Evaluation of chirp reversal power modulation sequence for contrast agent imaging

Abstract

Over the last decade, significant research effort has been focused on the use of chirp for contrast agent imaging because chirps are known to significantly increase imaging contrast-to-noise ratio (CNR). New imaging schemes, such as chirp reversal (CR), have been developed to improve contrast detection by increasing non-linear microbubble responses. In this study we evaluated the contrast enhancement efficiency of various chirped imaging sequences in combination with well-established imaging schemes such as power modulation (PM) and pulse inversion (PI). The imaging schemes tested were implemented on a fully programmable open scanner and evaluated by ultrasonically scanning (excitation frequency of 2.5 MHz; amplitude of 350 kPa) a tissue-mimicking flow phantom comprising a 4 mm diameter tube through which aqueous dispersions (dilution fraction of 1/2000) of the commercial ultrasound contrast agent, SonoVue(®) were continuously circulated. The recovery of non-linear microbubble responses after chirp compression requires the development and the optimization of a specific filter. A compression filter was therefore designed and used to compress and extract several non-linear components from the received microbubble responses. The results showed that using chirps increased the image CNR by approximately 10 dB, as compared to conventional Gaussian apodized sine burst excitation but degraded the axial resolution by a factor of 1.4, at -3 dB. We demonstrated that the highest CNR and contrast-to-noise ratio (CTR) were achievable when CR was combined with PM as compared to other imaging schemes such as PI.

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

“…Nevertheless, the efficiency of other multi-pulse approaches based on nonlinear detection processes, such as PI and CR, remains limited by the cMUT nonlinear behavior. Moreover, as previously reported in (Phillips 2001, Eckersley et al 2005, Novell et al 2014b), the combination of several approaches (e.g., PIPM, CRPM) increased the sensitivity for detecting nonlinear responses from microbubbles. In this context, our objective in this paper was to combine the BVM sequence with these approaches to (i) suppress the undesirable harmonic component generated by the cMUT, (ii) optimize contrast agent detection.…”

“…In the case of BVM and CR strategies, the recovery of more than one frequency component ( e.g., f 0 , 2f 0 ) improves contrast agent detection (Novell et al 2014b, Novell et al 2014a). Because the compression filter acts as a bandpass filter, it must be optimized for nonlinear contrast imaging.…”

“…Because the compression filter acts as a bandpass filter, it must be optimized for nonlinear contrast imaging. For this reason and as previously described in (Novell et al 2014b), the compression filter was composed of the sum of two chirps: one identical to the excitation signal ( i.e., x upf or x dnf ) centered at the fundamental frequency f 0 and another centered at the 2f 0 frequency to extract both the fundamental and the harmonic components. The frequency modulation of the 2f 0 component was also equal to $\frac{\Delta f}{\Delta t}=\frac{2\mathrm{MHz}}{4\mu \mathrm{s}}$.…”

“…In the past, CR and CRPM have been performed using traditional piezoelectric transducers (Novell et al 2014b). While successful at improving the signal strength from microbubbles, the narrow frequency bandwidth of piezoelectric transducers only excites microbubbles with a resonant frequency near the center frequency of the transducer; this represents a small fraction of the entire microbubble population.…”

Targeted ultrasound contrast agents for ultrasound molecular imaging and therapy

“…However, performance of these imaging methods is degraded because the excitation frequency is much higher than the resonance frequency of the MBs, attenuation is higher, and far-wall artefacts are a big challenge to overcome [112][113][114][115][116]. Therefore, improved imaging methods have been extensively studied, such as improved harmonic imaging methods [117][118][119], chirp coded excitation alone [120,121] or combined with pulse inversion [122,123]. Among the non-linear components of the MB response the SH signal has drawn much attention lately, due to its MB specificity and artefact-free characteristics.…”