Self-demodulation of high-frequency ultrasound

Abstract

High-frequency (>10 MHz) ultrasound is used in, e.g., small animal imaging or intravascular applications. Currently available ultrasound contrast agents (UCAs) have a suboptimal response for high frequencies. This study therefore investigates the nonlinear propagation effects in a high-frequency ultrasound field (25 MHz) and its use for standard UCA and diagnostic frequencies (1-3 MHz). Nonlinear mixing of two high-frequency carrier waves produces a low-frequency wave, known as the self-demodulation or parametric array effect. Hydrophone experiments showed that the self-demodulated field of a focused 25 MHz transducer (850 kPa source pressure) has an amplitude of 45 kPa at 1.5 MHz in water. Such pressure level is sufficient for UCA excitation. Experimental values are confirmed by numerical simulations using the Khokhlov-Zabolotskaya-Kuznetsov equation on a spatially convergent grid.

Acoustic droplet vaporization is initiated by superharmonic focusing

“…1, are available to calculate the nonlinear pressure in the absence of a droplet. We use a numerical axisymmetric implementation written in C programming language developed in-house (37). It has been validated with calibrated hydrophone measurements and was found to be able to represent the experimental pressure waveform with a precision of 10% (37).…”

“…We use a numerical axisymmetric implementation written in C programming language developed in-house (37). It has been validated with calibrated hydrophone measurements and was found to be able to represent the experimental pressure waveform with a precision of 10% (37). The result of the simulation is the pressure field at the acoustic focus as a function of the position, time, and frequency for any given transducer geometry (i.e., diameter and focal distance).…”

Subharmonic, Non-linear Fundamental and Ultraharmonic Imaging of Microbubble Contrast at High Frequencies

“…The S-D signal is a low-frequency signal component produced by weakly non-linear propagation of an ultrasound wave. It is proportional to the second time derivative of the squared envelope of the transmitted signal (Averkiou et al 1993;Berktay 1965;Daeichin et al 2012;Vos et al 2010). Rectangular envelopes were found to produce strong SH stimulation of the UCA.…”

Computation of nonlinear ultrasound fields using a linearized contrast source method

“…The presence of bubbles causes a strong increase of the nonlinear behavior of the solution, and imaging of the bubble population is either performed at a higher harmonic 9,10 or at the subharmonic, 11 i.e., half the fundamental frequency. In the context of contrast enhanced ultrasound, there has also been a recent interest in the phenomenon of self-demodulation, 12 which involves the frequency band below the fundamental. Exploitation of the possibilities of nonlinear acoustics requires the development of novel imaging modalities and devices.…”