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Alex Peek

Ultrasound Engineer

Email

apeek@apl.uw.edu

Education

B.S. Physics, University of California Santa Barbara, 2014

M.S. Materials Science and Engineering, University of Washington, 2017

Publications

2000-present and while at APL-UW

Inertial cavitation behaviors induced by nonlinear focused ultrasound pulses

Bawiec, C.R., P.B. Rosnitskiy, A.T. Peek, A.D. Maxwell, W. Kreider, G.R. Ter Haar, O.A. Sapozhnikov, V.A. Khokhlova, and T.D. Khokhlova, "Inertial cavitation behaviors induced by nonlinear focused ultrasound pulses," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 68, 2884-2895, doi:10.1109/TUFFC.2021.3073347, 2021.

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1 Sep 2021

Inertial cavitation induced by pulsed high-intensity focused ultrasound (pHIFU) has previously been shown to successfully permeabilize tumor tissue and enhance chemotherapeutic drug uptake. In addition to HIFU frequency, peak rarefactional pressure, and pulse duration, the threshold for cavitation-induced bioeffects has recently been correlated with asymmetric distortion caused by nonlinear propagation, diffraction and formation of shocks in the focal waveform, and therefore with the transducer F-number. To connect previously observed bioeffects with bubble dynamics and their attendant physical mechanisms, the dependence of inertial cavitation behavior on shock formation was investigated in transparent agarose gel phantoms using high-speed photography and passive cavitation detection (PCD). Agarose phantoms with concentrations ranging from 1.5% to 5% were exposed to 1-ms pulses using three transducers of the same aperture but different focal distances (F-numbers of 0.77, 1.02, and 1.52). Pulses had central frequencies of 1, 1.5, or 1.9 MHz and a range of peak rarefactional pressure at the focus varying within 1–18 MPa. Three distinct categories of bubble behavior were observed as the acoustic power increased: stationary near-spherical oscillation of individual bubbles, proliferation of multiple bubbles along the pHIFU beam axis, and fanned-out proliferation toward the transducer. Proliferating bubbles were only observed under strongly nonlinear or shock-forming conditions regardless of frequency, and only where the bubbles reached a certain threshold size range. In stiffer gels with higher agarose concentrations, the same pattern of cavitation behavior was observed, but the dimensions of proliferating clouds were smaller. These observations suggest mechanisms that may be involved in bubble proliferation: enhanced growth of bubbles under shock-forming conditions, subsequent shock scattering from the gel–bubble interface, causing an increase in the repetitive tension created by the acoustic wave, and the appearance of a new growing bubble in the proximal direction. Different behaviors corresponded to specific spectral characteristics in the PCD signals: broadband noise in all cases, narrow peaks of backscattered harmonics in the case of stationary bubbles, and broadened, shifted harmonic peaks in the case of proliferating bubbles. The shift in harmonic peaks can be interpreted as a Doppler shift from targets moving at speeds of up to 2 m/s, which correspond to the observed bubble proliferation speeds.

Phase-aberration correction for HIFU therapy using a multielement array and backscattering of nonlinear pulses

Thomas, G.P.L., T.D. Khokhlova, C.R. Bawiec, A.T. Peek, O.A. Sapozhnikov, M. O'Donnell, and V.A. Khokhlova, "Phase-aberration correction for HIFU therapy using a multielement array and backscattering of nonlinear pulses," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 68, 1040-1050, doi:0.1109/TUFFC.2020.3030890, 2021.

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1 Apr 2021

Phase aberrations induced by heterogeneities in body wall tissues introduce a shift and broadening of the high-intensity focused ultrasound (HIFU) focus, associated with decreased focal intensity. This effect is particularly detrimental for HIFU therapies that rely on shock front formation at the focus, such as boiling histotripsy (BH). In this article, an aberration correction method based on the backscattering of nonlinear ultrasound pulses from the focus is proposed and evaluated in tissue-mimicking phantoms. A custom BH system comprising a 1.5-MHz 256-element array connected to a Verasonics V1 engine was used as a pulse/echo probe. Pulse inversion imaging was implemented to visualize the second harmonic of the backscattered signal from the focus inside a phantom when propagating through an aberrating layer. Phase correction for each array element was derived from an aberration-correction method for ultrasound imaging that combines both the beamsum and the nearest neighbor correlation method and adapted it to the unique configuration of the array. The results were confirmed by replacing the target tissue with a fiber-optic hydrophone. Comparing the shock amplitude before and after phase-aberration correction showed that the majority of losses due to tissue heterogeneity were compensated, enabling fully developed shocks to be generated while focusing through aberrating layers. The feasibility of using a HIFU phased-array transducer as a pulse-echo probe in harmonic imaging mode to correct for phase aberrations was demonstrated.

Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications

Peek, A.T., C. Hunter, W. Kreider, T.D. Khokhlova, P.B. Rosnitskiy, P.V. Yuldashev, O.A. Sapozhnikov, and V.A. Khokhlova, "Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications," J. Acoust. Soc. Am., 148, 3569-3580, doi:10.1121/10.0002877, 2020.

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1 Dec 2020

Aberrations induced by soft tissue inhomogeneities often complicate high-intensity focused ultrasound (HIFU) therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel and ballistic gel was built to mimic alternating layers of water-based and lipid tissues characteristic of an abdominal body wall and to reproducibly distort HIFU fields. The density, sound speed, and attenuation coefficient of each material were measured using a homogeneous gel layer. A surface with random topographical features was designed as an interface between gel layers using a 2D Fourier spectrum approach and replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for linear and nonlinear beam focusing and compared to the corresponding distortion induced by an ex vivo porcine body wall of the same thickness. Both spatial shift and widening of the focal lobe were observed, as well as dramatic reduction in focal pressures caused by aberrations. The results suggest that the phantom produced levels of aberration that are similar to a real body wall and can serve as a research tool for studying HIFU effects as well as for developing algorithms for aberration correction.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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