In vitro comparative study of the performance of pulse sequences for ultrasound contrast imaging of the carotid artery

This study intends to identify the best pulse sequences for ultrasound contrast imaging of the carotid artery by comparing a fairly exhaustive list of pulse sequences reported in literature. A good candidate must provide sensitive detection of contrast microbubbles, efficient suppression of tissue echoes and prevent artifacts. Especially the far wall artifact caused by nonlinear propagation through contrast agent dramatically impairs the detection of microbubbles in any region located behind a vessel. Pulse inversion, amplitude modulation, chirp reversal, subharmonic imaging, pulse subtraction time delay, radial modulation and ring-down imaging are investigated in vitro. A focused single-element transducer used in pulse-echo mode transmits ultrasound waves and receive backscattered signals in the frequency range of 3 MHz to 8 MHz. Transmitted waveforms are designed with a low mechanical index between 0.1 and 0.2. In a water tank, the transducer acquires a single line through a thin-wall tube containing diluted (1:5000) contrast agent (SonoVue, Bracco). A thin metallic wire is placed in front of the tube and a piece of rubber is positioned behind the tube to evaluate the contrast-to-tissue ratio (CTR) and the contrast-to-artifact ratio (CAR), respectively. The contrast-to-noise ratio (CNR) is determined by comparing backscattered signals from contrast agent with that of pure water. Only subharmonic imaging and ring-down imaging are free from the far wall artifact. Summing CNR, CAR and CTR, ring-down imaging (Hansen et al., IEEE Trans. UFFC, vol. 58(2), 2011) turns out to be the best candidate (CNR+CAR+CTR = 18 dB).