations and in vitro experiments. The method is implemented in a programmable ultrasound system and tested in a tissue-mimicking phantom containing a vessel filled with diluted contrast agent. At a given depth, increasing the distance between the two transmitting elements increases the angle describing the propagation directions of the two wavefronts. As a result, the nonlinear interaction between the two broadcasted waves is reduced. We show experimentally that increasing the distance between the transmitting elements from 0.6 to 24 mm reduces the amplitude of the pseudoenhancement at the far wall of the vessel relative to true contrast signal amplitude in the vessel by 12 dB, therefore improving specificity in the contrast-enhanced image.

Generalization of Multipulse Transmission Techniques for Ultrasound Imaging

To increase the contrast-to-tissue ratio (CTR) in contrast imaging or the signal-to-noise ratio (SNR) in tissue harmonic imaging, many multipulse transmission techniques have been suggested. This article first recalls the various imaging techniques proposed in the literature and then presents a mathematical background to synthesize and generalize most of the multipulse ultrasound imaging techniques. The formulation presented can be used to predict the relative amplitude of the nonlinear components in each frequency band and to design new transmission sequences to either increase or decrease specified nonlinear components in each harmonic band. Simulation results on several multipulse techniques agree with the results from previous studies.

Circulatory bubble dynamics: from physical to biological aspects

Bubbles can form in the body during or after decompression from pressure exposures such as those undergone by scuba divers, astronauts, caisson and tunnel workers. Bubble growth and detachment physics then becomes significant in predicting and controlling the probability of these bubbles causing mechanical problems by blocking vessels, displacing tissues, or inducing an inflammatory cascade if they persist for too long in the body before being dissolved. By contrast to decompression induced bubbles whose site of initial formation and exact composition are debated, there are other instances of bubbles in the bloodstream which are well-defined. Gas emboli unwillingly introduced during surgical procedures and ultrasound microbubbles injected for use as contrast or drug delivery agents are therefore also discussed. After presenting the different ways that bubbles can end up in the human bloodstream, the general mathematical formalism related to the physics of bubble growth and detachment from decompression is reviewed. Bubble behavior in the bloodstream is then discussed, including bubble dissolution in blood, bubble rheology and biological interactions for the different cases of bubble and blood composition considered.

CREANUIS: a non-linear radiofrequency ultrasound image simulator

Nonlinear ultrasound methods are widely used in clinical applications for tissue or contrast harmonic imaging. Accurate non-linear imaging simulation tools are required in research studies for the development of new methods. However, in existing simulators, the possible inhomogeneity of the coefficient of non-linearity, which is generally observed in tissue and in particular when contrast agents are involved, has not yet been implemented. This article describes a new ultrasound simulator, called CREANUIS, devoted to the computation of B-mode images where both linear and non-linear propagation in media is considered, with a possible inhomogeneous coefficient of non-linearity. The resulting fundamental images, based on a spatially variant and non-linear point spread function, are in accordance with those obtained through the reference linear FieldII simulator, with computation time reduced by a factor of at least 1.8. Non-linear images of media exhibiting inhomogeneous coefficients of non-linearity are also provided. The simulation software can be freely downloaded from our website.

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.

Fundamental and second-harmonic ultrasound field computation of inhomogeneous nonlinear medium with a generalized angular spectrum method

The simulation of nonlinear propagation of ultrasound waves is typically based on the Kuznetsov-Zabolotskaya- Khokhlov equation. A set of simulators has been proposed in the literature but none of them takes into account a possible spatial 3-D variation of the nonlinear parameter in the investigated medium. This paper proposes a generalization of the angular spectrum method (GASM) including the spatial variation of the nonlinear parameter. The proposed method computes the evolution of the fundamental and second-harmonic waves in four dimensions (spatial 3-D and time). For validation purposes, the one-way fields produced by the GASM are first compared with those produced by established reference simulators and with experimental one-way fields in media with a homogeneous nonlinear parameter. The same simulations are repeated for media having an axial variation of the nonlinear parameter. The mean errors estimated in the focal region are less than 4.0% for the fundamental and 5.4% for the second harmonic in all cases. Finally, the fundamental and second-harmonic fields simulated for media having nonlinear parameter variations in the axial, lateral, and elevation directions, which cannot be simulated with other currently available methods, are presented. The new approach is also shown to yield a reduction in computation time by a factor of 13 with respect to the standard nonlinear simulator.

