Evaluation of an experimental system for the in vitro assessment of ultrasonic contrast agents

Stability of echogenic liposomes as a blood pool ultrasound contrast agent in a physiologic flow phantom

Echogenic liposomes (ELIP) are multifunctional ultrasound contrast agents (UCAs) with a lipid shell encapsulating both air and an aqueous core. ELIP are being developed for molecular imaging and image-guided therapeutic delivery. Stability of the echogenicity of ELIP in physiologic conditions is crucial to their successful translation to clinical use. In this study, we determined the effects of the surrounding media's dissolved air concentration, temperature transition and hydrodynamic pressure on the echogenicity of a chemically modified formulation of ELIP to promote stability and echogenicity. ELIP samples were diluted in porcine plasma or whole blood and pumped through a pulsatile flow system with adjustable hydrodynamic pressures and temperature. B-mode images were acquired using a clinical diagnostic scanner every 5 s for a total duration of 75 s. Echogenicity in porcine plasma was assessed as a function of total dissolved gas saturation. ELIP were added to plasma at room temperature (22 °C) or body temperature (37 °C) and pumped through a system maintained at 22 °C or 37 °C to study the effect of temperature transitions on ELIP echogenicity. Echogenicity at normotensive (120/80 mmHg) and hypertensive pressures (145/90 mmHg) was measured. ELIP were echogenic in plasma and whole blood at body temperature under normotensive to hypertensive pressures. Warming of samples from room temperature to body temperature did not alter echogenicity. However, in plasma cooled rapidly from body temperature to room temperature or in degassed plasma, ELIP lost echogenicity within 20 s at 120/80 mmHg. The stability of echogenicity of a modified ELIP formulation was determined in vitro at body temperature, physiologic gas concentration and throughout the physiologic pressure range. However, proper care should be taken to ensure that ELIP are not cooled rapidly from body temperature to room temperature as they will lose their echogenic properties. Further in vivo investigations will be needed to evaluate the optimal usage of ELIP as blood pool contrast agents.

Imaging of the ovine corpus luteum microcirculation with contrast ultrasound

Ultrasound contrast agents have been the subject of microvascular imaging research. The sheep corpus luteum (CL) is a microvascular tissue that provides a natural angiogenic and antiangiogenic process, which changes during the luteal phase of the estrous cycle of the ewe. It can also be controlled and monitored endocrinologically, providing a very attractive in vivo model for the study and development of microvascular measurement. The perfusion of the fully developed CL between days 8 and 12 of the estrous cycle was studied in six ewes. A Philips iU22 ultrasound scanner (Bothell, WA, USA) with the linear array probe L9-3 was used to capture contrast-enhanced images after an intravenous bolus injection of 2.4 mL SonoVue (Bracco S.P.A., Milan, Italy). Time-intensity curves of a region of interest inside the CL were formed from linearized image data. A lagged-normal model to simulate the compartmental kinetics of the microvascular flow was used to fit the data, and the wash-in time was measured. Good contrast enhancement was observed in the CLs of all animals and the wash-in time averaged at 5.5 s with 9% uncertainty. The regression coefficient was highly significant for all fits. These data correlated with stained endothelial area in the histology performed postmortem. Two ewes were injected with prostaglandin F2alpha to induce CL regression, which resulted in an increase of wash-in time after a few hours. The CL of the ewe is thus proposed as an ideal model for the study and development of microvascular measurements using contrast ultrasound. Our initial results demonstrate a highly reproducible model for the study of the microvascular hemodynamics in a range of tissues and organs.

The assessment of microvascular flow and tissue perfusion using ultrasound imaging

Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.

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.

Absolute measurement of ultrasonic backscatter from single microbubbles

Good quality acoustical experiments are needed to measure microbubble behavior. An absolute calibration of the transmitted ultrasound field is possible using a calibrated hydrophone, but characterization of the received ultrasound beam is a more elaborate process and is not described in the literature. A new system based on a hydrodynamically focused flow has been used to measure echoes from single microbubbles at well specified positions in the ultrasonic field. An experimental set-up was built around a commercial scanner (Sonos 5500, Philips Medical Systems) to measure the scatter from solid spheres with radii between 30 to 60 microm. The behavior of these linear scatterers is accurately predicted by theory and software was produced to incorporate a simulation of the experimental conditions. The calibration of a phased array transducer was achieved by quantifying the receiver's spectral sensitivity for the range of receive frequencies (1.2 to 4.5 MHz). Examples of echoes from the microbubble agent Definity are used to illustrate the implementation of the calibration technique.

