The use of microbubbles in Doppler ultrasound studies

Abstract

Ultrasound contrast agents (UCAs) are widely used in Doppler studies, either for simple echo enhancement purposes, or to increase the low signal-to-clutter ratio typical of microcirculation investigations. Common to all Doppler techniques, which are briefly reviewed in this paper, is the basic assumption that possible phase and amplitude changes in received echoes are only associated with UCA microbubble movements due to the drag force of blood. Actually, when UCAs are insonified, phenomena such as rupture, displacement due to radiation force, and acoustically driven deflation might influence the results of Doppler investigations. In this paper, we investigate the possible Doppler effects of such phenomena by means of a numerical simulation model and a special acousto-optical set-up which allows analysis of the behavior of individual microbubbles over relatively long time intervals. It is thus found that all phenomena produce evident Doppler effects in vitro, but that bubble displacement and deflation in particular, are not expected to significantly interfere with clinical measurements in standard conditions.

Flow Velocity Mapping Using Contrast Enhanced High-Frame-Rate Plane Wave Ultrasound and Image Tracking: Methods and Initial in Vitro and in Vivo Evaluation

“…The peak velocities obtained were 12.5, 46 and 68 cm/s; these are slightly higher than the analytical peak velocities derived from the measured flow rates obtained using a bucket and stopwatch, which were 10, 38 and 59 cm/s. The discrepancy is probably due to an imperfect setting of spectral Doppler angle and the broadening of the Doppler spectrum by microbubbles (Tortoli et al 2009). …”

Superharmonic microbubble Doppler effect in ultrasound therapy

“…Another distinct observation from previous studies is that their observed Doppler effect appeared as a broadening of the harmonic peak (Tortoli et al 2000, 2005) while we observed a distinct peak that can be quantified and characterized. The reasons are that previous studies examined the fundamental frequency and that the shifts were not due to continuous translation of the acoustic sources, but due to the inter-pulse microbubble movement during the off-time of the sequence (Tortoli et al 2009). Hence, whereas the instantaneous m s −1 velocities should translate into tens of kHz of frequency shift, the detected broadening was constrained to a few hundreds of Hz (Tortoli et al 2005, 2009).…”

“…The reasons are that previous studies examined the fundamental frequency and that the shifts were not due to continuous translation of the acoustic sources, but due to the inter-pulse microbubble movement during the off-time of the sequence (Tortoli et al 2009). Hence, whereas the instantaneous m s −1 velocities should translate into tens of kHz of frequency shift, the detected broadening was constrained to a few hundreds of Hz (Tortoli et al 2005, 2009). …”

“…at center frequencies within the range 2.5–8 MHz, pulse lengths of up to 10 cycles (or several microseconds) and pulse repetition frequencies on the order of kHz. Hence, the calculated velocities on the order of m s −1 were the instantaneous microbubble velocities during the on-time and the detected shifts were attributed to the inter-pulse movement of the cavitation nuclei (Tortoli et al 2009). In stark contrast with imaging applications, microbubble-mediated focused ultrasound therapies are typically conducted using low-frequency (<2 MHz) and long-pulse (>1 000 cycles) sonication (Ferrara et al 2007, Coussios and Roy 2008).…”

Special issue on microbubbles: from contrast enhancement to cancer therapy

“…As discussed in the paper by Tortoli et al [8] they are widely used in Doppler studies, especially in the microcirculation, to improve the signal-to-noise ratio. Interpretation of the microbubble signals can however be complicated not only by their non-linearity, but also by a variety of phenomena including destruction, deflation and displacement of microbubbles due to radiation force.…”