Anthony Novell

Exploitation of capacitive micromachined transducers for nonlinear ultrasound imaging

Capacitive micromachined ultrasonic transducers (CMUTs) present advantages such as wide frequency bandwidth, which could be further developed for nonlinear imaging. However, the driving electrostatic force induces a nonlinear behavior of the CMUT, thus generating undesirable harmonic components in the generated acoustic signal. Consequently, the use of CMUT for harmonic imaging (with or without contrast agents) becomes challenging. This paper suggests 2 compensation approaches, linear and nonlinear methods, to cancel unwanted nonlinear components. Furthermore, nonlinear responses from contrast agent were evaluated using CMUT in transmit before and after compensation. The results were compared with those obtained using a PZT transducer in transmit. Results showed that CMUT nonlinear behavior is highly influenced by the excitation to bias voltage ratio. Measurements of output pressure very close to the CMUT surface allow the estimation of optimal parameters for each compensation approach. Both methods showed a harmonic reduction higher than 20 dB when one element or several elements are excited. In addition, the study demonstrates that nonlinear approach seems to be more efficient because it is shown to be less sensitive to interelement variability and further avoids fundamental component deterioration. The results from contrast agent measurements showed that the responses obtained using CMUT elements in transmit with compensation were similar to those from PZT transducer excitation. This experimental study demonstrates the opportunity to use CMUT with traditional harmonic contrast imaging techniques.

Doxorubicin liposome-loaded microbubbles for contrast imaging and ultrasound-triggered drug delivery

Targeted drug delivery under image guidance is gaining more interest in the drug-delivery field. The use of microbubbles as contrast agents in diagnostic ultrasound provides new opportunities in noninvasive image-guided drug delivery. In the present study, the imaging and therapeutic properties of novel doxorubicin liposome-loaded microbubbles are evaluated. The results showed that at scanning settings (1.7 MHz and mechanical index 0.2), these microbubbles scatter sufficient signal for nonlinear ultrasound imaging and can thus be imaged in real time and be tracked in vivo. In vitro therapeutic evaluation showed that ultrasound at 1 MHz and pressures up to 600 kPa in combination with the doxorubicin liposomeloaded microbubbles induced 4-fold decrease of cell viability compared with treatment with free doxorubicin or doxorubicin liposome-loaded microbubbles alone. The therapeutic effectiveness is correlated to an ultrasound-triggered release of doxorubicin from the liposomes and an enhanced uptake of the free doxorubicin by glioblastoma cells. The results obtained demonstrate that the combination of ultrasound and the doxorubicin liposome-loaded microbubbles can provide a new method of noninvasive image-guided drug delivery.

In-Vivo Gene Delivery by Sonoporation: Recent Progress and Prospects

The increasing knowledge of cellular and molecular mechanisms of human diseases allows envisaging the gene therapy by sonoporation as an emerging and promising therapeutic alternative. Sonoporation combines the local application of ultrasound waves and the intravascular or intratissue administration of gas microbubbles. In such a way, the permeability of vessels and tissues to the poorly permeant molecules is transiently increased. Ultrasound based modality offers new opportunities since ultrasound can be easily focused on a target tissue or organ and hence gene delivery and expression should be limited to the insonified region. Consequently, it might be possible to develop an efficient and safe tissue- or organ-specific delivery method by microbubble targeting and focused ultrasound. This review focuses on the current knowledge of sonoporation fundamentals and mechanisms. The sonoporation procedure and current preclinical trials will be then presented. Finally, the new challenges of sonoporation will be discussed.

Microbubble attenuation and destruction: are they involved in sonoporation efficiency?

This technical note investigates the involvement of microbubble attenuation and destruction in sonoporation mechanisms. First, we evaluate sonoporation efficiency using Vevo Micromarker, and a comparison is made with BR14 and SonoVue microbubbles. Then, the acoustical properties of the microbubbles are measured to gain insight into the sonoporation mechanisms using a green fluorescent protein as a marker. Using glioblastoma cells, an unprecedented transfection rate of 70% is reached with Vevo Micromarker, corresponding to a 1.5-fold increase compared with the rate achieved with the other microbubbles. Moreover, attenuation and destruction were shown to be two key parameters in sonoporation efficiency.

Bleomycin delivery into cancer cells in vitro with ultrasound and SonoVue® or BR14® microbubbles

Abstract Background: Cell exposure to ultrasound (US) in the presence of contrast agent microbubbles (MBs) can result in cell sonoporation that can be exploited for drug or gene delivery. Anticancer drug bleomycin (BLM), used in sonoporation, can effectively eliminate tumor cells in vitro and in vivo. Nevertheless, sonoporation mechanism is not known, thus different US parameters and MB types are used. Recently, we proposed that efficiency of cell sonoporation can be related to the efficiency of MB sonodestruction. Purpose: We analyzed human tumor cells viability in response to BLM, US and MB treatment. Methods: Human glioblastoma astrocytoma (U-87 MG) or colon cancer (HCT-116) cells were exposed to US in the presence of BLM and either SonoVue® or BR14® MBs. MB sonodestruction was evaluated according to US signal attenuation. Results: Both HCT-116 and U-87 MG cell viability following US exposure decreased up to 30%. Decrease in cell viability followed similar tendency as MB sonodestruction, which suggests direct relationship between MB sonodestruction and BLM intracellular delivery. Conclusion: Sonoporation is a feasible method to deliver BLM in to several types of human cancer cell lines. Efficiency of cell sonoporation correlated well with MB sonodestruction, providing a possibility to optimize US parameters by measuring MB sonodestruction.