Anin vitrostudy of a microbubble contrast agent using a clinical ultrasound imaging system

Optimal insonation settings for contrast imaging are yet to be specified, mainly due to the lack of good understanding of the behaviour of the microbubbles. A satisfactory model that explains the behaviour of individual contrast agent scatterers has not yet been reported in the literature. An in vitro system based on a commercial scanner (ATL HDI3000) has been developed to investigate the backscatter of such agents. Suspensions of Definity were introduced in an anechoic tank. The frequency of transmitted ultrasound varied from 1 to 5 MHz, pulse period from 2 to 10 periods and peak negative acoustic pressure from 0.08 to 1.7 MPa. The backscatter at the fundamental and second harmonic frequency windows from the agent was normalized in terms of the corresponding components of backscatter from a blood mimicking fluid suspension. The agent provided a dominant resonance effect at 1.6 MHz transmit frequency. Second harmonic normalized backscatter averaged around 9 dB higher than the fundamental. The normalized fundamental backscatter intensity was linear with peak negative pressure. The second harmonic at resonance peaked at 0.5 MPa suggestive of bubble disruption above such pressure. The system proved capable of illustrating the ultrasonic behaviour of Definity in vitro, and the investigation suggested particular insonation conditions for optimal image enhancement using Definity.

The behaviour of individual contrast agent microbubbles

In recent years, our knowledge of the behaviour of ultrasonic microbubble contrast agents has improved substantially through in vitro experiments. However, there has been a tendency to use high concentrations of contrast agents in suspension, so that ultrasonic backscatter data are generated by a cloud of microbubbles. Such experiments involve a variety of assumptions with validity that is open to question. In addition, high concentrations of microbubbles cannot be used to understand the behaviour of individual microbubble scatterers. This paper proposes a technique that minimises the number of assumptions that need to be made to interpret in vitro experimental data. The basis of the technique is a dilute suspension of microbubbles that makes single scattering events distinguishable. A commercial scanner was used to collect radio frequency (RF) data from suspensions of two different contrast agents, Quantison and Definity. Backscatter data were collected over a range of acoustic pressures. It was found that Definity provided a constant number of scattering events per unit volume of suspension for almost all applied acoustic pressures. Quantison demonstrated an increasing number of scattering events per unit volume with increasing acoustic pressure. Below 0.6 MPa, Quantison scatterers were not individually detectable and provided levels of backscatter similar to those of a blood-mimicking fluid, which suggests that Quantison microbubbles had almost linear scattering behaviour. At acoustic pressures greater than 0.6 MPa, both agents appeared to provide echoes from free bubbles. The change in the number of scatterers per unit volume with acoustic pressure cannot be demonstrated using high concentrations of contrast agent.

Understanding the limitations of ultrasonic backscatter measurements from microbubble populations

Despite over ten years of in vitro investigations of ultrasound contrast agents, the level of understanding of their behaviour in ultrasound fields is limited. Several problems associated with these investigations, particular to the nature of contrast agents, are discussed. Using a commercial scanner the RF normalized backscatter of two different contrast agents (Definity and Quantison) was measured at different suspension concentrations and acoustic pressures. Both contrast agents scattered ultrasound nonlinearly and the backscatter showed a dependence on acoustic pressure. In order to assess the average behaviour of the agents across the range of acoustic pressures and microbubble concentrations the experimental data were fitted to a theoretically acceptable model using nonlinear regression analysis. The analysis showed that both the backscatter and the attenuation of the Quantison suspensions displayed a higher order of dependence on acoustic pressure than the Definity suspensions. It was also discovered that Quantison microbubbles did not demonstrate uniform behaviour across the acoustic pressure range. At lower acoustic pressures the behaviour could not follow a model similar to that which predicted the behaviour at higher acoustic pressures, which was mainly due to the fact that free bubbles were released in a fashion dependent on acoustic pressure. The fact that two different populations of scatterers exist in the same suspensions makes the assessment of the behaviour of the particular agent impossible with the high concentrations that are commonly used. Very low concentration suspensions whereby single scattering events can be monitored should be more useful. In conclusion, the approach of using high microbubble concentrations in order to investigate the properties of ultrasonic contrast agents is limited in that the results of such studies cannot be used to understand the behaviour of single microbubbles.

The dependence of ultrasound contrast agents backscatter on acoustic pressure: theory versus experiment

