Objectives
Ultrasound imaging using contrast agents is a promising tool for many clinical applications. Ultrasound contrast agents (UCA) can emit nonlinear harmonic signals. These nonlinear harmonic signals are filtered to distinguish microbubbles from surrounding tissues, based on the assumption that the propagation of ultrasound in tissue is linear, so nonlinear signals only come from UCA. However, in practice, the generated harmonics during wave propagation in tissue lead to a limited contrast-to-tissue ratio (CTR). This project aims at increasing the CTR in ultrasound contrast imaging, by designing transmission strategies.
Generalization of multi-pulse techniques
In order to increase the contrast-to-tissue ratio (CTR) in contrast imaging or the signal-to-noise ratio (SNR) in tissue harmonic imaging, many multi-pulse transmission techniques have been proposed. Here, a mathematical background to generalize most of the multi-pulse ultrasound imaging techniques is presented.
The resulting nonlinear components in each frequency band depend on the choice of parameter b and c. Simulation results on several multi-pulse techniques agree with the results given in previous literatures. The presented formulation can be used to predict the nonlinear components in each frequency band, and to design new transmissions.
Figure 1: Spectral analysis of the RF lines in the UCA region, from the one-pulse, pulse inversion (PI) [b=(1,-1),c=(1,1)], amplitude modulation (AM) [b=(1,0.5),c=(1,-2)] and pulse inversion amplitude modulation (PIAM) [b=(1,-0.5),c=(1,2)] images, spectra were normalized to the maximum spectral amplitude of the one-pulse image.
Influence of scatterer motion in multi-pulse techniques
Techniques based on multiple transmissions are generally based on the response of static scatterers inside the imaged region. However, scatterer motion, for example in blood vessels, has an inevitable influence on multi-pulse techniques, which can either upgrade or degrade the technique involved. This research investigates the influence of scatterer motion on multi-pulse techniques.
Simulations, in-vitro experiments from a single bubble and clouds of bubbles, and in-vivo experiments from white rats show that the phase shift of the echoes backscattered from bubbles depends on the transmissions’ phase shift, but also on the bubble motion which influences the efficiency of multi-pulse techniques: fundamental and second-harmonic amplitudes of the summed signal change periodically, exhibiting maximum or minimum values, according to scatterer motion. Furthermore, experimental results based on the second-harmonic inversion (SHI) technique
Figure 2: In-vitro experimental results from a single UCA bubble (BR14) moving in the axial direction. Average (dots) and standard deviation (bars) of the second-harmonic amplitude of SHI (normalized to the classic second-harmonic amplitude and expressed in dB) versus PRI between two 90° phase-shifted pulses (lower axis) or versus the bubble motion, normalized to the transmitted wavelength (upper axis).
Video of in-vitro experimental SHI images of a fluid phantom, with circulating UCA in the tube, acquired with different PRF. The nominal average flow velocity was 8.3 ± 0.7 cm/s.