2C-4 Chirp Reversal Ultrasound Contrast Imaging

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. 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 BR14 bubbles of radii from 1 mum to 5 mum. Chirps with center frequencies of 1.7 MHz and 50% bandwidth were transmitted with peak negative pressures ranging from 70 kPa to 200 kPa. Simulations showed that for larger bubbles (>2 mum), significant differences occur between upsweep chirp response and down sweep response at 1.7 MHz transmit frequency. Optical observations confirmed these results. From the optical radius-time curves, the larger bubbles showed different dynamics when upsweep or downsweep frequencies were used in transmission. Upsweep 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 (<2 mum) 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. 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