High frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) color flow (CF) imaging [1]. In this paper we build upon this work by examining the performance of SH CF as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1 mm diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity((TM)). The phantom results show that there is an identifiable pressure range where accurate SH CF and PWD velocity estimates can be made, above which velocity estimates are biased by radiation force effects and bubble disruption, and below which velocity estimates are degraded by poor SNR These results support the findings of the previous feasibility study. Further, the PWD data demonstrates the feasibility of high frequency SH PWD. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear and blood velocity estimates compared well with fundamental mode PWD. The ability to suppress tissue signals using SH signals may improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.