Karen E. Morgan

Experimental and theoretical evaluation of microbubble behavior: effect of transmitted phase and bubble size

Ultrasound contrast agents provide new opportunities to image vascular volume and flow rate directly. To accomplish this goal, new pulse sequences can be developed to detect specifically the presence of a microbubble or group of microbubbles. We consider a new scheme to detect the presence of contrast agents in the body by examining the effect of transmitted phase on the received echoes from single bubbles. In this study, three tools are uniquely combined to aid in the understanding of the effects of transmission parameters and bubble radius on the received echo. These tools allow for optical measurement of radial oscillations of single bubbles during insonation, acoustical study of echoes from single contrast agent bubbles, and the comparison of these experimental observations with theoretical predictions. A modified Herring equation with shell terms is solved for the time-dependent bubble radius and wall velocity, and these outputs are used to formulate the predicted echo from a single encapsulated bubble. The model is validated by direct comparison of the predicted radial oscillations with those measured optically. The transient bubble response is evaluated with a transducer excitation consisting of one-cycle pulses with a center frequency of 2.4-MHz. The experimental and theoretical results are in good agreement and predict that the transmission of two pulses with opposite polarity will yield similar time domain echoes with the first significant portion of the echo generated when the rarefactional half-cycle reaches the bubble.

Optical and acoustical observations of the effects of ultrasound on contrast agents

Optimal use of encapsulated microbubbles for ultrasound contrast agents and drug delivery requires an understanding of the complex set of phenomena that affect the contrast agent echo and persistence. With the use of a video microscopy system coupled to either an ultrasound flow phantom or a chamber for insonifying stationary bubbles, we show that ultrasound has significant effects on encapsulated microbubbles. In vitro studies show that a train of ultrasound pulses can alter the structure of an albumin-shelled bubble, initiate various mechanisms of bubble destruction or produce aggregation that changes the echo spectrum. In this analysis, changes observed optically are compared with those observed acoustically for both albumin and lipid-shelled agents. We show that, when insonified with a narrowband pulse at an acoustic pressure of several hundred kPa, a phospholipid-shelled bubble can undergo net radius fluctuations of at least 15%; and an albumin-shelled bubble initially demonstrates constrained expansion and contraction. If the albumin shell contains air, the shell may not initially experience surface tension; therefore, the echo changes more significantly with repeated pulsing. A set of observations of contrast agent destruction is presented, which includes the slow diffusion of gas through the shell and formation of a shell defect followed by rapid diffusion of gas into the surrounding liquid. These observations demonstrate that the low-solubility gas used in these agents can persist for several hundred milliseconds in solution. With the transmission of a high-pulse repetition rate and a low pressure, the echoes from, contrast agents can be affected by secondary radiation force. Secondary radiation force is an attractive force for these experimental conditions, creating aggregates with distinct echo characteristics and extended persistence. The scattered echo from an aggregate is several times stronger and more narrowband than echoes from individual bubbles.

A preliminary evaluation of the effects of primary and secondary radiation forces on acoustic contrast agents

Primary and secondary radiation forces result from pressure gradients in the incident and scattered ultrasonic fields. These forces and their dependence on experimental parameters are described, and the theory for primary radiation force is extended to consider a pulsed traveling wave. Both primary and secondary radiation forces are shown to have a significant effect on the flow of microbubbles through a small vessel during insonation. The primary radiation force produces displacement of microspheres across a 100 micron vessel radius for a small transmitted acoustic pressure. The displacement produced by primary radiation force is shown to display the expected linear dependence on the pulse repetition frequency and a nonlinear dependence on transmitted pressure. The secondary radiation force produces a reversible attraction and aggregation of microspheres with a significant attraction over a distance of approximately 100 microns. The magnitude of the secondary radiation force is proportional to the inverse of the squared separation distance, and thus two aggregates accelerate as they approach one another. We show that this force is sufficient to produce aggregates that remain intact for a physiologically appropriate shear rate. Brief interruption of acoustic transmission allows an immediate disruption of the aggregate.

The effect of the phase of transmission on contrast agent echoes

Ultrasound contrast agents consist of a gas bubble, encapsulated by a shell for stabilization. The shell dampens the fluctuations in the bubble radius when insonified. The detection of contrast microbubbles during a medical examination can indicate whether a region is perfused with blood. Here, the authors consider the effect of the phase of sonification signal on the backscatter by the bubble echo. By transmitting two short pulses of ultrasound with opposite phases, the authors demonstrate that a unique pair of echoes can be generated by a single microbubble, and that the properties of these echoes may be useful in the discrimination of bubble and tissue echoes. Specifically, the significant echo amplitude begins coincident with each transmitted rarefactional half-cycle, and the mean frequency of this echo depends on the transmitted phase. When rarefaction is transmitted first for a 2.25 MHz signal, the mean frequency is 0.8 MHz higher for an albumin-shelled bubble and 0.9 MHz higher for a lipid-shelled bubble. The experimental results agree with the predictions of the Gilmore-Akulichev equation.

Direct video-microscopic observation of the dynamic effects of medical ultrasound on ultrasound contrast microspheres.

RATIONALE AND OBJECTIVES Ultrasound can cause destruction of microbubble contrast agents used to enhance medical ultrasound imaging. This study sought to characterize the dynamics of this interaction by direct visual observation of microbubbles during insonification in vitro by a medical ultrasound imaging system. METHODS Video microscopy was used to observe air-filled sonicated albumin microspheres adsorbed to a solid support during insonation. RESULTS Deflation was not observed at lowest transmit power settings. At higher intensities, gas left the microparticle gradually, apparently dissolving into the surrounding medium. Deflation was slower for higher microsphere surface densities. Intermittent ultrasound imaging (0.5 Hz refresh rate) caused slower deflation than continuous imaging (33 Hz). CONCLUSIONS Higher concentrations of microbubbles, lower ultrasound transmit power settings, and intermittent imaging each can reduce the rate of destruction of microspheres resulting from medical ultrasound insonation.

