Exploitation of capacitive micromachined transducers for nonlinear ultrasound imaging

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) present advantages such as wide frequency bandwidth, which could be further developed for nonlinear imaging. However, the driving electrostatic force induces a nonlinear behavior of the CMUT, thus generating undesirable harmonic components in the generated acoustic signal. Consequently, the use of CMUT for harmonic imaging (with or without contrast agents) becomes challenging. This paper suggests 2 compensation approaches, linear and nonlinear methods, to cancel unwanted nonlinear components. Furthermore, nonlinear responses from contrast agent were evaluated using CMUT in transmit before and after compensation. The results were compared with those obtained using a PZT transducer in transmit. Results showed that CMUT nonlinear behavior is highly influenced by the excitation to bias voltage ratio. Measurements of output pressure very close to the CMUT surface allow the estimation of optimal parameters for each compensation approach. Both methods showed a harmonic reduction higher than 20 dB when one element or several elements are excited. In addition, the study demonstrates that nonlinear approach seems to be more efficient because it is shown to be less sensitive to interelement variability and further avoids fundamental component deterioration. The results from contrast agent measurements showed that the responses obtained using CMUT elements in transmit with compensation were similar to those from PZT transducer excitation. This experimental study demonstrates the opportunity to use CMUT with traditional harmonic contrast imaging techniques.

Wideband acoustic activation and detection of droplet vaporization events using a capacitive micromachined ultrasonic transducer

“…Indeed, upon certain excitation conditions (e.g., low frequency and high voltage), the acoustic wave emitted from the CMUT can be distorted by the presence of higher frequency components generated by the transducer. 34,35 Although this limitation is particularly deleterious for harmonic imaging, various approaches such as the use of predistorted waveforms, 34,36 the development of specific pulse schemes 35 or by the addition of a series impedance to the CMUT capacitance, 37 have been recently proposed to suppress the undesired harmonic components. Here, the ADV signal will not be affected by nonlinear distortion because it is received well below the excitation frequency.…”

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“…Tissue harmonic imaging is a widely used imaging mode [18][19][20][21][22]. The ultrasound system sends out bursts of ultrasound at the fundamental frequency.…”

“…Compared to traditional PZT transducers, CMUT is at a disadvantage in THI mode because of the nonlinear transmit property from its electrostatic actuation mechanism [19,20], where the actuation force (hence the generated acoustic pressure)…”

“…The method is heavily dependent on detailed CMUT transmit properties and its bias voltage, requiring complicated and frequent calibration. Subharmonic driving is also tried in [19], but because CMUT has a DC bias voltage, the emitted acoustic pulse still contains the sub-harmonic frequency content which becomes an additional interference.…”

“…For example, the Harmonic Imaging mode uses the second harmonic of the pulse to provide high resolution images [18][19][20][21][22]; the Power Mode Doppler visualizes the magnitude of Doppler signal, rather than the frequency 1 It is important to point out that in the PW mode, the Doppler effect does not come from the frequency shift of a single received echo pulse, since a short pulse is broadband, and therefore it is difficult to detect the small Doppler frequency shift (typically less than 100KHz). Besides, the frequency-dependent attenuation through the tissue complicates the task even more.…”