Chirp reversal ultrasound contrast imaging

Aim: We present a new contrast imaging approach based on chirps named chirp reversal contrast imaging. The technique consists of transmitting a first excitation signal being a chirp of increasing frequency with time (the so-called upsweep) and a second excitation signal, the downsweep, being a replica of the first signal, but time reversed with a sweep of decreasing frequency with time*. Method: Simulations and optical observations were carried out to explore the potential of the chirp reversal approach in detecting microbubbles. Simulations using a Rayleigh-Plesset equation were performed considering various microbubbles excited with chirps at 1.7 MHz center frequency and 50% bandwidth. Optical observations with the Brandaris camera were carried out using BR 14 bubbles of radii from I pm to 5 gm. Chirps with center frequencies of 1.7 MHz and 50% bandwidth were transmitted with peak negative pressures ranging from 70 kPa to 200 kPa. Results: Simulations showed that for larger bubbles (>2um), significant differences occur between upsweep chirp response and down sweep response at 1.7MHz transmit frequency. Optical observations confirmed these results. From the optical radius-time curves, the larger bubbles showed different dynamics when up-sweep or down-sweep frequencies were used in transmission. Up-sweep excitation chirps produce highly damped responses with large amplitude excursions whereas the response to downsweep chirps showed a pronounced resonance behavior with smaller amplitudes. Smaller bubbles (<2um) appear to be less sensitive to frequency sweep at 1.7 MHz transmit frequency. However, driven at a higher center frequency, smaller bubbles tend to be more sensitive. Conclusions: Experimental and theoretical data confirm that chirp reversal is feasible and can be used to detect contrast microbubbles and to improve the contrast to tissue ratio.