Extensions of nonlinear B/A parameter imaging methods for echo mode

This paper investigates nonlinear B/A parameter imaging in ultrasound echo mode. First, the B/A estimation methods which can be extended to echo mode are identified. The finite amplitude approaches are found to be excellent candidates to assess the nonlinear parameter because of their experimental simplicity, supported by a strong mathematical background. Second, three nonlinear coefficient measurement methods, thus far proposed for applications in homogeneous media, are extended to heterogeneous media. In particular, the simulations show that the extended comparative method (ECM) offers the best results when the probe diffraction effects are taken into consideration. The first experimental images obtained by applying the ECM for two different phantoms are also presented, showing the feasibility of B/A imaging.

The acoustic properties, centered on 20 MHZ, of an IEC agar-based tissue-mimicking material and its temperature, frequency and age dependence

The purpose of this study was to characterize the ultrasonic properties of agar-based tissue-mimicking materials (TMMs) at ultrasound frequencies centered around 20 MHz. The TMM acoustic properties measured are the amplitude attenuation coefficient alpha (dB cm(-1)MHz(-1)), the speed of sound (ms(-1)) and the backscattered power spectral density (distribution of power per unit frequency normalized to the total received power) characteristics of spectral slope (dB MHz(-1)), y-axis intercept (dB) and reflected power (dB). The acoustic properties are measured over a temperature range of 22 to 37 degrees C. An intercomparison of results between two independent ultrasound measurement laboratories is also presented. A longitudinal study of the acoustic properties over a period of two years is also detailed, and the effect of water immersion on the acoustic properties of TMM is measured. In addition, the physical parameters of mass density rho (kg m(-3)) and specific heat capacity C (J kg(-1) K(-1)) are included. The measurement techniques used were based on the substitution technique using both broadband and narrowband pulses centered on 20 MHz. Both the attenuation coefficient and speed of sound (both group and phase) showed good agreement with the expected values of 0.5 dB cm(-1) MHz(-1) and 1540 ms(-1), respectively, with average values over the three-year period of 0.49 dBcm(-1)MHz1 (SD +/- 0.05) and 1540.9 ms(-1) (SD +/- 8.7). These results also showed agreement between the two independent measurement laboratories. Speed of sound and attenuation coefficient were shown to change with temperature with rates of + 2.1 m s(-1) degrees C(-1) and -0.005 dB cm(-1) MHz(-1) degrees C(-1), respectively. Attenuation changed linearly with frequency at the high frequency range of 17 to 23 MHz, and speed of sound was found to be independent of frequency in this range. The spectral slope of relative backscattered power for the material increased with frequency at typically 1.5 dB MHz(-1). This compared favorably with theoretical spectral slope values, calculated for a variety of scatterer sizes, albeit at a lower frequency range. It is also noticed that, on extrapolation back to lower frequencies, the backscatter is comparable with that measured at 7 MHz. Overall, this non-commercial agar-based TMM is shown to perform as expected at the higher frequency range of 17 to 23 MHz and is seen to retain its acoustic properties of attenuation and speed of sound over a three-year period.

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.

Experimental investigation and theoretical modelling of the nonlinear acoustical behaviour of a liver tissue and comparison with a tissue mimicking hydrogel

Native harmonics generated by nonlinear distortion of ultrasound during propagation in a medium may cause misinterpretations in spectral analysis when studying contrast agents. The aim of this paper is to quantitatively evaluate nonlinear propagation effects of diagnostic ultrasound pulses in biological tissues and to assess whether a cellulose-based hydrogel can be a suitable material for tissue mimicking purposes. Hydrogel and pig liver tissue samples of various thicknesses were insonified in a through-transmission set-up, employing 2.25-MHz pulses with different mechanical index (MI) values (range 0.06-0.60). Second harmonic and first harmonic amplitudes were extracted from spectra of received signals and their ratio was then used to compare hydrogel and liver behaviours. Resulting trends are very similar for sample thicknesses up to 8 cm and highlight a significant increase in nonlinearity for MI > 0.3, for both liver and hydrogel. A numerical procedure was also employed to calculate pressure distribution along the beam axis: these theoretical results showed a very good agreement with experimental data in the low pressure range, though failed in predicting the MI threshold. In conclusion, the hydrogel resulted to be a suitable material for manufacturing tissue mimicking phantoms, in particular to study contrast agent behaviour with a "low power approach".

