Ultrasound tissue harmonic imaging using nonlinear chirp coded excitation: in vitro study and analysis

The nonlinear chirp tissue harmonic imaging (NC) has been proposed in our group for enhancing the signal-to-noise ratio (SNR) of the harmonic imaging and increasing the frame rate twice compared to the pulse inversion harmonic imaging (PI) by using single transmission for each scanline. Since it has not yet been implemented or evaluated, we demonstrated the feasibility of the NC, evaluated its performance, and compared with other harmonic imaging methods such as the PI without chirp (PIw/oC), the PI with the linear chirp (PI-LC), and the linear chirp (LC). We performed in vitro experiments with an SC1-6 convex array transducer (Alpinion, Republic of Korea) to evaluate and compare the four methods. To our best knowledge, this is the first report on the comparison of LC and NC with a multi-elements channel system. The LC and NC signals were designed with the same bandwidth which instantaneous frequency varies linearly and nonlinearly. The NC was expected to perform better in terms of suppression of the fundamental component than the LC since the overlapped time duration between fundamental and harmonic components in NC is shorter than that of the LC. In in-vitro experiments, NC method suppresses the fundamental component by about 7dB compared with the LC and PI-LC resulting in the better axial resolution.