Antonios N. Pouliopoulos

Acoustic particle palpation for measuring tissue elasticity.

We propose acoustic particle palpation-the use of sound to press a population of acoustic particles against an interface-as a method for measuring the qualitative and quantitative mechanical properties of materials. We tested the feasibility of this method by emitting ultrasound pulses across a tunnel of an elastic material filled with microbubbles. Ultrasound stimulated the microbubble cloud to move in the direction of wave propagation, press against the distal surface, and cause deformations relevant for elasticity measurements. Shear waves propagated away from the palpation site with a velocity that was used to estimate the materials Youngs modulus.

Rapid short-pulse sequences enhance the spatiotemporal uniformity of acoustically driven microbubble activity during flow conditions

Despite the promise of microbubble-mediated focused ultrasound therapies, in vivo findings have revealed over-treated and under-treated regions distributed throughout the focal volume. This poor distribution cannot be improved by conventional pulse shapes and sequences, due to their limited ability to control acoustic cavitation dynamics within the ultrasonic focus. This paper describes the design of a rapid short-pulse (RaSP) sequence which is comprised of short pulses separated by μs off-time intervals. Improved acoustic cavitation distribution was based on the hypothesis that microbubbles can freely move during the pulse off-times. Flowing SonoVue® microbubbles (flow velocity: 10 mm/s) were sonicated with a 0.5 MHz focused ultrasound transducer using RaSP sequences (peak-rarefactional pressures: 146-900 kPa, pulse repetition frequency: 1.25 kHz, and pulse lengths: 5-50 cycles). The distribution of cavitation activity was evaluated using passive acoustic mapping. RaSP sequences generated uniform distributions within the focus in contrast to long pulses (50 000 cycles) that produced non-uniform distributions. Fast microbubble destruction occurred for long pulses, whereas microbubble activity was sustained for longer durations for shorter pulses. High-speed microscopy revealed increased mobility in the direction of flow during RaSP sonication. In conclusion, RaSP sequences produced spatiotemporally uniform cavitation distributions and could result in efficient therapies by spreading cavitation throughout the treatment area.

Superharmonic microbubble Doppler effect in ultrasound therapy

Abstract 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  ×  104–5  ×  107 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.

Clustering dynamics of microbubbles exposed to low-pressure 1-MHz ultrasound

Ultrasound-driven microbubbles have been used in therapeutic applications to deliver drugs across capillaries and into cells or to dissolve blood clots. Yet the performance and safety of these applications have been difficult to control. Microbubbles exposed to ultrasound not only volumetrically oscillate, but also move due to acoustic radiation, or Bjerknes, forces. The purpose of this work was to understand the extent to which microbubbles moved and clustered due to secondary Bjerknes forces. A microbubble population was exposed to a 1-MHz ultrasound pulse with a peak-rarefactional pressure of 50-100 kPa and a pulse length of 20 ms. Microbubbles exposed to low-pressure therapeutic ultrasound were observed to cluster at clustering rates of 0.01-0.02 microbubbles per duration (in ms) per initial average inter-bubble distance (in μm), resulting in 1 to 3 clustered microbubbles per initial average inter-bubble distance (in μm). Higher pressures caused faster clustering rates and a larger number of clustered microbubbles. Experimental data revealed clustering time scales, cluster localizations, and cluster sizes that were in reasonable agreement with simulations using a time-averaged model at low pressures. This study demonstrates that clustering of microbubbles occurs within a few milliseconds and is likely to influence the distribution of stimuli produced in therapeutic applications.

Simultaneous Ultrasound Therapy and Monitoring of Microbubble-Seeded Acoustic Cavitation Using a Single-Element Transducer

Ultrasound-driven microbubble (MB) activity is used in therapeutic applications such as blood clot dissolution and targeted drug delivery. The safety and performance of these technologies are linked to the type and distribution of MB activities produced within the targeted area, but controlling and monitoring these activities in vivo and in real time has proven to be difficult. As therapeutic pulses are often milliseconds long, MB monitoring currently requires a separate transducer used in a passive reception mode. Here, we present a simple, inexpensive, integrated setup, in which a focused single-element transducer can perform ultrasound therapy and monitoring simultaneously. MBs were made to flow through a vessel-mimicking tube, placed within the transducer’s focus, and were sonicated with therapeutic pulses (peak rarefactional pressure: 75–827 kPa, pulse lengths:

Pulse inversion enhances the passive mapping of microbubble-based ultrasound therapy

200~\mu \text{s}

Rapid short-pulse (RaSP) sequences improve the distribution of drug delivery to the brain in vivo

and 20 ms). The MB-seeded acoustic emissions were captured using the same transducer. The received signals were separated from the therapeutic signal with a hybrid coupler and a high-pass filter. We discriminated the MB-generated cavitation signal from the primary acoustic field and characterized MB behavior in real time. The simplicity and versatility of our circuit could make existing single-element therapeutic transducers also act as cavitation detectors, allowing the production of compact therapeutic systems with real time monitoring capabilities.

Doppler passive acoustic mapping for monitoring microbubble velocities in ultrasound therapy

Therapeutic ultrasound combined with preformed circulating microbubbles has enabled non-invasive and targeted drug delivery into the brain, tumors, and blood clots. Monitoring the microbubble activity is essential for the success of such therapies; however, skull and tissues limit our ability to detect low acoustic signals. Here, we show that by emitting consecutive therapeutic pulses of inverse polarity, the sensitivity in the detection of weak bubble acoustic signals during blood-brain barrier opening is enhanced compared to therapeutic pulses of the same polarity. Synchronous passive mapping of the cavitation activity was conducted using delay-and-sum beamforming with absolute time delays, which offers superior spatial resolution compared to the existing asynchronous passive imaging techniques. Sonication with pulse inversion allowed filter-free suppression of the tissue signals by up to 8 dB in a tissue-mimicking phantom and by 7 dB in vivo, compared to exposure without pulse inversion, enabling enhanced passive mapping of microbubble activity. Both therapeutic schemes resulted in similar free-field microbubble activation in vitro and efficient blood-brain barrier opening in vivo.

Pulse inversion therapy for improved monitoring of blood-brain barrier opening

Focused ultrasound and microbubbles have been shown to locally and noninvasively open the blood-brain barrier. Despite encouraging results in human patients, several performance and safety features, such as poor drug distribution, high drug accumulation along vessels and small sites of red blood cell extravasation, have been unavoidable. We have recently developed a new ultrasound sequence — rapid short-pulse (RaSP) sequence — designed to suppress these adverse features by promoting safer modes of cavitation activity throughout capillaries. In our RaSP sequences, low-pressure short ultrasonic pulses are emitted at kHz pulse repetition frequencies (PRF) and grouped into bursts. We have shown in vitro that RaSP sequences prolong microbubble lifetime and increase their mobility, enhancing the distribution of acoustic cavitation activity. Here we evaluate the ability of RaSP sequences to improve the in vivo performance and safety of ultrasound-mediated drug delivery to the brain.