Harmonic imaging using multitone nonlinear coding

We present a new method that uses nonlinear properties of tissue to improve contrast-to-noise ratio. In our novel method, the acoustic source is activated with two tone-bursts (2.2 and 4.4 MHz), with specially designed polarization of the individual tone-burst. This new approach is called multitone nonlinear coding (MNC) because the choice of polarization of both tones (and their amplitudes), allowing optimization of the receiving properties, depends on the nonlinear properties of tissue. The calculations were done for two tone-bursts propagating in the tissue-like glossy medium with absorption of 7 Np/m x MHz. The method was experimentally verified by scanning the incident pulses propagating in soft tissue and by scanning the thread phantom immersed in water. The concept of the virtual fields was introduced to explain abilities and properties of pulse inversion and MNC and to compare the two methods. Comparison of the spatial field distribution obtained using MNC with the conventional harmonic imaging approach, in which the second harmonic is used to reconstruct the image, is presented. It was shown that, for the same peak pressure amplitude, the resulting mechanical index was about 40% lower for MNC, lateral resolution was 10% to 30% better and, what seems to be the most encouraging, the signal gain was up to eight times higher than pulse inversion.

Plane wave source with minimal harmonic distortion for investigating nonlinear acoustic properties

The objective of this investigation is to introduce and validate a practical ultrasound source to be used in the investigation of the nonlinear material properties of liquids and soft tissues studied in vitro. Methods based on the progressive distortion of finite amplitude ultrasonic waves in the low megahertz frequency range are most easily implemented under the assumption of plane wave propagation. However, achieving an approximately planar ultrasonic field over substantial propagation distances can be challenging. Furthermore, undesired harmonic distortion of the ultrasonic field prior to insonification of the specified region of interest represents another serious limitation. This paper introduces an approach based on the use of the ultrasonic field emanating from a stainless-steel delay line. Both simulation and direct experimental measurement demonstrate that such a field exhibits relatively planar wave fronts to a good approximation (such that a 3-mm-diam receiver would be exposed to no more than 3 dB of loss across its face) and is free from the significant harmonic distortion that would occur in a conventional water path.

Finite amplitude measurements of the nonlinear parameter B/A for liquid mixtures spanning a range relevant to tissue harmonic mode

The objective of this investigation was to measure the nonlinear parameter B/A using an enhanced finite amplitude distortion technique, based on nonlinear propagation effects analogous to those associated with tissue harmonic imaging. These measurements validate an improved method for measuring the nonlinear parameter B/A, the small-signal speed of sound, and the attenuation coefficient from a single set of ultrasonic measurements. To test the method, measurements were performed on 11 different mixtures of isopropyl alcohol (isopropanol) and water that span the range of concentrations from 0% to 100% isopropanol. Results for B/A ranging from approximately five to 11 were found to be reproducible and in good agreement with previously published values obtained using a thermodynamic method.

Study on the multiple scattering effects of ultrasound contrast agents

In this paper, the multiple scattering of interacting encapsulated microbubbles in suspensions is calculated using two novel approaches--Kargl's effective medium approach and Ye and Ding's approach of 2nd-order correction. Two types of contrast agents with bubbles of different sizes and concentrations are chosen for our numerical simulations. One is Albunex, which is depicted by Church and has a size range of several microns, and the other is sodium laureate solution (before fractionation) described by Soetanto and Chan and has an average size of 35.5 microm. The numerical results from these two approaches are compared with that from the linear approximation. It is found that the multiple scattering effects on attenuation and dispersion of sound in suspensions are evident in the cases of high bubble volume fractions, basically greater than the order of 1 x 10(-4), and much more obvious for larger bubbles with average size of tens of microns.

