Contrast improvement by combining pulse inversion with EMD and EEMD

Ultrasound nonlinear contrast imaging often has limited contrast because acoustic wave propagation in tissue is also nonlinear. In this study, a new ultrasound contrast imaging method which combines pulse-inversion technique with empirical mode decomposition (EMD) and the ensemble empirical mode decomposition (EEMD) in Hilbert-Huang transform (HHT) is explored to enhance the contrast between bubbles and surrounding tissues. The EMD is a method associated with HHT that allows decomposition of the data into a finite number of intrinsic modes (IMF). EEMD method further utilizes the ensemble of IMF with added white noise to better differentiate the nonlinear microbubble signal from the nonlinear tissue signal. This hypothesis was tested on pulse-inversion (PI) nonlinear imaging. Our results showed that the contrast-to-tissue ratio (CTR) in the fundamental band and the second harmonic band was improved by 10.2 and 4.3 dB after EEMD. However, echoes from contrast agent areas after decomposition may vary from the original data. By extracting microbubble component and suppressing background noise, it is demonstrated both numerically and experimentally that the EEMD can noticeably improve image contrast.