Dual high-frequency difference excitation for contrast detection

Abstract

Stimulating high-frequency nonlinear oscillations of ultrasound contrast agents is helpful to distinguish microbubbles from background tissues. Nevertheless, inefficiency of such oscillations from most commercially available contrast agents and intense attenuation of the resultant high-frequency harmonics limit microbubble detection with high-frequency ultrasound. To avoid this high-frequency nature, we devised and explored a dual-frequency difference excitation technique to induce efficiently low-frequency, rather than high-frequency, nonlinear scattering from microbubbles by using high-frequency ultrasound. The proposed excitation pulse is comprised of 2 high-frequency sinusoids with frequency difference subject to the microbubble resonance frequency. Its envelope, with frequency being the difference between the 2 frequencies, is used to stimulate nonlinear oscillation of microbubbles for the consonant low-frequency harmonic generation, whereas high-imaging resolution is retained because of narrow high-frequency transmit beams. Hydrophone measurements and phantom experiments of speckle-generating flow phantoms were performed to demonstrate the efficacy of the proposed technique. The results show that, especially when the envelope frequency is near the microbubbleiquests resonance frequency, the envelope of the proposed excitation pulse can induce significant nonlinear scattering from microbubbles, the induced nonlinear responses tend to increase with the pulse pressures, and up to 26 dB and 36 dB contrast-to-tissue ratios with second- and fourth-order nonlinear responses, respectively, can be obtained. Potential applications of this method include microbubble fragmentation and cavitation with high-frequency ultrasound.

Broadband Reduction of the Second Harmonic Distortion During Nonlinear Ultrasound Wave Propagation

“…The detection of UCA can be also improved by taking into consideration the inherent property of an insonified microbubble to generate a so-called subharmonic response, when the spectrum of the backscattered ultrasound echoes, comprise of components at the half of the fundamental frequency (Shankar et al 1998;Shi et al 1999). Yeh et al (2008) considered a dual high-frequency difference excitation technique for contrast detection. A highfrequency wave, arising in the propagating medium as a sum of two frequency excitation bursts, contains the spectral component close to the UCA resonance frequency and is used for the excitation waveform.…”

Self-demodulation of high-frequency ultrasound

“…[18][19][20][21] Due to a nonlinear response of bubbles, the two frequencies would produce sum-and-difference components, which can be detected. For example, two studies exploited the beat frequency f b of two carrier waves f 1 and f 2 , where f b Ӷ f 1 , f 2 .…”

Microbubble destruction by dual-high-frequency ultrasound excitation

“…We have previously proposed a dual-frequency excitation technique for efficiently inducing low-frequency nonlinear oscillation from microbubbles using high-frequency ultrasound [13]. In this study, the dual-frequency excitation technique was demonstrated for its effects on the microbubble destruction threshold under different insonation conditions.…”

“…Moreover, the image lateral resolution remains high because the microbubbles are insonified by the narrow highfrequency beam [13]. Because the proposed dual-frequency excitation scheme requires longer pulse length to excite microbubbles, the resultant axial resolution is poor.…”

“…specifically, the carrier waveform alternates between negative and positive values many times in one cycle of the modulation term (assuming a sufficiently large difference between ω 1 and ω 2 ), which creates carrier amplitude maxima from both the negative and positive regions of the modulation term, making the overall envelope frequency double the modulation frequency. note that excitation by an envelope component at the difference frequency serves as a low-frequency excitation to the microbubbles [13]. This low-frequency envelope component with a frequency equal to the difference of 2 high-frequency sinusoids can excite microbubbles, because most commercial contrast-agent microbubbles resonate at frequencies of about several megahertz [18].…”