Improved Contrast-Enhanced Ultrasound Imaging With Multiplane-Wave Imaging

Abstract

Contrast-enhanced ultrasound (CEUS) imaging has great potential for use in new ultrasound clinical applications such as myocardial perfusion imaging and abdominal lesion characterization. In CEUS imaging, contrast agents (i.e., microbubbles) are used to improve contrast between blood and tissue because of their high nonlinearity under low ultrasound pressure. However, the quality of CEUS imaging sometimes suffers from a low signal-to-noise ratio (SNR) in deeper imaging regions when a low mechanical index (MI) is used to avoid microbubble disruption, especially for imaging at off-resonance transmit frequencies. In this paper, we propose a new strategy of combining CEUS sequences with the recently proposed multiplane wave (MW) compounding method to improve the SNR of CEUS in deeper imaging regions without increasing MI or sacrificing frame rate. The MW-CEUS method emits multiple Hadamard-coded CEUS pulses in each transmission event (i.e., pulse-echo event). The received echo signals first undergo fundamental bandpass filtering (i.e., the filter is centered on the transmit frequency) to eliminate the microbubble’s second harmonic signals because they cannot be encoded by pulse inversion. The filtered signals are then Hadamard decoded and re-aligned in fast time to recover the signals as they would have been obtained using classic CEUS pulses, followed by designed recombination to cancel the linear tissue responses. The MW-CEUS method significantly improved contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR) of CEUS imaging by transmitting longer coded pulses. The image resolution was also preserved. The microbubble disruption ratio and motion artifacts in MW-CEUS were similar to those of classic CEUS imaging. In addition, the MW-CEUS sequence can be adapted to other transmission coding formats. These properties of MW-CEUS can potentially facilitate CEUS imaging for many clinical applications, especially assessing deep abdominal organs or the heart.

On Combination of Hadamard-Encoded Multipulses and Multiplane Wave Transmission in Contrast-Enhanced Ultrasound Imaging

“…As previously demonstrated in [10], MW-AM did not introduce extra microbubble disruption or decorrelation as compared to classic AM at low MI . It is expected that MW-HEM would behave similarly as MW-AM in the context of microbubble disruption and decorrelation.…”

“…Another possible reason for the higher CTR in MW-HEM is that the tissue residue signals in MW-HEM mainly existed at the 2 nd - harmonic frequency which could be largely suppressed by the fundamental bandpass filter [3]. For MW-AM, as analyzed in [10], the tissue residue signals contains nonlinear energy within the fundamental bandwidth which could not be removed by the fundamental bandpass filter. Such component increases with Hadamard coding orders, undermining the CTR improvement in MW-AM.…”

“…Such component increases with Hadamard coding orders, undermining the CTR improvement in MW-AM. Therefore, the Hadamard coding order selection in MW-AM is a tradeoff between microbubble signal improvement and tissue harmonic amplification [10]. In contrast, MW-HEM is more flexible in order selection to improve the CEUS image quality.…”

“…In addition to MW-HEM, HEM and classic AM implemented with coherent plane wave compounding, and MW-AM [10] images were also obtained as benchmarks. MI was set to 0.1.…”

“…In our previous work [10], we have described a novel strategy of combining classic CEUS sequences with MW compounding to increase the microbubble responses in plane-wave-based CEUS imaging. In this study, we will extend the combination strategy to HEM and MW transmissions following a similar strategy.…”