Adapting amplitude modulation to plane-wave non-linear Doppler imaging

High frame-rate plane-wave ultrasound allows one to combine Doppler and contrast-enhanced imaging without compromising the ensemble length, thus maintaining SNR and flow sensitivity. We have shown in previous work [1] that high frame-rate plane-wave contrast-enhanced imaging adds the vein and artery vessel morphology to microbubble perfusion imaging. However, our ability to differentiate perfusion from the tissue signal is still limited by the nonlinear propagation of ultrasound in tissue. One way to improve the contrast-to-tissue-ratio (CTR) could be to use a Doppler sequence based on amplitude modulation (AM), which compares the echoes from driving pulses of different amplitudes. As AM does not rely on phase inversion and is usually accomplished by apodisation of the transmit array, it often yields better tissue cancellation. In this work, we show that AM-Doppler (AMD) is feasible. We also demonstrate that AM performs much better than PI as the acoustic pressure is decreased below 200kPa due to the softening of the microbubble shell.