Doppler spectra from contrast agents crossing an ultrasound field

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.

The effect of size range on ultrasound-induced translations in microbubble populations

Microbubble translations driven by ultrasound-induced radiation forces can be beneficial for applications in ultrasound molecular imaging and drug delivery. Here, the effect of size range in microbubble populations on their translations is investigated experimentally and theoretically. The displacements within five distinct size-isolated microbubble populations are driven by a standard ultrasound-imaging probe at frequencies ranging from 3 to 7 MHz, and measured using the multi-gate spectral Doppler approach. Peak microbubble displacements, reaching up to 10 μm per pulse, are found to describe transient phenomena from the resonant proportion of each bubble population. The overall trend of the statistical behavior of the bubble displacements, quantified by the total number of identified displacements, reveals significant differences between the bubble populations as a function of the transmission frequency. A good agreement is found between the experiments and theory that includes a model parameter fit, which is further supported by separate measurements of individual microbubbles to characterize the viscoelasticity of their stabilizing lipid shell. These findings may help to tune the microbubble size distribution and ultrasound transmission parameters to optimize the radiation-force translations. They also demonstrate a simple technique to characterize the microbubble shell viscosity, the fitted model parameter, from freely floating microbubble populations using a standard ultrasound-imaging probe.

Microbubble Radiation Force-Induced Translation in Plane-Wave Versus Focused Transmission Modes

Due to the primary radiation force, microbubble displacement has been observed previously in the focal region of single-element and array ultrasound probes. This effect has been harnessed to increase the contact between the microbubbles and targeted endothelium for drug delivery and ultrasound molecular imaging. In this study, microbubble displacements associated with plane-wave (PW) transmission are thoroughly investigated and compared to those obtained in focused-wave (FW) transmission over a range of pulse repetition frequencies, burst lengths (BLs), peak negative pressures, and transmission frequencies. In PW mode, the displacements, depending upon the experimental conditions, are in some cases consistently higher (e.g., by 28%, when the longest BL was used at PRF = 4 kHz), and the axial displacements are spatially more uniform compared to FW mode. Statistical analysis on the measured displacements reveals a slightly different frequency dependence of statistical quantities compared to transient peak microbubble displacements, which may suggest the need to consider the size range within the tested microbubble population.

Ultrasound assessment of translation of microbubbles driven by acoustic radiation force in a channel filled with stationary fluid

In this report, a method is proposed to quantify the translation of ultrasound contrast agent (UCA) microbubbles driven by acoustic radiation for the detection of channels filled with stationary fluid. The authors subjected UCA microbubbles in a channel with diameters of 0.1 and 0.5 mm to ultrasound pulses with a center frequency of 14.4 MHz. The translational velocity of the UCA microbubbles increased with the sound pressure and pulse repetition frequency (PRF) of the transmitted ultrasound. The mean translational velocity reached 0.75 mm/s at a negative peak sound pressure of 2.76 MPa and a PRF of 2 kHz. This trend agreed with the theoretical prediction, which indicated that the translational velocity was proportional to the square of the sound pressure and the PRF. Furthermore, an experiment was carried out with a phantom that mimics tissue and found that the proposed method aided in detection of the channel, even in the case of a low contrast-echo to tissue-echo ratio. The authors expect to develop the proposed method into a technique for detecting lymph vessels.

Plane-Wave Contrast Imaging: A Radiation Force Point of View

Radiation force is known to produce microbubble axial displacements by an amount that depends on the transmit burst frequency, amplitude, and length, as well as the pulse repetition frequency (PRF). In the standard focused-imaging mode, the actual PRF experienced by each microbubble is low, because it is of the order of the frame rate (i.e., usually tens of Hertz). In the plane-wave imaging mode, however, the actual PRF is considerably higher, as it is equivalent to the transmit PRF (kiloHertz range). Furthermore, the radiation pressure is expected to be almost uniform over the field of view, and typically lower than the peak pressure experienced in the focused transmit (TX) mode. We have experimentally investigated the possible effects of radiation force in the plane-wave mode. Here, we report on preliminary findings that show that the acoustic radiation force is negligible only at lower TX levels. At higher TX amplitudes, the bubble displacements due to radiation force are comparable to those obtained for focused waves at the same PRF. In addition, the radiation force is nearly uniform over the field of view and increases as the TX burst central frequency approaches the resonance frequency of size-isolated microbubbles.

Combining Acoustic Trapping With Plane Wave Imaging for Localized Microbubble Accumulation in Large Vessels

The capability of accumulating microbubbles using ultrasound could be beneficial for enhancing targeted drug delivery. When microbubbles are used to deliver a therapeutic payload, there is a need to track them, for a localized release of the payload. In this paper, a method for localizing microbubble accumulation with fast image guidance is presented. A linear array transducer performed trapping of microbubble populations interleaved with plane wave imaging, through the use of a composite pulse sequence. The acoustic trap in the pressure field was created parallel with the direction of flow in a model of a vessel section. The acoustic trapping force resultant from the large gradients in the acoustic field was engendered to directly oppose the flowing microbubbles. This was demonstrated numerically with field simulations, and experimentally using an Ultrasound Array Research Platform II. SonoVue microbubbles at clinically relevant concentrations were pumped through a tissue-mimicking flow phantom and exposed to either the acoustic trap or a control ultrasonic field composed of a single-peak acoustic radiation force beam. Under the flow condition at a shear rate of 433 s^-1, the use of the acoustic trap led to lower speed estimations ( ) in the center of the acoustic field, and an enhancement of 71% ± 28%( ) in microbubble image brightness.

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.

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.

