Christian Coviello

Ultrasound‐Propelled Nanocups for Drug Delivery

Ultrasound-induced bubble activity (cavitation) has been recently shown to actively transport and improve the distribution of therapeutic agents in tumors. However, existing cavitation-promoting agents are micron-sized and cannot sustain cavitation activity over prolonged time periods because they are rapidly destroyed upon ultrasound exposure. A novel ultrasound-responsive single-cavity polymeric nanoparticle (nanocup) capable of trapping and stabilizing gas against dissolution in the bloodstream is reported. Upon ultrasound exposure at frequencies and intensities achievable with existing diagnostic and therapeutic systems, nanocups initiate and sustain readily detectable cavitation activity for at least four times longer than existing microbubble constructs in an in vivo tumor model. As a proof-of-concept of their ability to enhance the delivery of unmodified therapeutics, intravenously injected nanocups are also found to improve the distribution of a freely circulating IgG mouse antibody when the tumor is exposed to ultrasound. Quantification of the delivery distance and concentration of both the nanocups and coadministered model therapeutic in an in vitro flow phantom shows that the ultrasound-propelled nanocups travel further than the model therapeutic, which is itself delivered to hundreds of microns from the vessel wall. Thus nanocups offer considerable potential for enhanced drug delivery and treatment monitoring in oncological and other biomedical applications.

Passive acoustic mapping utilizing optimal beamforming in ultrasound therapy monitoring.

Passive acoustic mapping (PAM) is a promising imaging method that enables real-time three-dimensional monitoring of ultrasound therapy through the reconstruction of acoustic emissions passively received on an array of ultrasonic sensors. A passive beamforming method is presented that provides greatly improved spatial accuracy over the conventionally used time exposure acoustics (TEA) PAM reconstruction algorithm. Both the Capon beamformer and the robust Capon beamformer (RCB) for PAM are suggested as methods to reduce interference artifacts and improve resolution, which has been one of the experimental issues previously observed with TEA. Simulation results that replicate the experimental artifacts are shown to suggest that bubble interactions are the chief cause. Analysis is provided to show that these multiple bubble artifacts are generally not reduced by TEA, while Capon-based methods are able to reduce the artifacts. This is followed by experimental results from in vitro experiments and in vivo oncolytic viral therapy trials that show improved results in PAM, where RCB is able to more accurately localize the acoustic activity than TEA.

Passive acoustic mapping of magnetic microbubbles for cavitation enhancement and localization.

Magnetic targeting of microbubbles functionalized with superparamagnetic nanoparticles has been demonstrated previously for diagnostic (B-mode) ultrasound imaging and shown to enhance gene delivery in vitro and in vivo. In the present work, passive acoustic mapping (PAM) was used to investigate the potential of magnetic microbubbles for localizing and enhancing cavitation activity under focused ultrasound. Suspensions of magnetic microbubbles consisting of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), air and 10 nm diameter iron oxide nanoparticles were injected into a tissue mimicking phantom at different flow velocities (from 0 to 50 mm s(-1)) with or without an applied magnetic field. Microbubbles were excited using a 500 kHz single element focused transducer at peak negative focal pressures of 0.1-1.0 MPa, while a 64 channel imaging array passively recorded their acoustic emissions. Magnetic localization of microbubble-induced cavitation activity was successfully achieved and could be resolved using PAM as a shift in the spatial distribution and increases in the intensity and sustainability of cavitation activity under the influence of a magnetic field. Under flow conditions at shear rates of up to 100 s(-1) targeting efficacy was maintained. Application of a magnetic field was shown to consistently increase the energy of cavitation emissions by a factor of 2-5 times over the duration of exposures compared to the case without targeting, which was approximately equivalent to doubling the injected microbubble dose. These results suggest that magnetic targeting could be used to localize and increase the concentration of microbubbles and hence cavitation activity for a given systemic dose of microbubbles or ultrasound intensity.

Passive cavitation mapping with temporal sparsity constraint

The spatial resolution of cavitation maps generated from passive recordings of cavitation emissions is compromised by the bandlimited nature of the recordings. Deconvolution based on the assumption that cavitation consists of a sparse series of discrete events allows the recovery of frequency components that are not only outside the frequency band of the receivers, but may also have been attenuated by the medium before being detectable. In the current work, two sparse deconvolution techniques, matching pursuit and basis pursuit, were applied to simulated and experimental cavitation recordings before they were beamformed to provide passive maps of cavitation activity. Matching pursuit was shown to reduce the maximal diameter of the point spread function by almost a third, at the cost of greater susceptibility to inter-source interference. In contrast, although basis pursuit causes an almost 20% increase in the maximal diameter of the point spread function, its application to experimental data appears to enhance the ability of passive mapping to resolve multiple sources.

Thin-film sparse boundary array design for passive acoustic mapping during ultrasound therapy

A new 2-D hydrophone array for ultrasound therapy monitoring is presented, along with a novel algorithm for passive acoustic mapping using a sparse weighted aperture. The array is constructed using existing polyvinylidene fluoride (PVDF) ultrasound sensor technology, and is utilized for its broadband characteristics and its high receive sensitivity. For most 2-D arrays, high-resolution imagery is desired, which requires a large aperture at the cost of a large number of elements. The proposed arrays geometry is sparse, with elements only on the boundary of the rectangular aperture. The missing information from the interior is filled in using linear imaging techniques. After receiving acoustic emissions during ultrasound therapy, this algorithm applies an apodization to the sparse aperture to limit side lobes and then reconstructs acoustic activity with high spatiotemporal resolution. Experiments show verification of the theoretical point spread function, and cavitation maps in agar phantoms correspond closely to predicted areas, showing the validity of the array and methodology.