Experimental investigations have not fully explored the interaction between ultrasound beams and microbubble contrast agents. Moreover theoretical investigations have not solved the problem of the microbubble oscillation. A simple in-vitro system based on a commercial scanner (ATL UM9) was used to insonate (3 MHz transmission) diluted contrast suspensions of Definity and Quantison at different acoustic pressures (0.27-1.52 MPa). The experimental data were referred to a blood mimicking fluid in order to extract an estimate of their scattering cross-section. The results were compared with the solutions of the three main bubble oscillatidn models, Rayleigh-Plesset, Herring and Gilmore. Non-linear solutions of the above models were produced numerically using the Mathematica Package Software. The experiments showed that both agents provided a linear increase in scattering cross-section with increasing acoustic pressure. The thick shelled Quantison provided an increasing number of scatterers with increasing acoustic pressure, which proved that free bubbles leaked out of the shell. At high acoustic pressures both Quantison and Definity scattering cross-sections were almost identical, and were probably that of a free bubble. The Rayleigh-Plesset model provided a scattering cross-section almost independent of acoustic pressure. On the contrary the scattering cross-sections calculated by the Herring and Gilmore models solutions displayed a definite dependence on acoustic pressure of an order higher than one, which is slightly higher than the order of dependence exhibited by the experimental data. However, the increase of the experimentally measured scattering cross-section with acoustic pressure was sharper than the calculated one by the above two models. This is most probably due to the fact that the models simulated damped and not free bubble oscillations. In conclusion the Rayleigh-Plesset model was inadequate in describing the bubble oscillations even at small diagnostic acoustic pressures. The Herring and Gilmore models could simulate the dependence of the scattering cross-section of encapsulated microbubbles on acoustic pressure. However the contribution of free bubble oscillations has still to be modelled.

An in vitro system for the study of ultrasound contrast agents using a commercial imaging system

An in vitro system for the investigation of the behaviour of contrast microbubbles in an ultrasound field, that provides a full diagnostic range of settings, is yet to be presented in the literature. The evaluation of a good compromise of such a system is presented in this paper. It is based on (a) an HD13000 ATL scanner (Bothell, WA, USA) externally controlled by a PC and (b) on the use of well-defined reference materials. The suspensions of the reference ultrasonic scattering material are placed in an anechoic tank. The pulse length ranges from 2 to 10 cycles, the acoustic pressure from 0.08 to 1.8 MPa, the transmit frequency from 1 to 4.3 MHz, and the receive frequency from 1 to 8 MHz. The collection of 256 samples of RF data, at an offset distance from the transducer face, was performed at 20 MHz digitization rate, which corresponds to approximately 1 cm depth in water. Two particle suspensions are also presented for use as reference scatterers for contrast studies: (a) a suspension of Orgasol (ELF Atochem, Paris, France) particles (approximately 5 microm mean diameter) and (b) a suspension of Eccosphere (New Metals & Chemicals Ltd, Essex, UK) particles (approximately 50 microm mean diameter). A preliminary experiment with the contrast agent Definity (DuPont Pharmaceutical Co, Waltham, MA) showed that the above two materials are suitable for use as a reference for contrast backscatter.

Contrast agent stability: a continuous B-mode imaging approach

The stability of contrast agents in suspensions with various dissolved gas levels has not been reported in the literature. An in vitro investigation has been carried out that studied the combined effect of varying the acoustic pressure along with degassing the suspension environment. In this study, the contrast agents were introduced into suspensions with different oxygen concentration levels, and their relative performance was assessed in terms of decay rate of their backscatter echoes. The partial pressures of oxygen in those solutions ranged between 1.5 and 26 kPa. Two IV and one arterial contrast agents were used: Definity, Quantison, and Myomap. It was found that Quantison and Myomap released free bubbles at high acoustic pressure that also dissolved faster in degassed suspensions. The backscatter decay for Definity did not depend on the air content of the suspensions. The destruction of bubbles was dependent on acoustic pressure. Different backscatter performance was observed by different populations of bubbles of the last two agents. The physical quantity of "overall backscatter" (OB) was defined as the integral of the decay rate over time of the backscatter of the contrast suspensions, and improved significantly the understanding of the behaviour of the agents. A quantitative analysis of the backscatter properties of contrast agents using a continuous imaging approach was difficult to achieve. This is due to the fact that the backscatter in the field of view is representative of a bubble population affected by the ultrasound (US) field, but this bubble population is not representative of the contrast suspension in the whole tank. Single frame insonation is suggested to avoid the effects of decay due to the ultrasonic field, and to measure a tank-representative backscatter. The definition of OB was useful, however, in understanding the behaviour of the agents.

An in vitro comparison of ultrasonic contrast agents in solutions with varying air levels

The performance, in particular, the stability of ultrasound (US) contrast agents has yet to be assessed. An in vitro system has been set up to investigate the properties of ultrasonic contrast agents under different suspension conditions. This is designed to contribute to the optimal use of agents in clinical practice. In this study, the contrast agents were introduced into solutions of different oxygen concentration levels, as might be encountered in blood, and their relative performance was assessed in terms of decay in the solution environment. The partial pressures of oxygen in those solutions ranged between 1.5 and 26 kPa. Three IV and one arterial contrast agents were used: Levovist, DMP115, Quantison and Myomap. Levovist showed the highest sensitivity to oxygen concentration in the solution, and the other three proved tolerant for the above values of oxygen concentrations.