High contrast ultrasound imaging of very low frequency (100 kHz) modulated microbubbles

In applications such as liver or transcranial imaging, contrast-enhanced ultrasound (CEUS) imaging is challenged by high levels of patient-specific attenuation and physiological motion. To generate high contrast images of targets below highly attenuating tissues, we proposed and investigated the method of using very low frequency (100 kHz) ultrasound to modulate and image microbubbles at 2 or 5 MHz. Results of free field of imaging microbubbles in water indicate that it is feasible to use 100 kHz ultrasound to modulate the scattering amplitude of microbubbles. Images acquired in a phantom indicate that the proposed method provides significantly higher contrast-to-tissue ratio (22.7 to 24.6 dB) than amplitude modulation pulse inversion (10.4 to 11.9 dB). In addition, contrast degradation due to tissue motion was overcome by singular value decomposition-based clutter filter. Transcranial images obtained using 2 MHz imaging pulses show significant contrast enhancement (17.9 to 23.1 dB) at a depth of 3 cm when imaging through the temporal bone of the human skull.