Changes in the echoes from ultrasonic contrast agents with imaging parameters

Current harmonic imaging scanners transmit a narrowband signal that limits spatial resolution in order to differentiate the echoes from tissue from the echoes from microbubbles. Because spatial resolution is particularly important in applications, including mapping vessel density in tumors, we explore the use of wideband signals in contrast imaging. It is first demonstrated that microspheres can be destroyed using one or two pulses of ultrasound. Thus, temporal signal processing strategies that use the change in the echo over time can be used to differentiate echoes from bubbles and echoes from tissue. Echo parameters, including intensity and spectral shape for narrowband and wideband transmission, are then evaluated. Through these experiments, the echo intensity received from bubbles after wideband transmission is shown to be at least as large as that for narrowband transmission, and can be larger. In each case, the echo intensity increases in a nonlinear fashion in comparison with the transmitted signal intensity. Although the echo intensity at harmonic multiples of the transmitted wave center frequency can be larger for narrowband insonation, echoes received after wideband insonation demonstrate a broadband spectrum with significant amplitude over a very wide range of frequencies.

Experimental and theoretical analysis of individual contrast agent behavior

An improved understanding of contrast agent behavior may yield more sophisticated bubble detection techniques. In this study, the optical measurements of single bubble oscillations during insonation are compared directly to theoretical predictions. These results are then used to aid in the understanding of the effects of transmission and bubble parameters on the bubble oscillations and resulting received echoes. A Rayleigh-Plesset-like bubble equation with additional shell terms is solved for the time dependent bubble radius and wall velocity, and these outputs are also used to formulate the predicted echo from a single encapsulated bubble. The experimental and theoretical radius-time curves are in good agreement; with a consistent, predictable response from the lipid-shelled agent with varying amplitude, phase and length of the transmission pulses. The radius-time curves of the albumin-shelled agent Optison/sup TM/ are less predictable due to its asymmetric oscillations. Observations of the effects of transmitted phase and the corresponding predicted echoes are consistent with previous experimental results. These results demonstrate that the transmission of two pulses with opposite phases will yield similar time domain echoes with the echo from the pulse with rarefaction first (180/spl deg/) having a mean frequency that is higher than the compression first response (0/spl deg/).

Properties of contrast agents insonified at frequencies above 10 MHz

The authors compare the properties of contrast agents following insonation in the 3-7 MHz range with the properties observed at higher frequencies, in order to differentiate the properties associated with insonation near resonance. In addition, to map small blood vessels located in deeper tissues, such as those within the retina and within lymph nodes, contrast-enhanced imaging at high frequencies may be desirable. Increasing the echo intensity with a contrast agent may allow the use of a higher frequency transducer, and thus improve the spatial resolution of the vascular map. With these goals in mind, the authors explore the properties of ultrasonic echoes from contrast agents at 38 MHz and compare these properties to those at lower frequencies.

Action of microbubbles when insonified: experimental evidence

Primary and secondary Bjerknes forces result from pressure gradients in the incident and scattered ultrasonic fields. These forces and their dependence on experimental parameters are described. Both primary and secondary Bjerknes forces are shown to have a significant effect on the flow of microbubbles through a small vessel during insonation. The primary Bjerknes force produces displacement of microspheres across a 100 micron vessel radius for a small transmitted acoustic power. The secondary Bjerknes force produces a reversible attraction and aggregation of microspheres with a significant attraction over a distance of approximately 100 microns. The magnitude of the secondary Bjerknes force is proportional to the inverse of the squared separation distance, and thus two aggregates accelerate as they approach one another.

Ultrasound contrast agents phagocytosed by neutrophils demonstrate acoustic activity

Ultrasound contrast agents are microbubbles composed of a thin lipid or albumin shell filled with air or a high molecular weight gas. These microbubbles are used for contrast-enhanced ultrasound (CEU) assessment of organ perfusion. In regions of inflammation, microbubbles are phagocytosed intact by activated neutrophils adherent to the venular wall. The authors hypothesized that microbubbles remain acoustically active following phagocytosis. Accordingly, they assessed the physical responses of both phagocytosed and free microbubbles by direct microscopic observation during delivery of repetitive single pulses of ultrasound at various acoustic pressures. Insonation results in oscillation in the bubbles volume. Microbubbles were optically recorded during insonation with a high-speed imaging system and diameter-time curves were analyzed to determine the effect of phagocytosis. Phagocytosed microbubbles retained their acoustic activity, although the intracellular environment increased viscoelastic damping experienced by microbubbles. With a pulse of high acoustic intensity (>1 MPa), phagocytosed microbubbles expanded up to 500% of their initial radii, which occasionally resulted in neutrophil rupture. Primary radiation force displaced phagocytosed microbubbles a distance of 100 microns with an acoustic pressure of -240 kPa and a pulse repetition frequency of 10 kHz, thus providing further evidence of acoustic activity. The authors conclude that phagocytosed microbubbles exhibit viscoelastic damping and yet are susceptible to acoustic destruction. They can generate non-linear echoes on the same order of magnitude as free microbubbles. These results indicate that CEU may be used to identify and assess regions of inflammation by detecting acoustic signals from microbubbles that are phagocytosed by activated neutrophils. In addition, the rapid expansion of a microbubble at high acoustic pressure may present a means to rupture a neutrophil or drug capsule at a specific site, resulting in delivery of a drug.