Ultrasound attenuation in encapsulated microbubble suspensions: The multiple scattering effects

The phenomenon of image distortions caused by the multiple scattering (MS) effects of encapsulated microbubbles in ultrasoniqc imaging was experimentally found in previous studies (Soetanto and Chan 2000a), but its mechanism has not been fully understood. To study the MS effects of microbubbles in contrast imaging, two approaches are employed in this article--the effective medium approach initialized by Kargl (2002), which includes all the high-order rescattering of free bubbles, and the classic lowest-order approximation approach of Commander and Prosperetti (1989), which ignores the higher-order rescattering between bubbles. In this work, they are modified to model encapsulated microbubble suspensions, and the discrepancies in attenuation coefficients calculated by these two approaches, i.e., the higher-order rescattering of bubbles are defined as the measure of the MS effects of microbubbles. The intrinsic relations between the MS effects of microbubbles in suspensions and physical properties of the microbubbles, such as the bubble concentrations, sizes, and the shell thicknesses etc., are simulated and discussed. It is found that in suspensions for identical microbubbles >12 microm in size, the MS effects come to be significant when the bubble concentrations exceed 1 x 10(5) bubbles/mL. The MS effects of microbubbles with broad size spectrums are examined by simulating Soetanto and Chan's experiments. Also, the MS effects of UCAs in current ultrasonic imaging practice are discussed. The STARs and extinction cross-sections of different-sized individual encapsulated microbubbles are calculated for further investigations on the mechanism of the MS effects of UCAs.

Predicting backscatter characteristics from micron- and submicron-scale ultrasound contrast agents using a size-integration technique

In this paper, a computationally, inexpensive, size-integration method based on a modified Rayleigh-Plesset model is developed to predict backscatter spectra from groups of bubbles with various size distributions, incident acoustic amplitudes, and driving frequencies. The method was validated using experimentally measured spectra from contrast bubbles of various sizes: Optison, Levovist, ST68 microbubbles, and submicron bubbles. This method provides a computationally inexpensive means of examining backscatter spectrum from multiple bubbles, especially in predicting occurrence and relative amplitude of subharmonics and second harmonics.

Theoretical predictions of harmonic generation from submicron ultrasound contrast agents for nonlinear biomedical ultrasound imaging

Submicron ultrasound contrast agents have aroused attention for their significant promise in ultrasonic contrast/molecular imaging, targeted therapy and echo particle imaging velocimetry. However, nonlinear acoustic properties of submicron encapsulated gas bubbles for ultrasonic applications are still not clearly understood. In this paper, nonlinear acoustic emission characteristics from submicron bubbles were examined using a numerical study. The modified RP equation incorporating viscosity, acoustic radiation, thermal effects and encapsulated shell was used to study single bubble dynamics. Further, a size integration method, shown previously to be useful in prediction of backscatter spectra from groups of bubbles, was applied to analyse response from a bubble population. We show that bubbles with radii (200-500 nm) produce significant subharmonic and ultraharmonic components of the backscatter spectrum, while smaller bubbles (<200 nm) provide substantial second harmonic components. Additionally, nanoscale bubbles (<100 nm) produce very low backscatter amplitudes and thus may not be useful with the use of current ultrasound technology. Analysing optimal ultrasound driving pressures and bubbles size ranges for maximal subharmonic and ultraharmonic signals showed that sub and ultraharmonic mode nonlinear imaging methods may be potentially competitive for larger size bubbles (>200 nm) in providing proper contrast-to-tissue signal ratios.

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.

Pressure-dependent attenuation in ultrasound contrast agents

Although microbubble contrast agents are believed to respond differently under different driving-pressure amplitudes, few studies have been performed to extensively study the pressure-dependence of their properties. In this paper, attenuation coefficients of two contrast agents (Optison and Definity) were measured under different driving-pressure amplitudes using a narrowband incident pulse. The attenuation of both contrast agents was found to increase with increasing driving pressure. Simulations using the Rayleigh, Plesset, Noltingk, Neppiras, Poritsky (RPNNP) equation were performed to study this behavior. Simulation results show that significant harmonic generation at high driving-pressure amplitudes contributes to the higher attenuation. Other possible explanations for this behavior were also examined. Attenuation coefficients of two contrast agents were also measured using a broadband method. The results showed great inconsistency when the center frequency of the incident broadband pulse was changed, indicating that broadband techniques may not be suitable for contrast-agent attenuation measurements.