Tissue harmonic imaging with CMUTs

In this work, we report on the characterization of a CMUT probe (Capacitive Micromachined Ultrasound Transducer) for Tissue Harmonic Imaging (THI). The intrinsic nonlinear behavior of the CMUT probe was first investigated. Matched electrical waveforms were transmitted to limit the impact of the transmit response distortion. With the implemented method, we demonstrated higher performances through in-vitro harmonic imaging.

Simulations and measurements of 3-D ultrasonic fields radiated by phased-array transducers using the westervelt equation

The purpose of this work is to validate, by comparing numerical and experimental results, the ability of the Westervelt equation to predict the behavior of ultrasound beams generated by phased-array transducers. To this end, the full Westervelt equation is solved numerically and the results obtained are compared with experimental measurements. The numerical implementation of the Westervelt equation is performed using the explicit finite-difference time-domain method on a three-dimensional Cartesian grid. The validation of the developed numerical code is first carried out by using experimental data obtained for two different focused circular transducers in the regimes of small-amplitude and finite-amplitude excitations. Then, the comparison of simulated and measured ultrasonic fields is extended to the case of a modified 32-element array transducer. It is shown that the developed code is capable of correctly predicting the behavior of the main lobe and the grating lobes in the cases of zero and nonzero steering angles for both the fundamental and the second-harmonic components.

SECOND HARMONIC AND SUBHARMONIC FOR NON-LINEAR WIDEBAND CONTRAST IMAGING USING A CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCER ARRAY

When insonified with suitable ultrasound excitation, contrast microbubbles generate various non-linear scattered components, such as the second harmonic (2H) and the subharmonic (SH). In this study, we exploit the wide frequency bandwidth of capacitive micromachined ultrasonic transducers (CMUTs) to enhance the response from ultrasound contrast agents by selective imaging of both the 2H and SH components simultaneously. To this end, contrast images using the pulse inversion method were recorded with a 64-element CMUT linear array connected to an open scanner. In comparison to imaging at 2H alone, the wideband imaging including both the 2H and SH contributions provided up to 130% and 180% increases in the signal-to-noise and contrast-to-tissue ratios, respectively. The wide-frequency band of CMUTs offers new opportunities for improved ultrasound contrast agent imaging.

Focused ultrasound influence on calcein-loaded thermosensitive stealth liposomes.

Abstract Focused ultrasound (FUS) is a versatile technology for non-invasive thermal therapies in oncology. Indeed, this technology has great potential for local heat-mediated drug delivery from thermosensitive liposomes (TSLs), thus improving therapeutic efficacy and reducing toxicity profiles. In the present study we evaluated the influence of FUS parameters on the release of calcein from TSLs used to model a hydrophilic drug. Quantitative calcein release from TSLs (DPPC/CHOL/DSPE-PEG2000: 90/5/5) and non-thermosensitive liposomes (NTSLs) (DPPC/CHOL/DSPE-PEG2000: 65/30/5) was measured by spectrofluorimetry after both water bath and FUS-induced in vitro heating. The heating of TSLs at 42 °C in a water bath resulted in a maximum calcein release of 45%. No additional calcein release was observed at temperatures above 42 °C. A similar percentage of calcein release was achieved when TSLs were exposed to 1 MHz sinusoidal waves at peak negative pressure of 1.5 MPa, 40% duty cycle, for 10 min (i.e. above 42 °C). No release was detected when NTSLs were heated in a water bath. For both TSLs and NTSLs, the calcein release was increased by more than 10% for acoustic pressures ranging from 1.5 MPa to 2 MPa. This additional release was attributed to the mechanical stress generated by FUS, which was sufficient to disrupt the liposomal membrane. Furthermore, analysis of cryo-TEM images showed a significant decrease in liposome size (14%) induced by the thermal effect, whereas the liposome diameter remained unaffected by the FUS-triggered non-thermal effects.

Optimization of contrast-to-tissue ratio by frequency adaptation in pulse inversion imaging

Contrast imaging has significantly improved clinical diagnosis by increasing the contrast-to-tissue ratio after microbubble injection. Pulse inversion imaging is the most commonly used contrast imaging technique because it greatly increases the contrast-to-tissue ratio by extracting microbubble nonlinearities. The main purpose of our study was to propose an automatic technique providing the best contrast- to-tissue ratio throughout the experiment. For reasons of simplicity, we proposed maximizing the contrast-to-tissue ratio with an appropriate choice of the transmit frequency. The contrast-to-tissue ratio was maximized by a closed-loop system including the pulse inversion technique. An algorithm based on gradient descent provided iterative determination of the optimal transmit frequency. The optimization method converged quickly after six iterations. This optimal control method is easy to implement and it optimizes the contrast-to-tissue ratio by adaptively selecting the transmit frequency.