Optimization of multi-pulse sequences for nonlinear contrast agent imaging using a cMUT array

Abstract

Capacitive micromachined ultrasonic transducer (cMUT) technology provides advantages such as wide frequency bandwidth, which can be exploited for contrast agent imaging. Nevertheless, the efficiency of traditional multi-pulse imaging schemes, such as pulse inversion (PI), remains limited because of the intrinsic nonlinear character of cMUTs. Recently, a new contrast imaging sequence, called bias voltage modulation sequence (BVM), had been specifically developed for cMUTs to suppress their unwanted nonlinear behavior. In this study, we propose to optimize contrast agent detection by combining the BVM sequence with PI and/or chirp reversal (CR). An aqueous dispersion of lipid encapsulated microbubbles was exposed to several combinations of multi-pulse imaging sequences. Approaches were evaluated in vitro using 9 inter-connected elements of a cMUT linear array (excitation frequency of 4 MHz; peak negative pressure of 100 kPa). For sequences using chirp excitations, a specific compression filter was designed to compress and extract several nonlinear components from the received microbubble responses. A satisfactory cancellation of the nonlinear signal from the source is achieved when BVM is combined with PI and CR. In comparison with PI and CR imaging modes alone, using sequences incorporating BVM increases the contrast-to-tissue ratio by 10.0 dB and 4.6 dB, respectively. Furthermore, the combination of BVM with CR and PI results in a significant increase of the contrast-to-noise ratio (+29 dB). This enhancement is attributed to the use of chirps as excitation signals and the improved preservation of several nonlinear components contained within the contrast agent response.

Dual-frequency acoustic droplet vaporization detection for medical imaging

“…It is important to note that the ‘pulse-high, listen-low’ scheme was designed to exclude microbubble signals. However, many proven microbubble specific imaging techniques for vascular applications, such as pulse inversion [6], amplitude modulation [7], and chirp reversal [9] are capable of generating CTRs in the same range as seen here for droplets [57]. As droplets continue to gain momentum as theranostic agents, the ability to map their conversion to microbubbles in real-time using the dual frequency technique could concurrently become an important tool in clinical practice.…”

Visualization of Microvascular Angiogenesis Using Dual-Frequency Contrast-Enhanced Acoustic Angiography: A Review

“…CMUTs are microfabricated devices that can be produced with a greater bandwidth than piezoelectric transducers and may provide an alternative solution to achieve the ultra-wide bandwidth needed for acoustic angiography. CMUTs have been previously utilized for ultra-wideband imaging of other multi-frequency events, such as acoustic droplet vaporization (Novell et al 2016), although they have inherent nonlinear behavior that requires extra processing to separate transmitted content from that scattered by microbubbles (Novell et al 2015).…”

Evaluation of capacitive micromachined ultrasonic transducers for passive monitoring of microbubble-assisted ultrasound therapies

“…21 In this condition, the modulation of the bias voltage is an interesting option that can be exploited to reduce the nonlinear CMUT behavior. 24 However, reducing the bias voltage would result in the deterioration of the signal-to-noise ratio (SNR). Thus, a compromise between acceptable nonlinearity and CMUT performance must be found.…”