Quantitative analysis of subharmonic imaging using microbubbles in contrast imaging

Second-harmonic imaging (SecHI) has been widely used to improve the contrast of microbubbles with respect to tissue since microbubbles have a large second-harmonic response. Unlike tissue, microbubbles can also have a response at subharmonic frequency. In order to take advantage of subharmonics in contrast imaging, the image quality of subharmonic imaging (SubHI) and SecHI are analyzed through quantitative comparisons. Nonlinear tissue and bubble responses are simulated for numerical analysis. SubHI and SecHI modes are implemented on a Logiq 9 scanner (GE Healthcare, Milwaukee, WI, USA). Images of a flow phantom (ATS laboratories, CT, USA) with Sonazoid (GE Healthcare, Oslo, Norway) microbubbles are presented. The contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR) are calculated with variations of the bubble concentration and the depth of the vessel tube. The experimental results agree well with the simulations. The CTR at the subharmonic frequency can be higher than the value in second-harmonics. For deep-lying bubbles, the CTR for SubHI is 15 dB higher than for SecHI. Although the SNR at the subharmonic frequency can be lower than that at the second-harmonic frequency, it is suggested that SubHI with high blood-to-tissue contrast may be useful when the sensitivity is a major concern.