Hadamard-encoded multi-pulses for contrast-enhanced ultrasound imaging
The development of 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, the contrast agents (i.e., microbubbles) are used to improve the contrast between blood and tissue because they reflect high nonlinear energy under low ultrasound pressure. Several classic CEUS pulse sequences are widely used such as pulse inversion, amplitude modulation, and pulse inversion amplitude modulation combined. They generally involve two individual pulses which are emitted separately in two transmission events (i.e., pulse-echo events). However, single pulse emission with low acoustic pressure may result in low signal-to-noise ratio (SNR), especially in deeper imaging region. In this paper, we propose a new CEUS pulse sequence by combining Hadamard-encoded multi-pulses (HEM) with fundamental frequency bandpass filter (i.e., filter centered on transmit frequency). HEM consecutively emits multi-pulses encoded by a 2nd-order Hadamard matrix in each of the two transmission events rather than individual pulses in two separate events. The microbubble responses can thus be improved by the longer transmit pulse and the tissue harmonics can be suppressed by the fundamental frequency filter, leading to significantly improved SNR and contrast-to-tissue ratio (CTR). In addition, the fast polarity change between consecutive coded pulse emissions excites strong nonlinear microbubble echoes, further enhancing the CEUS image quality. The spatial resolution of HEM image is compromised as compared to other CEUS methods due to the longer transmit pulses and the lower imaging frequency (i.e., fundamental frequency). However, the resolution loss was shown to be negligible and could be offset by the significantly enhanced CTR and SNR. These properties of HEM can potentially facilitate robust CEUS imaging for a wide spectrum of clinical applications, especially for deep abdominal organs and heart.