Effective medium approach to linear acoustics in bubbly liquids

Linear wave propagation through a bubbly liquid has seen a resurgence of interest because of proposed "corrections" to the lowest-order approximation of an effective wave number obtained from Foldy's exact multiple scattering theory [Foldy, Phys. Rev. 67, 107 (1945)]. An alternative approach to wave propagation through a bubbly liquid reduces the governing equations for a two-phase medium to an effective medium. Based on this approach, Commander and Prosperetti [J. Acoust. Soc. Am. 85, 732 (1989)] derive an expression for the lowest-order approximation to an effective wave number. At this level of approximation the bubbles interact with only the mean acoustic field without higher-order rescattering. That is, the field scattered from a bubble may interact with one or more new bubbles in the distribution, but a portion of that scattered field may not be scattered back to any previous bubble. The current article shows that modifications to the results of Commander and Prosperetti lead to a new expression for the effective wave number, which properly accounts for all higher orders of multiple scattering.

High-resolution functional imaging with ultrasound contrast agents based on RF processing in an in vivo kidney experiment

Knowledge of the relative tissue perfusion distribution is valuable in the diagnosis of numerous diseases. Techniques for the assessment of the relative perfusion distribution, based on ultrasound (US) contrast agents, have several advantages compared to established nuclear techniques. These are, among others, a better spatial and temporal resolution, the lack of exposure of the patient to ionizing radiation and the relatively low cost. In the present study, US radiofrequency (RF) image sequences are acquired, containing the signal intensity changes associated with the transit of a bolus contrast agent through the microvasculature of a dog kidney. The primary objective is to explore the feasibility of calculating functional images with high spatial resolution. The functional images characterize the transit of the contrast agent bolus and represent distributions of peak time, peak value, transit time, peak area, wash-in rate and wash-out decay constant. For the evaluation of the method, dog experiments were performed under optimized conditions where motion artefacts were minimized and an IA injection of the contrast agent Levovist was employed. It was demonstrated that processing of RF signals obtained with a 3.5-MHz echo system can provide functional images with a high spatial resolution of 2 mm in axial resolution, 2 to 5 mm in lateral resolution and a slice thickness of 2 mm. The functional images expose several known aspects of kidney perfusion, like perfusion heterogeneity of the kidney cortex and a different peripheral cortical perfusion compared to the inner cortex. Based on the findings of the present study, and given the results of complimentary studies, it is likely that the functional images reflect the relative perfusion distribution of the kidney.

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.

The experimental investigation of ultrasonic properties for a sonicated contrast agent and its application in biomedicine

The ultrasonic properties of a promising ultrasound (US) contrast agent, named SDA (sonicated dextrose albumin) are reported in this paper. SDA is a suspension of stable microencapsulated gas bubbles with average diameter 2.0 microm prepared from sonicated dextrose albumin. The ultrasonic linear and nonlinear parameters, such as acoustic velocity, sound attenuation and acoustic nonlinearity parameter B/A of SDA, as a function of its bubble concentration from 1.0 x 10(7) to 2.05 x 10(8) microbubbles/mL in the frequency range of 2-6 MHz are measured in vitro. The sound attenuation coefficients over 2-6 MHz are linearly proportional to the bubble concentration and frequency. It is important to point out that the acoustic nonlinearity parameter B/A for SDA has a very large value that nonlinearly increases with the increase of bubble concentration.

Source prebiasing for improved second harmonic bubble-response imaging

The use of the second harmonic bandwidth in order to improve the contrast enhancement of vascular space provided by microbubble echo contrast is well established. A significant obstacle to improving on the contrast advantage of the second harmonic bandwidth arises from the linear response of tissue to the finite amplitude distortion produced second harmonic in the beam. A scheme in which the source wave contains a second harmonic component designed to cancel out the second harmonic produced by finite amplitude distortion in the focal region was computationally investigated. This prebiasing scheme was found to offer significant reductions in the amplitude of the second harmonic in the focal region. These reductions were found in both the homogeneous tissue path case and in the inhomogeneous tissue path case. The resulting clinical potential of source prebiasing is discussed. Also, it was observed that the inhomogeneous focusing of the finite amplitude distortion-produced second harmonic was significantly better than that of a same frequency fundamental with an identical homogeneous path focal profile.