Detection of Contrast Agents: Plane Wave Versus Focused Transmission

Ultrasound contrast agent (UCA) imaging provides a cost-effective diagnostic tool to assess tissue perfusion and vascular pathologies. However, excessive transmission (TX) levels may negatively impact both uniform diffusion and survival rates of contrast agents, limiting their density and thus their echogenicity. Contrast detection methods with both high sensitivity and low-contrast destruction rate are thus essential to maintain diagnostic capabilities. Plane-wave TX with a high number of compounding angles has been suggested to produce good quality images at pressure levels that do not destroy UCA. In this paper, we performed a quantitative evaluation of detection efficacy of flowing UCA with either traditional focused scanning or ultrafast plane-wave imaging. Amplitude modulation (AM) at nondestructive pressure levels was implemented in the ULA-OP ultrasound research platform. The influence of the number of compounding angles, peak-negative pressure, and flow speed on the final image quality was investigated. Results show that the images obtained by compounding multiple angled plane waves offer a greater contrast (up to a 12-dB increase) with respect to focused AM. This increase is attributed mainly to noise reduction caused by the coherent summation in the compounding step. Additionally, we show that highly sensitive detection is already achieved with a limited compounding number ($N <; 16$), thus suggesting the feasibility of continuous contrast monitoring at a high frame rate. This capability is essential to properly detect contrast agents flowing at high speed, as an excessive angle compounding is shown to be destructive for the contrast signal, as the UCA motion quickly causes loss of correlation between consecutive echoes.

I. Introduction

In the 1980s, it was believed that ultrasound contrast agents (UCAs) could be sufficiently detected and imaged with the conventional imaging methods nowadays referred to as fundamental imaging. Later, it was acknowledged that newer imaging techniques based on specific properties of the UCA proved to be more sensitive. In general, these new characteristics involve nonlinear and transient characteristics of contrast agents that appear at the high end of the diagnostic acoustic intensity [1]. Imaging modalities used today for UCA detection are, besides fundamental imaging, harmonic imaging, most notably second harmonic imaging, and subharmonic imaging; multipulse imaging, such as pulse-inversion; and Doppler-based methods, such as power Doppler. Some advanced modalities may combine two or more strategies to improve the performance, as is the case in harmonic power Doppler and pulse inversion Doppler. The methods are either destructive [high mechanical index (MI)] or nondestructive (low MI). The destructive method results in a very good contrast to tissue ratio, but requires triggered imaging and is therefore not often used. So, imaging of UCAs is currently performed at a low MI [2].