On the interaction between ultrasound and contrast agents during Doppler investigations

Doppler Passive Acoustic Mapping

In therapeutic ultrasound using microbubbles, it is essential to drive the microbubbles into the correct type of activity and the correct location to produce the desired biological response. Although passive acoustic mapping (PAM) is capable of locating where microbubble activities are generated, it is well known that microbubbles rapidly move within the ultrasound beam. We propose a technique that can image microbubble movement by estimating their velocities within the focal volume. Microbubbles embedded within a wall-less channel of a tissue-mimicking material were sonicated using 1-MHz focused ultrasound. The acoustic emissions generated by the microbubbles were captured with a linear array (L7-4). PAM with robust Capon beamforming was used to localize the microbubble acoustic emissions. We spectrally analyzed the time trace of each position and isolated the higher harmonics. Microbubble velocity maps were constructed from the position-dependent Doppler shifts at different time points during sonication. Microbubbles moved primarily away from the transducer at velocities on the order of 1 m/s due to primary acoustic radiation forces, producing a time-dependent velocity distribution. We detected microbubble motion both away and toward the receiving array, revealing the influence of acoustic radiation forces and fluid motion due to the ultrasound exposure. High-speed optical images confirmed the acoustically measured microbubble velocities. Doppler PAM enables passive estimation of microbubble motion and may be useful in therapeutic applications, such as drug delivery across the blood-brain barrier, sonoporation, sonothrombolysis, and drug release.

Concepts and Tradeoffs in Velocity Estimation With Plane-Wave Contrast-Enhanced Doppler

While long Doppler ensembles are, in principle, beneficial for velocity estimates, short acoustic pulses must be used in microbubble contrast-enhanced (CE) Doppler to mitigate microbubble destruction. This introduces inherent tradeoffs in velocity estimates with autocorrelators, which are studied here. A model of the autocorrelation function adapted to the microbubble Doppler signal accounting for transit time, the echo frequency uncertainty, and contrast-agent destruction is derived and validated in vitro. It is further demonstrated that a local measurement of the center frequency of the microbubble echo is essential in order to avoid significant bias in velocity estimates arising from the linear and nonlinear frequency-dependent scattering of microbubbles and compensate for the inherent speckle nature of the received echo frequency. For these reasons, broadband Doppler estimators (2-D autocorrelator and Radon projection) are better suited than simpler narrow-band estimators (1-D autocorrelator and 1-D Fourier transform) for CE flow assessment. A case study of perfusion in a VX-2 carcinoma using CE plane-wave Doppler is also shown. We demonstrate that even when considering all uncertainties associated with microbubble-related decorrelation (destruction, pulse bandwidth, transit time, and flow gradient) and the need for real-time imaging, a coefficient of variation of 4% on the axial velocity is achievable with plane-wave imaging.

Superharmonic microbubble Doppler effect in ultrasound therapy

The introduction of microbubbles in focused ultrasound therapies has enabled a diverse range of non-invasive technologies: sonoporation to deliver drugs into cells, sonothrombolysis to dissolve blood clots, and blood-brain barrier opening to deliver drugs into the brain. Current methods for passively monitoring the microbubble dynamics responsible for these therapeutic effects can identify the cavitation position by passive acoustic mapping and cavitation mode by spectral analysis. Here, we introduce a new feature that can be monitored: microbubble effective velocity. Previous studies have shown that echoes from short imaging pulses had a Doppler shift that was produced by the movement of microbubbles. Therapeutic pulses are longer (>1 000 cycles) and thus produce a larger alteration of microbubble distribution due to primary and secondary acoustic radiation force effects which cannot be monitored using pulse-echo techniques. In our experiments, we captured and analyzed the Doppler shift during long therapeutic pulses using a passive cavitation detector. A population of microbubbles (5  ×  10(4)-5  ×  10(7) microbubbles ml(-1)) was embedded in a vessel (inner diameter: 4 mm) and sonicated using a 0.5 MHz focused ultrasound transducer (peak-rarefactional pressure: 75-366 kPa, pulse length: 50 000 cycles or 100 ms) within a water tank. Microbubble acoustic emissions were captured with a coaxially aligned 7.5 MHz passive cavitation detector and spectrally analyzed to measure the Doppler shift for multiple harmonics above the 10th harmonic (i.e. superharmonics). A Doppler shift was observed on the order of tens of kHz with respect to the primary superharmonic peak and is due to the axial movement of the microbubbles. The position, amplitude and width of the Doppler peaks depended on the acoustic pressure and the microbubble concentration. Higher pressures increased the effective velocity of the microbubbles up to 3 m s(-1), prior to the onset of broadband emissions, which is an indicator for high magnitude inertial cavitation. Although the microbubble redistribution was shown to persist for the entire sonication period in dense populations, it was constrained to the first few milliseconds in lower concentrations. In conclusion, superharmonic microbubble Doppler effects can provide a quantitative measure of effective velocities of a sonicated microbubble population and could be used for monitoring ultrasound therapy in real-time.

