Optimization of chirp reversal for ultrasound contrast imaging

Chirp reversal consists of transmitting first a chirp with upsweep frequencies (UPF) then a second chirp with downsweep frequencies (DNF). We have shown that contrast bubbles react differently to these chirps allowing the use of chirp reversal for ultrasound contrast imaging. The aim of the current study is to explore how chirp reversal can be optimized in terms of transmit frequency, bubble size, acoustic pressure and frequency bandwidth. Simulations were carried out to determine ultrasound parameters that provide largest differences in bubble's response to UPF and DNF chirps. The transmit frequency was varied from 0.5 MHz to 8 MHz, the bubble radius from 0.4 mu m to 8 mu m, the acoustic pressure from 10 kPa to 200 kPa and the frequency bandwidth from 30% to 65 %. Scattered pressures were calculated using a modified Rayleigh-Plesset equation. To evaluate the performance of chirp reversal for bubble detection, the echo from an UPF chirp was correlated to the time reversed echo from a DNF chirp for each scanning parameter. Experimental verification was carried out with the high speed camera Brandaris and BR.14 microbubbles (Bracco Research, Geneva) using frequencies from 1.3 to 2.8 MHz and pressures from 70 to 150 kPa. The correlation function showed maximum amplitude for bubbles at resonance and for bubbles far away from resonance. However a marked decrease in the correlation function amplitude occurred for bubbles that are around 62%, 80% and 142% of the resonance size, where the bubbles around 142% of the resonance size exhibited stronger decorrelation. Similar trend is seen as a function of frequency. Frequencies below the resonance frequency of the bubble induce a larger decorrelation than frequencies above the resonance frequency. The increase of transmitted bandwidth from 30% to 65% widens the decorrelation trough indicating the contribution of a large number of bubbles. Higher acoustic pressures, decrease further the amplitude of the correlation function especially for frequencies below the resonance frequency. This is likely attributed to extensive nonlinear activity of those bubbles that are below resonance. The Brandaris recordings comfort these simulations where bubbles above and below the resonance size provide different responses to UPF and DNF chirps. Transmit frequencies below resonance frequency provide larger decorrelation than frequencies above resonance. Bubbles oscillating at resonance and bubbles oscillating far away from resonance are not sensitive to a change in the frequency sweep of the chirp. Simulations and optical measurements allow for an optimization of chirp reversal and demonstrate a potential application for contrast imaging when appropriate scanning parameters are selected.