Analysis of index modulation in microembolic Doppler signals part I: radiation force as a new hypothesis-simulations

The purpose of this study was to reveal the cause of frequency modulation (FM) present in microembolic Doppler ultrasound signals. This novel explanation should help the development of sensitive microembolus discrimination techniques. We suggest that the frequency modulation detected is caused by the ultrasonic radiation force (URF) acting directly on microemboli. The frequency modulation and the imposed displacement were calculated using a numerical dynamic model. By setting simulation parameters with practical values, it was possible to reproduce most microembolic frequency modulation signatures. The most interesting findings in this study were that: (1) the ultrasound radiation force acting on a gaseous microembolus and its corresponding cumulative displacement were far higher than those obtained for a solid microembolus, and that is encouraging for discrimination purposes; and 2) the calculated frequency modulation indices (FMIs) (≈20 kHz) were in good agreement with literature results. By taking into account the URF, the flow pulsatility, the beam-to-flow angle and both the velocity and the ultrasound beam profiles, it was possible to explain all erratic FM signatures of a microbubble. Finally, by measuring FMI from simulated Doppler signals and by using a constant threshold of 1 KHz, it was possible to discriminate gaseous from solid microemboli with ease.

A consistent tissue attenuation coefficient estimator using bubble harmonic echoes

The ultrasonic property of soft tissue can be quantified by its attenuation coefficient α. Traditionally the backscattering signal of tissue is used to estimate α. To improve precision, a large number of spatially independent samples of tissue echoes are required for averaging. In this paper, we propose a new estimation method, which makes use of microbubbles to provide temporally independent samples for averaging. It is easier for temporal sampling to maintain ergodicity and provide a large number of independent samples for statistical averaging. A stochastic model for the harmonic signals of an ideal bubble attenuated by tissue is derived based on Kuc's and Miller's works. An estimator of α is then presented. This estimator is consistent and could be biased because of the unknown squarelaw relation between the second and fundamental harmonics for non-ideal bubble oscillation. In experimental works, we design a simplified phantom for demonstrating the performance of the proposed estimator. It is shown that both first and second harmonics can estimate α consistently. However, the interference of the tissue backscattering signal may cause additional estimation error using the first harmonic.

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).

Different effects of microbubble destruction and translation in Doppler measurements

In flow measurements in which microbubbles are involved, the amplitude and phase of the received echo signal are noticeably influenced by the transmitted ultrasound intensity. Previous studies have shown that, when such intensity is progressively increased, the Doppler spectrum is accordingly distorted, i.e., it is asymmetrically broadened toward the negative frequency side. Such deformation has been attributed to radiation force, which pushes the microbubbles into the sound propagation direction, thus yielding additional phase delays in the received echoes. However, the possible contribution of microbubble destruction to this spectral deformation has not been considered yet. In this paper, this issue is investigated by analyzing the experimental spectra produced by two different types of microbubbles suspended in a moving fluid and insonified in pulsed wave (PW) mode at programmable pulse repetition frequency (PRF) and pressure. Conditions are created in which either the radiation force or the destruction mechanism is expected to be dominant. Effects produced by the two phenomena on the Doppler spectrum are shown to be different. When the PRF is low (2 kHz), so that, according to theoretical simulations, the radiation force effect is negligible, a 26 dB noise floor increase is observed for a 13 dB pressure increment. For a higher PRF (16 kHz), the same pressure increase not only affects the noise floor, but also causes the bubbles to deviate from their original streamlines, yielding a Doppler bandwidth increase by a factor of 5. It is concluded that asymmetrical spectral broadening is mainly due to radiation force, and microbubble destruction mainly results in an increased noise floor without affecting the spectral shape.

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.

Volume scattering of distributed microbubbles and its influence on blood flow estimation

In recent years, microbubble contrast agents have become a potential adjunct in Doppler ultrasound diagnosis. In this paper, we show that volume scattering makes the effective band in Doppler spectrum shift downward after injection of microbubbles. Because the insonified volume comprises a collection of distributed microbubbles, the statistical properties such as the autocorrelation function and ensemble average power spectrum of the echoes from a collection of distributed microbubbles were derived first. It can be observed that, beyond a critical frequency, the theoretical volume backscattering cross section derived from the ensemble average power spectrum of microbubbles decreases with frequency. On the contrary, the volume backscattering cross section of red cells increases with frequency. Using two-dimensional (2-D) Fourier transform, the variation in Doppler spectrum caused by different volume backscattering cross section can be demonstrated, and the consequential downward shifts of the estimated Doppler parameters (e.g., the mean and maximum Doppler shifts, and the variance of Doppler power spectrum) after microbubble injection are shown. In addition, it can be observed that the variation gets larger as the transmitted bandwidth increases. And, the variations in Doppler parameters estimated with experimental data are presented to verify the theoretical deviations.

On the interaction between ultrasound and contrast agents during Doppler investigations

Knowledge of interaction mechanisms between ultrasound (US) and contrast agents (CA) suspended in blood is important for a correct interpretation of clinical investigation results. Experiments performed in different laboratories have shown that, as a consequence of primary radiation force, CA tend to move away from the US transducer. Accordingly, Doppler spectra produced by particles suspended in moving water turn out to be significantly altered from what is theoretically expected. The purpose of this paper is twofold. First, an original model describing the bubble dynamics as the outcome of the balance between US radiation force and fluid drag force is validated for the case in which bubbles are suspended in blood. The high fluid viscosity is shown to prevent significant bubble deviations from the unperturbed fluid streamlines so that, in large vessels, a residual spectral distortion may exist only at the highest intensity levels permitted by current regulations. Finally, the relative importance and differences between the effect of primary radiation force and streaming mechanisms that, in principle, could lead to similar effects, are discussed.