Visualizing the Tumor Microvasculature With a Nonlinear Plane-Wave Doppler Imaging Scheme Based on Amplitude Modulation

Imaging with ultrasonic plane waves enables the combination of Doppler and microbubble contrast-enhanced imaging without compromising the Doppler ensemble length, as is the case for conventional line-by-line imaging, thus maintaining flow sensitivity. This permits the separation of conduit flow in large vessels from the perfusion background and the presentation of velocity estimates in real-time. However, the ability to differentiate perfusion from the tissue signal is limited by the contrast-to-tissue (CTR) ratio achieved with the contrast-enhanced pulsing sequence, independently of the acquisition length. One way to improve the CTR is to use a Doppler sequence based on amplitude modulation instead of one based on pulse inversion. In this work, we discuss how amplitude modulation can be adapted to Doppler processing. We show that amplitude modulation Doppler, like pulse inversion Doppler, can separate the signal of moving tissue from that of moving microbubbles, while achieving a better contrast-to-tissue ratio than pulse inversion Doppler, both in vitro and in vivo. Both amplitude modulation Doppler and pulse inversion Doppler yield similar velocity estimates when the bandwidth of the RF echo is properly compensated. Finally, we demonstrate how amplitude modulation Doppler can be used to reveal both the conduit flow and the capillary perfusion at high frame rates in an in vivo tumor.

Combined perfusion and doppler imaging using plane-wave nonlinear detection and microbubble contrast agents

Plane-wave imaging offers image acquisition rates at the pulse repetition frequency, effectively increasing the imaging frame rates by up to two orders of magnitude over conventional line-by-line imaging. This form of acquisition can be used to achieve very long ensemble lengths in nonlinear modes such as pulse inversion Doppler, which enables new imaging trade-offs that were previously unattainable. We first demonstrate in this paper that the coherence of microbubble signals under repeated exposure to acoustic pulses of low mechanical index can be as high as 204 ± 5 pulses, which is long enough to allow an accurate power Doppler measurement. We then show that external factors, such as tissue acceleration, restrict the detection of perfusion at the capillary level with linear Doppler, even if long Doppler ensembles are considered. Hence, perfusion at the capillary level can only be detected with ultrasound through combined microbubbles and Doppler imaging. Finally, plane-wave contrast-enhanced power and color Doppler are performed on a rabbit kidney in vivo as a proof of principle. We establish that long pulse-inversion Doppler sequences and conventional wall-filters can create an image that simultaneously resolves both the vascular morphology of veins and arteries, and perfusion at the capillary level with frame rates above 100 Hz.

Effect of microbubble contrast on intracranial blood flow velocity assessed by transcranial Doppler

PURPOSE

Ultrasound contrast agents (UCA) salvage a considerable number of transcranial Doppler (TCD) exams which would have failed because of poor bone window. UCA bolus injection causes an undesirable increase in measured blood flow velocity (BFV). The effect of UCA continuous infusion on measured BFV has not been investigated, and some in vitro experiments suggest that gain reduction during UCA administration may also influence measured BFV. This study aimed to investigate the effect of UCA continuous infusion on BFV measured by TCD and the influence of gain reduction on these measurements in a clinical setting.

METHODS

The right middle cerebral artery of ten patients with optimal bone window was insonated using a 2 MHz probe. UCA were administered using an infusion pump. BFV was measured (1) at baseline, (2) during UCA infusion, (3) during UCA infusion with gain reduction, and (4) after UCA wash-out phase. Gain reduction was based on the agreement between two neurosonographers on the degree of gain reduction necessary to restore baseline Doppler signal intensity (DSI). Actual DSI was estimated offline by analysis of raw data.