Polymeric Cups for Cavitation-mediated Delivery of Oncolytic Vaccinia Virus

Oncolytic viruses (OV) could become the most powerful and selective cancer therapies. However, the limited transport of OV into and throughout tumors following intravenous injection means their clinical administration is often restricted to direct intratumoral dosing. Application of physical stimuli, such as focused ultrasound, offers a means of achieving enhanced mass transport. In particular, shockwaves and microstreaming resulting from the instigation of an ultrasound-induced event known as inertial cavitation can propel OV hundreds of microns. We have recently developed a polymeric cup formulation which, when delivered intravenously, provides the nuclei for instigation of sustained inertial cavitation events within tumors. Here we report that exposure of tumors to focused ultrasound after intravenous coinjection of cups and oncolytic vaccinia virus , leads to substantial and significant increases in activity. When cavitation was instigated within SKOV-3 or HepG2 xenografts, reporter gene expression from vaccinia virus was enhanced 1,000-fold (P < 0.0001) or 10,000-fold (P < 0.001), respectively. Similar increases in the number of vaccinia virus genomes recovered from tumors were also observed. In survival studies, the application of cup mediated cavitation to a vaccinia virus expressing a prodrug converting enzyme provided significant (P < 0.05) retardation of tumor growth. This technology could improve the clinical utility of all biological therapeutics including OV.

Sum-of-harmonics method for improved narrowband and broadband signal quantification during passive monitoring of ultrasound therapies

Passive Acoustic Mapping (PAM) enables real-time monitoring of ultrasound therapies by beamforming acoustic emissions emanating from the ultrasound focus. Reconstruction of the narrowband or broadband acoustic emissions component enables mapping of different physical phenomena, with narrowband emissions arising from non-linear propagation and scattering, non-inertial cavitation or tissue boiling, and broadband (generally, of significantly lower amplitude) indicating inertial cavitation. Currently, accurate classification of the received signals based on pre-defined frequency-domain comb filters cannot be guaranteed because varying levels of leakage occur as a function of signal amplitude and the choice of windowing function. This work presents a time-domain parametric model aimed at enabling accurate estimation of the amplitude of time-varying narrowband components in the presence of broadband signals. Conversely, the method makes it possible to recover a weak broadband signal in the presence of a dominant harmonic or other narrowband component. Compared to conventional comb filtering, the proposed sum-of-harmonics method enables PAM of cavitation sources that better reflect their physical location and extent.

Discrete polynomial transform representation using binary matrices and flow diagrams

This paper presents a new method for computing discrete polynomial transforms. The method is shown for the Hermite, binomial, and Laguerre transforms. The new method factors Pascals matrix into binary matrices. Constructing the flow diagrams for the transform matrices requires only additions and N-2 multipliers for N-point Hermite and binomial transforms, and 2N multipliers for an N-point Laguerre transform. The method involves a three-stage process where stages 1 and 3 are identical for all three transforms.

Real-time three-dimensional passive cavitation detection for clinical high intensity focused ultrasound systems

Bubble activity during High Intensity Focussed Ultrasound (HIFU) surgery has been linked with desirable effects, such as an enhanced heat deposition caused by inertial cavitation, and undesirable effects, such as lesion migration caused by boiling bubbles. There is presently no reliable way of achieving spatiotemporal monitoring of cavitation activity during clinical HIFU treatments. In the present work, a near-acoustically- transparent two-dimensional 32-element PVDF array was designed and mounted on the therapy transducer of a clinical HIFU device (Model JC200, Chongqing Haifu) to enable detection of acoustic emissions arising from cavitation during therapy. The signal detected by each of the elements was digitized and processed in real time on a Graphical Processing Unit (GPU), and beamformed using our previously described Passive Acoustic Mapping (PAM) algorithm to produce real-time three-dimensional (3D) maps of cavitation activity with a frame rate in excess of 5 Hz. The system was initially validated in agar-based tissue-mimicking materials, demonstrating that the displayed volume of cavitation activity agreed with predictions based on in situ pressure calibrations. The system was further validated during clinical HIFU treatments of kidney tumour, liver tumour and uterine fibroid ablation, and was found to enable accurate localization of the HIFU focus at sub-lesioning intensities.

Robust Capon beamforming for passive cavitation mapping during high‐intensity focused ultrasound therapy.

A passive method that uses time exposure acoustics (TEA) to map inertial cavitation activity in real time during therapeutic ultrasound exposure was recently presented [Gyongy and Coussios, IEEE Trans. Biomed. Eng. 57, 48–56 (2010)]. While this approach provides sub‐millimetric spatial resolution transversely to the high‐intensity focused ultrasound (HIFU) beam, its axial resolution requires improvement. Furthermore, the TEA method is expected to have a lower overall resolution and diminished ability to reject interference and noise compared to an adaptive beamforming approach. To address these limitations, we propose the use of the adaptive, robust Capon beamformer (RCB), which has been previously shown in the context of active, 2‐D ultrasound imaging to provide high resolution, good interference suppression, and robustness against steering vector errors. Using a multi‐element tomographic cavitation sensor being developed for quality assessment of clinical HIFU transducers, the effectiveness of RCB is de...