Phase in bubble echoes due to nonlinear response improves contrast in ultrasound images

The image quality of contrast-enhanced ultrasound (CEUS) is limited by the tissue harmonics generated from the nonlinear propagation of sound waves, even at low amplitudes. Amplitude modulation (AM) is typically implemented by firing a full aperture and two complimentary half apertures. A phase difference (ΔΦΑΜ) between the echoes from the full and half pulse has been observed which is lower for tissue than bubble echoes. This phase difference may be used as a segmentation criterion for tissues and bubbles to improve contrast to tissue ratio (CTR) of CEUS images. This technique may be further expanded by considering also the phase difference (ΔΦhalf) between the complementary half amplitude pulses of the AM scheme (ΔΦhalf). This additional phase difference results from the spatial differences of the complementary half amplitude fields. We considered the change in ΔΦhalf when using various aperture patterns for AM. We observed that ΔΦhalf increases as the complementary fields become spatially dissimilar. For certain aperture patterns, we found that the phase difference between the echoes of the complementary apertures was greater than that between the full and half pulse (ΔΦhalf i> ΔΦΑΜ) and that it further contributed to the image CTR.