RESULTS

BFV measured during UCA infusion with no gain adjustment was significantly higher than baseline BFV [peak systolic velocity (PSV): 85.1 ± 19.7 vs. 74.4 ± 19.7 cm/s, p < 0.0001; Mean velocity (MV): 56.5 ± 11.8 vs. 50.2 ± 12.3 cm/s, p < 0.0001]. BFV measured during UCA infusion with gain reduction was not significantly higher than baseline BFV (PSV: 74.3 ± 18.9 vs. 74.4 ± 19.4 cm/s, p = 0.8; MV: 49.4 ± 11.0 vs. 50.2 ± 12.3 cm/s, p = 0.8). Actual DSI during UCA infusion with gain reduction was not significantly higher than baseline DSI (13 ± 1 vs. 13 ± 1 dB).

CONCLUSION

This study shows that UCA continuous infusion leads to an increase in measured BFV which may be counteracted by reducing Doppler gain thus restoring pre-contrast DSI.

Effects of acoustic radiation force on the binding efficiency of BR55, a VEGFR2-specific ultrasound contrast agent

This work describes an in vivo study analyzing the effect of acoustic radiation force (ARF) on the binding of BR55 VEGFR2-specific contrast-agent microbubbles in a model of prostatic adenocarcinoma in rat. A commercial ultrasound system was modified by implementing high duty-cycle 3.5-MHz center frequency ARF bursts in a scanning configuration. This enabled comparing the effects of ARF on binding in tumor and healthy tissue effectively in the same field of view. Bubble binding was established by measuring late-phase enhancement in amplitude modulation (AM) contrast-specific imaging mode (4 MHz, 150 kPa) 10 min after agent injection when the unbound bubbles were cleared from the circulation. Optimal experimental conditions, such as agent concentration (0.4 × 10(8)-1.6 × 10(8) bubbles/kg), acoustic pressure amplitude (26-51 kPa) and duty-cycle (20%-95%) of the ARF bursts, were evaluated in their ability to enhance binding in tumor without significantly increasing binding in healthy tissue. Using the optimal conditions (38 kPa peak-negative pressure, 95% duty cycle), ARF-assisted binding of BR55 improved significantly in tumor (by a factor of 7) at a lower agent dose compared with binding without ARF, and it had an insignificant effect on binding in healthy tissue. Thus, the high binding specificity of BR55 microbubbles for targeting VEGFR2 present at sites of active angiogenesis was confirmed by this study. Therefore, it is believed that based on the results obtained in this work, ultrasound molecular imaging using target-specific contrast-agent microbubbles should preferably be performed in combination with ARF.

Contrast-enhanced and targeted ultrasound

Ultrasonic imaging is becoming the most popular medical imaging modality, owing to the low price per examination and its safety. However, blood is a poor scatterer of ultrasound waves at clinical diagnostic transmit frequencies. For perfusion imaging, markers have been designed to enhance the contrast in B-mode imaging. These so-called ultrasound contrast agents consist of microscopically small gas bubbles encapsulated in biodegradable shells. In this review, the physical principles of ultrasound contrast agent microbubble behavior and their adjustment for drug delivery including sonoporation are described. Furthermore, an outline of clinical imaging applications of contrast-enhanced ultrasound is given. It is a challenging task to quantify and predict which bubble phenomenon occurs under which acoustic condition, and how these phenomena may be utilized in ultrasonic imaging. Aided by high-speed photography, our improved understanding of encapsulated microbubble behavior will lead to more sophisticated detection and delivery techniques. More sophisticated methods use quantitative approaches to measure the amount and the time course of bolus or reperfusion curves, and have shown great promise in revealing effective tumor responses to anti-angiogenic drugs in humans before tumor shrinkage occurs. These are beginning to be accepted into clinical practice. In the long term, targeted microbubbles for molecular imaging and eventually for directed anti-tumor therapy are expected to be tested.

