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Randy Williams

Postdoctoral Scholar






2000-present and while at APL-UW

The histotripsy spectrum: Differences and similarities in techniques and instrumentation

Williams, R.P., J.C. Simon, V.A. Khokhlova, O.A. Sapozhnikov, and T.D. Khokhlova, "The histotripsy spectrum: Differences and similarities in techniques and instrumentation," Int. J. Hyperthermia, 40, doi:10.1080/02656736.2023.2233720, 2023.

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17 Jul 2023

Since its inception about two decades ago, histotripsy — a non-thermal mechanical tissue ablation technique — has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Dual-mode 1D linear ultrasound array for image-guided drug delivery enhancement without ultrasound contrast agents

Williams, R.P., M.M. Karzova, P.V. Yuldashev, A.Z. Kaloev, F.A. Nartov, V.A. Khokhlova, B.W. Cunitz, K.P. Morrison, and T.D. Khokhlova, "Dual-mode 1D linear ultrasound array for image-guided drug delivery enhancement without ultrasound contrast agents," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 70, 693-707, doi:10.1109/TUFFC.2023.3268603, 2023.

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19 Apr 2023

Pulsed high-intensity focused ultrasound (pHIFU) uses nonlinearly distorted millisecond-long ultrasound pulses of moderate intensity to induce inertial cavitation in tissue without administration of contrast agents. The resulting mechanical disruption permeabilizes the tissue and enhances the diffusion of systemically administered drugs. This is especially beneficial for tissues with poor perfusion such as pancreatic tumors. Here we characterize the performance of a dual-mode ultrasound array designed for image-guided pHIFU therapies in producing inertial cavitation and ultrasound imaging. The 64-element linear array (1.071 MHz, aperture of 14.8 mm x 51.2 mm, and pitch of 0.8 mm) with elevational focal length of 50 mm was driven by the Verasonics V-1 ultrasound system with extended burst option. The attainable focal pressures and electronic steering range in linear and nonlinear operating regimes (relevant to pHIFU treatments) were characterized through hydrophone measurements, acoustic holography, and numerical simulations. The steering range at ±10% from the nominal focal pressure was found to be ±±6 mm axially and ±11 mm azimuthally. Focal waveforms with shock fronts of up to 45 MPa, and peak negative pressures up to 9 MPa were achieved at focusing distances of 38–75 mm from the array. Cavitation behaviors induced by isolated 1 ms pHIFU pulses in optically transparent agarose gel phantoms were observed by high-speed photography across a range of excitation amplitudes and focal distances. For all focusing configurations the appearance of sparse, stationary cavitation bubbles occurred at the same P_ threshold of 2 MPa. As the output level increased, a qualitative change in cavitation behavior occurred, to pairs and sets of proliferating bubbles. The pressure P_ at which this transition was observed corresponded to substantial nonlinear distortion and shock formation in the focal region and was thus dependent on the focal distance of the beam ranging within 3–4 MPa for azimuthal F-numbers of 0.74 to 1.5. The array was capable of B-mode imaging at 1.5 MHz of centimeter-sized targets in phantoms and in vivo pig tissues at depths of 3 cm to 7 cm, relevant to pHIFU applications in abdominal targets.

An air-coupled electrostatic ultrasound transducer using a MEMS microphone architecture

Niu, X., Z. Liu, Y. Meng, C.M. Hodges, R.P. Williams, and N.A. Hall, "An air-coupled electrostatic ultrasound transducer using a MEMS microphone architecture," J. Microelectromechan. Syst., 31, 813-819, doi:10.1109/JMEMS.2022.3190714, 2022.

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20 Jul 2022

We demonstrate the transmission of ultrasound in air using a device that resembles a MEMS microphone in its construction. The device has a 1 mm diameter diaphragm and uses a perforated backplate for electrostatic actuation. The device is driven with large amplitude AC signals, with peak values that exceed the pull-in voltage of the diaphragm. In doing so, relatively large diaphragm displacements are achieved, as the diaphragm oscillations traverse the complete diaphragm-backplate gap. Large amplitude diaphragm vibration is advantageous for high SPL applications in air, as sound pressure is directly proportional to diaphragm displacement for a given operating frequency. Transient diaphragm displacement waveforms are measured in response to tone-burst waveforms ranging in frequency from 4 kHz to 97 kHz. Resultant acoustic pressure waveforms in air are made using a broadband microphone.

More Publications

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