Dual-frequency transducer for nonlinear contrast agent imaging

Abstract

Detection of high-order nonlinear components issued from microbubbles has emerged as a sensitive method for contrast agent imaging. Nevertheless, the detection of these high-frequency components, including the third, fourth, and fifth harmonics, remains challenging because of the lack of transducer sensitivity and bandwidth. In this context, we propose a new design of imaging transducer based on a simple fabrication process for high-frequency nonlinear imaging. The transducer is composed of two elements: the outer low-frequency (LF) element was centered at 4 MHz and used in transmit mode, whereas the inner high-frequency (HF) element centered at 14 MHz was used in receive mode. The center element was pad-printed using a lead zirconate titanate (PZT) paste. The outer element was molded using a commercial PZT, and curved porous unpoled PZT was used as backing. Each piezoelectric element was characterized to determine the electromechanical performance with thickness coupling factor around 45%. After the assembly of the two transducer elements, hydrophone measurements (electroacoustic responses and radiation patterns) were carried out and demonstrated a large bandwidth (70% at -3 dB) of the HF transducer. Finally, the transducer was evaluated for contrast agent imaging using contrast agent microbubbles. The results showed that harmonic components (up to the sixth harmonic) of the microbubbles were successfully detected. Moreover, images from a flow phantom were acquired and demonstrated the potential of the transducer for high-frequency nonlinear contrast imaging.

Superharmonic imaging with chirp coded excitation: filtering spectrally overlapped harmonics

“…In this study, the SNR and penetration were improved by using coded excitation and increasing the total transmitted energy, but the available transducer bandwidth is still the biggest limitation for the superharmonic imaging. The commercial ultrasound probes do not have sufficient bandwidth; however the research on transducer and CMUT technology is focusing on increasing the transducer bandwidth and reception sensitivity [9]- [11], [15], [17], [18], [53], [54]. With the availability of very broad bandwidth (>150%) and dual frequency transducers, the superharmonic imaging can become a standard modality for high resolution imaging in the future.…”

“…By using a similar transducer and receiver pair working at a higher frequency range, such as the transducer designed by Guiroy et al [11], the benefits of the FChT filtering technique become increasingly significant. When the excitation frequency is increased, more harmonics will be generated due to nonlinear propagation at the same acoustic pressure level and depth.…”

“…Matte et al showed that the topology and frequency range of these interleaved transducers can be optimized for superharmonic imaging [10]. Use of dual frequency transducers is another option for superharmonic imaging, where Guiroy et al developed a transducer for imaging of high-order nonlinear harmonics generated by microbubbles [11]. The capacitive micromachined ultrasound transducers (CMUTs) can easily achieve > 120% fractional bandwidths, but usually have poor performance in terms of the output distortion and power [12]- [15].…”

Dual-Frequency Piezoelectric Transducers for Contrast Enhanced Ultrasound Imaging

“…Several similar transducers have recently been reported. A mechanically-scanned transducer with two concentric elements operating at 4 MHz and 14 MHz was demonstrated by Guiroy et al [ 70 ]. Li et al have alternatively demonstrated a micromachined PMN-PT 1–3 composite based dual-frequency (17.5/35 MHz) transducer ( Figure 4 ) for harmonic imaging [ 71 ].…”

Adaptive windowing in contrast-enhanced intravascular ultrasound imaging

“…Several other dual-frequency transducers for microbubble contrast agent imaging have recently been fabricated by our group as well as others [23–27]. Forming images from echoes near 30 MHz produces high resolution images of microbubbles.…”