Microfoam formation in a capillary

The ultrasound-induced formation of bubble clusters may be of interest as a therapeutic means. If the clusters behave as one entity, i.e., one mega-bubble, its ultrasonic manipulation towards a boundary is straightforward and quick. If the clusters can be forced to accumulate to a microfoam, entire vessels might be blocked on purpose using an ultrasound contrast agent and a sound source. In this paper, we analyse how ultrasound contrast agent clusters are formed in a capillary and what happens to the clusters if sonication is continued, using continuous driving frequencies in the range 1-10 MHz. Furthermore, we show high-speed camera footage of microbubble clustering phenomena. We observed the following stages of microfoam formation within a dense population of microbubbles before ultrasound arrival. After the sonication started, contrast microbubbles collided, forming small clusters, owing to secondary radiation forces. These clusters coalesced within the space of a quarter of the ultrasonic wavelength, owing to primary radiation forces. The resulting microfoams translated in the direction of the ultrasound field, hitting the capillary wall, also owing to primary radiation forces. We have demonstrated that as soon as the bubble clusters are formed and as long as they are in the sound field, they behave as one entity. At our acoustic settings, it takes seconds to force the bubble clusters to positions approximately a quarter wavelength apart. It also just takes seconds to drive the clusters towards the capillary wall. Subjecting an ultrasound contrast agent of given concentration to a continuous low-amplitude signal makes it cluster to a microfoam of known position and known size, allowing for sonic manipulation.

Method for microbubble characterization using primary radiation force

Medical ultrasound contrast agents (UCAs) have evolved from straight image enhancers to pathophysiological markers and drug delivery vehicles. However, the exact dynamic behavior of the encapsulated bubbles composing UCAs is still not entirely known. In this article, we propose to characterize full populations of UCAs, by looking at the translational effects of ultrasound radiation force on each bubble in a diluted population. The setup involves a sensitive, fully programmable transmitter/receiver and two unconventional, real-time display modes. Such display modes are used to measure the displacements produced by irradiation at frequencies in the range 2-8 MHz and pressures between 150 kPa and 1.5 MPa. The behavior of individual bubbles freely moving in a water tank is clearly observed, and it is shown that it depends on the bubble physical dimensions as well as on the viscoelastic properties of the encapsulation. A new method also is distilled that estimates the viscoelastic properties of bubble encapsulation by fitting the experimental bubble velocities to values simulated by a numerical model based on the modified Herring equation and the Bjerknes force. The fit results are a shear modulus of 18 MPa and a viscosity of 0.23 Pas for a thermoplastic PVC-AN shell. Phospholipid shell elasticity and friction parameter of the experimental contrast agent are estimated as 0.8 N/m and 1 10(-7) kg/s, respectively (shear modulus of 32 MPa and viscosity of 0.19 Pas, assuming 4-nm shell thickness).

Optical observations of acoustical radiation force effects on individual air bubbles

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 microm to 79 microm was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 microm. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position.

Ultrasound-induced encapsulated microbubble phenomena

When encapsulated microbubbles are subjected to high-amplitude ultrasound, the following phenomena have been reported: oscillation, translation, coalescence, fragmentation, sonic cracking and jetting. In this paper, we explain these phenomena, based on theories that were validated for relatively big, free (not encapsulated) gas bubbles. These theories are compared with high-speed optical observations of insonified contrast agent microbubbles. Furthermore, the potential clinical applications of the bubble-ultrasound interaction are explored. We conclude that most of the results obtained are consistent with free gas bubble theory. Similar to cavitation theory, the number of fragments after bubble fission is in agreement with the dominant spherical harmonic oscillation mode. Remarkable are our observations of jetting through contrast agent microbubbles. The pressure at the tip of a jet is high enough to penetrate any human cell. Hence, liquid jets may act as remote-controlled microsyringes, delivering a drug to a region-of-interest. Encapsulated microbubbles have (potential) clinical applications in both diagnostics and therapeutics.

Microbubble ultrasound contrast agents: a review

The superior scattering properties of gas bubbles compared with blood cells have made microbubble ultrasound contrast agents important tools in ultrasound diagnosis. Over the past 2 years they have become the focus of a wide and rapidly expanding field of research, with their benefits being repeatedly demonstrated, both in ultrasound image enhancement, and more recently in drug and gene delivery applications. However, despite considerable investigation, their behaviour is by no means fully understood and, while no definite evidence of harmful effects has been obtained, there remain some concerns as to their safety. In this review the existing theoretical and experimental evidence is examined in order to clarify the extent to which contrast agents are currently understood and to identify areas for future research. In particular the disparity between the conditions considered in theoretical models and those encountered both in vitro, and more importantly in vivo is discussed, together with the controversy regarding the risk of harmful bio-effects.